Add context to E0084, E0517, E0518

[ExpHP]

A small diagnostic enhancement to get my feet wet. Please scrutinize!

This modifies errors E0084, E0517, and E0518 to include both the annotation and the annotated item. All of these errors already had labels; I moved the label to the other span, and rephrased it as necessary.

Fixes #45886

[ExpHP]

Oops, just noticed these are missing tildes (but the test still passes?)

[ExpHP]

surveying nearby examples it seems I simply shouldn't include the "note" messages in this test anyways? (we just want to know that the error is triggered here?)

[estebank]

@ExpHP if you run build/<ENV>/stage1/bin/rustc src/test/compile-fail/attr-usage-repr.rs you will see the actual output. The test as it was previously was incorrect, as it wasn't asserting the label in that line. It should have been a //~ NOTE: not a struct there. Please change it here and elsewhere to avoid regressions and actually test this change.

[estebank]

r=me after replacing all the // with //~ NOTE: in the tests. Also, could yo paste in a comment the visual output (before and after preferably) before we merge?

[ExpHP]

Before

error[E0518]: attribute should be applied to function
  --> examples/E0518.rs:14:1
   |
14 | #[inline(never)] //~ ERROR E0518
   | ^^^^^^^^^^^^^^^^ requires a function

error[E0517]: attribute should be applied to struct or union
  --> examples/E0517.rs:14:1
   |
14 | #[repr(packed)] //~ requires a struct
   | ^^^^^^^^^^^^^^^ requires a struct or union

error[E0084]: unsupported representation for zero-variant enum
  --> examples/E0084.rs:12:1
   |
12 | enum Foo {}  //~ ERROR E0084
   | ^^^^^^^^^^^ unsupported enum representation

After

error[E0518]: attribute should be applied to function
  --> examples/E0518.rs:14:1
   |
14 |   #[inline(never)] //~ ERROR E0518
   |   ^^^^^^^^^^^^^^^^
15 | / impl Foo {       //~ not a function
16 | | }
   | |_- not a function

error[E0517]: attribute should be applied to struct or union
  --> examples/E0517.rs:14:1
   |
14 | #[repr(packed)] //~ requires a struct
   | ^^^^^^^^^^^^^^^
15 | enum Foo2 {Bar, Baz} //~ ERROR E0517
   | -------------------- not a struct or union

error[E0084]: unsupported representation for zero-variant enum
  --> examples/E0084.rs:11:1
   |
11 | #[repr(i32)] //~ unsupported enum representation
   | ^^^^^^^^^^^^
12 | enum Foo {}  //~ ERROR E0084
   | ----------- zero-variant enum

(In color)

Saved terminal output with ansi escape sequences:

$ # before
$ curl -s https://gist.githubusercontent.com/ExpHP/31fda7073c66462f633219a87e14c99a/raw/8cb74221ca2e7f0f138e85931541f1c754d81ffd/before-ansi

$ # after
$ curl -s https://gist.githubusercontent.com/ExpHP/88beefb1d1dfe8cbd21ad8883358b438/raw/95143504316b7dfe0c3c17f50684352546f99cf1/examples-ansi

[estebank]

@bors r+ rollup

[bors]

📌 Commit ead9ac3 has been approved by estebank