"patterns are not allowed to reset the default binding mode" suggestion means nothing to me

[shepmaster]

Code

use std::collections::BTreeMap;

fn demo(map: BTreeMap<(usize, usize), char>) {
    map.iter().filter(|(&(_x, _y), &_c)| false);
}

Current output

error: patterns are not allowed to reset the default binding mode in edition 2024
 --> src/lib.rs:4:24
  |
4 |     map.iter().filter(|(&(_x, _y), &_c)| false);
  |                        -^^^^^^^^^^^^^^^
  |                        |
  |                        help: desugar the match ergonomics: `&`

Desired output

error: patterns are not allowed to reset the default binding mode in edition 2024
 --> src/lib.rs:4:24
  |
4 |     map.iter().filter(|(&(_x, _y), &_c)| false);
  |                        -^^^^^^^^^^^^^^^
  |                        |
  |                        help: add an `&` at the beginning of this pattern

Rationale and extra context

Presented with this error, I was completely stumped. I have no idea what the suggestion is attempting to get me to do.

I do not think the word "desugar" is useful here because it's not obvious to me (a) that there was sugar and (b) what the sugaring was. Without that knowledge, the suggestion is non-actionable.

After minutes of reading/reducing/reproducing the problem, I finally realized that that the - part of the -^^^^^^^^^^^^^^^ was pointing to the parenthesis and not to the &, so I tried adding it and it compiled.

Additionally, I worry that "default binding mode" is not an established / well-known terminology so there's not much meat from that message for me to go on either.

Rust Version

rustc 1.85.0-nightly (c44b3d50f 2024-12-03)
binary: rustc
commit-hash: c44b3d50fea96a3e0417e8264c16ea21a0a3fca2
commit-date: 2024-12-03
host: aarch64-apple-darwin
release: 1.85.0-nightly
LLVM version: 19.1.4

[traviscross]

cc @Nadrieril @Jules-Bertholet

[dianne]

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