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]>

3 files changed, 59 insertions, 12 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);
diff --git a/crates/ra_syntax/tests/data/parser/inline/0069_match_arm.rs b/crates/ra_syntax/tests/data/parser/inline/0069_match_arm.rs
index 2c0e88414..b23a12500 100644
--- a/crates/ra_syntax/tests/data/parser/inline/0069_match_arm.rs
+++ b/crates/ra_syntax/tests/data/parser/inline/0069_match_arm.rs
@@ -2,5 +2,7 @@ fn foo() {
     match () {
         _ => (),
         X | Y if Z => (),
+        | X | Y if Z => (),
+        | X => (),
     };
 }
diff --git a/crates/ra_syntax/tests/data/parser/inline/0069_match_arm.txt b/crates/ra_syntax/tests/data/parser/inline/0069_match_arm.txt
index 013d1716a..f862b1268 100644
--- a/crates/ra_syntax/tests/data/parser/inline/0069_match_arm.txt
+++ b/crates/ra_syntax/tests/data/parser/inline/0069_match_arm.txt
@@ -1,5 +1,5 @@
-ROOT@[0; 78)
-  FN_DEF@[0; 77)
+ROOT@[0; 125)
+  FN_DEF@[0; 124)
     FN_KW@[0; 2)
     WHITESPACE@[2; 3)
     NAME@[3; 6)
@@ -8,18 +8,18 @@ ROOT@[0; 78)
       L_PAREN@[6; 7)
       R_PAREN@[7; 8)
     WHITESPACE@[8; 9)
-    BLOCK@[9; 77)
+    BLOCK@[9; 124)
       L_CURLY@[9; 10)
       WHITESPACE@[10; 15)
-      EXPR_STMT@[15; 75)
-        MATCH_EXPR@[15; 74)
+      EXPR_STMT@[15; 122)
+        MATCH_EXPR@[15; 121)
           MATCH_KW@[15; 20)
           WHITESPACE@[20; 21)
           TUPLE_EXPR@[21; 23)
             L_PAREN@[21; 22)
             R_PAREN@[22; 23)
           WHITESPACE@[23; 24)
-          MATCH_ARM_LIST@[24; 74)
+          MATCH_ARM_LIST@[24; 121)
             L_CURLY@[24; 25)
             WHITESPACE@[25; 34)
             MATCH_ARM@[34; 41)
@@ -58,9 +58,51 @@ ROOT@[0; 78)
                 L_PAREN@[65; 66)
                 R_PAREN@[66; 67)
             COMMA@[67; 68)
-            WHITESPACE@[68; 73)
-            R_CURLY@[73; 74)
-        SEMI@[74; 75)
-      WHITESPACE@[75; 76)
-      R_CURLY@[76; 77)
-  WHITESPACE@[77; 78)
+            WHITESPACE@[68; 77)
+            MATCH_ARM@[77; 95)
+              PIPE@[77; 78)
+              WHITESPACE@[78; 79)
+              BIND_PAT@[79; 80)
+                NAME@[79; 80)
+                  IDENT@[79; 80) "X"
+              WHITESPACE@[80; 81)
+              PIPE@[81; 82)
+              WHITESPACE@[82; 83)
+              BIND_PAT@[83; 84)
+                NAME@[83; 84)
+                  IDENT@[83; 84) "Y"
+              WHITESPACE@[84; 85)
+              IF_KW@[85; 87)
+              WHITESPACE@[87; 88)
+              PATH_EXPR@[88; 89)
+                PATH@[88; 89)
+                  PATH_SEGMENT@[88; 89)
+                    NAME_REF@[88; 89)
+                      IDENT@[88; 89) "Z"
+              WHITESPACE@[89; 90)
+              FAT_ARROW@[90; 92)
+              WHITESPACE@[92; 93)
+              TUPLE_EXPR@[93; 95)
+                L_PAREN@[93; 94)
+                R_PAREN@[94; 95)
+            COMMA@[95; 96)
+            WHITESPACE@[96; 105)
+            MATCH_ARM@[105; 114)
+              PIPE@[105; 106)
+              WHITESPACE@[106; 107)
+              BIND_PAT@[107; 108)
+                NAME@[107; 108)
+                  IDENT@[107; 108) "X"
+              WHITESPACE@[108; 109)
+              FAT_ARROW@[109; 111)
+              WHITESPACE@[111; 112)
+              TUPLE_EXPR@[112; 114)
+                L_PAREN@[112; 113)
+                R_PAREN@[113; 114)
+            COMMA@[114; 115)
+            WHITESPACE@[115; 120)
+            R_CURLY@[120; 121)
+        SEMI@[121; 122)
+      WHITESPACE@[122; 123)
+      R_CURLY@[123; 124)
+  WHITESPACE@[124; 125)