Rollup merge of #90856 - ken-matsui:suggestion-to-wrap-vec-allocator-api-in-tuple, r=davidtwco

Suggestion to wrap inner types using 'allocator_api' in tuple

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

Closes https://github.com/rust-lang/rust/issues/83250

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

```diff
 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
```

src/test/ui/stability-attribute/suggest-vec-allocator-api.rs

@@ -0,0 +1,9 @@
+fn main() {
+    let _: Vec<u8, _> = vec![]; //~ ERROR use of unstable library feature 'allocator_api'
+    #[rustfmt::skip]
+    let _: Vec<
+        String,
+        _> = vec![]; //~ ERROR use of unstable library feature 'allocator_api'
+    let _ = Vec::<u16, _>::new(); //~ ERROR use of unstable library feature 'allocator_api'
+    let _boxed: Box<u32, _> = Box::new(10); //~ ERROR use of unstable library feature 'allocator_api'
+}

src/test/ui/stability-attribute/suggest-vec-allocator-api.stderr

@@ -0,0 +1,49 @@
+error[E0658]: use of unstable library feature 'allocator_api'
+  --> $DIR/suggest-vec-allocator-api.rs:2:20
+   |
+LL |     let _: 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
+
+error[E0658]: use of unstable library feature 'allocator_api'
+  --> $DIR/suggest-vec-allocator-api.rs:6:9
+   |
+LL |         _> = vec![];
+   |         ^
+   |
+   = 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
+help: consider wrapping the inner types in tuple
+   |
+LL ~     let _: Vec<(
+LL +         String,
+LL ~         _)> = vec![];
+   |
+
+error[E0658]: use of unstable library feature 'allocator_api'
+  --> $DIR/suggest-vec-allocator-api.rs:8:26
+   |
+LL |     let _boxed: Box<u32, _> = Box::new(10);
+   |                          ^
+   |
+   = 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
+
+error[E0658]: use of unstable library feature 'allocator_api'
+  --> $DIR/suggest-vec-allocator-api.rs:7:24
+   |
+LL |     let _ = Vec::<u16, _>::new();
+   |                   -----^
+   |                   |
+   |                   help: consider wrapping the inner types in tuple: `(u16, _)`
+   |
+   = 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
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0658`.