Merge #93

93: Support leading pipe in match arms r=matklad a=DJMcNab

This adds support for match arms of the form:
```rust
<...>
| X | Y => <...>,
| X => <...>,
| 1..2 => <...>,
etc
```

# Implementation discussion

This just naïvely 'eats' a leading pipe if one is available. The equivalent line in the reference `libsyntax` is in [`parse_arm`](https://github.com/rust-lang/rust/blob/441519536c8bd138e8c651743249acd6814747a1/src/libsyntax/parse/parser.rs#L3552).

As noted in the comment linked above, this feature was formally introduced as a result of rust-lang/rfcs#1925. This feature is in active use in the [`rust-analyzer` codebase](https://github.com/matklad/rust-analyzer/blob/c87fcb4ea5874a7307c1d9d1192e923f3ae2c922/crates/ra_syntax/src/syntax_kinds/generated.rs#L231)

I have added some tests for this feature, but maybe more would be required

EDIT: Always looking for feedback - is this PR description over-engineered?

Co-authored-by: Daniel McNab <[email protected]>

1 files changed, 3 insertions, 0 deletions

diff --git a/crates/ra_syntax/src/grammar/expressions/atom.rs b/crates/ra_syntax/src/grammar/expressions/atom.rs
index f01df56bc..a720d255f 100644
--- a/crates/ra_syntax/src/grammar/expressions/atom.rs
+++ b/crates/ra_syntax/src/grammar/expressions/atom.rs
@@ -319,10 +319,13 @@ pub(crate) fn match_arm_list(p: &mut Parser) {
 //     match () {
 //         _ => (),
 //         X | Y if Z => (),
+//         | X | Y if Z => (),
+//         | X => (),
 //     };
 // }
 fn match_arm(p: &mut Parser) -> BlockLike {
     let m = p.start();
+    p.eat(PIPE);
     patterns::pattern_r(p, TokenSet::EMPTY);
     while p.eat(PIPE) {
         patterns::pattern(p);