Decide on Rule 4 variant for match ergonomics

[traviscross]

In our 2024-06-26 design meeting on match ergonomics (part 3):

• Design meeting: Match ergonomics 2024 part 3 lang-team#268

...we decided on a slate on rules:

• Rule 1: When the DBM (default binding mode) is not move (whether or not behind a reference), writing mut on a binding is an error.
• Rule 2: When a reference pattern matches against a reference, do not update the DBM.
• Rule 3: If we've previously matched against a shared reference in the scrutinee (or against a ref DBM under Rule 4, or against a mutable reference treated as a shared one or a ref mut DBM treated as a ref one under Rule 5), set the DBM to ref whenever we would otherwise set it to ref mut.
• Rule 4: If an & or &mut pattern is being matched against a non-reference type and if the DBM is ref or ref mut, match the pattern against the DBM as though it were a type.
• Rule 5: If an & pattern is being matched against a mutable reference type (or against a ref mut DBM under Rule 4), act as if the type were a shared reference instead (or that the ref mut DBM is a ref DBM instead).

In discussion toward the end of and after the meeting, there emerged some potentially good reasons to adopt a slight variation of Rule 4. The motivation is that, under Rule 4, this is an error:

let [&mut x] = &mut [&T]; //~ ERROR

(T here is a unit struct that implements Copy.)

That's kind of awkward and unfortunate, because without the &mut in the pattern, we get a type of &mut &T, so we really want adding &mut to get us a &T, since, in all other cases, that's what happens.

We can make this work with a small tweak to Rule 4 (which we'll call the extended Rule 4):

• Rule 4 (extended): If an & pattern is being matched against a non-reference type or an &mut pattern is being matched against a shared reference type or a non-reference type, and if the DBM is ref or ref mut, match the pattern against the DBM as though it were a type.

(Emphasis highlights the diff.)

We've analyzed this now, and think it's OK and a good tweak. We nominate to confirm this sounds good to the team.

If adopted, we'll revise Rule 4 to be the extended version.

Tracking:

• Tracking Issue for match ergonomics 2024 (RFC 3627) #123076

@rustbot labels +T-lang +I-lang-nominated +C-discussion

cc @Jules-Bertholet @Nadrieril

[traviscross]

@rustbot labels -I-lang-nominated

We discussed this in the triage meeting today, 2024-07-03, and we resolved via meeting consensus, with tmandry, nikomatsakis, Josh, and myself present, to adopt the extended version of Rule 4 as stated above as Rule 4, and to amend our meeting consensus from the 2024-06-26 design meeting accordingly.

The next step would be for the RFC and the experimental implementation to be updated according to Rules 1-5 as amended above.