Tracking issue for RFC #2175: or-patterns in if / while let expressions

[aturon]

This is a tracking issue for the RFC "or-patterns in if / while let expressions" (rust-lang/rfcs#2175).

Steps:

• Implement the RFC (cc @rust-lang/compiler -- can anyone write up mentoring instructions?)
• Adjust documentation (Stabilize #![feature(if_while_or_patterns)] #57532)
• Stabilization PR (Stabilize #![feature(if_while_or_patterns)] #57532)

[zackmdavis]

This seems likely to be a rather straightforward change: currently parse_arm calls parse_pats to parse the |-separated patterns of a match arm, whereas parse_if_let_expr just calls parse_pat.

I'd like to take this.

[zackmdavis]

Arguably we should retitle the issue and RFC to make it clear that the ultimately approved proposal actually includes let itself (in addition to if let and while let).

[zackmdavis]

This seems likely to be a rather straightforward change: currently [...]

Er, this remark was terribly naïve: the change to the parser might be that simple, but then the IfLet (&c.) AST nodes have to hold a vector rather than a single pattern, and much of the code that handles them (e.g., HIR lowering) needs to change correspondingly.

[nikomatsakis]

@zackmdavis

Er, this remark was terribly naïve: the change to the parser might be that simple, but then the IfLet (&c.) AST nodes have to hold a vector rather than a single pattern, and much of the code that handles them (e.g., HIR lowering) needs to change correspondingly.

Sounds about right. but it should still be something that can be done purely in HIR lowering, right? I would think so.

[zackmdavis]

something that can be done purely in HIR lowering, right?

Visitors, pretty-printing, and resolution are also affected.

[nikomatsakis]

Sounds about right. I guess what I meant is "after HIR lowering, I wouldn't expect much other code to care"

[zackmdavis]

(I was actively working on this today, but would seem to have been pre-empted by #48490.)

[dtolnay]

Does optional leading vert need to be supported here for consistency with #44101?

if let
    | A(x)
    | B(x) = expr
{
    /* ... */
}

@mdinger