Ensure 'let mut ;' where ':pat' is banned.

src/libsyntax/parse/parser/pat.rs

@@ -384,6 +384,7 @@ impl<'a> Parser<'a> {
         })
     }
 
+    /// Parse a mutable binding with the `mut` token already eaten.
     fn parse_pat_ident_mut(&mut self) -> PResult<'a, PatKind> {
         let mut_span = self.prev_span;
 
@@ -393,6 +394,14 @@ impl<'a> Parser<'a> {
 
         self.recover_additional_muts();
 
+        // Make sure we don't allow e.g. `let mut $p;` where `$p:pat`.
+        if let token::Interpolated(ref nt) = self.token.kind {
+             if let token::NtPat(_) = **nt {
+                 self.expected_ident_found().emit();
+             }
+        }
+
+        // Parse the pattern we hope to be an identifier.
         let mut pat = self.parse_pat(Some("identifier"))?;
 
         // Add `mut` to any binding in the parsed pattern.

src/test/ui/parser/mut-patterns.rs

@@ -32,4 +32,12 @@ pub fn main() {
     let mut W(mut a, W(b, W(ref c, W(d, B { box f }))))
     //~^ ERROR `mut` must be attached to each individual binding
         = W(0, W(1, W(2, W(3, B { f: Box::new(4u8) }))));
+
+    // Make sure we don't accidentally allow `mut $p` where `$p:pat`.
+    macro_rules! foo {
+        ($p:pat) => {
+            let mut $p = 0; //~ ERROR expected identifier, found `x`
+        }
+    }
+    foo!(x);
 }

src/test/ui/parser/mut-patterns.stderr

@@ -64,5 +64,14 @@ error: `mut` must be attached to each individual binding
 LL |     let mut W(mut a, W(b, W(ref c, W(d, B { box f }))))
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: add `mut` to each binding: `W(mut a, W(mut b, W(ref c, W(mut d, B { box mut f }))))`
 
-error: aborting due to 9 previous errors
+error: expected identifier, found `x`
+  --> $DIR/mut-patterns.rs:39:21
+   |
+LL |             let mut $p = 0;
+   |                     ^^ expected identifier
+...
+LL |     foo!(x);
+   |     -------- in this macro invocation
+
+error: aborting due to 10 previous errors