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Wrong suggestion for const well-formedness of bool #122395

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slanterns opened this issue Mar 12, 2024 · 3 comments · Fixed by #122802
Closed

Wrong suggestion for const well-formedness of bool #122395

slanterns opened this issue Mar 12, 2024 · 3 comments · Fixed by #122802
Labels
A-const-generics Area: const generics (parameters and arguments) A-diagnostics Area: Messages for errors, warnings, and lints const-generics-bad-diagnostics An error is correctly emitted, but is confusing, for `min_const_generics`. F-generic_const_exprs `#![feature(generic_const_exprs)]` T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@slanterns
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slanterns commented Mar 12, 2024 •
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Code

#![feature(generic_const_exprs)]

fn bb<const N: bool>() {}

fn b<const N: bool>() {
    bb::<{!N}>();
}

fn main() {}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=6c33a1495d719fbc2d8efe30177c3ca4

Current output

help: try adding a `where` bound using this expression: `where [(); {!N}]:`

Desired output

`where [(); !N as usize]:`

Rationale and extra context

The well-formedness shouldn't be written as where [(); {!N}]: when !N is a bool, which doesn't compile. Should cast it to an integral type.

Other cases

No response

Rust Version

1.78.0-nightly (2024-03-11 4a0cc881dcc4d800f106)

Anything else?

No response
@slanterns slanterns 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. labels Mar 12, 2024
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rustbot commented Mar 12, 2024

Error: Label const-generics-bad-diagnostics can only be set by Rust team members

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #t-infra on Zulip.

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@rustbot rustbot added A-const-generics Area: const generics (parameters and arguments) F-generic_const_exprs `#![feature(generic_const_exprs)]` labels Mar 12, 2024
@compiler-errors compiler-errors added the const-generics-bad-diagnostics An error is correctly emitted, but is confusing, for `min_const_generics`. label Mar 12, 2024
@estebank estebank mentioned this issue Mar 21, 2024
Merged
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Mar 25, 2024
@matthiaskrgr
Rollup merge of rust-lang#122802 - estebank:unconstrained-generic-con…
5c5d36c 
…st, r=Nadrieril

Provide structured suggestion for unconstrained generic constant

```
error: unconstrained generic constant
  --> $DIR/const-argument-if-length.rs:18:10
   |
LL |     pad: [u8; is_zst::<T>()],
   |          ^^^^^^^^^^^^^^^^^^^
   |
help: try adding a `where` bound
   |
LL | pub struct AtLeastByte<T: ?Sized> where [(); is_zst::<T>()]: {
   |                                   ++++++++++++++++++++++++++
```

Detect when the constant expression isn't `usize` and suggest casting:

```
error: unconstrained generic constant
 --> f300.rs:6:10
  |
6 |     bb::<{!N}>();
  |          ^^^^
-Ztrack-diagnostics: created at compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs:3539:36
  |
help: try adding a `where` bound
  |
5 | fn b<const N: bool>() where [(); {!N} as usize]: {
  |                       ++++++++++++++++++++++++++
```

Fix rust-lang#122395.
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Mar 25, 2024
@matthiaskrgr
Rollup merge of rust-lang#122802 - estebank:unconstrained-generic-con…
449db0e 
…st, r=Nadrieril

Provide structured suggestion for unconstrained generic constant

```
error: unconstrained generic constant
  --> $DIR/const-argument-if-length.rs:18:10
   |
LL |     pad: [u8; is_zst::<T>()],
   |          ^^^^^^^^^^^^^^^^^^^
   |
help: try adding a `where` bound
   |
LL | pub struct AtLeastByte<T: ?Sized> where [(); is_zst::<T>()]: {
   |                                   ++++++++++++++++++++++++++
```

Detect when the constant expression isn't `usize` and suggest casting:

```
error: unconstrained generic constant
 --> f300.rs:6:10
  |
6 |     bb::<{!N}>();
  |          ^^^^
-Ztrack-diagnostics: created at compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs:3539:36
  |
help: try adding a `where` bound
  |
5 | fn b<const N: bool>() where [(); {!N} as usize]: {
  |                       ++++++++++++++++++++++++++
```

Fix rust-lang#122395.
bors added a commit to rust-lang-ci/rust that referenced this issue Mar 25, 2024
@bors
Auto merge of rust-lang#122802 - estebank:unconstrained-generic-const…
dda2372 
…, r=Nadrieril

Provide structured suggestion for unconstrained generic constant

```
error: unconstrained generic constant
  --> $DIR/const-argument-if-length.rs:18:10
   |
LL |     pad: [u8; is_zst::<T>()],
   |          ^^^^^^^^^^^^^^^^^^^
   |
help: try adding a `where` bound
   |
LL | pub struct AtLeastByte<T: ?Sized> where [(); is_zst::<T>()]: {
   |                                   ++++++++++++++++++++++++++
```

Detect when the constant expression isn't `usize` and suggest casting:

```
error: unconstrained generic constant
 --> f300.rs:6:10
  |
6 |     bb::<{!N}>();
  |          ^^^^
-Ztrack-diagnostics: created at compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs:3539:36
  |
help: try adding a `where` bound
  |
5 | fn b<const N: bool>() where [(); {!N} as usize]: {
  |                       ++++++++++++++++++++++++++
```

Fix rust-lang#122395.
@bors bors closed this as completed in 6b24fdf Mar 25, 2024
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Labels
A-const-generics Area: const generics (parameters and arguments) A-diagnostics Area: Messages for errors, warnings, and lints const-generics-bad-diagnostics An error is correctly emitted, but is confusing, for `min_const_generics`. F-generic_const_exprs `#![feature(generic_const_exprs)]` T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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Provide structured suggestion for unconstrained generic constant estebank/rust
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@compiler-errors @slanterns @rustbot