Commit 895267c916

mlugg <mlugg@mlugg.co.uk>
2024-08-12 00:16:06
frontend: incremental progress
This commit makes more progress towards incremental compilation, fixing some crashes in the frontend. Notably, it fixes the regressions introduced by #20964. It also cleans up the "outdated file root" mechanism, by virtue of deleting it: we now detect outdated file roots just after updating ZIR refs, and re-scan their namespaces.
1 parent 2b05e85
src/codegen/c.zig
@@ -2585,7 +2585,7 @@ pub fn genTypeDecl(
                 const ty = Type.fromInterned(index);
                 _ = try renderTypePrefix(.flush, global_ctype_pool, zcu, writer, global_ctype, .suffix, .{});
                 try writer.writeByte(';');
-                const file_scope = ty.typeDeclInstAllowGeneratedTag(zcu).?.resolveFull(ip).file;
+                const file_scope = ty.typeDeclInstAllowGeneratedTag(zcu).?.resolveFile(ip);
                 if (!zcu.fileByIndex(file_scope).mod.strip) try writer.print(" /* {} */", .{
                     ty.containerTypeName(ip).fmt(ip),
                 });
src/codegen/llvm.zig
@@ -1959,7 +1959,7 @@ pub const Object = struct {
                     );
                 }
 
-                const file = try o.getDebugFile(ty.typeDeclInstAllowGeneratedTag(zcu).?.resolveFull(ip).file);
+                const file = try o.getDebugFile(ty.typeDeclInstAllowGeneratedTag(zcu).?.resolveFile(ip));
                 const scope = if (ty.getParentNamespace(zcu).unwrap()) |parent_namespace|
                     try o.namespaceToDebugScope(parent_namespace)
                 else
@@ -2137,7 +2137,7 @@ pub const Object = struct {
                 const name = try o.allocTypeName(ty);
                 defer gpa.free(name);
 
-                const file = try o.getDebugFile(ty.typeDeclInstAllowGeneratedTag(zcu).?.resolveFull(ip).file);
+                const file = try o.getDebugFile(ty.typeDeclInstAllowGeneratedTag(zcu).?.resolveFile(ip));
                 const scope = if (ty.getParentNamespace(zcu).unwrap()) |parent_namespace|
                     try o.namespaceToDebugScope(parent_namespace)
                 else
@@ -2772,7 +2772,7 @@ pub const Object = struct {
     fn makeEmptyNamespaceDebugType(o: *Object, ty: Type) !Builder.Metadata {
         const zcu = o.pt.zcu;
         const ip = &zcu.intern_pool;
-        const file = try o.getDebugFile(ty.typeDeclInstAllowGeneratedTag(zcu).?.resolveFull(ip).file);
+        const file = try o.getDebugFile(ty.typeDeclInstAllowGeneratedTag(zcu).?.resolveFile(ip));
         const scope = if (ty.getParentNamespace(zcu).unwrap()) |parent_namespace|
             try o.namespaceToDebugScope(parent_namespace)
         else
src/Zcu/PerThread.zig
@@ -39,7 +39,6 @@ pub fn astGenFile(
     pt: Zcu.PerThread,
     file: *Zcu.File,
     path_digest: Cache.BinDigest,
-    old_root_type: InternPool.Index,
 ) !void {
     dev.check(.ast_gen);
     assert(!file.mod.isBuiltin());
@@ -299,25 +298,15 @@ pub fn astGenFile(
         file.status = .astgen_failure;
         return error.AnalysisFail;
     }
-
-    if (old_root_type != .none) {
-        // The root of this file must be re-analyzed, since the file has changed.
-        comp.mutex.lock();
-        defer comp.mutex.unlock();
-
-        log.debug("outdated file root type: {}", .{old_root_type});
-        try zcu.outdated_file_root.put(gpa, old_root_type, {});
-    }
 }
 
 const UpdatedFile = struct {
-    file_index: Zcu.File.Index,
     file: *Zcu.File,
     inst_map: std.AutoHashMapUnmanaged(Zir.Inst.Index, Zir.Inst.Index),
 };
 
-fn cleanupUpdatedFiles(gpa: Allocator, updated_files: *std.ArrayListUnmanaged(UpdatedFile)) void {
-    for (updated_files.items) |*elem| elem.inst_map.deinit(gpa);
+fn cleanupUpdatedFiles(gpa: Allocator, updated_files: *std.AutoArrayHashMapUnmanaged(Zcu.File.Index, UpdatedFile)) void {
+    for (updated_files.values()) |*elem| elem.inst_map.deinit(gpa);
     updated_files.deinit(gpa);
 }
 
@@ -328,143 +317,166 @@ pub fn updateZirRefs(pt: Zcu.PerThread) Allocator.Error!void {
     const gpa = zcu.gpa;
 
     // We need to visit every updated File for every TrackedInst in InternPool.
-    var updated_files: std.ArrayListUnmanaged(UpdatedFile) = .{};
+    var updated_files: std.AutoArrayHashMapUnmanaged(Zcu.File.Index, UpdatedFile) = .{};
     defer cleanupUpdatedFiles(gpa, &updated_files);
     for (zcu.import_table.values()) |file_index| {
         const file = zcu.fileByIndex(file_index);
         const old_zir = file.prev_zir orelse continue;
         const new_zir = file.zir;
-        try updated_files.append(gpa, .{
-            .file_index = file_index,
+        const gop = try updated_files.getOrPut(gpa, file_index);
+        assert(!gop.found_existing);
+        gop.value_ptr.* = .{
             .file = file,
             .inst_map = .{},
-        });
-        const inst_map = &updated_files.items[updated_files.items.len - 1].inst_map;
-        try Zcu.mapOldZirToNew(gpa, old_zir.*, new_zir, inst_map);
+        };
+        if (!new_zir.hasCompileErrors()) {
+            try Zcu.mapOldZirToNew(gpa, old_zir.*, file.zir, &gop.value_ptr.inst_map);
+        }
     }
 
-    if (updated_files.items.len == 0)
+    if (updated_files.count() == 0)
         return;
 
     for (ip.locals, 0..) |*local, tid| {
         const tracked_insts_list = local.getMutableTrackedInsts(gpa);
-        for (tracked_insts_list.view().items(.@"0"), 0..) |*tracked_inst, tracked_inst_unwrapped_index| {
-            for (updated_files.items) |updated_file| {
-                const file_index = updated_file.file_index;
-                if (tracked_inst.file != file_index) continue;
-
-                const file = updated_file.file;
-                const old_zir = file.prev_zir.?.*;
-                const new_zir = file.zir;
-                const old_tag = old_zir.instructions.items(.tag);
-                const old_data = old_zir.instructions.items(.data);
-                const inst_map = &updated_file.inst_map;
-
-                const old_inst = tracked_inst.inst;
-                const tracked_inst_index = (InternPool.TrackedInst.Index.Unwrapped{
-                    .tid = @enumFromInt(tid),
-                    .index = @intCast(tracked_inst_unwrapped_index),
-                }).wrap(ip);
-                tracked_inst.inst = inst_map.get(old_inst) orelse {
-                    // Tracking failed for this instruction. Invalidate associated `src_hash` deps.
-                    log.debug("tracking failed for %{d}", .{old_inst});
-                    try zcu.markDependeeOutdated(.{ .src_hash = tracked_inst_index });
-                    continue;
-                };
+        for (tracked_insts_list.viewAllowEmpty().items(.@"0"), 0..) |*tracked_inst, tracked_inst_unwrapped_index| {
+            const file_index = tracked_inst.file;
+            const updated_file = updated_files.get(file_index) orelse continue;
 
-                if (old_zir.getAssociatedSrcHash(old_inst)) |old_hash| hash_changed: {
-                    if (new_zir.getAssociatedSrcHash(tracked_inst.inst)) |new_hash| {
-                        if (std.zig.srcHashEql(old_hash, new_hash)) {
-                            break :hash_changed;
-                        }
-                        log.debug("hash for (%{d} -> %{d}) changed: {} -> {}", .{
-                            old_inst,
-                            tracked_inst.inst,
-                            std.fmt.fmtSliceHexLower(&old_hash),
-                            std.fmt.fmtSliceHexLower(&new_hash),
-                        });
+            const file = updated_file.file;
+
+            if (file.zir.hasCompileErrors()) {
+                // If we mark this as outdated now, users of this inst will just get a transitive analysis failure.
+                // Ultimately, they would end up throwing out potentially useful analysis results.
+                // So, do nothing. We already have the file failure -- that's sufficient for now!
+                continue;
+            }
+            const old_inst = tracked_inst.inst.unwrap() orelse continue; // we can't continue tracking lost insts
+            const tracked_inst_index = (InternPool.TrackedInst.Index.Unwrapped{
+                .tid = @enumFromInt(tid),
+                .index = @intCast(tracked_inst_unwrapped_index),
+            }).wrap(ip);
+            const new_inst = updated_file.inst_map.get(old_inst) orelse {
+                // Tracking failed for this instruction. Invalidate associated `src_hash` deps.
+                log.debug("tracking failed for %{d}", .{old_inst});
+                tracked_inst.inst = .lost;
+                try zcu.markDependeeOutdated(.{ .src_hash = tracked_inst_index });
+                continue;
+            };
+            tracked_inst.inst = InternPool.TrackedInst.MaybeLost.ZirIndex.wrap(new_inst);
+
+            const old_zir = file.prev_zir.?.*;
+            const new_zir = file.zir;
+            const old_tag = old_zir.instructions.items(.tag);
+            const old_data = old_zir.instructions.items(.data);
+
+            if (old_zir.getAssociatedSrcHash(old_inst)) |old_hash| hash_changed: {
+                if (new_zir.getAssociatedSrcHash(new_inst)) |new_hash| {
+                    if (std.zig.srcHashEql(old_hash, new_hash)) {
+                        break :hash_changed;
                     }
-                    // The source hash associated with this instruction changed - invalidate relevant dependencies.
-                    try zcu.markDependeeOutdated(.{ .src_hash = tracked_inst_index });
+                    log.debug("hash for (%{d} -> %{d}) changed: {} -> {}", .{
+                        old_inst,
+                        new_inst,
+                        std.fmt.fmtSliceHexLower(&old_hash),
+                        std.fmt.fmtSliceHexLower(&new_hash),
+                    });
                 }
+                // The source hash associated with this instruction changed - invalidate relevant dependencies.
+                try zcu.markDependeeOutdated(.{ .src_hash = tracked_inst_index });
+            }
 
-                // If this is a `struct_decl` etc, we must invalidate any outdated namespace dependencies.
-                const has_namespace = switch (old_tag[@intFromEnum(old_inst)]) {
-                    .extended => switch (old_data[@intFromEnum(old_inst)].extended.opcode) {
-                        .struct_decl, .union_decl, .opaque_decl, .enum_decl => true,
-                        else => false,
-                    },
+            // If this is a `struct_decl` etc, we must invalidate any outdated namespace dependencies.
+            const has_namespace = switch (old_tag[@intFromEnum(old_inst)]) {
+                .extended => switch (old_data[@intFromEnum(old_inst)].extended.opcode) {
+                    .struct_decl, .union_decl, .opaque_decl, .enum_decl => true,
                     else => false,
-                };
-                if (!has_namespace) continue;
-
-                var old_names: std.AutoArrayHashMapUnmanaged(InternPool.NullTerminatedString, void) = .{};
-                defer old_names.deinit(zcu.gpa);
-                {
-                    var it = old_zir.declIterator(old_inst);
-                    while (it.next()) |decl_inst| {
-                        const decl_name = old_zir.getDeclaration(decl_inst)[0].name;
-                        switch (decl_name) {
-                            .@"comptime", .@"usingnamespace", .unnamed_test, .decltest => continue,
-                            _ => if (decl_name.isNamedTest(old_zir)) continue,
-                        }
-                        const name_zir = decl_name.toString(old_zir).?;
-                        const name_ip = try zcu.intern_pool.getOrPutString(
-                            zcu.gpa,
-                            pt.tid,
-                            old_zir.nullTerminatedString(name_zir),
-                            .no_embedded_nulls,
-                        );
-                        try old_names.put(zcu.gpa, name_ip, {});
+                },
+                else => false,
+            };
+            if (!has_namespace) continue;
+
+            var old_names: std.AutoArrayHashMapUnmanaged(InternPool.NullTerminatedString, void) = .{};
+            defer old_names.deinit(zcu.gpa);
+            {
+                var it = old_zir.declIterator(old_inst);
+                while (it.next()) |decl_inst| {
+                    const decl_name = old_zir.getDeclaration(decl_inst)[0].name;
+                    switch (decl_name) {
+                        .@"comptime", .@"usingnamespace", .unnamed_test, .decltest => continue,
+                        _ => if (decl_name.isNamedTest(old_zir)) continue,
                     }
+                    const name_zir = decl_name.toString(old_zir).?;
+                    const name_ip = try zcu.intern_pool.getOrPutString(
+                        zcu.gpa,
+                        pt.tid,
+                        old_zir.nullTerminatedString(name_zir),
+                        .no_embedded_nulls,
+                    );
+                    try old_names.put(zcu.gpa, name_ip, {});
                 }
-                var any_change = false;
-                {
-                    var it = new_zir.declIterator(tracked_inst.inst);
-                    while (it.next()) |decl_inst| {
-                        const decl_name = new_zir.getDeclaration(decl_inst)[0].name;
-                        switch (decl_name) {
-                            .@"comptime", .@"usingnamespace", .unnamed_test, .decltest => continue,
-                            _ => if (decl_name.isNamedTest(new_zir)) continue,
-                        }
-                        const name_zir = decl_name.toString(new_zir).?;
-                        const name_ip = try zcu.intern_pool.getOrPutString(
-                            zcu.gpa,
-                            pt.tid,
-                            new_zir.nullTerminatedString(name_zir),
-                            .no_embedded_nulls,
-                        );
-                        if (!old_names.swapRemove(name_ip)) continue;
-                        // Name added
-                        any_change = true;
-                        try zcu.markDependeeOutdated(.{ .namespace_name = .{
-                            .namespace = tracked_inst_index,
-                            .name = name_ip,
-                        } });
+            }
+            var any_change = false;
+            {
+                var it = new_zir.declIterator(new_inst);
+                while (it.next()) |decl_inst| {
+                    const decl_name = new_zir.getDeclaration(decl_inst)[0].name;
+                    switch (decl_name) {
+                        .@"comptime", .@"usingnamespace", .unnamed_test, .decltest => continue,
+                        _ => if (decl_name.isNamedTest(new_zir)) continue,
                     }
-                }
-                // The only elements remaining in `old_names` now are any names which were removed.
-                for (old_names.keys()) |name_ip| {
+                    const name_zir = decl_name.toString(new_zir).?;
+                    const name_ip = try zcu.intern_pool.getOrPutString(
+                        zcu.gpa,
+                        pt.tid,
+                        new_zir.nullTerminatedString(name_zir),
+                        .no_embedded_nulls,
+                    );
+                    if (!old_names.swapRemove(name_ip)) continue;
+                    // Name added
                     any_change = true;
                     try zcu.markDependeeOutdated(.{ .namespace_name = .{
                         .namespace = tracked_inst_index,
                         .name = name_ip,
                     } });
                 }
+            }
+            // The only elements remaining in `old_names` now are any names which were removed.
+            for (old_names.keys()) |name_ip| {
+                any_change = true;
+                try zcu.markDependeeOutdated(.{ .namespace_name = .{
+                    .namespace = tracked_inst_index,
+                    .name = name_ip,
+                } });
+            }
 
-                if (any_change) {
-                    try zcu.markDependeeOutdated(.{ .namespace = tracked_inst_index });
-                }
+            if (any_change) {
+                try zcu.markDependeeOutdated(.{ .namespace = tracked_inst_index });
             }
         }
     }
 
-    for (updated_files.items) |updated_file| {
+    try ip.rehashTrackedInsts(gpa, pt.tid);
+
+    for (updated_files.keys(), updated_files.values()) |file_index, updated_file| {
         const file = updated_file.file;
-        const prev_zir = file.prev_zir.?;
-        file.prev_zir = null;
-        prev_zir.deinit(gpa);
-        gpa.destroy(prev_zir);
+        if (file.zir.hasCompileErrors()) {
+            // Keep `prev_zir` around: it's the last non-error ZIR.
+            // Don't update the namespace, as we have no new data to update *to*.
+        } else {
+            const prev_zir = file.prev_zir.?;
+            file.prev_zir = null;
+            prev_zir.deinit(gpa);
+            gpa.destroy(prev_zir);
+
+            // For every file which has changed, re-scan the namespace of the file's root struct type.
+            // These types are special-cased because they don't have an enclosing declaration which will
+            // be re-analyzed (causing the struct's namespace to be re-scanned). It's fine to do this
+            // now because this work is fast (no actual Sema work is happening, we're just updating the
+            // namespace contents). We must do this after updating ZIR refs above, since `scanNamespace`
+            // will track some instructions.
+            try pt.updateFileNamespace(file_index);
+        }
     }
 }
 
@@ -473,6 +485,8 @@ pub fn updateZirRefs(pt: Zcu.PerThread) Allocator.Error!void {
 pub fn ensureFileAnalyzed(pt: Zcu.PerThread, file_index: Zcu.File.Index) Zcu.SemaError!void {
     const file_root_type = pt.zcu.fileRootType(file_index);
     if (file_root_type != .none) {
+        // The namespace is already up-to-date thanks to the `updateFileNamespace` calls at the
+        // start of this update. We just have to check whether the type itself is okay!
         const file_root_type_cau = pt.zcu.intern_pool.loadStructType(file_root_type).cau.unwrap().?;
         return pt.ensureCauAnalyzed(file_root_type_cau);
     } else {
@@ -493,7 +507,6 @@ pub fn ensureCauAnalyzed(pt: Zcu.PerThread, cau_index: InternPool.Cau.Index) Zcu
 
     const anal_unit = InternPool.AnalUnit.wrap(.{ .cau = cau_index });
     const cau = ip.getCau(cau_index);
-    const inst_info = cau.zir_index.resolveFull(ip);
 
     log.debug("ensureCauAnalyzed {d}", .{@intFromEnum(cau_index)});
 
@@ -516,12 +529,9 @@ pub fn ensureCauAnalyzed(pt: Zcu.PerThread, cau_index: InternPool.Cau.Index) Zcu
         _ = zcu.outdated_ready.swapRemove(anal_unit);
     }
 
-    // TODO: this only works if namespace lookups in Sema trigger `ensureCauAnalyzed`, because
-    // `outdated_file_root` information is not "viral", so we need that a namespace lookup first
-    // handles the case where the file root is not an outdated *type* but does have an outdated
-    // *namespace*. A more logically simple alternative may be for a file's root struct to register
-    // a dependency on the file's entire source code (hash). Alternatively, we could make sure that
-    // these are always handled first in an update. Actually, that's probably the best option.
+    const inst_info = cau.zir_index.resolveFull(ip) orelse return error.AnalysisFail;
+
+    // TODO: document this elsewhere mlugg!
     // For my own benefit, here's how a namespace update for a normal (non-file-root) type works:
     // `const S = struct { ... };`
     // We are adding or removing a declaration within this `struct`.
@@ -535,16 +545,12 @@ pub fn ensureCauAnalyzed(pt: Zcu.PerThread, cau_index: InternPool.Cau.Index) Zcu
     //   * we basically do `scanDecls`, updating the namespace as needed
     //   * TODO: optimize this to make sure we only do it once a generation i guess?
     // * so everyone lived happily ever after
-    const file_root_outdated = switch (cau.owner.unwrap()) {
-        .type => |ty| zcu.outdated_file_root.swapRemove(ty),
-        .nav, .none => false,
-    };
 
     if (zcu.fileByIndex(inst_info.file).status != .success_zir) {
         return error.AnalysisFail;
     }
 
-    if (!cau_outdated and !file_root_outdated) {
+    if (!cau_outdated) {
         // We can trust the current information about this `Cau`.
         if (zcu.failed_analysis.contains(anal_unit) or zcu.transitive_failed_analysis.contains(anal_unit)) {
             return error.AnalysisFail;
@@ -571,10 +577,13 @@ pub fn ensureCauAnalyzed(pt: Zcu.PerThread, cau_index: InternPool.Cau.Index) Zcu
 
     const sema_result: SemaCauResult = res: {
         if (inst_info.inst == .main_struct_inst) {
-            const changed = try pt.semaFileUpdate(inst_info.file, cau_outdated);
+            // Note that this is definitely a *recreation* due to outdated, because
+            // this instruction indicates that `cau.owner` is a `type`, which only
+            // reaches here if `cau_outdated`.
+            try pt.recreateFileRoot(inst_info.file);
             break :res .{
-                .invalidate_decl_val = changed,
-                .invalidate_decl_ref = changed,
+                .invalidate_decl_val = true,
+                .invalidate_decl_ref = true,
             };
         }
 
@@ -690,8 +699,8 @@ pub fn ensureFuncBodyAnalyzed(pt: Zcu.PerThread, maybe_coerced_func_index: Inter
         zcu.potentially_outdated.swapRemove(anal_unit);
 
     if (func_outdated) {
-        dev.check(.incremental);
         _ = zcu.outdated_ready.swapRemove(anal_unit);
+        dev.check(.incremental);
         zcu.deleteUnitExports(anal_unit);
         zcu.deleteUnitReferences(anal_unit);
     }
@@ -920,12 +929,9 @@ fn createFileRootStruct(
     return wip_ty.finish(ip, new_cau_index.toOptional(), namespace_index);
 }
 
-/// Re-analyze the root type of a file on an incremental update.
-/// If `type_outdated`, the struct type itself is considered outdated and is
-/// reconstructed at a new InternPool index. Otherwise, the namespace is just
-/// re-analyzed. Returns whether the decl's tyval was invalidated.
-/// Returns `error.AnalysisFail` if the file has an error.
-fn semaFileUpdate(pt: Zcu.PerThread, file_index: Zcu.File.Index, type_outdated: bool) Zcu.SemaError!bool {
+/// Recreate the root type of a file after it becomes outdated. A new struct type
+/// is constructed at a new InternPool index, reusing the namespace for efficiency.
+fn recreateFileRoot(pt: Zcu.PerThread, file_index: Zcu.File.Index) Zcu.SemaError!void {
     const zcu = pt.zcu;
     const ip = &zcu.intern_pool;
     const file = zcu.fileByIndex(file_index);
@@ -934,48 +940,58 @@ fn semaFileUpdate(pt: Zcu.PerThread, file_index: Zcu.File.Index, type_outdated:
 
     assert(file_root_type != .none);
 
-    log.debug("semaFileUpdate mod={s} sub_file_path={s} type_outdated={}", .{
+    log.debug("recreateFileRoot mod={s} sub_file_path={s}", .{
         file.mod.fully_qualified_name,
         file.sub_file_path,
-        type_outdated,
     });
 
     if (file.status != .success_zir) {
         return error.AnalysisFail;
     }
 
-    if (type_outdated) {
-        // Invalidate the existing type, reusing its namespace.
-        const file_root_type_cau = ip.loadStructType(file_root_type).cau.unwrap().?;
-        ip.removeDependenciesForDepender(
-            zcu.gpa,
-            InternPool.AnalUnit.wrap(.{ .cau = file_root_type_cau }),
-        );
-        ip.remove(pt.tid, file_root_type);
-        _ = try pt.createFileRootStruct(file_index, namespace_index);
-        return true;
-    }
-
-    // Only the struct's namespace is outdated.
-    // Preserve the type - just scan the namespace again.
+    // Invalidate the existing type, reusing its namespace.
+    const file_root_type_cau = ip.loadStructType(file_root_type).cau.unwrap().?;
+    ip.removeDependenciesForDepender(
+        zcu.gpa,
+        InternPool.AnalUnit.wrap(.{ .cau = file_root_type_cau }),
+    );
+    ip.remove(pt.tid, file_root_type);
+    _ = try pt.createFileRootStruct(file_index, namespace_index);
+}
 
-    const extended = file.zir.instructions.items(.data)[@intFromEnum(Zir.Inst.Index.main_struct_inst)].extended;
-    const small: Zir.Inst.StructDecl.Small = @bitCast(extended.small);
+/// Re-scan the namespace of a file's root struct type on an incremental update.
+/// The file must have successfully populated ZIR.
+/// If the file's root struct type is not populated (the file is unreferenced), nothing is done.
+/// This is called by `updateZirRefs` for all updated files before the main work loop.
+/// This function does not perform any semantic analysis.
+fn updateFileNamespace(pt: Zcu.PerThread, file_index: Zcu.File.Index) Allocator.Error!void {
+    const zcu = pt.zcu;
 
-    var extra_index: usize = extended.operand + @typeInfo(Zir.Inst.StructDecl).Struct.fields.len;
-    extra_index += @intFromBool(small.has_fields_len);
-    const decls_len = if (small.has_decls_len) blk: {
-        const decls_len = file.zir.extra[extra_index];
-        extra_index += 1;
-        break :blk decls_len;
-    } else 0;
-    const decls = file.zir.bodySlice(extra_index, decls_len);
+    const file = zcu.fileByIndex(file_index);
+    assert(file.status == .success_zir);
+    const file_root_type = zcu.fileRootType(file_index);
+    if (file_root_type == .none) return;
 
-    if (!type_outdated) {
-        try pt.scanNamespace(namespace_index, decls);
-    }
+    log.debug("updateFileNamespace mod={s} sub_file_path={s}", .{
+        file.mod.fully_qualified_name,
+        file.sub_file_path,
+    });
 
-    return false;
+    const namespace_index = Type.fromInterned(file_root_type).getNamespaceIndex(zcu);
+    const decls = decls: {
+        const extended = file.zir.instructions.items(.data)[@intFromEnum(Zir.Inst.Index.main_struct_inst)].extended;
+        const small: Zir.Inst.StructDecl.Small = @bitCast(extended.small);
+
+        var extra_index: usize = extended.operand + @typeInfo(Zir.Inst.StructDecl).Struct.fields.len;
+        extra_index += @intFromBool(small.has_fields_len);
+        const decls_len = if (small.has_decls_len) blk: {
+            const decls_len = file.zir.extra[extra_index];
+            extra_index += 1;
+            break :blk decls_len;
+        } else 0;
+        break :decls file.zir.bodySlice(extra_index, decls_len);
+    };
+    try pt.scanNamespace(namespace_index, decls);
 }
 
 /// Regardless of the file status, will create a `Decl` if none exists so that we can track
@@ -1052,7 +1068,7 @@ fn semaCau(pt: Zcu.PerThread, cau_index: InternPool.Cau.Index) !SemaCauResult {
     const anal_unit = InternPool.AnalUnit.wrap(.{ .cau = cau_index });
 
     const cau = ip.getCau(cau_index);
-    const inst_info = cau.zir_index.resolveFull(ip);
+    const inst_info = cau.zir_index.resolveFull(ip) orelse return error.AnalysisFail;
     const file = zcu.fileByIndex(inst_info.file);
     const zir = file.zir;
 
@@ -1944,6 +1960,9 @@ const ScanDeclIter = struct {
                 const cau, const nav = if (existing_cau) |cau_index| cau_nav: {
                     const nav_index = ip.getCau(cau_index).owner.unwrap().nav;
                     const nav = ip.getNav(nav_index);
+                    if (nav.name != name) {
+                        std.debug.panic("'{}' vs '{}'", .{ nav.name.fmt(ip), name.fmt(ip) });
+                    }
                     assert(nav.name == name);
                     assert(nav.fqn == fqn);
                     break :cau_nav .{ cau_index, nav_index };
@@ -2011,7 +2030,7 @@ fn analyzeFnBody(pt: Zcu.PerThread, func_index: InternPool.Index) Zcu.SemaError!
 
     const anal_unit = InternPool.AnalUnit.wrap(.{ .func = func_index });
     const func = zcu.funcInfo(func_index);
-    const inst_info = func.zir_body_inst.resolveFull(ip);
+    const inst_info = func.zir_body_inst.resolveFull(ip) orelse return error.AnalysisFail;
     const file = zcu.fileByIndex(inst_info.file);
     const zir = file.zir;
 
@@ -2097,7 +2116,7 @@ fn analyzeFnBody(pt: Zcu.PerThread, func_index: InternPool.Index) Zcu.SemaError!
     };
     defer inner_block.instructions.deinit(gpa);
 
-    const fn_info = sema.code.getFnInfo(func.zirBodyInstUnordered(ip).resolve(ip));
+    const fn_info = sema.code.getFnInfo(func.zirBodyInstUnordered(ip).resolve(ip) orelse return error.AnalysisFail);
 
     // Here we are performing "runtime semantic analysis" for a function body, which means
     // we must map the parameter ZIR instructions to `arg` AIR instructions.
src/codegen.zig
@@ -98,7 +98,7 @@ pub fn generateLazyFunction(
     debug_output: DebugInfoOutput,
 ) CodeGenError!Result {
     const zcu = pt.zcu;
-    const file = Type.fromInterned(lazy_sym.ty).typeDeclInstAllowGeneratedTag(zcu).?.resolveFull(&zcu.intern_pool).file;
+    const file = Type.fromInterned(lazy_sym.ty).typeDeclInstAllowGeneratedTag(zcu).?.resolveFile(&zcu.intern_pool);
     const target = zcu.fileByIndex(file).mod.resolved_target.result;
     switch (target_util.zigBackend(target, false)) {
         else => unreachable,
src/Compilation.zig
@@ -3081,7 +3081,7 @@ pub fn totalErrorCount(comp: *Compilation) u32 {
         for (zcu.failed_analysis.keys()) |anal_unit| {
             const file_index = switch (anal_unit.unwrap()) {
                 .cau => |cau| zcu.namespacePtr(ip.getCau(cau).namespace).file_scope,
-                .func => |ip_index| zcu.funcInfo(ip_index).zir_body_inst.resolveFull(ip).file,
+                .func => |ip_index| (zcu.funcInfo(ip_index).zir_body_inst.resolveFull(ip) orelse continue).file,
             };
             if (zcu.fileByIndex(file_index).okToReportErrors()) {
                 total += 1;
@@ -3091,11 +3091,13 @@ pub fn totalErrorCount(comp: *Compilation) u32 {
             }
         }
 
-        if (zcu.intern_pool.global_error_set.getNamesFromMainThread().len > zcu.error_limit) {
-            total += 1;
+        for (zcu.failed_codegen.keys()) |nav| {
+            if (zcu.navFileScope(nav).okToReportErrors()) {
+                total += 1;
+            }
         }
 
-        for (zcu.failed_codegen.keys()) |_| {
+        if (zcu.intern_pool.global_error_set.getNamesFromMainThread().len > zcu.error_limit) {
             total += 1;
         }
     }
@@ -3114,7 +3116,13 @@ pub fn totalErrorCount(comp: *Compilation) u32 {
         }
     }
 
-    return @as(u32, @intCast(total));
+    if (comp.module) |zcu| {
+        if (total == 0 and zcu.transitive_failed_analysis.count() > 0) {
+            @panic("Transitive analysis errors, but none actually emitted");
+        }
+    }
+
+    return @intCast(total);
 }
 
 /// This function is temporally single-threaded.
@@ -3214,7 +3222,7 @@ pub fn getAllErrorsAlloc(comp: *Compilation) !ErrorBundle {
         for (zcu.failed_analysis.keys(), zcu.failed_analysis.values()) |anal_unit, error_msg| {
             const file_index = switch (anal_unit.unwrap()) {
                 .cau => |cau| zcu.namespacePtr(ip.getCau(cau).namespace).file_scope,
-                .func => |ip_index| zcu.funcInfo(ip_index).zir_body_inst.resolveFull(ip).file,
+                .func => |ip_index| (zcu.funcInfo(ip_index).zir_body_inst.resolveFull(ip) orelse continue).file,
             };
 
             // Skip errors for AnalUnits within files that had a parse failure.
@@ -3243,7 +3251,8 @@ pub fn getAllErrorsAlloc(comp: *Compilation) !ErrorBundle {
                 }
             }
         }
-        for (zcu.failed_codegen.values()) |error_msg| {
+        for (zcu.failed_codegen.keys(), zcu.failed_codegen.values()) |nav, error_msg| {
+            if (!zcu.navFileScope(nav).okToReportErrors()) continue;
             try addModuleErrorMsg(zcu, &bundle, error_msg.*, &all_references);
         }
         for (zcu.failed_exports.values()) |value| {
@@ -3608,10 +3617,9 @@ fn performAllTheWorkInner(
                     // Pre-load these things from our single-threaded context since they
                     // will be needed by the worker threads.
                     const path_digest = zcu.filePathDigest(file_index);
-                    const old_root_type = zcu.fileRootType(file_index);
                     const file = zcu.fileByIndex(file_index);
                     comp.thread_pool.spawnWgId(&astgen_wait_group, workerAstGenFile, .{
-                        comp, file, file_index, path_digest, old_root_type, zir_prog_node, &astgen_wait_group, .root,
+                        comp, file, file_index, path_digest, zir_prog_node, &astgen_wait_group, .root,
                     });
                 }
             }
@@ -3649,6 +3657,7 @@ fn performAllTheWorkInner(
         }
         try reportMultiModuleErrors(pt);
         try zcu.flushRetryableFailures();
+
         zcu.sema_prog_node = main_progress_node.start("Semantic Analysis", 0);
         zcu.codegen_prog_node = main_progress_node.start("Code Generation", 0);
     }
@@ -4283,7 +4292,6 @@ fn workerAstGenFile(
     file: *Zcu.File,
     file_index: Zcu.File.Index,
     path_digest: Cache.BinDigest,
-    old_root_type: InternPool.Index,
     prog_node: std.Progress.Node,
     wg: *WaitGroup,
     src: Zcu.AstGenSrc,
@@ -4292,7 +4300,7 @@ fn workerAstGenFile(
     defer child_prog_node.end();
 
     const pt: Zcu.PerThread = .{ .zcu = comp.module.?, .tid = @enumFromInt(tid) };
-    pt.astGenFile(file, path_digest, old_root_type) catch |err| switch (err) {
+    pt.astGenFile(file, path_digest) catch |err| switch (err) {
         error.AnalysisFail => return,
         else => {
             file.status = .retryable_failure;
@@ -4323,7 +4331,7 @@ fn workerAstGenFile(
             // `@import("builtin")` is handled specially.
             if (mem.eql(u8, import_path, "builtin")) continue;
 
-            const import_result, const imported_path_digest, const imported_root_type = blk: {
+            const import_result, const imported_path_digest = blk: {
                 comp.mutex.lock();
                 defer comp.mutex.unlock();
 
@@ -4338,8 +4346,7 @@ fn workerAstGenFile(
                     comp.appendFileSystemInput(fsi, res.file.mod.root, res.file.sub_file_path) catch continue;
                 };
                 const imported_path_digest = pt.zcu.filePathDigest(res.file_index);
-                const imported_root_type = pt.zcu.fileRootType(res.file_index);
-                break :blk .{ res, imported_path_digest, imported_root_type };
+                break :blk .{ res, imported_path_digest };
             };
             if (import_result.is_new) {
                 log.debug("AstGen of {s} has import '{s}'; queuing AstGen of {s}", .{
@@ -4350,7 +4357,7 @@ fn workerAstGenFile(
                     .import_tok = item.data.token,
                 } };
                 comp.thread_pool.spawnWgId(wg, workerAstGenFile, .{
-                    comp, import_result.file, import_result.file_index, imported_path_digest, imported_root_type, prog_node, wg, sub_src,
+                    comp, import_result.file, import_result.file_index, imported_path_digest, prog_node, wg, sub_src,
                 });
             }
         }
@@ -6443,7 +6450,8 @@ fn buildOutputFromZig(
 
     try comp.updateSubCompilation(sub_compilation, misc_task_tag, prog_node);
 
-    assert(out.* == null);
+    // Under incremental compilation, `out` may already be populated from a prior update.
+    assert(out.* == null or comp.incremental);
     out.* = try sub_compilation.toCrtFile();
 }
 
src/InternPool.zig
@@ -65,19 +65,49 @@ pub const single_threaded = builtin.single_threaded or !want_multi_threaded;
 pub const TrackedInst = extern struct {
     file: FileIndex,
     inst: Zir.Inst.Index,
-    comptime {
-        // The fields should be tightly packed. See also serialiation logic in `Compilation.saveState`.
-        assert(@sizeOf(@This()) == @sizeOf(FileIndex) + @sizeOf(Zir.Inst.Index));
-    }
+
+    pub const MaybeLost = extern struct {
+        file: FileIndex,
+        inst: ZirIndex,
+        pub const ZirIndex = enum(u32) {
+            /// Tracking failed for this ZIR instruction. Uses of it should fail.
+            lost = std.math.maxInt(u32),
+            _,
+            pub fn unwrap(inst: ZirIndex) ?Zir.Inst.Index {
+                return switch (inst) {
+                    .lost => null,
+                    _ => @enumFromInt(@intFromEnum(inst)),
+                };
+            }
+            pub fn wrap(inst: Zir.Inst.Index) ZirIndex {
+                return @enumFromInt(@intFromEnum(inst));
+            }
+        };
+        comptime {
+            // The fields should be tightly packed. See also serialiation logic in `Compilation.saveState`.
+            assert(@sizeOf(@This()) == @sizeOf(FileIndex) + @sizeOf(ZirIndex));
+        }
+    };
+
     pub const Index = enum(u32) {
         _,
-        pub fn resolveFull(tracked_inst_index: TrackedInst.Index, ip: *const InternPool) TrackedInst {
+        pub fn resolveFull(tracked_inst_index: TrackedInst.Index, ip: *const InternPool) ?TrackedInst {
             const tracked_inst_unwrapped = tracked_inst_index.unwrap(ip);
             const tracked_insts = ip.getLocalShared(tracked_inst_unwrapped.tid).tracked_insts.acquire();
-            return tracked_insts.view().items(.@"0")[tracked_inst_unwrapped.index];
+            const maybe_lost = tracked_insts.view().items(.@"0")[tracked_inst_unwrapped.index];
+            return .{
+                .file = maybe_lost.file,
+                .inst = maybe_lost.inst.unwrap() orelse return null,
+            };
         }
-        pub fn resolve(i: TrackedInst.Index, ip: *const InternPool) Zir.Inst.Index {
-            return i.resolveFull(ip).inst;
+        pub fn resolveFile(tracked_inst_index: TrackedInst.Index, ip: *const InternPool) FileIndex {
+            const tracked_inst_unwrapped = tracked_inst_index.unwrap(ip);
+            const tracked_insts = ip.getLocalShared(tracked_inst_unwrapped.tid).tracked_insts.acquire();
+            const maybe_lost = tracked_insts.view().items(.@"0")[tracked_inst_unwrapped.index];
+            return maybe_lost.file;
+        }
+        pub fn resolve(i: TrackedInst.Index, ip: *const InternPool) ?Zir.Inst.Index {
+            return (i.resolveFull(ip) orelse return null).inst;
         }
 
         pub fn toOptional(i: TrackedInst.Index) Optional {
@@ -120,7 +150,11 @@ pub fn trackZir(
     tid: Zcu.PerThread.Id,
     key: TrackedInst,
 ) Allocator.Error!TrackedInst.Index {
-    const full_hash = Hash.hash(0, std.mem.asBytes(&key));
+    const maybe_lost_key: TrackedInst.MaybeLost = .{
+        .file = key.file,
+        .inst = TrackedInst.MaybeLost.ZirIndex.wrap(key.inst),
+    };
+    const full_hash = Hash.hash(0, std.mem.asBytes(&maybe_lost_key));
     const hash: u32 = @truncate(full_hash >> 32);
     const shard = &ip.shards[@intCast(full_hash & (ip.shards.len - 1))];
     var map = shard.shared.tracked_inst_map.acquire();
@@ -132,12 +166,11 @@ pub fn trackZir(
         const entry = &map.entries[map_index];
         const index = entry.acquire().unwrap() orelse break;
         if (entry.hash != hash) continue;
-        if (std.meta.eql(index.resolveFull(ip), key)) return index;
+        if (std.meta.eql(index.resolveFull(ip) orelse continue, key)) return index;
     }
     shard.mutate.tracked_inst_map.mutex.lock();
     defer shard.mutate.tracked_inst_map.mutex.unlock();
     if (map.entries != shard.shared.tracked_inst_map.entries) {
-        shard.mutate.tracked_inst_map.len += 1;
         map = shard.shared.tracked_inst_map;
         map_mask = map.header().mask();
         map_index = hash;
@@ -147,7 +180,7 @@ pub fn trackZir(
         const entry = &map.entries[map_index];
         const index = entry.acquire().unwrap() orelse break;
         if (entry.hash != hash) continue;
-        if (std.meta.eql(index.resolveFull(ip), key)) return index;
+        if (std.meta.eql(index.resolveFull(ip) orelse continue, key)) return index;
     }
     defer shard.mutate.tracked_inst_map.len += 1;
     const local = ip.getLocal(tid);
@@ -161,7 +194,7 @@ pub fn trackZir(
             .tid = tid,
             .index = list.mutate.len,
         }).wrap(ip);
-        list.appendAssumeCapacity(.{key});
+        list.appendAssumeCapacity(.{maybe_lost_key});
         entry.release(index.toOptional());
         return index;
     }
@@ -205,12 +238,91 @@ pub fn trackZir(
         .tid = tid,
         .index = list.mutate.len,
     }).wrap(ip);
-    list.appendAssumeCapacity(.{key});
+    list.appendAssumeCapacity(.{maybe_lost_key});
     map.entries[map_index] = .{ .value = index.toOptional(), .hash = hash };
     shard.shared.tracked_inst_map.release(new_map);
     return index;
 }
 
+pub fn rehashTrackedInsts(
+    ip: *InternPool,
+    gpa: Allocator,
+    /// TODO: maybe don't take this? it doesn't actually matter, only one thread is running at this point
+    tid: Zcu.PerThread.Id,
+) Allocator.Error!void {
+    // TODO: this function doesn't handle OOM well. What should it do?
+    //       Indeed, what should anyone do when they run out of memory?
+
+    // We don't lock anything, as this function assumes that no other thread is
+    // accessing `tracked_insts`. This is necessary because we're going to be
+    // iterating the `TrackedInst`s in each `Local`, so we have to know that
+    // none will be added as we work.
+
+    // Figure out how big each shard need to be and store it in its mutate `len`.
+    for (ip.shards) |*shard| shard.mutate.tracked_inst_map.len = 0;
+    for (ip.locals) |*local| {
+        // `getMutableTrackedInsts` is okay only because no other thread is currently active.
+        // We need the `mutate` for the len.
+        for (local.getMutableTrackedInsts(gpa).viewAllowEmpty().items(.@"0")) |tracked_inst| {
+            if (tracked_inst.inst == .lost) continue; // we can ignore this one!
+            const full_hash = Hash.hash(0, std.mem.asBytes(&tracked_inst));
+            const shard = &ip.shards[@intCast(full_hash & (ip.shards.len - 1))];
+            shard.mutate.tracked_inst_map.len += 1;
+        }
+    }
+
+    const Map = Shard.Map(TrackedInst.Index.Optional);
+
+    const arena_state = &ip.getLocal(tid).mutate.arena;
+
+    // We know how big each shard must be, so ensure we have the capacity we need.
+    for (ip.shards) |*shard| {
+        const want_capacity = std.math.ceilPowerOfTwo(u32, shard.mutate.tracked_inst_map.len * 5 / 3) catch unreachable;
+        const have_capacity = shard.shared.tracked_inst_map.header().capacity; // no acquire because we hold the mutex
+        if (have_capacity >= want_capacity) {
+            @memset(shard.shared.tracked_inst_map.entries[0..have_capacity], .{ .value = .none, .hash = undefined });
+            continue;
+        }
+        var arena = arena_state.promote(gpa);
+        defer arena_state.* = arena.state;
+        const new_map_buf = try arena.allocator().alignedAlloc(
+            u8,
+            Map.alignment,
+            Map.entries_offset + want_capacity * @sizeOf(Map.Entry),
+        );
+        const new_map: Map = .{ .entries = @ptrCast(new_map_buf[Map.entries_offset..].ptr) };
+        new_map.header().* = .{ .capacity = want_capacity };
+        @memset(new_map.entries[0..want_capacity], .{ .value = .none, .hash = undefined });
+        shard.shared.tracked_inst_map.release(new_map);
+    }
+
+    // Now, actually insert the items.
+    for (ip.locals, 0..) |*local, local_tid| {
+        // `getMutableTrackedInsts` is okay only because no other thread is currently active.
+        // We need the `mutate` for the len.
+        for (local.getMutableTrackedInsts(gpa).viewAllowEmpty().items(.@"0"), 0..) |tracked_inst, local_inst_index| {
+            if (tracked_inst.inst == .lost) continue; // we can ignore this one!
+            const full_hash = Hash.hash(0, std.mem.asBytes(&tracked_inst));
+            const hash: u32 = @truncate(full_hash >> 32);
+            const shard = &ip.shards[@intCast(full_hash & (ip.shards.len - 1))];
+            const map = shard.shared.tracked_inst_map; // no acquire because we hold the mutex
+            const map_mask = map.header().mask();
+            var map_index = hash;
+            const entry = while (true) : (map_index += 1) {
+                map_index &= map_mask;
+                const entry = &map.entries[map_index];
+                if (entry.acquire() == .none) break entry;
+            };
+            const index = TrackedInst.Index.Unwrapped.wrap(.{
+                .tid = @enumFromInt(local_tid),
+                .index = @intCast(local_inst_index),
+            }, ip);
+            entry.hash = hash;
+            entry.release(index.toOptional());
+        }
+    }
+}
+
 /// Analysis Unit. Represents a single entity which undergoes semantic analysis.
 /// This is either a `Cau` or a runtime function.
 /// The LSB is used as a tag bit.
@@ -728,7 +840,7 @@ const Local = struct {
         else => @compileError("unsupported host"),
     };
     const Strings = List(struct { u8 });
-    const TrackedInsts = List(struct { TrackedInst });
+    const TrackedInsts = List(struct { TrackedInst.MaybeLost });
     const Maps = List(struct { FieldMap });
     const Caus = List(struct { Cau });
     const Navs = List(Nav.Repr);
@@ -959,6 +1071,14 @@ const Local = struct {
                     mutable.list.release(new_list);
                 }
 
+                pub fn viewAllowEmpty(mutable: Mutable) View {
+                    const capacity = mutable.list.header().capacity;
+                    return .{
+                        .bytes = mutable.list.bytes,
+                        .len = mutable.mutate.len,
+                        .capacity = capacity,
+                    };
+                }
                 pub fn view(mutable: Mutable) View {
                     const capacity = mutable.list.header().capacity;
                     assert(capacity > 0); // optimizes `MultiArrayList.Slice.items`
@@ -996,7 +1116,6 @@ const Local = struct {
             fn header(list: ListSelf) *Header {
                 return @ptrFromInt(@intFromPtr(list.bytes) - bytes_offset);
             }
-
             pub fn view(list: ListSelf) View {
                 const capacity = list.header().capacity;
                 assert(capacity > 0); // optimizes `MultiArrayList.Slice.items`
@@ -11000,7 +11119,6 @@ pub fn getOrPutTrailingString(
     shard.mutate.string_map.mutex.lock();
     defer shard.mutate.string_map.mutex.unlock();
     if (map.entries != shard.shared.string_map.entries) {
-        shard.mutate.string_map.len += 1;
         map = shard.shared.string_map;
         map_mask = map.header().mask();
         map_index = hash;
src/Sema.zig
@@ -999,7 +999,7 @@ fn analyzeBodyInner(
         // The hashmap lookup in here is a little expensive, and LLVM fails to optimize it away.
         if (build_options.enable_logging) {
             std.log.scoped(.sema_zir).debug("sema ZIR {s} %{d}", .{ sub_file_path: {
-                const file_index = block.src_base_inst.resolveFull(&zcu.intern_pool).file;
+                const file_index = block.src_base_inst.resolveFile(&zcu.intern_pool);
                 const file = zcu.fileByIndex(file_index);
                 break :sub_file_path file.sub_file_path;
             }, inst });
@@ -2873,7 +2873,7 @@ fn createTypeName(
         .anon => {}, // handled after switch
         .parent => return block.type_name_ctx,
         .func => func_strat: {
-            const fn_info = sema.code.getFnInfo(ip.funcZirBodyInst(sema.func_index).resolve(ip));
+            const fn_info = sema.code.getFnInfo(ip.funcZirBodyInst(sema.func_index).resolve(ip) orelse return error.AnalysisFail);
             const zir_tags = sema.code.instructions.items(.tag);
 
             var buf: std.ArrayListUnmanaged(u8) = .{};
@@ -5487,7 +5487,7 @@ fn failWithBadMemberAccess(
         .Enum => "enum",
         else => unreachable,
     };
-    if (agg_ty.typeDeclInst(zcu)) |inst| if (inst.resolve(ip) == .main_struct_inst) {
+    if (agg_ty.typeDeclInst(zcu)) |inst| if ((inst.resolve(ip) orelse return error.AnalysisFail) == .main_struct_inst) {
         return sema.fail(block, field_src, "root struct of file '{}' has no member named '{}'", .{
             agg_ty.fmt(pt), field_name.fmt(ip),
         });
@@ -6041,8 +6041,7 @@ fn zirCImport(sema: *Sema, parent_block: *Block, inst: Zir.Inst.Index) CompileEr
         return sema.fail(&child_block, src, "C import failed: {s}", .{@errorName(err)});
 
     const path_digest = zcu.filePathDigest(result.file_index);
-    const old_root_type = zcu.fileRootType(result.file_index);
-    pt.astGenFile(result.file, path_digest, old_root_type) catch |err|
+    pt.astGenFile(result.file, path_digest) catch |err|
         return sema.fail(&child_block, src, "C import failed: {s}", .{@errorName(err)});
 
     // TODO: register some kind of dependency on the file.
@@ -7778,7 +7777,7 @@ fn analyzeCall(
         // the AIR instructions of the callsite. The callee could be a generic function
         // which means its parameter type expressions must be resolved in order and used
         // to successively coerce the arguments.
-        const fn_info = ics.callee().code.getFnInfo(module_fn.zir_body_inst.resolve(ip));
+        const fn_info = ics.callee().code.getFnInfo(module_fn.zir_body_inst.resolve(ip) orelse return error.AnalysisFail);
         try ics.callee().inst_map.ensureSpaceForInstructions(gpa, fn_info.param_body);
 
         var arg_i: u32 = 0;
@@ -7823,7 +7822,7 @@ fn analyzeCall(
         // each of the parameters, resolving the return type and providing it to the child
         // `Sema` so that it can be used for the `ret_ptr` instruction.
         const ret_ty_inst = if (fn_info.ret_ty_body.len != 0)
-            try sema.resolveInlineBody(&child_block, fn_info.ret_ty_body, module_fn.zir_body_inst.resolve(ip))
+            try sema.resolveInlineBody(&child_block, fn_info.ret_ty_body, module_fn.zir_body_inst.resolve(ip) orelse return error.AnalysisFail)
         else
             try sema.resolveInst(fn_info.ret_ty_ref);
         const ret_ty_src: LazySrcLoc = .{ .base_node_inst = module_fn.zir_body_inst, .offset = .{ .node_offset_fn_type_ret_ty = 0 } };
@@ -8210,7 +8209,7 @@ fn instantiateGenericCall(
     const fn_nav = ip.getNav(generic_owner_func.owner_nav);
     const fn_cau = ip.getCau(fn_nav.analysis_owner.unwrap().?);
     const fn_zir = zcu.namespacePtr(fn_cau.namespace).fileScope(zcu).zir;
-    const fn_info = fn_zir.getFnInfo(generic_owner_func.zir_body_inst.resolve(ip));
+    const fn_info = fn_zir.getFnInfo(generic_owner_func.zir_body_inst.resolve(ip) orelse return error.AnalysisFail);
 
     const comptime_args = try sema.arena.alloc(InternPool.Index, args_info.count());
     @memset(comptime_args, .none);
@@ -9416,7 +9415,7 @@ fn zirFunc(
             break :cau generic_owner_nav.analysis_owner.unwrap().?;
         } else sema.owner.unwrap().cau;
         const fn_is_exported = exported: {
-            const decl_inst = ip.getCau(func_decl_cau).zir_index.resolve(ip);
+            const decl_inst = ip.getCau(func_decl_cau).zir_index.resolve(ip) orelse return error.AnalysisFail;
             const zir_decl = sema.code.getDeclaration(decl_inst)[0];
             break :exported zir_decl.flags.is_export;
         };
@@ -26125,7 +26124,7 @@ fn zirVarExtended(
         const addrspace_src = block.src(.{ .node_offset_var_decl_addrspace = 0 });
 
         const decl_inst, const decl_bodies = decl: {
-            const decl_inst = sema.getOwnerCauDeclInst().resolve(ip);
+            const decl_inst = sema.getOwnerCauDeclInst().resolve(ip) orelse return error.AnalysisFail;
             const zir_decl, const extra_end = sema.code.getDeclaration(decl_inst);
             break :decl .{ decl_inst, zir_decl.getBodies(extra_end, sema.code) };
         };
@@ -26354,7 +26353,7 @@ fn zirFuncFancy(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!A
                 break :decl_inst cau.zir_index;
             } else sema.getOwnerCauDeclInst(); // not an instantiation so we're analyzing a function declaration Cau
 
-            const zir_decl = sema.code.getDeclaration(decl_inst.resolve(&mod.intern_pool))[0];
+            const zir_decl = sema.code.getDeclaration(decl_inst.resolve(&mod.intern_pool) orelse return error.AnalysisFail)[0];
             if (zir_decl.flags.is_export) {
                 break :cc .C;
             }
@@ -35505,7 +35504,7 @@ fn semaBackingIntType(pt: Zcu.PerThread, struct_type: InternPool.LoadedStructTyp
         break :blk accumulator;
     };
 
-    const zir_index = struct_type.zir_index.unwrap().?.resolve(ip);
+    const zir_index = struct_type.zir_index.unwrap().?.resolve(ip) orelse return error.AnalysisFail;
     const extended = zir.instructions.items(.data)[@intFromEnum(zir_index)].extended;
     assert(extended.opcode == .struct_decl);
     const small: Zir.Inst.StructDecl.Small = @bitCast(extended.small);
@@ -36120,7 +36119,7 @@ fn semaStructFields(
     const cau_index = struct_type.cau.unwrap().?;
     const namespace_index = ip.getCau(cau_index).namespace;
     const zir = zcu.namespacePtr(namespace_index).fileScope(zcu).zir;
-    const zir_index = struct_type.zir_index.unwrap().?.resolve(ip);
+    const zir_index = struct_type.zir_index.unwrap().?.resolve(ip) orelse return error.AnalysisFail;
 
     const fields_len, const small, var extra_index = structZirInfo(zir, zir_index);
 
@@ -36343,7 +36342,7 @@ fn semaStructFieldInits(
     const cau_index = struct_type.cau.unwrap().?;
     const namespace_index = ip.getCau(cau_index).namespace;
     const zir = zcu.namespacePtr(namespace_index).fileScope(zcu).zir;
-    const zir_index = struct_type.zir_index.unwrap().?.resolve(ip);
+    const zir_index = struct_type.zir_index.unwrap().?.resolve(ip) orelse return error.AnalysisFail;
     const fields_len, const small, var extra_index = structZirInfo(zir, zir_index);
 
     var comptime_err_ret_trace = std.ArrayList(LazySrcLoc).init(gpa);
@@ -36477,7 +36476,7 @@ fn semaUnionFields(pt: Zcu.PerThread, arena: Allocator, union_ty: InternPool.Ind
     const ip = &zcu.intern_pool;
     const cau_index = union_type.cau;
     const zir = zcu.namespacePtr(union_type.namespace).fileScope(zcu).zir;
-    const zir_index = union_type.zir_index.resolve(ip);
+    const zir_index = union_type.zir_index.resolve(ip) orelse return error.AnalysisFail;
     const extended = zir.instructions.items(.data)[@intFromEnum(zir_index)].extended;
     assert(extended.opcode == .union_decl);
     const small: Zir.Inst.UnionDecl.Small = @bitCast(extended.small);
src/Type.zig
@@ -3437,7 +3437,7 @@ pub fn typeDeclSrcLine(ty: Type, zcu: *Zcu) ?u32 {
         },
         else => return null,
     };
-    const info = tracked.resolveFull(&zcu.intern_pool);
+    const info = tracked.resolveFull(&zcu.intern_pool) orelse return null;
     const file = zcu.fileByIndex(info.file);
     assert(file.zir_loaded);
     const zir = file.zir;
src/Zcu.zig
@@ -162,12 +162,6 @@ outdated: std.AutoArrayHashMapUnmanaged(AnalUnit, u32) = .{},
 /// Such `AnalUnit`s are ready for immediate re-analysis.
 /// See `findOutdatedToAnalyze` for details.
 outdated_ready: std.AutoArrayHashMapUnmanaged(AnalUnit, void) = .{},
-/// This contains a set of struct types whose corresponding `Cau` may not be in
-/// `outdated`, but are the root types of files which have updated source and
-/// thus must be re-analyzed. If such a type is only in this set, the struct type
-/// index may be preserved (only the namespace might change). If its owned `Cau`
-/// is also outdated, the struct type index must be recreated.
-outdated_file_root: std.AutoArrayHashMapUnmanaged(InternPool.Index, void) = .{},
 /// This contains a list of AnalUnit whose analysis or codegen failed, but the
 /// failure was something like running out of disk space, and trying again may
 /// succeed. On the next update, we will flush this list, marking all members of
@@ -2025,7 +2019,7 @@ pub const LazySrcLoc = struct {
     pub fn resolveBaseNode(base_node_inst: InternPool.TrackedInst.Index, zcu: *Zcu) struct { *File, Ast.Node.Index } {
         const ip = &zcu.intern_pool;
         const file_index, const zir_inst = inst: {
-            const info = base_node_inst.resolveFull(ip);
+            const info = base_node_inst.resolveFull(ip) orelse @panic("TODO: resolve source location relative to lost inst");
             break :inst .{ info.file, info.inst };
         };
         const file = zcu.fileByIndex(file_index);
@@ -2148,7 +2142,6 @@ pub fn deinit(zcu: *Zcu) void {
     zcu.potentially_outdated.deinit(gpa);
     zcu.outdated.deinit(gpa);
     zcu.outdated_ready.deinit(gpa);
-    zcu.outdated_file_root.deinit(gpa);
     zcu.retryable_failures.deinit(gpa);
 
     zcu.test_functions.deinit(gpa);
@@ -2355,8 +2348,6 @@ fn markTransitiveDependersPotentiallyOutdated(zcu: *Zcu, maybe_outdated: AnalUni
 pub fn findOutdatedToAnalyze(zcu: *Zcu) Allocator.Error!?AnalUnit {
     if (!zcu.comp.incremental) return null;
 
-    if (true) @panic("TODO: findOutdatedToAnalyze");
-
     if (zcu.outdated.count() == 0 and zcu.potentially_outdated.count() == 0) {
         log.debug("findOutdatedToAnalyze: no outdated depender", .{});
         return null;
@@ -2381,87 +2372,57 @@ pub fn findOutdatedToAnalyze(zcu: *Zcu) Allocator.Error!?AnalUnit {
         return zcu.outdated_ready.keys()[0];
     }
 
-    // Next, we will see if there is any outdated file root which was not in
-    // `outdated`. This set will be small (number of files changed in this
-    // update), so it's alright for us to just iterate here.
-    for (zcu.outdated_file_root.keys()) |file_decl| {
-        const decl_depender = AnalUnit.wrap(.{ .decl = file_decl });
-        if (zcu.outdated.contains(decl_depender)) {
-            // Since we didn't hit this in the first loop, this Decl must have
-            // pending dependencies, so is ineligible.
-            continue;
-        }
-        if (zcu.potentially_outdated.contains(decl_depender)) {
-            // This Decl's struct may or may not need to be recreated depending
-            // on whether it is outdated. If we analyzed it now, we would have
-            // to assume it was outdated and recreate it!
-            continue;
-        }
-        log.debug("findOutdatedToAnalyze: outdated file root decl '{d}'", .{file_decl});
-        return decl_depender;
-    }
-
-    // There is no single AnalUnit which is ready for re-analysis. Instead, we
-    // must assume that some Decl with PO dependencies is outdated - e.g. in the
-    // above example we arbitrarily pick one of A or B. We should select a Decl,
-    // since a Decl is definitely responsible for the loop in the dependency
-    // graph (since you can't depend on a runtime function analysis!).
+    // There is no single AnalUnit which is ready for re-analysis. Instead, we must assume that some
+    // Cau with PO dependencies is outdated -- e.g. in the above example we arbitrarily pick one of
+    // A or B. We should select a Cau, since a Cau is definitely responsible for the loop in the
+    // dependency graph (since IES dependencies can't have loops). We should also, of course, not
+    // select a Cau owned by a `comptime` declaration, since you can't depend on those!
 
-    // The choice of this Decl could have a big impact on how much total
-    // analysis we perform, since if analysis concludes its tyval is unchanged,
-    // then other PO AnalUnit may be resolved as up-to-date. To hopefully avoid
-    // doing too much work, let's find a Decl which the most things depend on -
-    // the idea is that this will resolve a lot of loops (but this is only a
-    // heuristic).
+    // The choice of this Cau could have a big impact on how much total analysis we perform, since
+    // if analysis concludes any dependencies on its result are up-to-date, then other PO AnalUnit
+    // may be resolved as up-to-date. To hopefully avoid doing too much work, let's find a Decl
+    // which the most things depend on - the idea is that this will resolve a lot of loops (but this
+    // is only a heuristic).
 
     log.debug("findOutdatedToAnalyze: no trivial ready, using heuristic; {d} outdated, {d} PO", .{
         zcu.outdated.count(),
         zcu.potentially_outdated.count(),
     });
 
-    const Decl = {};
-
-    var chosen_decl_idx: ?Decl.Index = null;
-    var chosen_decl_dependers: u32 = undefined;
-
-    for (zcu.outdated.keys()) |depender| {
-        const decl_index = switch (depender.unwrap()) {
-            .decl => |d| d,
-            .func => continue,
-        };
-
-        var n: u32 = 0;
-        var it = zcu.intern_pool.dependencyIterator(.{ .decl_val = decl_index });
-        while (it.next()) |_| n += 1;
+    const ip = &zcu.intern_pool;
 
-        if (chosen_decl_idx == null or n > chosen_decl_dependers) {
-            chosen_decl_idx = decl_index;
-            chosen_decl_dependers = n;
-        }
-    }
+    var chosen_cau: ?InternPool.Cau.Index = null;
+    var chosen_cau_dependers: u32 = undefined;
 
-    for (zcu.potentially_outdated.keys()) |depender| {
-        const decl_index = switch (depender.unwrap()) {
-            .decl => |d| d,
-            .func => continue,
-        };
+    inline for (.{ zcu.outdated.keys(), zcu.potentially_outdated.keys() }) |outdated_units| {
+        for (outdated_units) |unit| {
+            const cau = switch (unit.unwrap()) {
+                .cau => |cau| cau,
+                .func => continue, // a `func` definitely can't be causing the loop so it is a bad choice
+            };
+            const cau_owner = ip.getCau(cau).owner;
 
-        var n: u32 = 0;
-        var it = zcu.intern_pool.dependencyIterator(.{ .decl_val = decl_index });
-        while (it.next()) |_| n += 1;
+            var n: u32 = 0;
+            var it = ip.dependencyIterator(switch (cau_owner.unwrap()) {
+                .none => continue, // there can be no dependencies on this `Cau` so it is a terrible choice
+                .type => |ty| .{ .interned = ty },
+                .nav => |nav| .{ .nav_val = nav },
+            });
+            while (it.next()) |_| n += 1;
 
-        if (chosen_decl_idx == null or n > chosen_decl_dependers) {
-            chosen_decl_idx = decl_index;
-            chosen_decl_dependers = n;
+            if (chosen_cau == null or n > chosen_cau_dependers) {
+                chosen_cau = cau;
+                chosen_cau_dependers = n;
+            }
         }
     }
 
-    log.debug("findOutdatedToAnalyze: heuristic returned Decl {d} ({d} dependers)", .{
-        chosen_decl_idx.?,
-        chosen_decl_dependers,
+    log.debug("findOutdatedToAnalyze: heuristic returned Cau {d} ({d} dependers)", .{
+        @intFromEnum(chosen_cau.?),
+        chosen_cau_dependers,
     });
 
-    return AnalUnit.wrap(.{ .decl = chosen_decl_idx.? });
+    return AnalUnit.wrap(.{ .cau = chosen_cau.? });
 }
 
 /// During an incremental update, before semantic analysis, call this to flush all values from
@@ -2583,7 +2544,7 @@ pub fn mapOldZirToNew(
                     break :inst unnamed_tests.items[unnamed_test_idx];
                 },
                 _ => inst: {
-                    const name_nts = new_decl.name.toString(old_zir).?;
+                    const name_nts = new_decl.name.toString(new_zir).?;
                     const name = new_zir.nullTerminatedString(name_nts);
                     if (new_decl.name.isNamedTest(new_zir)) {
                         break :inst named_tests.get(name) orelse continue;
@@ -3093,7 +3054,7 @@ pub fn navSrcLoc(zcu: *const Zcu, nav_index: InternPool.Nav.Index) LazySrcLoc {
 
 pub fn navSrcLine(zcu: *Zcu, nav_index: InternPool.Nav.Index) u32 {
     const ip = &zcu.intern_pool;
-    const inst_info = ip.getNav(nav_index).srcInst(ip).resolveFull(ip);
+    const inst_info = ip.getNav(nav_index).srcInst(ip).resolveFull(ip).?;
     const zir = zcu.fileByIndex(inst_info.file).zir;
     const inst = zir.instructions.get(@intFromEnum(inst_info.inst));
     assert(inst.tag == .declaration);
@@ -3106,7 +3067,7 @@ pub fn navValue(zcu: *const Zcu, nav_index: InternPool.Nav.Index) Value {
 
 pub fn navFileScopeIndex(zcu: *Zcu, nav: InternPool.Nav.Index) File.Index {
     const ip = &zcu.intern_pool;
-    return ip.getNav(nav).srcInst(ip).resolveFull(ip).file;
+    return ip.getNav(nav).srcInst(ip).resolveFile(ip);
 }
 
 pub fn navFileScope(zcu: *Zcu, nav: InternPool.Nav.Index) *File {
@@ -3115,6 +3076,6 @@ pub fn navFileScope(zcu: *Zcu, nav: InternPool.Nav.Index) *File {
 
 pub fn cauFileScope(zcu: *Zcu, cau: InternPool.Cau.Index) *File {
     const ip = &zcu.intern_pool;
-    const file_index = ip.getCau(cau).zir_index.resolveFull(ip).file;
+    const file_index = ip.getCau(cau).zir_index.resolveFile(ip);
     return zcu.fileByIndex(file_index);
 }