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local variable is never mutated regression for field and array accesses #25540

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local variable is never mutated regression for field and array accesses#25540
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mlugg
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bugObserved behavior contradicts documented or intended behaviorfrontendTokenization, parsing, AstGen, Sema, and Liveness.regressionIt worked in a previous version of Zig, but stopped working.
Milestone
0.16.0

Description

@squeek502
squeek502
opened on Oct 11, 2025

Zig Version

0.16.0-dev.699+529aa9f27

Steps to Reproduce and Observed Behavior

I've run into this a few times now and thought I was going crazy. Locally, everything would be fine but CI would fail immediately with local variable is never mutated. One reproduction:

fn foo() void {
    var target_path_w: []const u8 = "foo";
    _ = target_path_w.len;
}

With zig1, it hits this error:

> zig1.exe ..\lib build-obj -ofmt=c -Mroot=test.zig
test.zig:2:9: error: local variable is never mutated
    var target_path_w: []const u8 = "foo";
        ^~~~~~~~~~~~~
test.zig:2:9: note: consider using 'const'

With a stage3 compiler, it does not:

> zig build-obj -ofmt=c -Mroot=test.zig

Note that it's not related to the function being unused or something like that. If you add a test block:

test "foo" {
    foo();
}

it passes:

> zig test test.zig
All 1 tests passed.

Somewhat surprisingly, zig1 seems to have the more correct behavior, as in each case I've hit, the error seems applicable.

Expected Behavior

zig1 and the stage3 compiler to agree on when things are never mutated.

Activity

  1. squeek502
    added
    bugObserved behavior contradicts documented or intended behavior
    on Oct 11, 2025

  2. squeek502 commented on Oct 11, 2025

    @squeek502
    squeek502
    on Oct 11, 2025
    MemberAuthor

    This is actually #25154 (review), so it regressed in #25154 and zig1.wasm hasn't been updated since.

    If there was a follow-up issue made, then this should be closed as a duplicate of that. If not, then I guess this can serve as that follow-up issue.

    cc @mlugg

  3. squeek502
    changed the title [-]`local variable is never mutated` mismatch between zig1 and zig (stage3)[/-] [+]`local variable is never mutated` regression, mismatch between zig1 and zig (stage3)[/+] on Oct 11, 2025
  4. squeek502
    added
    frontendTokenization, parsing, AstGen, Sema, and Liveness.
    regressionIt worked in a previous version of Zig, but stopped working.
    on Oct 11, 2025

  5. mlugg commented on Oct 11, 2025

    @mlugg
    mlugg
    on Oct 11, 2025
    Member

    Ah yes, thank you @squeek502; indeed I forgot to make that issue, so this can serve as it.

  6. mlugg
    added this to the 0.16.0 milestone on Oct 11, 2025
  7. mlugg
    changed the title [-]`local variable is never mutated` regression, mismatch between zig1 and zig (stage3)[/-] [+]`local variable is never mutated` regression for field and array accesses[/+] on Oct 11, 2025
  8. mlugg
    self-assigned this
    on Oct 11, 2025

  9. mlugg commented on Jan 30, 2026

    @mlugg
    mlugg
    on Jan 30, 2026
    Member

    Migrated to https://codeberg.org/ziglang/zig/issues/31049

  10. mlugg
    closed this as completedon Jan 30, 2026
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bugObserved behavior contradicts documented or intended behaviorfrontendTokenization, parsing, AstGen, Sema, and Liveness.regressionIt worked in a previous version of Zig, but stopped working.

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