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]

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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...