Don't recursively expand undefined macros #12197
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This led to infinite recursion when compiling a macro which inside used an undefined macro. Instead some dummy expression is returned for the result of expansion (rather than the macro itself). Closes rust-lang#11692
| fn main() { | ||
| print!(test!()); | ||
| //~^ ERROR: macro undefined: 'test' | ||
| //~^^ ERROR: macro undefined: 'test' |
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Why would this print the error twice?
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I didn't actually quite figure that out. This has to do with constructing a nested parser that doesn't have the same macro context as the outer context, but at least one of the errors is for the sub-parser complaining about an unknown macro, and the second error message may come from the original parser parsing the expression again?
I figured going from infinity instances of 'macro undefined' to two was at least somewhat better. Fixing the two prints I think would require a better understanding of macro parsing (which may be a reason to reject this patch).
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The double error occurs because of how format_args!() is defined. If the first argument is not a string literal then it emits an error but also expands to the first argument. So then the macro is attempted to be expanded again, and fails.
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I just noticed that you can trigger similar behaviour with the code: It seems there is a fundamental issue with returning the unexpanded expression from expand_expr() |
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Also, this does not happen with a simple macro created by macro_rules!() |
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Closing in favor of #12370 |
Closes #11692. Instead of returning the original expression, a dummy expression (with identical span) is returned. This prevents infinite loops of failed expansions as well as odd double error messages in certain situations. This is a slightly better fix than #12197, because it does not produce a double error and also fixes a few other cases where an infinite loop could happen. This does not fix the other issue in #11692 (non-builtin macros not being recognised when expanded inside macros), which I think should be moved into a separate issue.
fix: Fix import insertion inserting after last comment in a file
fix [`missing_docs_in_private_items`] on some proc macros fixes: rust-lang#12197 --- changelog: [`missing_docs_in_private_items`] support manually search for docs as fallback method
This led to infinite recursion when compiling a macro which inside used an
undefined macro. Instead some dummy expression is returned for the result of
expansion (rather than the macro itself).
Closes #11692