Closed
Description
Code
fn main() {
let ptr: *const dyn Fn() = &|| ();
_ = ptr as *const dyn Fn(u8);
}Current output
error[E0308]: mismatched types
--> src/main.rs:3:9
|
3 | _ = ptr as *const dyn Fn(u8);
| ^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found `(u8,)`
|
= note: expected trait object `dyn Fn()`
found trait object `dyn Fn(u8)`
Desired output
error[E0308]: invalid cast from `*const dyn Fn()` to `*const dyn Fn(u8)`
--> src/main.rs:3:9
|
3 | _ = ptr as *const dyn Fn(u8);
| ^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found `(u8,)`
|
= note: `Fn()` and `Fn(u8)` might have different vtables, leading to unsoundness
= note: if you are absolutely sure you are using the resulting value safely, use `transmute`
Rationale and extra context
In #120248 I made checks for casting pointers to trait objects more strict: now they need to have the same generic arguments (this is due to the fact that for the cast to be safe, we need vtables to match).
The check is however made in a bit of a crude way, which results in this "mismatched types" diagnostic.
We should modify it so that the intent of the error is clear.
Rust Version
Reproduces on both stable 1.81.0 and 1.83.0-nightly (2024-09-05 9c01301).
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Area: Messages for errors, warnings, and lintsArea: Messages for errors, warnings, and lintsArea: raw pointers, MaybeUninit, NonNullArea: raw pointers, MaybeUninit, NonNullArea: trait objects, vtable layoutArea: trait objects, vtable layoutDiagnostics: Confusing error or lint that should be reworked.Diagnostics: Confusing error or lint that should be reworked.Diagnostics: An error or lint that doesn't give enough information about the problem at hand.Diagnostics: An error or lint that doesn't give enough information about the problem at hand.Relevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.Relevant to the types team, which will review and decide on the PR/issue.Relevant to the types team, which will review and decide on the PR/issue.