Code
fn main1() {
let bad_idx = 0u8;
let foo = [1, 2, 3][bad_idx.into()] as i32;
}
fn main2() {
let bad_idx = 0u8;
let foo = 0 + [1, 2, 3][bad_idx.into()];
}
fn main3() {
let bad_idx = 0u8;
let foo = [1, 2, 3][bad_idx.into()];
}Current output
Compiling playground v0.0.1 (/playground)
error[E0282]: type annotations needed
--> src/lib.rs:3:15
|
3 | let foo = [1, 2, 3][bad_idx.into()] as i32;
| ^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type
error[E0284]: type annotations needed
--> src/lib.rs:8:17
|
8 | let foo = 0 + [1, 2, 3][bad_idx.into()];
| ^ cannot infer type
|
= note: cannot satisfy `<i32 as Add<_>>::Output == _`
error[E0283]: type annotations needed
--> src/lib.rs:13:33
|
13 | let foo = [1, 2, 3][bad_idx.into()];
| ^^^^
|
= note: cannot satisfy `_: From<u8>`
= note: required for `u8` to implement `Into<_>`
help: try using a fully qualified path to specify the expected types
|
13 - let foo = [1, 2, 3][bad_idx.into()];
13 + let foo = [1, 2, 3][<u8 as Into<T>>::into(bad_idx)];
|
Some errors have detailed explanations: E0282, E0283, E0284.
For more information about an error, try `rustc --explain E0282`.
error: could not compile `playground` (lib) due to 3 previous errors
Desired output
For the first two functions, it should correctly identify the `into()` as the issue, instead of incorrectly highlighting a larger surrounding expression.
For the third, it's not even clear to me why it *doesn't* work. (`u8` does implement into for `usize`.) But, at least the problem location is clear, so I know what to fix.
Rationale and extra context
When I encountered this in the wild, it was more similar to the second form (with arithmetic), so the exact problem was very obscured. Trying to add explict casts (like the first form) didn't help.
Other cases
Rust Version
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=5a2136568a8f77eaaddd8190c4879464
Version listed is 1.97.0-nightly (2026-05-18 9eb3be26b46eccea1de7), I don't think I can run commandline stuff there
Anything else?
No response
Code
Current output
Desired output
Rationale and extra context
When I encountered this in the wild, it was more similar to the second form (with arithmetic), so the exact problem was very obscured. Trying to add explict casts (like the first form) didn't help.
Other cases
Rust Version
Anything else?
No response