trace_macro: Show both the macro call and its expansion. #42072.

[jorendorff]

See #42072 for the initial motivation behind this.

The change is not the minimal fix, but I want this behavior almost every time I use trace_macros.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @pnkfelix (or someone else) soon.

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[shepmaster]

Thanks! We'll get @pnkfelix on the case soon!

[estebank]

Thanks for the PR!

For some time now notes accepts multiline strings. Given that there's some duplication, I'm wondering wether it'd be nice to have output along the lines of:

note: trace_macro
  --> $DIR/trace-macro.rs:14:5
   |
14 |     println!("Hello, World!");
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: expanding this macro call to:
           `println! { "Hello, World!" }`
           `print ! ( concat ! ( "Hello, World!" , "/n" ) )`
           `$crate :: io :: _print ( format_args ! ( concat ! ( "Hello, World!" , "/n" ) )
           )`

[jorendorff]

In the particular test in this patch, the TokenStreams mostly fit on one line each, but I suspect that's rare in practice.

Here's a noisier example:

note: trace_macro
 --> main.rs:5:5
  |
5 |     assert_eq!(vec![1, 2, 3,].len(), 3);
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: expanding `assert_eq! { vec ! [ 1 , 2 , 3 , ] . len (  ) , 3 }`
  = note: to `{
          match ( & vec!(1 , 2 , 3 ,).len() , & 3 ) {
          ( left_val , right_val ) => {
          if ! ( * left_val == * right_val ) {
          panic ! (
          "assertion failed: `(left == right)` \
                                     (left: `{:?}`, right: `{:?}`)"
          , left_val , right_val ) } } } }`
  = note: expanding `panic! { "assertion failed: `(left == right)` \
                                     (left: `{:?}`, right: `{:?}`)"
          , left_val , right_val }`
  = note: to `{
          $crate :: rt :: begin_panic_fmt (
          & format_args ! (
          "assertion failed: `(left == right)` (left: `{:?}`, right: `{:?}`)" , left_val
          , right_val ) , {
          static _FILE_LINE : ( & 'static str , u32 ) = ( file ! (  ) , line ! (  ) ) ;
          & _FILE_LINE } ) }`

Note also that the second expanding: note here is not redundant with the immediately preceding to: note. Of course when it is redundant, we could certainly eliminate it.

[estebank]

@bors r+

@jorendorff can you file a ticket to handle that case?

[bors]

📌 Commit f8b66a0 has been approved by estebank

[bors]

⌛ Testing commit f8b66a0 with merge 5469410...

[bors]

💔 Test failed - status-appveyor

[Mark-Simulacrum]

@bors retry

• appveyor timed out; not sure why though, nothing in log is immediately suspicious.

[bors]

⌛ Testing commit f8b66a0 with merge 9337ad5...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: estebank
Pushing 9337ad5 to master...