Compiler crash: unreachable reached instead of runtime reference to comptime var error in codegen lowerPtr #30088

Zig Version

0.16.0-dev.1503+738d2be9d

Steps to Reproduce and Observed Behavior

Attempt to compile the following repro.zig

pubfnmain()void{constblah=comptimeblk:{varbuf:u8=0;break:blk.{.a=&buf};};@import("std").debug.print("{any}\n",.{blah});}

With a debug build of the compiler:

$ zig build-exe repro.zig
Compiler crash context:
(no context)
thread 6370 panic: reached unreachable code
[...]/zig/src/codegen.zig:742:56: 0x28f6a46 in lowerPtr (main.zig)
 .arr_elem, .comptime_field, .comptime_alloc => unreachable,
 ^
[...]/zig/src/codegen.zig:444:29: 0x23dbe8f in generateSymbol (main.zig)
 .ptr => try lowerPtr(bin_file, pt, src_loc, val.toIntern(), w, reloc_parent, 0),
 ^
[...]/zig/src/link/Dwarf.zig:2130:35: 0x4219deb in blockValue (main.zig)
 try codegen.generateSymbol(
 ^
[...]/zig/src/link/Dwarf.zig:4579:59: 0x45e6496 in updateContainerTypeWriterError (main.zig)
 try wip_nav.blockValue(ty_src_loc, .fromInterned(field_init));
 ^
[...]/zig/src/link/Dwarf.zig:4383:48: 0x3d5c046 in updateContainerType (main.zig)
 return dwarf.updateContainerTypeWriterError(pt, type_index) catch |err| switch (err) {
 ^
[...]/zig/src/link/Elf/ZigObject.zig:1718:59: 0x33303fd in updateContainerType (main.zig)
 if (self.dwarf) |*dwarf| try dwarf.updateContainerType(pt, ty);
 ^
[...]/zig/src/link/Elf.zig:1710:53: 0x3330c23 in updateContainerType (main.zig)
 return self.zigObjectPtr().?.updateContainerType(pt, ty) catch |err| switch (err) {
 ^
[...]/zig/src/link.zig:787:91: 0x2cde4f3 in updateContainerType (main.zig)
 return @as(*tag.Type(), @fieldParentPtr("base", base)).updateContainerType(pt, ty);
 ^
[...]/zig/src/link.zig:1549:43: 0x2843282 in doZcuTask (main.zig)
 lf.updateContainerType(pt, ty) catch |err| switch (err) {
 ^
[...]/zig/src/link/Queue.zig:318:23: 0x28209a7 in flushTaskQueue (main.zig)
 link.doZcuTask(comp, tid, task);
 ^
[...]/lib/std/Thread/Pool.zig:180:50: 0x2821153 in runFn (std.zig)
 @call(.auto, func, .{id.?} ++ closure.arguments);
 ^
[...]/lib/std/Thread/Pool.zig:293:27: 0x22b6d20 in worker (std.zig)
 runnable.runFn(runnable, id);
 ^
[...]/lib/std/Thread.zig:559:13: 0x20dac90 in callFn__anon_198523 (std.zig)
 @call(.auto, f, args);
 ^
[...]/lib/std/Thread.zig:1535:30: 0x1f6a500 in entryFn (std.zig)
 return callFn(f, self.fn_args);
 ^
[...]/lib/std/os/linux/x86_64.zig:105:5: 0x1cdc725 in clone (std.zig)
 asm volatile (
 ^
Error: nu::shell::core_dumped
 ×ばつ External command core dumped
 ╭─[entry #11:1:1]
 1 │ zig build-exe repro.zig
 · ─┬─
 · ╰── core dumped with SIGABRT (6)
 ╰────

Something about breaking with a struct allows this to slip through the cracks, since applying either

< break :blk .{ .a = &buf };
---
> break :blk .{ &buf };

< break :blk .{ .a = &buf };
---
> break :blk &buf;

produces the correct error.

In the inciting code this ended up happening due to a slice that wasn't dereferenced to an array that erroneously worked in 0.14.

Expected Behavior

The compiler not to crash and report the usual runtime reference to comptime memory.

#31403 doesn't close this because the underlying bug still exists. #31403 just masked it by (for now) disabling lowering struct default values into DWARF. As of #31403 you can reproduce a crash by doing this instead:

constT=T:{varbuf:u8=0;break:T@TypeOf(.{.ptr=&buf});};comptime{_=T;}

(For any compiler devs: the struct type being created by the anonymous struct initialization is having its ptr field made comptime with value &buf, which is invalid because &buf is not a "runtime value", which default field values are required to be. The field needs to be made non-comptime in that case! See similar logic in Sema.arrayInitAnon, look for values_no_comptime.)

#31403 doesn't close this because the underlying bug still exists. #31403 just masked it by (for now) disabling lowering struct default values into DWARF.

Ah sorry I misinterpreted the issue, I'll give fixing it a try :)