Implement RFC 1925

src/doc/unstable-book/src/language-features/match-beginning-vert.md

@@ -0,0 +1,21 @@
+# `match_beginning_vert`
+
+The tracking issue for this feature is [#44101].
+
+With this feature enabled, you are allowed to add a '|' to the beginning of a
+match arm:
+
+```rust
+#![feature(match_beginning_vert)]
+
+enum Foo { A, B }
+
+fn main() {
+    let x = Foo::A;
+    match x {
+        | A | B => {},
+    }
+}
+```
+
+[#44101]: https://github.com/rust-lang/rust/issues/44101

src/libsyntax/ast.rs

@@ -810,6 +810,7 @@ pub struct Arm {
     pub pats: Vec<P<Pat>>,
     pub guard: Option<P<Expr>>,
     pub body: P<Expr>,
+    pub beginning_vert: Option<Span>, // For RFC 1925 feature gate
 }
 
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]

src/libsyntax/ext/build.rs

@@ -878,7 +878,8 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
             attrs: vec![],
             pats,
             guard: None,
-            body: expr
+            body: expr,
+            beginning_vert: None,
         }
     }
 

src/libsyntax/feature_gate.rs

@@ -379,6 +379,9 @@ declare_features! (
 
     // allow `#[must_use]` on functions (RFC 1940)
     (active, fn_must_use, "1.21.0", Some(43302)),
+
+    // allow '|' at beginning of match arms (RFC 1925)
+    (active, match_beginning_vert, "1.21.0", Some(44101)),
 );
 
 declare_features! (
@@ -1435,6 +1438,14 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
         visit::walk_expr(self, e);
     }
 
+    fn visit_arm(&mut self, arm: &'a ast::Arm) {
+        if let Some(span) = arm.beginning_vert {
+            gate_feature_post!(&self, match_beginning_vert,
+                               span,
+                               "Use of a '|' at the beginning of a match arm is experimental")
+        }
+    }
+
     fn visit_pat(&mut self, pattern: &'a ast::Pat) {
         match pattern.node {
             PatKind::Slice(_, Some(_), ref last) if !last.is_empty() => {

src/libsyntax/fold.rs

@@ -344,12 +344,14 @@ pub fn fold_thin_attrs<T: Folder>(attrs: ThinVec<Attribute>, fld: &mut T) -> Thi
     fold_attrs(attrs.into(), fld).into()
 }
 
-pub fn noop_fold_arm<T: Folder>(Arm {attrs, pats, guard, body}: Arm, fld: &mut T) -> Arm {
+pub fn noop_fold_arm<T: Folder>(Arm {attrs, pats, guard, body, beginning_vert}: Arm,
+    fld: &mut T) -> Arm {
     Arm {
         attrs: fold_attrs(attrs, fld),
         pats: pats.move_map(|x| fld.fold_pat(x)),
         guard: guard.map(|x| fld.fold_expr(x)),
         body: fld.fold_expr(body),
+        beginning_vert,
     }
 }
 

src/libsyntax/parse/parser.rs

@@ -3155,6 +3155,12 @@ impl<'a> Parser<'a> {
         maybe_whole!(self, NtArm, |x| x);
 
         let attrs = self.parse_outer_attributes()?;
+        // Allow a '|' before the pats (RFC 1925)
+        let beginning_vert = if self.eat(&token::BinOp(token::Or)) {
+            Some(self.prev_span)
+        } else {
+            None
+        };
         let pats = self.parse_pats()?;
         let guard = if self.eat_keyword(keywords::If) {
             Some(self.parse_expr()?)
@@ -3178,6 +3184,7 @@ impl<'a> Parser<'a> {
             pats,
             guard,
             body: expr,
+            beginning_vert,
         })
     }
 

src/test/compile-fail/feature-gate-match_beginning_vert.rs

@@ -0,0 +1,36 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[allow(dead_code)]
+enum Foo {
+    A,
+    B,
+    C,
+    D,
+    E,
+}
+use Foo::*;
+
+fn main() {
+    let x = Foo::A;
+    match x {
+        | A => println!("A"),
+        //~^ ERROR: Use of a '|' at the beginning of a match arm is experimental (see issue #44101)
+        | B | C => println!("BC!"),
+        //~^ ERROR: Use of a '|' at the beginning of a match arm is experimental (see issue #44101)
+        | _ => {},
+        //~^ ERROR: Use of a '|' at the beginning of a match arm is experimental (see issue #44101)
+    };
+    match x {
+        A | B | C => println!("ABC!"),
+        _ => {},
+    };
+}
+