initial implementation of or-pattern parsing

Initial implementation of parsing or-patterns e.g., `Some(Foo | Bar)`.
This is a partial implementation of RFC 2535.

Author: dlrobertson

src/doc/unstable-book/src/language-features/or-patterns.md

@@ -0,0 +1,36 @@
+# `or_patterns`
+
+The tracking issue for this feature is: [#54883]
+
+[#54883]: https://github.com/rust-lang/rust/issues/54883
+
+------------------------
+
+The `or_pattern` language feature allows `|` to be arbitrarily nested within
+a pattern, for example, `Some(A(0) | B(1 | 2))` becomes a valid pattern.
+
+## Examples
+
+```rust,ignore
+#![feature(or_patterns)]
+
+pub enum Foo {
+    Bar,
+    Baz,
+    Quux,
+}
+
+pub fn example(maybe_foo: Option<Foo>) {
+    match maybe_foo {
+        Some(Foo::Bar | Foo::Baz) => {
+            println!("The value contained `Bar` or `Baz`");
+        }
+        Some(_) => {
+            println!("The value did not contain `Bar` or `Baz`");
+        }
+        None => {
+            println!("The value was `None`");
+        }
+    }
+}
+```

src/librustc/hir/mod.rs

@@ -978,7 +978,8 @@ pub enum PatKind {
     TupleStruct(QPath, HirVec<P<Pat>>, Option<usize>),
 
     /// An or-pattern `A | B | C`.
-    Or(Vec<P<Pat>>),
+    /// Invariant: `pats.len() >= 2`.
+    Or(HirVec<P<Pat>>),
 
     /// A path pattern for an unit struct/variant or a (maybe-associated) constant.
     Path(QPath),

src/librustc/hir/print.rs

@@ -4,7 +4,7 @@ use syntax::source_map::{SourceMap, Spanned};
 use syntax::parse::ParseSess;
 use syntax::print::pp::{self, Breaks};
 use syntax::print::pp::Breaks::{Consistent, Inconsistent};
-use syntax::print::pprust::{self, Comments, PrintState, SeparatorSpacing};
+use syntax::print::pprust::{self, Comments, PrintState};
 use syntax::symbol::kw;
 use syntax::util::parser::{self, AssocOp, Fixity};
 use syntax_pos::{self, BytePos, FileName};
@@ -1688,8 +1688,7 @@ impl<'a> State<'a> {
                 self.s.word("}");
             }
             PatKind::Or(ref pats) => {
-                let spacing = SeparatorSpacing::Both;
-                self.strsep("|", spacing, Inconsistent, &pats[..], |s, p| s.print_pat(&p))?;
+                self.strsep("|", true, Inconsistent, &pats[..], |s, p| s.print_pat(&p));
             }
             PatKind::Tuple(ref elts, ddpos) => {
                 self.popen();

src/librustc_mir/build/matches/mod.rs

@@ -658,9 +658,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 }
             }
             PatternKind::Or { ref pats } => {
-                // FIXME(#47184): extract or handle `pattern_user_ty` somehow
                 for pat in pats {
-                    self.visit_bindings(&pat, &pattern_user_ty.clone(), f);
+                    self.visit_bindings(&pat, pattern_user_ty.clone(), f);
                 }
             }
         }

src/librustc_mir/hair/pattern/_match.rs

@@ -75,6 +75,9 @@
 ///                     D((r_1, p_(i,2), .., p_(i,n)))
 ///                     D((r_2, p_(i,2), .., p_(i,n)))
 ///
+///     Note that the OR-patterns are not always used directly in Rust, but are used to derive
+///     the exhaustive integer matching rules, so they're written here for posterity.
+///
 /// The algorithm for computing `U`
 /// -------------------------------
 /// The algorithm is inductive (on the number of columns: i.e., components of tuple patterns).

src/librustc_mir/hair/pattern/mod.rs

@@ -176,7 +176,8 @@ pub enum PatternKind<'tcx> {
         suffix: Vec<Pattern<'tcx>>,
     },
 
-    /// or-pattern
+    /// An or-pattern, e.g. `p | q`.
+    /// Invariant: `pats.len() >= 2`.
     Or {
         pats: Vec<Pattern<'tcx>>,
     },

src/librustc_typeck/check/_match.rs

@@ -313,7 +313,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             PatKind::Or(ref pats) => {
                 let expected_ty = self.structurally_resolved_type(pat.span, expected);
                 for pat in pats {
-                    self.check_pat_walk(pat, expected, def_bm, false);
+                    self.check_pat_walk(pat, expected, def_bm, discrim_span);
                 }
                 expected_ty
             }

src/libsyntax/ast.rs

@@ -650,6 +650,7 @@ pub enum PatKind {
     TupleStruct(Path, Vec<P<Pat>>),
 
     /// An or-pattern `A | B | C`.
+    /// Invariant: `pats.len() >= 2`.
     Or(Vec<P<Pat>>),
 
     /// A possibly qualified path pattern.

src/libsyntax/feature_gate.rs

@@ -559,6 +559,9 @@ declare_features! (
     // Allows `impl Trait` to be used inside type aliases (RFC 2515).
     (active, type_alias_impl_trait, "1.38.0", Some(63063), None),
 
+    // Allows the use of or-patterns, e.g. `0 | 1`.
+    (active, or_patterns, "1.38.0", Some(54883), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
@@ -571,6 +574,7 @@ pub const INCOMPLETE_FEATURES: &[Symbol] = &[
     sym::impl_trait_in_bindings,
     sym::generic_associated_types,
     sym::const_generics,
+    sym::or_patterns,
     sym::let_chains,
 ];
 
@@ -2443,6 +2447,7 @@ pub fn check_crate(krate: &ast::Crate,
     gate_all!(let_chains_spans, let_chains, "`let` expressions in this position are experimental");
     gate_all!(async_closure_spans, async_closure, "async closures are unstable");
     gate_all!(yield_spans, generators, "yield syntax is experimental");
+    gate_all!(or_pattern_spans, or_patterns, "or-patterns syntax is experimental");
 
     let visitor = &mut PostExpansionVisitor {
         context: &ctx,

src/libsyntax/parse/mod.rs

@@ -66,6 +66,8 @@ pub struct ParseSess {
     // Places where `yield e?` exprs were used and should be feature gated.
     pub yield_spans: Lock<Vec<Span>>,
     pub injected_crate_name: Once<Symbol>,
+    // Places where or-patterns e.g. `Some(Foo | Bar)` were used and should be feature gated.
+    pub or_pattern_spans: Lock<Vec<Span>>,
 }
 
 impl ParseSess {
@@ -96,6 +98,7 @@ impl ParseSess {
             async_closure_spans: Lock::new(Vec::new()),
             yield_spans: Lock::new(Vec::new()),
             injected_crate_name: Once::new(),
+            or_pattern_spans: Lock::new(Vec::new()),
         }
     }