Open
Description
Code
fn main() {
let arr: [u32; 3] = [0, 1, 2];
let idx = 0i32;
println!("{}", arr[idx.into()]);
}Current output
error[E0282]: type annotations needed
--> src/main.rs:4:20
|
4 | println!("{}", arr[idx.into()]);
| ^^^^^^^^^^^^^^^ cannot infer type of the type parameter `T` declared on the associated function `new_display`
|
= note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider specifying the generic argument
|
4 | println!("{}", arr[idx.into()]::<T>);
| +++++
For more information about this error, try `rustc --explain E0282`.Desired output
I'm too new to Rust to say exactly, but:
- It should explain where the problem is (the issue is the
idx.into()expression, not the outer expression it is highlighting - It should not suggest a "fix" of adding
::<T>, which is not syntactically valid here
Rationale and extra context
Naively, one one expect that the requirement of array access being usize means an into() would work here. Clearly, that's not the case but I'm still not sure why.
Other cases
fn main() {
let arr: [u32; 3] = [0, 1, 2];
let idx = 0i32;
let val = arr[idx.into()];
println!("{}", val);
}still gives bad advice, although less catastrophically:
error[E0282]: type annotations needed
--> src/main.rs:4:9
|
4 | let val = arr[idx.into()];
| ^^^
|
help: consider giving `val` an explicit type
|
4 | let val: _ = arr[idx.into()];
| +++
For more information about this error, try `rustc --explain E0282`.
Only when you take that bad advice, but with an explicit type instead of _, do you get a useful message:
fn main() {
let arr: [u32; 3] = [0, 1, 2];
let idx = 0i32;
let val: u32 = arr[idx.into()];
println!("{}", val);
}error[E0282]: type annotations needed
--> src/main.rs:4:28
|
4 | let val: u32 = arr[idx.into()];
| ^^^^
|
help: try using a fully qualified path to specify the expected types
|
4 | let val: u32 = arr[<i32 as Into<T>>::into(idx)];
| +++++++++++++++++++++++ ~
For more information about this error, try `rustc --explain E0282`.
Anything else?
No response