stage2: unreachable in checkComptimeVarStore

[mitchellh]

Zig Version

0.10.0-dev.3900+ab4b26d8a

Steps to Reproduce

const std = @import("std");

test {
    const Widget = union(enum) { a: u0 };

    const block: Widget = block: {
        const info = @typeInfo(Widget).Union;
        inline for (info.fields) |field| {
            if (std.mem.eql(u8, "anything", field.name)) {
                switch (field.field_type) {
                    u0 => break :block @unionInit(Widget, field.name, 0),
                    else => unreachable,
                }
            }
        }

        unreachable;
    };

    _ = block;
}

zig test repro.zig -- fails
zig test repro.zig -fstage1 -- passes

Note: the u0 can be any type it appears...

Further, if you reduce it further and remove the switch, you actually get a "store to comptime variable depends on runtime condition" rather than a crash on unreachable. So there are perhaps two bugs here, with one masking the other.

Expected Behavior

Tests pass (to match stage1)

Actual Behavior

thread 2093712 panic: reached unreachable code
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:18378:9: 0x1966903 in checkComptimeVarStore (zig)
        unreachable;
        ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:24313:35: 0x196168b in storePtrVal (zig)
    try sema.checkComptimeVarStore(block, src, mut_kit.decl_ref_mut);
                                  ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:24238:33: 0x171b4b7 in storePtr2 (zig)
            try sema.storePtrVal(block, src, ptr_val, operand_val, elem_ty);
                                ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:24164:26: 0x1b77683 in storePtr (zig)
    return sema.storePtr2(block, src, ptr, src, uncasted_operand, src, .store);
                         ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:4444:25: 0x194b45b in zirStore (zig)
    return sema.storePtr(block, sema.src, ptr, value);
                        ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:1044:34: 0x171115f in analyzeBodyInner (zig)
                try sema.zirStore(block, inst);
                                 ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:626:45: 0x157d1ff in analyzeBodyBreak (zig)
    const break_inst = sema.analyzeBodyInner(block, body) catch |err| switch (err) {
                                            ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:1280:65: 0x171436b in analyzeBodyInner (zig)
                const opt_break_data = try sema.analyzeBodyBreak(block, inline_body);
                                                                ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:4850:34: 0x1b6d627 in resolveBlockBody (zig)
        if (sema.analyzeBodyInner(child_block, body)) |_| {
                                 ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:4833:33: 0x1956327 in zirBlock (zig)
    return sema.resolveBlockBody(parent_block, src, &child_block, body, inst, &label.merges);
                                ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:1239:69: 0x1713b17 in analyzeBodyInner (zig)
                if (!block.is_comptime) break :blk try sema.zirBlock(block, inst);
                                                                    ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:609:30: 0x170376b in analyzeBody (zig)
    _ = sema.analyzeBodyInner(block, body) catch |err| switch (err) {
                             ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Module.zig:5570:21: 0x15687d7 in analyzeFnBody (zig)
    sema.analyzeBody(&inner_block, fn_info.body) catch |err| switch (err) {
                    ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Module.zig:4271:40: 0x1394863 in ensureFuncBodyAnalyzed (zig)
            var air = mod.analyzeFnBody(func, sema_arena) catch |err| switch (err) {
                                       ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:25967:36: 0x1c35b73 in ensureFuncBodyAnalyzed (zig)
    sema.mod.ensureFuncBodyAnalyzed(func) catch |err| {
                                   ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:28086:40: 0x1a077cb in resolveInferredErrorSet (zig)
        try sema.ensureFuncBodyAnalyzed(ies.func);
                                       ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:26246:49: 0x1959ae3 in analyzeIsNonErrComptimeOnly (zig)
                try sema.resolveInferredErrorSet(block, src, ies);
                                                ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:15135:60: 0x1958d43 in zirTry (zig)
    const is_non_err = try sema.analyzeIsNonErrComptimeOnly(parent_block, operand_src, err_union);
                                                           ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:1365:67: 0x1715b47 in analyzeBodyInner (zig)
                if (!block.is_comptime) break :blk try sema.zirTry(block, inst);
                                                                  ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:626:45: 0x157d1ff in analyzeBodyBreak (zig)
    const break_inst = sema.analyzeBodyInner(block, body) catch |err| switch (err) {
                                            ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:591:50: 0x1b3bf97 in resolveBody (zig)
    const break_data = (try sema.analyzeBodyBreak(block, body)) orelse
                                                 ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:5588:46: 0x18c38b3 in zirCall (zig)
        const resolved = try sema.resolveBody(block, args_body[arg_start..arg_end], inst);
                                             ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:725:62: 0x1704fe7 in analyzeBodyInner (zig)
            .call                         => try sema.zirCall(block, inst),
                                                             ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Sema.zig:609:30: 0x170376b in analyzeBody (zig)
    _ = sema.analyzeBodyInner(block, body) catch |err| switch (err) {
                             ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Module.zig:5570:21: 0x15687d7 in analyzeFnBody (zig)
    sema.analyzeBody(&inner_block, fn_info.body) catch |err| switch (err) {
                    ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Module.zig:4271:40: 0x1394863 in ensureFuncBodyAnalyzed (zig)
            var air = mod.analyzeFnBody(func, sema_arena) catch |err| switch (err) {
                                       ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Compilation.zig:2976:42: 0x1392fdf in processOneJob (zig)
            module.ensureFuncBodyAnalyzed(func) catch |err| switch (err) {
                                         ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Compilation.zig:2914:30: 0x12b43ff in performAllTheWork (zig)
            try processOneJob(comp, work_item);
                             ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/Compilation.zig:2254:31: 0x12b1333 in update (zig)
    try comp.performAllTheWork(main_progress_node);
                              ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/main.zig:3348:20: 0x12d715f in updateModule (zig)
    try comp.update();
                   ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/main.zig:3022:17: 0x109f577 in buildOutputType (zig)
    updateModule(gpa, comp, hook) catch |err| switch (err) {
                ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/main.zig:236:31: 0x102df6f in mainArgs (zig)
        return buildOutputType(gpa, arena, args, .zig_test);
                              ^
/home/mitchellh/code/go/src/github.com/ziglang/zig/src/stage1.zig:48:24: 0x102d423 in main (zig)
        stage2.mainArgs(gpa, arena, args) catch unreachable;
                       ^
???:?:?: 0xffff74bf73f3 in ??? (???)
???:?:?: 0xffff74bf74cf in ??? (???)
../sysdeps/aarch64/start.S:81:0: 0x7ba3af in ??? (../sysdeps/aarch64/start.S)

[Vexu]

Here's a small patch that should turn that crash into a (likely invalid) compile error:

diff --git a/src/Sema.zig b/src/Sema.zig
index 15e891ef8..737f572e2 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -5218,6 +5218,10 @@ fn zirBreak(sema: *Sema, start_block: *Block, inst: Zir.Inst.Index) CompileError
                 if (block.runtime_cond == null and block.runtime_loop == null) {
                     block.runtime_cond = start_block.runtime_cond orelse start_block.runtime_loop;
                     block.runtime_loop = start_block.runtime_loop;
+                    if (block.runtime_cond == null and block.runtime_loop == null) {
+                        const block_data = sema.code.instructions.items(.data)[zir_block].pl_node;
+                        block.runtime_cond = block_data.src();
+                    }
                 }
                 return inst;
             }

Maybe it'll help in finding out what is causing the crash?

[mitchellh]

Thanks. I’ll give it a shot. Note you labeled this stage1 but this passes with stage1 and fails with stage2 so this should be labeled stage2.

[mitchellh]

Thanks @Vexu I've minimized it.

const std = @import("std");

test {
    const Widget = union(enum) { a: u0 };

    const block: Widget = block: {
        const info = @typeInfo(Widget).Union;
        inline for (info.fields) |field| {
            if (std.mem.eql(u8, "anything", field.name)) {
                switch (field.field_type) {
                    u0 => break :block @unionInit(Widget, field.name, 0),
                    else => unreachable,
                }
            }
        }

        unreachable;
    };

    _ = block;
}

[mitchellh]

Note: the u0 can be any type it appears...

Further, if you reduce it further and remove the switch, you actually get a "store to comptime variable depends on runtime condition" rather than a crash on unreachable. So there are perhaps two bugs here, with one masking the other.