Use diagnostics for trace_macro instead of println

[estebank]

When using trace_macro, use span_labels instead of println:

note: trace_macro
--> $DIR/trace-macro.rs:14:5
|
14 | println!("Hello, World!");
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: expands to `println!{"Hello, World!"}`
= note: expands to `print!{ concat ! ("Hello, World!","\n")}`

Fix#22597.

[rust-highfive]

r? @nikomatsakis

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[estebank]

CC @brson@jonathandturner

[nikomatsakis]

Do we have any tests of this feature? Maybe make a src/test/ui test?

[durka]

That's... a ton of extra noise, compared to:

println! { "" }
print! { concat ! ( "" , "\n" ) }

especially if a macro has multiple expansion steps. What's the concrete benefit to users of the feature, here? Can you at least get rid of the "macro in external crate" bit?

[nikomatsakis]

@durka line numbers seem like a win, but if they're just repeated N times, perhaps less useful that way. We could presumably make these attached notes without spans, right? (I've never actually used this utility...)

[durka]

Maybe it's possible to attach all the expansion steps to one span? Like this:

note: trace_macro
--> ../../src/test/run-pass/log_syntax-trace_macros-macro-locations.rs:27:5
|
27 | println!("");
| ^^^^^^^^^^^^^ expands to `println! { "" }`
| ^^^^^^^^^^^^^ expands to `print! { concat ! ( "" , "\n" ) }`

That would in fact be better than the current output, where you have to wade through all of it to find a particular macro's expansion. The rightward drift still bugs me a bit.

[nikomatsakis]

@durka yes that was roughly what I had in mind, but probably more like

note: trace_macro
--> ../../src/test/run-pass/log_syntax-trace_macros-macro-locations.rs:27:5
|
27 | println!("");
| ^^^^^^^^^^^^^ tracing macro expansion here
|
| note: expands to `println! { "" }`
| note: expands to `print! { concat ! ( "" , "\n" ) }`

[arielb1]

@estebank - just a friendly ping to keep this on your radar

[estebank]

Updated.

[nikomatsakis]

@bors r+

[bors]

📌 Commit 8c9ad8d has been approved by nikomatsakis

[bors]

⌛ Testing commit 8c9ad8d with merge bedd7da...

[bors]

☔ The latest upstream changes (presumably #41846) made this pull request unmergeable. Please resolve the merge conflicts.