f3f8e758f2
Use more precise span for E0282 in cast expressions This pull request fixes #85586. The example code given there: ```rust fn main() { let a = [1, 2, 3].iter().sum(); let b = (a + 1) as usize; } ``` currently produces ``` error[E0282]: type annotations needed --> issue-85586.rs:3:13 | 3 | let b = (a + 1) as usize; | ^^^^^^^^^^^^^^^^ cannot infer type | = note: type must be known at this point error: aborting due to previous error ``` even though the type of the entire cast expression quite clearly should be `usize`. The error is in the cast's left-hand side, which is made explicit by the changes in this PR: ``` error[E0282]: type annotations needed --> issue-85586.rs:3:13 | 3 | let b = (a + 1) as usize; | ^^^^^^^ cannot infer type | = note: type must be known at this point error: aborting due to previous error ```
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.