783efd29ae
Check that const parameters of trait methods have compatible types This PR fixes #86820. The problem is that this currently passes the type checker: ```rust trait Tr { fn foo<const N: u8>(self) -> u8; } impl Tr for f32 { fn foo<const N: bool>(self) -> u8 { 42 } } ``` i.e. the type checker fails to check whether const parameters in `impl` methods have the same type as the corresponding declaration in the trait. With my changes, I get, for the above code: ``` error[E0053]: method `foo` has an incompatible const parameter type for trait --> test.rs:6:18 | 6 | fn foo<const N: bool>(self) -> u8 { 42 } | ^ | note: the const parameter `N` has type `bool`, but the declaration in trait `Tr::foo` has type `u8` --> test.rs:2:18 | 2 | fn foo<const N: u8>(self) -> u8; | ^ error: aborting due to previous error ``` This fixes #86820, where an ICE happens later on because the trait method is declared with a const parameter of type `u8`, but the `impl` uses one of type `usize`: > `expected int of size 8, but got size 1` |
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For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.