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Manish Goregaokar
b285d68f36
Note numeric literals that can never fit in an expected type re https://github.com/rust-lang/rust/pull/72380#discussion_r438289385 Given the toy code ```rust fn is_positive(n: usize) { n > -1_isize; } ``` We currently get a type mismatch error like the following: ``` error[E0308]: mismatched types --> src/main.rs:2:9 | 2 | n > -1_isize; | ^^^^^^^^ expected `usize`, found `isize` | help: you can convert an `isize` to `usize` and panic if the converted value wouldn't fit | 2 | n > (-1_isize).try_into().unwrap(); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ``` But clearly, `-1` can never fit into a `usize`, so the suggestion will always panic. A more useful message would tell the user that the value can never fit in the expected type: ``` error[E0308]: mismatched types --> test.rs:2:9 | 2 | n > -1_isize; | ^^^^^^^^ expected `usize`, found `isize` | note: `-1_isize` can never fit into `usize` --> test.rs:2:9 | 2 | n > -1_isize; | ^^^^^^^^ ``` Which is what this commit implements. I only added this check for negative literals because - Currently we can only perform such a check for literals (constant value propagation is outside the scope of the typechecker at this point) - A lint error for out-of-range numeric literals is already emitted IMO it makes more sense to put this check in librustc_lint, but as far as I can tell the typecheck pass happens before the lint pass, so I've added it here. r? @estebank