Suggestion to wrap inner types using 'allocator_api' in tuple

[ken-matsui]

This PR provides a suggestion to wrap the inner types in tuple when being along with 'allocator_api'.

Closes #83250

fn main() {
    let _vec: Vec<u8, _> = vec![]; //~ ERROR use of unstable library feature 'allocator_api'
}

 error[E0658]: use of unstable library feature 'allocator_api'
   --> $DIR/suggest-vec-allocator-api.rs:2:23
    |
 LL |     let _vec: Vec<u8, _> = vec![];
-   |                       ^
+   |                   ----^
+   |                   |
+   |                   help: consider wrapping the inner types in tuple: `(u8, _)`
    |
    = note: see issue #32838 <https://github.com/rust-lang/rust/issues/32838> for more information
    = help: add `#![feature(allocator_api)]` to the crate attributes to enable

[rust-highfive]

r? @davidtwco

(rust-highfive has picked a reviewer for you, use r? to override)

[leonardo-m]

I don't understand what it's suggesting. Perhaps to replace Vec<u8, _> with Vec<u8, (_, _)> ?

[fbstj]

I don't understand what it's suggesting. Perhaps to replace Vec<u8, _> with Vec<u8, (_, _)> ?

looking at #83250 makes it more obvious that it is suggesting Vec<(u8, _)>; ie not passing anything as the second (allocator) type to avoid the alocator_api entirely. maybe the generated help: should attempt to represent the expected output rather than the <(_,_)> which is confusing

[davidtwco]

This is looking much better, thanks for addressing those comments, a few more changes.

[davidtwco]

Looks good, thanks for your work on this.

[davidtwco]

@bors r+

[bors]

📌 Commit 57494f7 has been approved by davidtwco

[damccull]

Awesome. Thanks for getting to this even before I found out lol. Rust team is awesome.