Incorrect diagnostic when the feature isn't enabled

[ChrisPardy]

Compiling tte following code using the old capture logic

#![feature(rustc_attrs)]

struct S(i32);

fn foo() {
    let b = Box::new(S(10));
    
    let c = #[rustc_capture_analysis] || {
        let _b = b.0;
    };
    
    c();
}

prints the following diagnostic when compiled(view on playground):

note: Min Capture b[] -> ImmBorrow
  --> src/lib.rs:9:39
   |
9  |       let c = #[rustc_capture_analysis] || {
   |  _______________________________________^
10 | |         let _b = b.0;
   | |                  ^^^ b[] captured as ImmBorrow here
11 | |     };
   | |_____^ b[] used here

It should instead look something like:

note: Min Capture b[] -> ImmBorrow
  --> /users/cpardy/example2.rs:10:18
   |
10 |         let _b = b.0;
   |                  ^^^

[arora-aman]

This only looks weird when debugging the compiler using #[rustc_capture_analysis] and indicates that the compiler didn't find a complete use of that path and was essentially faked. Users won't be impacted as fallback will be implemented by #8