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Compiler fails to report len for static A: [T; _] #135010

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workingjubilee opened this issue Jan 2, 2025 · 1 comment · Fixed by #135044
Closed

Compiler fails to report len for static A: [T; _] #135010

workingjubilee opened this issue Jan 2, 2025 · 1 comment · Fixed by #135044
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A-diagnostics Area: Messages for errors, warnings, and lints F-generic_arg_infer Using `_` as a const argument: #![feature(generic_arg_infer)]` T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@workingjubilee
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workingjubilee commented Jan 2, 2025 •
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Code

static A: [u8; _] = [12, 23, 34, 45];

Current output

Compiling playground v0.0.1 (/playground)
error: in expressions, `_` can only be used on the left-hand side of an assignment
 --> src/lib.rs:1:16
  |
1 | static A: [u8; _] = [12, 23, 34, 45];
  |                ^ `_` not allowed here

error[E0658]: using `_` for array lengths is unstable
 --> src/lib.rs:1:16
  |
1 | static A: [u8; _] = [12, 23, 34, 45];
  |                ^
  |
  = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information

For more information about this error, try `rustc --explain E0658`.
error: could not compile `playground` (lib) due to 2 previous errors

Desired output

Compiling playground v0.0.1 (/playground)
error: `_` is not a valid array length for a static item
 --> src/lib.rs:1:16
  |
1 | static A: [u8; _] = [12, 23, 34, 45];
  |                ^ `_` not allowed here
  |
  = help: you meant 4

For more information about this error, try `rustc --explain E0658`.
error: could not compile `playground` (lib) due to 2 previous errors

Rationale and extra context

rationale: @BoxyUwU expected it to do so.

Other cases

We can also get this:

  = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information
  = help: add `#![feature(generic_arg_infer)]` to the crate attributes to enable
  = note: this compiler was built on 2025-01-01; consider upgrading it if it is out of date

Rust Version

$ rustc --version --verbose
rustc 1.85.0-nightly (45d11e51b 2025-01-01)
binary: rustc
commit-hash: 45d11e51bb66c2deb63a006fe3953c4b6fbc50c2
commit-date: 2025-01-01
host: x86_64-unknown-linux-gnu
release: 1.85.0-nightly
LLVM version: 19.1.6

Anything else?

No response
@workingjubilee workingjubilee added A-diagnostics Area: Messages for errors, warnings, and lints T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. F-generic_arg_infer Using `_` as a const argument: #![feature(generic_arg_infer)]` labels Jan 2, 2025
@compiler-errors
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I'll look into this. Seems pretty involved.

@compiler-errors compiler-errors self-assigned this Jan 2, 2025
@compiler-errors compiler-errors mentioned this issue Jan 2, 2025
Merged
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Jan 3, 2025
@matthiaskrgr
Rollup merge of rust-lang#135044 - compiler-errors:better-infer-sugge…
58a5b51 
…stions-in-const, r=oli-obk

Improve infer (`_`) suggestions in `const`s and `static`s

Fixes rust-lang#135010.

This PR does a few things to (imo) greatly improve the error message when users write something like `static FOO: [i32; _] = [1, 2, 3]`.

Firstly, it adapts the recovery code for when we encounter `_` in a const/static to work a bit more like `fn foo() -> _`, and removes the somewhat redundant query `diagnostic_only_typeck`.

Secondly, it changes the lowering for `[T; _]` to always lower under the `feature(generic_arg_infer)` logic to `ConstArgKind::Infer`. We still issue the feature error, so it's not doing anything *observable* on the good path, but it does mean that we no longer erroneously interpret `[T; _]`'s array length as a `_` **wildcard expression** (à la destructuring assignment, like `(_, y) = expr`).

Lastly it makes the suggestions verbose and fixes (well, suppresses) a bug with stashing and suggestions.

r? oli-obk
@bors bors closed this as completed in 12cc9b4 Jan 4, 2025
rust-timer added a commit to rust-lang-ci/rust that referenced this issue Jan 4, 2025
@rust-timer
Unrolled build for rust-lang#135044
6d75392 
Rollup merge of rust-lang#135044 - compiler-errors:better-infer-suggestions-in-const, r=oli-obk

Improve infer (`_`) suggestions in `const`s and `static`s

Fixes rust-lang#135010.

This PR does a few things to (imo) greatly improve the error message when users write something like `static FOO: [i32; _] = [1, 2, 3]`.

Firstly, it adapts the recovery code for when we encounter `_` in a const/static to work a bit more like `fn foo() -> _`, and removes the somewhat redundant query `diagnostic_only_typeck`.

Secondly, it changes the lowering for `[T; _]` to always lower under the `feature(generic_arg_infer)` logic to `ConstArgKind::Infer`. We still issue the feature error, so it's not doing anything *observable* on the good path, but it does mean that we no longer erroneously interpret `[T; _]`'s array length as a `_` **wildcard expression** (à la destructuring assignment, like `(_, y) = expr`).

Lastly it makes the suggestions verbose and fixes (well, suppresses) a bug with stashing and suggestions.

r? oli-obk
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A-diagnostics Area: Messages for errors, warnings, and lints F-generic_arg_infer Using `_` as a const argument: #![feature(generic_arg_infer)]` T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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Improve infer (_) suggestions in consts and statics compiler-errors/rust
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