rustc suggests a syntax error (for x in &mut mut y)

[davidben]

This may be a duplicate of issue #57431; I don't understand Rust well enough yet to be sure.

When using chunks_exact instead of chunks_exact_mut and then mutating through the returned slices, rustc complains and suggests iterating over &mut mut chunks in the loop, which appears to be a syntax error.

I tried this code:

fn main() {
    let mut v = vec![1, 2, 3, 4, 5, 6, 7];
    let mut chunks = v.chunks_exact(2);
    for chunk in &mut chunks {
        chunk[0] += 1;
    }
    println!("v = {:?}; remainder = {:?}", v, chunks.remainder());
}

I expected to see this happen:

The actual bug is chunks_exact should be chunks_exact_mut (and then switch remainder to into_remainder to match). That's probably a tall order for the compiler to suggest, but the error message should definitely not suggest a syntax error.

Instead, this happened:

warning: variable does not need to be mutable
 --> src/main.rs:2:9
  |
2 |     let mut v = vec![1, 2, 3, 4, 5, 6, 7];
  |         ----^
  |         |
  |         help: remove this `mut`
  |
  = note: `#[warn(unused_mut)]` on by default

error[E0594]: cannot assign to `chunk[_]` which is behind a `&` reference
 --> src/main.rs:5:9
  |
4 |     for chunk in &mut chunks {
  |                  ----------- help: consider changing this to be a mutable reference: `&mut mut chunks`
5 |         chunk[0] += 1;
  |         ^^^^^^^^^^^^^ `chunk` is a `&` reference, so the data it refers to cannot be written

error: aborting due to previous error

Applying the suggestion results in a syntax error:

error: expected expression, found keyword `mut`
 --> src/main.rs:4:23
  |
4 |     for chunk in &mut mut chunks {
  |                       ^^^ expected expression

error: aborting due to previous error

Meta

rustc --version --verbose:
rustc 1.40.0 (73528e3 2019-12-16)
binary: rustc
commit-hash: 73528e3
commit-date: 2019-12-16
host: x86_64-unknown-linux-gnu
release: 1.40.0
LLVM version: 9.0

[rylev]

This no longer reproduces. Running on 1.48.0 I get the following diagnostics:

warning: variable does not need to be mutable
 --> src/main.rs:2:9
  |
2 |     let mut v = vec![1, 2, 3, 4, 5, 6, 7];
  |         ----^
  |         |
  |         help: remove this `mut`
  |
  = note: `#[warn(unused_mut)]` on by default

error[E0594]: cannot assign to `chunk[_]` which is behind a `&` reference
 --> src/main.rs:5:9
  |
4 |     for chunk in &mut chunks {
  |                  ----------- this iterator yields `&` references
5 |         chunk[0] += 1;
  |         ^^^^^^^^^^^^^ `chunk` is a `&` reference, so the data it refers to cannot be written