Auto merge of #36767 - jseyfried:enforce_rfc_1560_shadowing, r=nrc

Enforce the shadowing restrictions from RFC 1560 for today's macros

This PR enforces a weakened version of the shadowing restrictions from RFC 1560. More specifically,
 - If a macro expansion contains a `macro_rules!` macro definition that is used outside of the expansion, the defined macro may not shadow an existing macro.
 - If a macro expansion contains a `#[macro_use] extern crate` macro import that is used outside of the expansion, the imported macro may not shadow an existing macro.

This is a [breaking-change]. For example,
```rust
macro_rules! m { () => {} }
macro_rules! n { () => {
    macro_rules! m { () => {} } //< This shadows an existing macro.
    m!(); //< This is inside the expansion that generated `m`'s definition, so it is OK.
} }
n!();
m!(); //< This use of `m` is outside the expansion, so it causes the shadowing to be an error.
```

r? @nrc

1 files changed, 1 insertions, 2 deletions

diff --git a/src/libsyntax_ext/lib.rs b/src/libsyntax_ext/lib.rs
index bd9f1cf0d77..6e4f3dde4bd 100644
--- a/src/libsyntax_ext/lib.rs
+++ b/src/libsyntax_ext/lib.rs
@@ -51,13 +51,12 @@ pub mod deriving;
 use std::rc::Rc;
 use syntax::ast;
 use syntax::ext::base::{MacroExpanderFn, NormalTT, IdentTT, MultiModifier};
-use syntax::ext::hygiene::Mark;
 use syntax::ext::tt::macro_rules::MacroRulesExpander;
 use syntax::parse::token::intern;
 
 pub fn register_builtins(resolver: &mut syntax::ext::base::Resolver, enable_quotes: bool) {
     let mut register = |name, ext| {
-        resolver.add_ext(Mark::root(), ast::Ident::with_empty_ctxt(intern(name)), Rc::new(ext));
+        resolver.add_ext(ast::Ident::with_empty_ctxt(intern(name)), Rc::new(ext));
     };
 
     register("macro_rules", IdentTT(Box::new(MacroRulesExpander), None, false));