1.17 nightly Compiler suggests wrong syntax `mut mut var`

[yasammez]

When trying to compile

fn main() {
    let mut buf = &[1, 2, 3, 4];
    let mut dst = &[0, 0, 0, 0];
    for (x, y) in buf.iter_mut().zip(dst) {
        *y = *x
    }
}

the compiler rustc 1.17.0-nightly (3da4023 2017-03-24) throws an error together with a helpful suggestion

error: cannot borrow immutable borrowed content `*buf` as mutable
 --> <anon>:4:19
  |
2 |     let mut buf = &[1, 2, 3, 4];
  |         ------- consider changing this to `mut mut buf`
3 |     let mut dst = &[0, 0, 0, 0];
4 |     for (x, y) in buf.iter_mut().zip(dst) {
  |                   ^^^ cannot borrow as mutable

Here, the mut mut buf is obviously very very wrong. The stable and beta compilers don't suffer from this issue (they just provide no „consider …“ message at all).

[hanna-kruppe]

I think this was introduced in #40445

[therealprof]

Also other incorrect mut changes are suggested:

error: cannot borrow immutable borrowed content `*parent` as mutable
   --> src/parser.rs:175:29
    |
174 |                         if let Some (&Node::RegularNode(ref parent)) = nodestack.last() {
    |                                                         ---------- consider changing this to `mut ref parent`
175 |                             parent.add_child (Node::RegularNode(node));
    |                             ^^^^^^ cannot borrow as mutable

which is also not valid and should be ref mut instead.

This happens with:

rustc 1.17.0-nightly (134c4a0 2017-03-20)

[cuviper]

#40859 shows a similar bad suggestion even on stable. So maybe that problem has existed for a while, and the cases here just weren't making suggestions at all until recent nightlies.