Change `&` pat to only work with &T, and `&mut` with &mut T.

This implements RFC 179 by making the pattern `&<pat>` require matching
against a variable of type `&T`, and introducing the pattern `&mut
<pat>` which only works with variables of type `&mut T`.

The pattern `&mut x` currently parses as `&(mut x)` i.e. a pattern match
through a `&T` or a `&mut T` that binds the variable `x` to have type
`T` and to be mutable. This should be rewritten as follows, for example,

    for &mut x in slice.iter() {

becomes

    for &x in slice.iter() {
        let mut x = x;

Due to this, this is a

[breaking-change]

Closes #20496.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libsyntax/ext/deriving/generic/mod.rs b/src/libsyntax/ext/deriving/generic/mod.rs
index 8863de8757b..d522c346fa0 100644
--- a/src/libsyntax/ext/deriving/generic/mod.rs
+++ b/src/libsyntax/ext/deriving/generic/mod.rs
@@ -940,7 +940,7 @@ impl<'a> MethodDef<'a> {
                                                                          &**variant,
                                                                          self_arg_name,
                                                                          ast::MutImmutable);
-                    (cx.pat(sp, ast::PatRegion(p)), idents)
+                    (cx.pat(sp, ast::PatRegion(p, ast::MutImmutable)), idents)
                 };
 
                 // A single arm has form (&VariantK, &VariantK, ...) => BodyK