mut keyword alone unexpectedly dereferences value in pattern matching

[yescallop]

I tried this code:

let [a] = &[()];
let [mut b] = &[()];
b = a;

I expected to see this happen: the mutable binding b has type &(), and thus the compilation succeeds without any error.

Instead, this happened: the mutable binding b has type (), and the compilation fails with error:

error[E0308]: mismatched types
  --> src\lib.rs:80:9
   |
79 |     let [mut b] = &[()];
   |          ----- expected due to the type of this binding
80 |     b = a;
   |         ^ expected `()`, found `&()`
   |
help: consider dereferencing the borrow
   |
80 |     b = *a;
   |         +

Meta

rustc --version --verbose:

rustc 1.68.0-nightly (37d7de337 2022-12-12)
binary: rustc
commit-hash: 37d7de337903a558dbeb1e82c844fe915ab8ff25
commit-date: 2022-12-12
host: x86_64-pc-windows-msvc
release: 1.68.0-nightly
LLVM version: 15.0.6

[RedDocMD]

I don't think this is a bug. a resolves to &() because of match ergonomics while b is matched to () because b is explicitly marked as mut and so match ergonomics don't apply.

[Antikyth]

I don't think this is a bug. a resolves to &() because of match ergonomics while b is matched to () because b is explicitly marked as mut and so match ergonomics don't apply.

Are the match ergonomic rules listed somewhere I can learn more about? When they do or don't apply, as you say, isn't very intuitive to me, if it's explained somewhere I would like to learn more about it :)

[RedDocMD]

The Match Ergonomics RFC is a very good read on this. If you're in a hurry, you can start reading from here.

[yescallop]

I also notice that this issue was last mentioned by @varkor at #64586 (comment).

If I understand the match ergonomics RFC correctly, the code above currently gets desugared to:

let &[ref a] = &[()];
let &[mut b] = &[()]; // The default binding mode `ref` gets overridden by `mut`.
b = a;

which is quite a bit unintuitive. The desugared code I'd expect should be like this:

let &[ref a] = &[()];
let mut b = match &[()] {
    &[ref b] => b,
};
b = a;

I believe that both this issue and #64586 can be and ought to be fixed by a future RFC, in which we should probably consider another approach other than the current one where the default binding mode can be unexpectedly overridden.

[Nadrieril]

Hi! I was hoping to get this fixed on the new edition. I initially had a complex proposal but turns out the issue is pretty straightforward:

• it's easy to detect patterns that use mut in this surprising way (did it here for a crater run);
• it's straightforward to change a pattern so it doesn't rely on that (simply desugar the match ergonomics, i.e. add & and ref as appropriate);
• it's easy to make mut behave like it should in the new edition.

Moreover, the lang team has expressed general agreement to solve this for 2024 (contingent on details being ironed out). But, I'm finding no bandwidth to work on this. So, if anyone wants to take over this, please do. I'm happy to answer questions if you're not entirely clueless about what this would involve (mostly patience to go through the processes, which I don't have enough of).

And if not, well we can take our time and get it for 2027.