Resolve the nested inner macros correctly

[edwardw]

Currently, the inner macro in a nested macro invocation is not properly resolved which prevents nested macro invocation from working. If we are to support such invocation, this issue ought to be fixed.

cc #11692. #9323 is also relevant with regard to the macro scope rules in general.

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Example from #13876:

The expression println!(format!("test")) produces the following error:

<anon>:18:18: 18:24 error: macro undefined: 'format'
<anon>:18         println!(format!("test"))
                           ^~~~~~
<anon>:18:18: 18:33 error: format argument must be a string literal.
<anon>:18         println!(format!("test"))
                           ^~~~~~~~~~~~~~~
error: aborting due to 2 previous errors

[sfackler]

cc me

[huonw]

Could you give a code example of this to aid future triagers?

[edwardw]

Certainly.

echo 'fn main() { print!(format!("BAM")); }' | rustc -

should compile but it doesn't right now. Note

echo 'fn main() { print!(concat!("foo", " ", "bar\n")); }' | rustc -

does work but only because concat! is one of the handful builtin macros defined in syntax::ext::base::syntax_expander_table.

[alexcrichton]

I'm not sure that print!(format!("test")) should be valid because you'd have to execute the format string at compile-time (which can run arbitrary code).

The only reason that print!(concat!(..)) works is because special handler code was added. @jbclements also had concerns (#9740 (comment)) about expanding nested macros in terms of hygiene.

[huonw]

@alexcrichton it currently fails with a "undefined macro: format" error.

[jbclements]

This would be an appropriate place for partial expansion, if we had it. Specifically: expand the argument until it's clear that it is or isn't a string.

[steveklabnik]

Looks like this may be fixed:

hello.rs:2:14: 2:28 error: expected a literal
hello.rs:2     println!(format!("lol"));
                        ^~~~~~~~~~~~~~

[KalitaAlexey]

Hi @steveklabnik,

It works. Why this issue still opened?

[Mark-Simulacrum]

Yeah, I think that we've solved this.