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Don't recursively expand undefined macros #12197

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Don't recursively expand undefined macros #12197

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6 changes: 5 additions & 1 deletion src/libsyntax/ext/expand.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -64,7 +64,11 @@ pub fn expand_expr(e: @ast::Expr, fld: &mut MacroExpander) -> @ast::Expr {
extnamestr.get()));

// let compilation continue
return e;
return @ast::Expr {
id: ast::DUMMY_NODE_ID,
node: ast::ExprLogLevel,
span: e.span,
}
}
Some(&NormalTT(ref expandfun, exp_span)) => {
fld.cx.bt_push(ExpnInfo {
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20 changes: 20 additions & 0 deletions src/test/compile-fail/issue-11692.rs
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@@ -0,0 +1,20 @@
// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

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.

//~^^^ ERROR: format argument must be a string literal

concat!(test!());
//~^ ERROR: macro undefined: 'test'
//~^^ ERROR: expected a literal
}