94cbefb52a
Fix non-capturing closure return type coercion Fixes #88097. For the example given there: ```rust fn peculiar() -> impl Fn(u8) -> u8 { return |x| x + 1 } ``` which incorrectly reports an error, I noticed something weird in the debug log: ``` DEBUG rustc_typeck::check::coercion coercion::try_find_coercion_lub([closure@test.rs:2:12: 2:21], [closure@test.rs:2:12: 2:21], exprs=1 exprs) ``` Apparently, `try_find_coercion_lub()` thinks that the LUB for two closure types always has to be a function pointer (which explains the `expected closure, found fn pointer` error in #88097). There is one corner case where that isn't true, though — namely, when the two closure types are equal, in which case the trivial LUB is the type itself. This PR fixes this by inserting an explicit check for type equality in `try_find_coercion_lub()`.
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.