Provide context for macro expansions which result in unparsed tokens.

[whipsch]

Addresses issue #22425. See src/test/compile-fail/macro-incomplete-parse.rs for a relevant test:

macro-incomplete-parse.rs:15:9: 15:10 error: macro expansion ignores token `,` and any following
macro-incomplete-parse.rs:15         , //~ ERROR macro expansion ignores token `,`
                                     ^
macro-incomplete-parse.rs:27:1: 27:17 note: caused by the macro expansion here; the usage of `ignored_item` is likely invalid in this context
macro-incomplete-parse.rs:27 ignored_item!();
                             ^~~~~~~~~~~~~~~~
macro-incomplete-parse.rs:20:14: 20:15 error: macro expansion ignores token `,` and any following
macro-incomplete-parse.rs:20     () => ( 1, 2 ) //~ ERROR macro expansion ignores token `,`
                                          ^
macro-incomplete-parse.rs:30:5: 30:21 note: caused by the macro expansion here; the usage of `ignored_expr` is likely invalid in this context
macro-incomplete-parse.rs:30     ignored_expr!();
                                 ^~~~~~~~~~~~~~~~
macro-incomplete-parse.rs:24:14: 24:15 error: macro expansion ignores token `,` and any following
macro-incomplete-parse.rs:24     () => ( 1, 2 ) //~ ERROR macro expansion ignores token `,`
                                          ^
macro-incomplete-parse.rs:32:9: 32:23 note: caused by the macro expansion here; the usage of `ignored_pat` is likely invalid in this context
macro-incomplete-parse.rs:32         ignored_pat!() => (),
                                     ^~~~~~~~~~~~~~

This does not address the case of improper expansion inside of an impl { } as seen in issue #21607.

I'm not sure if the note text is ideal, but it can be refined if needed.

[rust-highfive]

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

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[alexcrichton]

Thanks! Could you update the test case as well with this error message? I believe //~ NOTE or something similar should provide a good assertion that it's being emitted.

Also, could you rebase out the merge commit in the middle?

[whipsch]

//~ NOTE was indeed correct.

@alexcrichton should be good to go

[alexcrichton]

Thanks! r? @kmcallister or @huonw or @sfackler (probably shoulda done that earlier as well..)

[rprichard]

When macro invocations expand to other macro invocations, I believe this fix only prints one of the call sites (either the immediate site or the earliest site, probably). Is that right? There will be a complete backtrace in the ExtCtxt::backtrace() value -- maybe we ought to print it?

It seems there's a family of issues whose root cause is that we don't print a macro backtrace while we're parsing -- only after parsing/expansion has attached backtraces to spans. I'm wondering if they all have a single fix -- when we print a parsing error, also print the current ExtCtxt::backtrace().

Other issues include:

• No expansion backtrace for macro where contents does not parse #23243
• an invalid arg format_args! reference of writeln! and println! (and builtin macros’?) diagnostics are terrible #23459
• an unused format_args! argument

[huonw]

I'm wondering if they all have a single fix -- when we print a parsing error, also print the current ExtCtxt::backtrace().

That sounds like a good idea to me, although the specific information offered by this patch is good too.

[huonw]

@bors r+

(We can add/swap in a more general approach in future.)

[bors]

📌 Commit ab32154 has been approved by huonw

[bors]

⌛ Testing commit ab32154 with merge aecd165...

[bors]

💔 Test failed - auto-mac-32-opt

[sfackler]

@bors: retry

[bors]

⌛ Testing commit ab32154 with merge ce97c19...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt