Commit c9c080a187

Andrew Kelley <andrew@ziglang.org>
2024-09-26 23:24:18
embrace panic helpers
Introduces `std.builtin.Panic` which is a complete interface for panicking. Provide `std.debug.FormattedPanic` and `std.debug.SimplePanic` and let the user choose, or make their own.
1 parent fcfbedc
lib/compiler_rt/common.zig
@@ -75,11 +75,14 @@ pub const gnu_f16_abi = switch (builtin.cpu.arch) {
 
 pub const want_sparc_abi = builtin.cpu.arch.isSPARC();
 
-// Avoid dragging in the runtime safety mechanisms into this .o file,
-// unless we're trying to test compiler-rt.
-pub fn panic(cause: std.builtin.PanicCause, error_return_trace: ?*std.builtin.StackTrace, ret_addr: ?usize) noreturn {
+// Avoid dragging in the runtime safety mechanisms into this .o file, unless
+// we're trying to test compiler-rt.
+pub const Panic = if (builtin.is_test) std.debug.FormattedPanic else struct {};
+
+/// To be deleted after zig1.wasm is updated.
+pub fn panic(msg: []const u8, error_return_trace: ?*std.builtin.StackTrace, ret_addr: ?usize) noreturn {
     if (builtin.is_test) {
-        std.debug.defaultPanic(cause, error_return_trace, ret_addr orelse @returnAddress());
+        std.debug.defaultPanic(msg, error_return_trace, ret_addr orelse @returnAddress());
     } else {
         unreachable;
     }
lib/std/debug/FormattedPanic.zig
@@ -0,0 +1,45 @@
+//! This namespace is the default one used by the Zig compiler to emit various
+//! kinds of safety panics, due to the logic in `std.builtin.Panic`.
+//!
+//! Since Zig does not have interfaces, this file serves as an example template
+//! for users to provide their own alternative panic handling.
+//!
+//! As an alternative, see `std.debug.SimplePanic`.
+
+const std = @import("../std.zig");
+
+/// Dumps a stack trace to standard error, then aborts.
+///
+/// Explicit calls to `@panic` lower to calling this function.
+pub const call: fn ([]const u8, ?*std.builtin.StackTrace, ?usize) noreturn = std.debug.defaultPanic;
+
+pub fn sentinelMismatch(expected: anytype, found: @TypeOf(expected)) noreturn {
+    @branchHint(.cold);
+    std.debug.panicExtra(null, @returnAddress(), "sentinel mismatch: expected {any}, found {any}", .{
+        expected, found,
+    });
+}
+
+pub fn unwrapError(ert: ?*std.builtin.StackTrace, err: anyerror) noreturn {
+    @branchHint(.cold);
+    std.debug.panicExtra(ert, @returnAddress(), "attempt to unwrap error: {s}", .{@errorName(err)});
+}
+
+pub fn outOfBounds(index: usize, len: usize) noreturn {
+    @branchHint(.cold);
+    std.debug.panicExtra(null, @returnAddress(), "index out of bounds: index {d}, len {d}", .{ index, len });
+}
+
+pub fn startGreaterThanEnd(start: usize, end: usize) noreturn {
+    @branchHint(.cold);
+    std.debug.panicExtra(null, @returnAddress(), "start index {d} is larger than end index {d}", .{ start, end });
+}
+
+pub fn inactiveUnionField(active: anytype, accessed: @TypeOf(active)) noreturn {
+    @branchHint(.cold);
+    std.debug.panicExtra(null, @returnAddress(), "access of union field '{s}' while field '{s}' is active", .{
+        @tagName(accessed), @tagName(active),
+    });
+}
+
+pub const messages = std.debug.SimplePanic.messages;
lib/std/debug/SimplePanic.zig
@@ -0,0 +1,86 @@
+//! This namespace is the default one used by the Zig compiler to emit various
+//! kinds of safety panics, due to the logic in `std.builtin.Panic`.
+//!
+//! Since Zig does not have interfaces, this file serves as an example template
+//! for users to provide their own alternative panic handling.
+//!
+//! As an alternative, see `std.debug.FormattedPanic`.
+
+const std = @import("../std.zig");
+
+/// Prints the message to stderr without a newline and then traps.
+///
+/// Explicit calls to `@panic` lower to calling this function.
+pub fn call(msg: []const u8, ert: ?*std.builtin.StackTrace, ra: ?usize) noreturn {
+    @branchHint(.cold);
+    _ = ert;
+    _ = ra;
+    std.debug.lockStdErr();
+    const stderr = std.io.getStdErr();
+    stderr.writeAll(msg) catch {};
+    @trap();
+}
+
+pub fn sentinelMismatch(expected: anytype, found: @TypeOf(expected)) noreturn {
+    _ = found;
+    call("sentinel mismatch", null, null);
+}
+
+pub fn unwrapError(ert: ?*std.builtin.StackTrace, err: anyerror) noreturn {
+    _ = ert;
+    _ = err;
+    call("attempt to unwrap error", null, null);
+}
+
+pub fn outOfBounds(index: usize, len: usize) noreturn {
+    _ = index;
+    _ = len;
+    call("index out of bounds", null, null);
+}
+
+pub fn startGreaterThanEnd(start: usize, end: usize) noreturn {
+    _ = start;
+    _ = end;
+    call("start index is larger than end index", null, null);
+}
+
+pub fn inactiveUnionField(active: anytype, accessed: @TypeOf(active)) noreturn {
+    _ = accessed;
+    call("access of inactive union field", null, null);
+}
+
+pub const messages = struct {
+    pub const reached_unreachable = "reached unreachable code";
+    pub const unwrap_null = "attempt to use null value";
+    pub const cast_to_null = "cast causes pointer to be null";
+    pub const incorrect_alignment = "incorrect alignment";
+    pub const invalid_error_code = "invalid error code";
+    pub const cast_truncated_data = "integer cast truncated bits";
+    pub const negative_to_unsigned = "attempt to cast negative value to unsigned integer";
+    pub const integer_overflow = "integer overflow";
+    pub const shl_overflow = "left shift overflowed bits";
+    pub const shr_overflow = "right shift overflowed bits";
+    pub const divide_by_zero = "division by zero";
+    pub const exact_division_remainder = "exact division produced remainder";
+    pub const integer_part_out_of_bounds = "integer part of floating point value out of bounds";
+    pub const corrupt_switch = "switch on corrupt value";
+    pub const shift_rhs_too_big = "shift amount is greater than the type size";
+    pub const invalid_enum_value = "invalid enum value";
+    pub const for_len_mismatch = "for loop over objects with non-equal lengths";
+    pub const memcpy_len_mismatch = "@memcpy arguments have non-equal lengths";
+    pub const memcpy_alias = "@memcpy arguments alias";
+    pub const noreturn_returned = "'noreturn' function returned";
+
+    /// To be deleted after zig1.wasm is updated.
+    pub const inactive_union_field = "access of inactive union field";
+    /// To be deleted after zig1.wasm is updated.
+    pub const sentinel_mismatch = "sentinel mismatch";
+    /// To be deleted after zig1.wasm is updated.
+    pub const unwrap_error = "attempt to unwrap error";
+    /// To be deleted after zig1.wasm is updated.
+    pub const index_out_of_bounds = "index out of bounds";
+    /// To be deleted after zig1.wasm is updated.
+    pub const start_index_greater_than_end = "start index is larger than end index";
+    /// To be deleted after zig1.wasm is updated.
+    pub const unreach = reached_unreachable;
+};
lib/std/builtin.zig
@@ -761,11 +761,10 @@ pub const TestFn = struct {
     func: *const fn () anyerror!void,
 };
 
-/// This function type is used by the Zig language code generation and
-/// therefore must be kept in sync with the compiler implementation.
-pub const PanicFn = fn (PanicCause, ?*StackTrace, ?usize) noreturn;
+/// Deprecated, use the `Panic` namespace instead.
+pub const PanicFn = fn ([]const u8, ?*StackTrace, ?usize) noreturn;
 
-/// The entry point for auto-generated calls by the compiler.
+/// Deprecated, use the `Panic` namespace instead.
 pub const panic: PanicFn = if (@hasDecl(root, "panic"))
     root.panic
 else if (@hasDecl(root, "os") and @hasDecl(root.os, "panic"))
@@ -773,143 +772,28 @@ else if (@hasDecl(root, "os") and @hasDecl(root.os, "panic"))
 else
     std.debug.defaultPanic;
 
-/// This data structure is used by the Zig language code generation and
-/// therefore must be kept in sync with the compiler implementation.
-pub const PanicCause = union(enum) {
-    reached_unreachable,
-    unwrap_null,
-    cast_to_null,
-    incorrect_alignment,
-    invalid_error_code,
-    cast_truncated_data,
-    negative_to_unsigned,
-    integer_overflow,
-    shl_overflow,
-    shr_overflow,
-    divide_by_zero,
-    exact_division_remainder,
-    inactive_union_field: InactiveUnionField,
-    integer_part_out_of_bounds,
-    corrupt_switch,
-    shift_rhs_too_big,
-    invalid_enum_value,
-    sentinel_mismatch_usize: SentinelMismatchUsize,
-    sentinel_mismatch_other,
-    unwrap_error: anyerror,
-    index_out_of_bounds: IndexOutOfBounds,
-    start_index_greater_than_end: StartIndexGreaterThanEnd,
-    for_len_mismatch,
-    memcpy_len_mismatch,
-    memcpy_alias,
-    noreturn_returned,
-    explicit_call: []const u8,
-    sentinel_mismatch_isize: SentinelMismatchIsize,
-
-    pub const IndexOutOfBounds = struct {
-        index: usize,
-        len: usize,
-    };
-
-    pub const StartIndexGreaterThanEnd = struct {
-        start: usize,
-        end: usize,
-    };
-
-    pub const SentinelMismatchUsize = struct {
-        expected: usize,
-        found: usize,
-    };
-
-    pub const SentinelMismatchIsize = struct {
-        expected: isize,
-        found: isize,
-    };
-
-    pub const InactiveUnionField = struct {
-        active: []const u8,
-        accessed: []const u8,
-    };
-};
-
-pub fn panicSentinelMismatch(expected: anytype, found: @TypeOf(expected)) noreturn {
-    @branchHint(.cold);
-    if (builtin.zig_backend == .stage2_riscv64) {
-        // https://github.com/ziglang/zig/issues/21519
-        @trap();
-    }
-    switch (@typeInfo(@TypeOf(expected))) {
-        .int => |int| switch (int.signedness) {
-            .unsigned => if (int.bits <= @bitSizeOf(usize)) panic(.{ .sentinel_mismatch_usize = .{
-                .expected = expected,
-                .found = found,
-            } }, null, @returnAddress()),
-            .signed => if (int.bits <= @bitSizeOf(isize)) panic(.{ .sentinel_mismatch_isize = .{
-                .expected = expected,
-                .found = found,
-            } }, null, @returnAddress()),
-        },
-        .@"enum" => |info| switch (@typeInfo(info.tag_type)) {
-            .int => |int| switch (int.signedness) {
-                .unsigned => if (int.bits <= @bitSizeOf(usize)) panic(.{ .sentinel_mismatch_usize = .{
-                    .expected = @intFromEnum(expected),
-                    .found = @intFromEnum(found),
-                } }, null, @returnAddress()),
-                .signed => if (int.bits <= @bitSizeOf(isize)) panic(.{ .sentinel_mismatch_isize = .{
-                    .expected = @intFromEnum(expected),
-                    .found = @intFromEnum(found),
-                } }, null, @returnAddress()),
-            },
-            else => comptime unreachable,
-        },
-        else => {},
-    }
-    panic(.sentinel_mismatch_other, null, @returnAddress());
-}
-
-pub fn panicUnwrapError(ert: ?*StackTrace, err: anyerror) noreturn {
-    @branchHint(.cold);
-    if (builtin.zig_backend == .stage2_riscv64) {
-        // https://github.com/ziglang/zig/issues/21519
-        @trap();
-    }
-    panic(.{ .unwrap_error = err }, ert, @returnAddress());
-}
-
-pub fn panicOutOfBounds(index: usize, len: usize) noreturn {
-    @branchHint(.cold);
-    if (builtin.zig_backend == .stage2_riscv64) {
-        // https://github.com/ziglang/zig/issues/21519
-        @trap();
-    }
-    panic(.{ .index_out_of_bounds = .{
-        .index = index,
-        .len = len,
-    } }, null, @returnAddress());
-}
-
-pub fn panicStartGreaterThanEnd(start: usize, end: usize) noreturn {
-    @branchHint(.cold);
-    if (builtin.zig_backend == .stage2_riscv64) {
-        // https://github.com/ziglang/zig/issues/21519
-        @trap();
-    }
-    panic(.{ .start_index_greater_than_end = .{
-        .start = start,
-        .end = end,
-    } }, null, @returnAddress());
-}
-
-pub fn panicInactiveUnionField(active: anytype, accessed: @TypeOf(active)) noreturn {
-    @branchHint(.cold);
-    if (builtin.zig_backend == .stage2_riscv64) {
-        // https://github.com/ziglang/zig/issues/21519
-        @trap();
-    }
-    panic(.{ .inactive_union_field = .{
-        .active = @tagName(active),
-        .accessed = @tagName(accessed),
-    } }, null, @returnAddress());
-}
+/// This namespace is used by the Zig compiler to emit various kinds of safety
+/// panics. These can be overridden by making a public `Panic` namespace in the
+/// root source file.
+pub const Panic: type = if (@hasDecl(root, "Panic"))
+    root.Panic
+else if (std.builtin.zig_backend == .stage2_riscv64)
+    std.debug.SimplePanic // https://github.com/ziglang/zig/issues/21519
+else
+    std.debug.FormattedPanic;
+
+/// To be deleted after zig1.wasm is updated.
+pub const panicSentinelMismatch = Panic.sentinelMismatch;
+/// To be deleted after zig1.wasm is updated.
+pub const panicUnwrapError = Panic.unwrapError;
+/// To be deleted after zig1.wasm is updated.
+pub const panicOutOfBounds = Panic.outOfBounds;
+/// To be deleted after zig1.wasm is updated.
+pub const panicStartGreaterThanEnd = Panic.startGreaterThanEnd;
+/// To be deleted after zig1.wasm is updated.
+pub const panicInactiveUnionField = Panic.inactiveUnionField;
+/// To be deleted after zig1.wasm is updated.
+pub const panic_messages = Panic.messages;
 
 pub noinline fn returnError(st: *StackTrace) void {
     @branchHint(.unlikely);
lib/std/debug.zig
@@ -21,6 +21,9 @@ pub const SelfInfo = @import("debug/SelfInfo.zig");
 pub const Info = @import("debug/Info.zig");
 pub const Coverage = @import("debug/Coverage.zig");
 
+pub const FormattedPanic = @import("debug/FormattedPanic.zig");
+pub const SimplePanic = @import("debug/SimplePanic.zig");
+
 /// Unresolved source locations can be represented with a single `usize` that
 /// corresponds to a virtual memory address of the program counter. Combined
 /// with debug information, those values can be converted into a resolved
@@ -408,10 +411,16 @@ pub fn assertReadable(slice: []const volatile u8) void {
     for (slice) |*byte| _ = byte.*;
 }
 
+/// By including a call to this function, the caller gains an error return trace
+/// secret parameter, making `@errorReturnTrace()` more useful. This is not
+/// necessary if the function already contains a call to an errorable function
+/// elsewhere.
+pub fn errorReturnTraceHelper() anyerror!void {}
+
 /// Equivalent to `@panic` but with a formatted message.
 pub fn panic(comptime format: []const u8, args: anytype) noreturn {
     @branchHint(.cold);
-
+    errorReturnTraceHelper() catch unreachable;
     panicExtra(@errorReturnTrace(), @returnAddress(), format, args);
 }
 
@@ -437,7 +446,7 @@ pub fn panicExtra(
             break :blk &buf;
         },
     };
-    std.builtin.panic(.{ .explicit_call = msg }, trace, ret_addr);
+    std.builtin.Panic.call(msg, trace, ret_addr);
 }
 
 /// Non-zero whenever the program triggered a panic.
@@ -448,11 +457,9 @@ var panicking = std.atomic.Value(u8).init(0);
 /// This is used to catch and handle panics triggered by the panic handler.
 threadlocal var panic_stage: usize = 0;
 
-// Dumps a stack trace to standard error, then aborts.
-//
-// This function avoids a dependency on formatted printing.
+/// Dumps a stack trace to standard error, then aborts.
 pub fn defaultPanic(
-    cause: std.builtin.PanicCause,
+    msg: []const u8,
     error_return_trace: ?*const std.builtin.StackTrace,
     first_trace_addr: ?usize,
 ) noreturn {
@@ -471,18 +478,6 @@ pub fn defaultPanic(
         @trap();
     }
 
-    if (builtin.zig_backend == .stage2_riscv64) {
-        var buffer: [1000]u8 = undefined;
-        var i: usize = 0;
-        i += fmtPanicCause(buffer[i..], cause);
-        buffer[i] = '\n';
-        i += 1;
-        const msg = buffer[0..i];
-        lockStdErr();
-        io.getStdErr().writeAll(msg) catch {};
-        @trap();
-    }
-
     switch (builtin.os.tag) {
         .freestanding => {
             @trap();
@@ -490,14 +485,10 @@ pub fn defaultPanic(
         .uefi => {
             const uefi = std.os.uefi;
 
-            var buffer: [1000]u8 = undefined;
-            var i: usize = 0;
-            i += fmtBuf(buffer[i..], "panic: ");
-            i += fmtPanicCause(buffer[i..], cause);
-            i += fmtBuf(buffer[i..], "\r\n\x00");
-
             var utf16_buffer: [1000]u16 = undefined;
-            const len = std.unicode.utf8ToUtf16Le(&utf16_buffer, buffer[0..i]) catch 0;
+            const len_minus_3 = std.unicode.utf8ToUtf16Le(&utf16_buffer, msg) catch 0;
+            utf16_buffer[len_minus_3][0..3].* = .{ '\r', '\n', 0 };
+            const len = len_minus_3 + 3;
             const exit_msg = utf16_buffer[0 .. len - 1 :0];
 
             // Output to both std_err and con_out, as std_err is easier
@@ -521,15 +512,11 @@ pub fn defaultPanic(
         },
         .cuda, .amdhsa => std.posix.abort(),
         .plan9 => {
-            var buffer: [1000]u8 = undefined;
-            comptime assert(buffer.len > std.os.plan9.ERRMAX);
-            var i: usize = 0;
-            i += fmtPanicCause(buffer[i..], cause);
-            buffer[i] = '\n';
-            i += 1;
-            const len = @min(i, std.os.plan9.ERRMAX - 1);
-            buffer[len] = 0;
-            std.os.plan9.exits(buffer[0..len :0]);
+            var status: [std.os.plan9.ERRMAX]u8 = undefined;
+            const len = @min(msg.len, status.len - 1);
+            @memcpy(status[0..len], msg[0..len]);
+            status[len] = 0;
+            std.os.plan9.exits(status[0..len :0]);
         },
         else => {},
     }
@@ -548,26 +535,18 @@ pub fn defaultPanic(
             _ = panicking.fetchAdd(1, .seq_cst);
 
             {
-                // This code avoids a dependency on formatted printing, the writer interface,
-                // and limits to only 1 syscall made to print the panic message to stderr.
-                var buffer: [0x1000]u8 = undefined;
-                var i: usize = 0;
+                lockStdErr();
+                defer unlockStdErr();
+
+                const stderr = io.getStdErr().writer();
                 if (builtin.single_threaded) {
-                    i += fmtBuf(buffer[i..], "panic: ");
+                    stderr.print("panic: ", .{}) catch posix.abort();
                 } else {
-                    i += fmtBuf(buffer[i..], "thread ");
-                    i += fmtInt10(buffer[i..], std.Thread.getCurrentId());
-                    i += fmtBuf(buffer[i..], " panic: ");
+                    const current_thread_id = std.Thread.getCurrentId();
+                    stderr.print("thread {} panic: ", .{current_thread_id}) catch posix.abort();
                 }
-                i += fmtPanicCause(buffer[i..], cause);
-                buffer[i] = '\n';
-                i += 1;
-                const msg = buffer[0..i];
+                stderr.print("{s}\n", .{msg}) catch posix.abort();
 
-                lockStdErr();
-                defer unlockStdErr();
-
-                io.getStdErr().writeAll(msg) catch posix.abort();
                 if (error_return_trace) |t| dumpStackTrace(t.*);
                 dumpCurrentStackTrace(first_trace_addr orelse @returnAddress());
             }
@@ -588,108 +567,6 @@ pub fn defaultPanic(
     posix.abort();
 }
 
-pub fn fmtPanicCause(buffer: []u8, cause: std.builtin.PanicCause) usize {
-    var i: usize = 0;
-
-    switch (cause) {
-        .reached_unreachable => i += fmtBuf(buffer[i..], "reached unreachable code"),
-        .unwrap_null => i += fmtBuf(buffer[i..], "attempt to use null value"),
-        .cast_to_null => i += fmtBuf(buffer[i..], "cast causes pointer to be null"),
-        .incorrect_alignment => i += fmtBuf(buffer[i..], "incorrect alignment"),
-        .invalid_error_code => i += fmtBuf(buffer[i..], "invalid error code"),
-        .cast_truncated_data => i += fmtBuf(buffer[i..], "integer cast truncated bits"),
-        .negative_to_unsigned => i += fmtBuf(buffer[i..], "attempt to cast negative value to unsigned integer"),
-        .integer_overflow => i += fmtBuf(buffer[i..], "integer overflow"),
-        .shl_overflow => i += fmtBuf(buffer[i..], "left shift overflowed bits"),
-        .shr_overflow => i += fmtBuf(buffer[i..], "right shift overflowed bits"),
-        .divide_by_zero => i += fmtBuf(buffer[i..], "division by zero"),
-        .exact_division_remainder => i += fmtBuf(buffer[i..], "exact division produced remainder"),
-        .inactive_union_field => |info| {
-            i += fmtBuf(buffer[i..], "access of union field '");
-            i += fmtBuf(buffer[i..], info.accessed);
-            i += fmtBuf(buffer[i..], "' while field '");
-            i += fmtBuf(buffer[i..], info.active);
-            i += fmtBuf(buffer[i..], "' is active");
-        },
-        .integer_part_out_of_bounds => i += fmtBuf(buffer[i..], "integer part of floating point value out of bounds"),
-        .corrupt_switch => i += fmtBuf(buffer[i..], "switch on corrupt value"),
-        .shift_rhs_too_big => i += fmtBuf(buffer[i..], "shift amount is greater than the type size"),
-        .invalid_enum_value => i += fmtBuf(buffer[i..], "invalid enum value"),
-        .sentinel_mismatch_usize => |mm| {
-            i += fmtBuf(buffer[i..], "sentinel mismatch: expected ");
-            i += fmtInt10(buffer[i..], mm.expected);
-            i += fmtBuf(buffer[i..], ", found ");
-            i += fmtInt10(buffer[i..], mm.found);
-        },
-        .sentinel_mismatch_isize => |mm| {
-            i += fmtBuf(buffer[i..], "sentinel mismatch: expected ");
-            i += fmtInt10s(buffer[i..], mm.expected);
-            i += fmtBuf(buffer[i..], ", found ");
-            i += fmtInt10s(buffer[i..], mm.found);
-        },
-        .sentinel_mismatch_other => i += fmtBuf(buffer[i..], "sentinel mismatch"),
-        .unwrap_error => |err| {
-            if (builtin.zig_backend == .stage2_riscv64) {
-                // https://github.com/ziglang/zig/issues/21519
-                i += fmtBuf(buffer[i..], "attempt to unwrap error");
-                return i;
-            }
-            i += fmtBuf(buffer[i..], "attempt to unwrap error: ");
-            i += fmtBuf(buffer[i..], @errorName(err));
-        },
-        .index_out_of_bounds => |oob| {
-            i += fmtBuf(buffer[i..], "index ");
-            i += fmtInt10(buffer[i..], oob.index);
-            i += fmtBuf(buffer[i..], " exceeds length ");
-            i += fmtInt10(buffer[i..], oob.len);
-        },
-        .start_index_greater_than_end => |oob| {
-            i += fmtBuf(buffer[i..], "start index ");
-            i += fmtInt10(buffer[i..], oob.start);
-            i += fmtBuf(buffer[i..], " exceeds end index ");
-            i += fmtInt10(buffer[i..], oob.end);
-        },
-        .for_len_mismatch => i += fmtBuf(buffer[i..], "for loop over objects with non-equal lengths"),
-        .memcpy_len_mismatch => i += fmtBuf(buffer[i..], "@memcpy arguments have non-equal lengths"),
-        .memcpy_alias => i += fmtBuf(buffer[i..], "@memcpy arguments alias"),
-        .noreturn_returned => i += fmtBuf(buffer[i..], "'noreturn' function returned"),
-        .explicit_call => |msg| i += fmtBuf(buffer[i..], msg),
-    }
-
-    return i;
-}
-
-fn fmtBuf(out_buf: []u8, s: []const u8) usize {
-    @memcpy(out_buf[0..s.len], s);
-    return s.len;
-}
-
-fn fmtInt10s(out_buf: []u8, integer_value: isize) usize {
-    if (integer_value < 0) {
-        out_buf[0] = '-';
-        return 1 + fmtInt10(out_buf[1..], @abs(integer_value));
-    } else {
-        return fmtInt10(out_buf, @abs(integer_value));
-    }
-}
-
-fn fmtInt10(out_buf: []u8, integer_value: usize) usize {
-    var tmp_buf: [50]u8 = undefined;
-    var i: usize = tmp_buf.len;
-    var a: usize = integer_value;
-
-    while (true) {
-        i -= 1;
-        tmp_buf[i] = '0' + @as(u8, @intCast(a % 10));
-        a /= 10;
-        if (a == 0) break;
-    }
-
-    const result = tmp_buf[i..];
-    @memcpy(out_buf[0..result.len], result);
-    return result.len;
-}
-
 /// Must be called only after adding 1 to `panicking`. There are three callsites.
 fn waitForOtherThreadToFinishPanicking() void {
     if (panicking.fetchSub(1, .seq_cst) != 1) {
src/crash_report.zig
@@ -152,16 +152,12 @@ fn writeFilePath(file: *Zcu.File, writer: anytype) !void {
     try writer.writeAll(file.sub_file_path);
 }
 
-pub fn compilerPanic(
-    cause: std.builtin.PanicCause,
-    error_return_trace: ?*std.builtin.StackTrace,
-    maybe_ret_addr: ?usize,
-) noreturn {
+pub fn compilerPanic(msg: []const u8, error_return_trace: ?*std.builtin.StackTrace, maybe_ret_addr: ?usize) noreturn {
     @branchHint(.cold);
     PanicSwitch.preDispatch();
     const ret_addr = maybe_ret_addr orelse @returnAddress();
     const stack_ctx: StackContext = .{ .current = .{ .ret_addr = ret_addr } };
-    PanicSwitch.dispatch(error_return_trace, stack_ctx, cause);
+    PanicSwitch.dispatch(error_return_trace, stack_ctx, msg);
 }
 
 /// Attaches a global SIGSEGV handler
@@ -212,7 +208,7 @@ fn handleSegfaultPosix(sig: i32, info: *const posix.siginfo_t, ctx_ptr: ?*anyopa
         else => .not_supported,
     };
 
-    PanicSwitch.dispatch(null, stack_ctx, .{ .explicit_call = error_msg });
+    PanicSwitch.dispatch(null, stack_ctx, error_msg);
 }
 
 const WindowsSegfaultMessage = union(enum) {
@@ -335,7 +331,7 @@ const PanicSwitch = struct {
         // it's happening and print a message.
         var panic_state: *volatile PanicState = &panic_state_raw;
         if (panic_state.awaiting_dispatch) {
-            dispatch(null, .{ .current = .{ .ret_addr = null } }, .{ .explicit_call = "Panic while preparing callstack" });
+            dispatch(null, .{ .current = .{ .ret_addr = null } }, "Panic while preparing callstack");
         }
         panic_state.awaiting_dispatch = true;
     }
@@ -355,17 +351,17 @@ const PanicSwitch = struct {
     pub fn dispatch(
         trace: ?*const std.builtin.StackTrace,
         stack_ctx: StackContext,
-        panic_cause: std.builtin.PanicCause,
+        msg: []const u8,
     ) noreturn {
         var panic_state: *volatile PanicState = &panic_state_raw;
         debug.assert(panic_state.awaiting_dispatch);
         panic_state.awaiting_dispatch = false;
         nosuspend switch (panic_state.recover_stage) {
-            .initialize => goTo(initPanic, .{ panic_state, trace, stack_ctx, panic_cause }),
-            .report_stack => goTo(recoverReportStack, .{ panic_state, trace, stack_ctx, panic_cause }),
-            .release_mutex => goTo(recoverReleaseMutex, .{ panic_state, trace, stack_ctx, panic_cause }),
-            .release_ref_count => goTo(recoverReleaseRefCount, .{ panic_state, trace, stack_ctx, panic_cause }),
-            .abort => goTo(recoverAbort, .{ panic_state, trace, stack_ctx, panic_cause }),
+            .initialize => goTo(initPanic, .{ panic_state, trace, stack_ctx, msg }),
+            .report_stack => goTo(recoverReportStack, .{ panic_state, trace, stack_ctx, msg }),
+            .release_mutex => goTo(recoverReleaseMutex, .{ panic_state, trace, stack_ctx, msg }),
+            .release_ref_count => goTo(recoverReleaseRefCount, .{ panic_state, trace, stack_ctx, msg }),
+            .abort => goTo(recoverAbort, .{ panic_state, trace, stack_ctx, msg }),
             .silent_abort => goTo(abort, .{}),
         };
     }
@@ -374,7 +370,7 @@ const PanicSwitch = struct {
         state: *volatile PanicState,
         trace: ?*const std.builtin.StackTrace,
         stack: StackContext,
-        panic_cause: std.builtin.PanicCause,
+        msg: []const u8,
     ) noreturn {
         // use a temporary so there's only one volatile store
         const new_state = PanicState{
@@ -399,8 +395,6 @@ const PanicSwitch = struct {
             const current_thread_id = std.Thread.getCurrentId();
             stderr.print("thread {} panic: ", .{current_thread_id}) catch goTo(releaseMutex, .{state});
         }
-        var buffer: [1000]u8 = undefined;
-        const msg = buffer[0..std.debug.fmtPanicCause(&buffer, panic_cause)];
         stderr.print("{s}\n", .{msg}) catch goTo(releaseMutex, .{state});
 
         state.recover_stage = .report_stack;
@@ -416,9 +410,9 @@ const PanicSwitch = struct {
         state: *volatile PanicState,
         trace: ?*const std.builtin.StackTrace,
         stack: StackContext,
-        panic_cause: std.builtin.PanicCause,
+        msg: []const u8,
     ) noreturn {
-        recover(state, trace, stack, panic_cause);
+        recover(state, trace, stack, msg);
 
         state.recover_stage = .release_mutex;
         const stderr = io.getStdErr().writer();
@@ -441,9 +435,9 @@ const PanicSwitch = struct {
         state: *volatile PanicState,
         trace: ?*const std.builtin.StackTrace,
         stack: StackContext,
-        panic_cause: std.builtin.PanicCause,
+        msg: []const u8,
     ) noreturn {
-        recover(state, trace, stack, panic_cause);
+        recover(state, trace, stack, msg);
         goTo(releaseMutex, .{state});
     }
 
@@ -459,9 +453,9 @@ const PanicSwitch = struct {
         state: *volatile PanicState,
         trace: ?*const std.builtin.StackTrace,
         stack: StackContext,
-        panic_cause: std.builtin.PanicCause,
+        msg: []const u8,
     ) noreturn {
-        recover(state, trace, stack, panic_cause);
+        recover(state, trace, stack, msg);
         goTo(releaseRefCount, .{state});
     }
 
@@ -487,9 +481,9 @@ const PanicSwitch = struct {
         state: *volatile PanicState,
         trace: ?*const std.builtin.StackTrace,
         stack: StackContext,
-        panic_cause: std.builtin.PanicCause,
+        msg: []const u8,
     ) noreturn {
-        recover(state, trace, stack, panic_cause);
+        recover(state, trace, stack, msg);
 
         state.recover_stage = .silent_abort;
         const stderr = io.getStdErr().writer();
@@ -513,9 +507,8 @@ const PanicSwitch = struct {
         state: *volatile PanicState,
         trace: ?*const std.builtin.StackTrace,
         stack: StackContext,
-        panic_cause: std.builtin.PanicCause,
+        msg: []const u8,
     ) void {
-        var buffer: [1000]u8 = undefined;
         switch (state.recover_verbosity) {
             .message_and_stack => {
                 // lower the verbosity, and restore it at the end if we don't panic.
@@ -523,7 +516,6 @@ const PanicSwitch = struct {
 
                 const stderr = io.getStdErr().writer();
                 stderr.writeAll("\nPanicked during a panic: ") catch {};
-                const msg = buffer[0..std.debug.fmtPanicCause(&buffer, panic_cause)];
                 stderr.writeAll(msg) catch {};
                 stderr.writeAll("\nInner panic stack:\n") catch {};
                 if (trace) |t| {
@@ -538,7 +530,6 @@ const PanicSwitch = struct {
 
                 const stderr = io.getStdErr().writer();
                 stderr.writeAll("\nPanicked while dumping inner panic stack: ") catch {};
-                const msg = buffer[0..std.debug.fmtPanicCause(&buffer, panic_cause)];
                 stderr.writeAll(msg) catch {};
                 stderr.writeAll("\n") catch {};
 
src/Sema.zig
@@ -2566,7 +2566,7 @@ pub fn failWithOwnedErrorMsg(sema: *Sema, block: ?*Block, err_msg: *Zcu.ErrorMsg
         std.debug.print("compile error during Sema:\n", .{});
         var error_bundle = wip_errors.toOwnedBundle("") catch @panic("out of memory");
         error_bundle.renderToStdErr(.{ .ttyconf = .no_color });
-        crash_report.compilerPanic(.{ .explicit_call = "unexpected compile error occurred" }, null, null);
+        crash_report.compilerPanic("unexpected compile error occurred", null, null);
     }
 
     if (block) |start_block| {
@@ -5810,6 +5810,8 @@ fn zirPanic(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void
     const src = block.nodeOffset(inst_data.src_node);
     const msg_inst = try sema.resolveInst(inst_data.operand);
 
+    // `panicWithMsg` would perform this coercion for us, but we can get a better
+    // source location if we do it here.
     const coerced_msg = try sema.coerce(block, Type.slice_const_u8, msg_inst, block.builtinCallArgSrc(inst_data.src_node, 0));
 
     if (block.is_comptime) {
@@ -5822,7 +5824,7 @@ fn zirPanic(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void
         sema.branch_hint = .cold;
     }
 
-    try callPanic(sema, block, src, .explicit_call, coerced_msg, .@"@panic");
+    try sema.panicWithMsg(block, src, coerced_msg, .@"@panic");
 }
 
 fn zirTrap(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void {
@@ -7303,33 +7305,6 @@ fn callBuiltin(
     );
 }
 
-const PanicCauseTag = @typeInfo(std.builtin.PanicCause).@"union".tag_type.?;
-
-fn callPanic(
-    sema: *Sema,
-    block: *Block,
-    call_src: LazySrcLoc,
-    tag: PanicCauseTag,
-    payload: Air.Inst.Ref,
-    call_operation: CallOperation,
-) !void {
-    const pt = sema.pt;
-    const zcu = pt.zcu;
-    if (!zcu.backendSupportsFeature(.panic_fn)) {
-        _ = try block.addNoOp(.trap);
-        return;
-    }
-    const panic_cause_ty = try pt.getBuiltinType("PanicCause");
-    const panic_cause = try unionInitFromEnumTag(sema, block, call_src, panic_cause_ty, @intFromEnum(tag), payload);
-    try preparePanic(sema, block, call_src);
-    const panic_fn = Air.internedToRef(zcu.panic_func_index);
-    const err_return_trace = try sema.getErrorReturnTrace(block);
-    const opt_usize_ty = try pt.optionalType(.usize_type);
-    const null_usize = try pt.nullValue(opt_usize_ty);
-    const args: [3]Air.Inst.Ref = .{ panic_cause, err_return_trace, Air.internedToRef(null_usize.toIntern()) };
-    try sema.callBuiltin(block, call_src, panic_fn, .auto, &args, call_operation);
-}
-
 const CallOperation = enum {
     call,
     @"@call",
@@ -14233,7 +14208,11 @@ fn maybeErrorUnwrap(
             .panic => {
                 const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
                 const msg_inst = try sema.resolveInst(inst_data.operand);
-                try callPanic(sema, block, operand_src, .explicit_call, msg_inst, .@"@panic");
+
+                const panic_fn = try pt.getBuiltinInnerType("Panic", "call");
+                const err_return_trace = try sema.getErrorReturnTrace(block);
+                const args: [3]Air.Inst.Ref = .{ msg_inst, err_return_trace, .null_value };
+                try sema.callBuiltin(block, operand_src, panic_fn, .auto, &args, .@"safety check");
                 return true;
             },
             else => unreachable,
@@ -17380,7 +17359,9 @@ fn analyzeArithmetic(
 
     if (block.wantSafety() and want_safety and scalar_tag == .int) {
         if (zcu.backendSupportsFeature(.safety_checked_instructions)) {
-            if (air_tag != air_tag_safe) try sema.preparePanicIntegerOverflow(block, src);
+            if (air_tag != air_tag_safe) {
+                _ = try sema.preparePanicId(block, src, .integer_overflow);
+            }
             return block.addBinOp(air_tag_safe, casted_lhs, casted_rhs);
         } else {
             const maybe_op_ov: ?Air.Inst.Tag = switch (air_tag) {
@@ -21683,16 +21664,6 @@ fn zirTagName(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Air
     const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
     const src = block.nodeOffset(inst_data.src_node);
     const operand = try sema.resolveInst(inst_data.operand);
-    return analyzeTagName(sema, block, src, operand_src, operand);
-}
-
-fn analyzeTagName(
-    sema: *Sema,
-    block: *Block,
-    src: LazySrcLoc,
-    operand_src: LazySrcLoc,
-    operand: Air.Inst.Ref,
-) CompileError!Air.Inst.Ref {
     const operand_ty = sema.typeOf(operand);
     const pt = sema.pt;
     const zcu = pt.zcu;
@@ -27663,7 +27634,7 @@ fn explainWhyTypeIsNotPacked(
     }
 }
 
-fn preparePanic(sema: *Sema, block: *Block, src: LazySrcLoc) !void {
+fn prepareSimplePanic(sema: *Sema, block: *Block, src: LazySrcLoc) !void {
     const pt = sema.pt;
     const zcu = pt.zcu;
 
@@ -27694,30 +27665,33 @@ fn preparePanic(sema: *Sema, block: *Block, src: LazySrcLoc) !void {
             .val = .none,
         } });
     }
-
-    if (zcu.panic_cause_type == .none) {
-        const panic_cause_ty = try pt.getBuiltinType("PanicCause");
-        try panic_cause_ty.resolveFields(pt);
-        zcu.panic_cause_type = panic_cause_ty.toIntern();
-        zcu.panic_cause_tag_type = panic_cause_ty.unionTagType(zcu).?.toIntern();
-    }
 }
 
-fn preparePanicIntegerOverflow(sema: *Sema, block: *Block, src: LazySrcLoc) !void {
+/// Backends depend on panic decls being available when lowering safety-checked
+/// instructions. This function ensures the panic function will be available to
+/// be called during that time.
+fn preparePanicId(sema: *Sema, block: *Block, src: LazySrcLoc, panic_id: Zcu.PanicId) !InternPool.Nav.Index {
     const pt = sema.pt;
     const zcu = pt.zcu;
-    try preparePanic(sema, block, src);
-    if (zcu.panic_cause_integer_overflow == .none) {
-        const union_val = try pt.unionValue(
-            Type.fromInterned(zcu.panic_cause_type),
-            try pt.enumValueFieldIndex(
-                Type.fromInterned(zcu.panic_cause_tag_type),
-                @intFromEnum(PanicCauseTag.integer_overflow),
-            ),
-            Value.void,
-        );
-        zcu.panic_cause_integer_overflow = try pt.refValue(union_val.toIntern());
-    }
+    const gpa = sema.gpa;
+    if (zcu.panic_messages[@intFromEnum(panic_id)].unwrap()) |x| return x;
+
+    try sema.prepareSimplePanic(block, src);
+
+    const panic_messages_ty = try pt.getBuiltinType("panic_messages");
+    const msg_nav_index = (sema.namespaceLookup(
+        block,
+        LazySrcLoc.unneeded,
+        panic_messages_ty.getNamespaceIndex(zcu),
+        try zcu.intern_pool.getOrPutString(gpa, pt.tid, @tagName(panic_id), .no_embedded_nulls),
+    ) catch |err| switch (err) {
+        error.AnalysisFail => @panic("std.builtin.panic_messages is corrupt"),
+        error.GenericPoison, error.ComptimeReturn, error.ComptimeBreak => unreachable,
+        error.OutOfMemory => |e| return e,
+    }).?;
+    try sema.ensureNavResolved(src, msg_nav_index);
+    zcu.panic_messages[@intFromEnum(panic_id)] = msg_nav_index.toOptional();
+    return msg_nav_index;
 }
 
 fn addSafetyCheck(
@@ -27725,7 +27699,7 @@ fn addSafetyCheck(
     parent_block: *Block,
     src: LazySrcLoc,
     ok: Air.Inst.Ref,
-    panic_cause_tag: PanicCauseTag,
+    panic_id: Zcu.PanicId,
 ) !void {
     const gpa = sema.gpa;
     assert(!parent_block.is_comptime);
@@ -27743,7 +27717,7 @@ fn addSafetyCheck(
 
     defer fail_block.instructions.deinit(gpa);
 
-    try sema.safetyPanic(&fail_block, src, panic_cause_tag);
+    try sema.safetyPanic(&fail_block, src, panic_id);
     try sema.addSafetyCheckExtra(parent_block, ok, &fail_block);
 }
 
@@ -27812,6 +27786,29 @@ fn addSafetyCheckExtra(
     parent_block.instructions.appendAssumeCapacity(block_inst);
 }
 
+fn panicWithMsg(sema: *Sema, block: *Block, src: LazySrcLoc, msg_inst: Air.Inst.Ref, operation: CallOperation) !void {
+    const pt = sema.pt;
+    const zcu = pt.zcu;
+
+    if (!zcu.backendSupportsFeature(.panic_fn)) {
+        _ = try block.addNoOp(.trap);
+        return;
+    }
+
+    try sema.prepareSimplePanic(block, src);
+
+    const panic_func = zcu.funcInfo(zcu.panic_func_index);
+    const panic_fn = try sema.analyzeNavVal(block, src, panic_func.owner_nav);
+    const null_stack_trace = Air.internedToRef(zcu.null_stack_trace);
+
+    const opt_usize_ty = try pt.optionalType(.usize_type);
+    const null_ret_addr = Air.internedToRef((try pt.intern(.{ .opt = .{
+        .ty = opt_usize_ty.toIntern(),
+        .val = .none,
+    } })));
+    try sema.callBuiltin(block, src, panic_fn, .auto, &.{ msg_inst, null_stack_trace, null_ret_addr }, operation);
+}
+
 fn addSafetyCheckUnwrapError(
     sema: *Sema,
     parent_block: *Block,
@@ -27849,7 +27846,7 @@ fn safetyPanicUnwrapError(sema: *Sema, block: *Block, src: LazySrcLoc, err: Air.
     if (!zcu.backendSupportsFeature(.panic_fn)) {
         _ = try block.addNoOp(.trap);
     } else {
-        const panic_fn = try pt.getBuiltin("panicUnwrapError");
+        const panic_fn = try pt.getBuiltinInnerType("Panic", "unwrapError");
         const err_return_trace = try sema.getErrorReturnTrace(block);
         const args: [2]Air.Inst.Ref = .{ err_return_trace, err };
         try sema.callBuiltin(block, src, panic_fn, .auto, &args, .@"safety check");
@@ -27866,7 +27863,7 @@ fn addSafetyCheckIndexOob(
 ) !void {
     assert(!parent_block.is_comptime);
     const ok = try parent_block.addBinOp(cmp_op, index, len);
-    return addSafetyCheckCall(sema, parent_block, src, ok, "panicOutOfBounds", &.{ index, len });
+    return addSafetyCheckCall(sema, parent_block, src, ok, "outOfBounds", &.{ index, len });
 }
 
 fn addSafetyCheckInactiveUnionField(
@@ -27878,7 +27875,7 @@ fn addSafetyCheckInactiveUnionField(
 ) !void {
     assert(!parent_block.is_comptime);
     const ok = try parent_block.addBinOp(.cmp_eq, active_tag, wanted_tag);
-    return addSafetyCheckCall(sema, parent_block, src, ok, "panicInactiveUnionField", &.{ active_tag, wanted_tag });
+    return addSafetyCheckCall(sema, parent_block, src, ok, "inactiveUnionField", &.{ active_tag, wanted_tag });
 }
 
 fn addSafetyCheckSentinelMismatch(
@@ -27919,7 +27916,7 @@ fn addSafetyCheckSentinelMismatch(
         break :ok try parent_block.addBinOp(.cmp_eq, expected_sentinel, actual_sentinel);
     };
 
-    return addSafetyCheckCall(sema, parent_block, src, ok, "panicSentinelMismatch", &.{
+    return addSafetyCheckCall(sema, parent_block, src, ok, "sentinelMismatch", &.{
         expected_sentinel, actual_sentinel,
     });
 }
@@ -27953,7 +27950,7 @@ fn addSafetyCheckCall(
     if (!zcu.backendSupportsFeature(.panic_fn)) {
         _ = try fail_block.addNoOp(.trap);
     } else {
-        const panic_fn = try pt.getBuiltin(func_name);
+        const panic_fn = try pt.getBuiltinInnerType("Panic", func_name);
         try sema.callBuiltin(&fail_block, src, panic_fn, .auto, args, .@"safety check");
     }
 
@@ -27961,8 +27958,10 @@ fn addSafetyCheckCall(
 }
 
 /// This does not set `sema.branch_hint`.
-fn safetyPanic(sema: *Sema, block: *Block, src: LazySrcLoc, panic_cause_tag: PanicCauseTag) CompileError!void {
-    try callPanic(sema, block, src, panic_cause_tag, .void_value, .@"safety check");
+fn safetyPanic(sema: *Sema, block: *Block, src: LazySrcLoc, panic_id: Zcu.PanicId) CompileError!void {
+    const msg_nav_index = try sema.preparePanicId(block, src, panic_id);
+    const msg_inst = try sema.analyzeNavVal(block, src, msg_nav_index);
+    try sema.panicWithMsg(block, src, msg_inst, .@"safety check");
 }
 
 fn emitBackwardBranch(sema: *Sema, block: *Block, src: LazySrcLoc) !void {
@@ -33456,7 +33455,7 @@ fn analyzeSlice(
         assert(!block.is_comptime);
         try sema.requireRuntimeBlock(block, src, runtime_src.?);
         const ok = try block.addBinOp(.cmp_lte, start, end);
-        try sema.addSafetyCheckCall(block, src, ok, "panicStartGreaterThanEnd", &.{ start, end });
+        try sema.addSafetyCheckCall(block, src, ok, "startGreaterThanEnd", &.{ start, end });
     }
     const new_len = if (by_length)
         try sema.coerce(block, Type.usize, uncasted_end_opt, end_src)
src/Zcu.zig
@@ -210,17 +210,40 @@ all_type_references: std.ArrayListUnmanaged(TypeReference) = .empty,
 /// Freelist of indices in `all_type_references`.
 free_type_references: std.ArrayListUnmanaged(u32) = .empty,
 
+panic_messages: [PanicId.len]InternPool.Nav.Index.Optional = .{.none} ** PanicId.len,
 /// The panic function body.
 panic_func_index: InternPool.Index = .none,
 null_stack_trace: InternPool.Index = .none,
-panic_cause_type: InternPool.Index = .none,
-panic_cause_tag_type: InternPool.Index = .none,
-panic_cause_integer_overflow: InternPool.Index = .none,
 
 generation: u32 = 0,
 
 pub const PerThread = @import("Zcu/PerThread.zig");
 
+pub const PanicId = enum {
+    reached_unreachable,
+    unwrap_null,
+    cast_to_null,
+    incorrect_alignment,
+    invalid_error_code,
+    cast_truncated_data,
+    negative_to_unsigned,
+    integer_overflow,
+    shl_overflow,
+    shr_overflow,
+    divide_by_zero,
+    exact_division_remainder,
+    integer_part_out_of_bounds,
+    corrupt_switch,
+    shift_rhs_too_big,
+    invalid_enum_value,
+    for_len_mismatch,
+    memcpy_len_mismatch,
+    memcpy_alias,
+    noreturn_returned,
+
+    pub const len = @typeInfo(PanicId).@"enum".fields.len;
+};
+
 pub const GlobalErrorSet = std.AutoArrayHashMapUnmanaged(InternPool.NullTerminatedString, void);
 
 pub const CImportError = struct {