implement edition-specific :pat behavior for 2015/18

compiler/rustc_parse/src/parser/expr.rs

@@ -1,4 +1,4 @@
-use super::pat::{GateOr, PARAM_EXPECTED};
+use super::pat::{GateOr, RecoverComma, PARAM_EXPECTED};
 use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
 use super::{BlockMode, Parser, PathStyle, Restrictions, TokenType};
 use super::{SemiColonMode, SeqSep, TokenExpectType};
@@ -1729,7 +1729,7 @@ impl<'a> Parser<'a> {
     /// The `let` token has already been eaten.
     fn parse_let_expr(&mut self, attrs: AttrVec) -> PResult<'a, P<Expr>> {
         let lo = self.prev_token.span;
-        let pat = self.parse_top_pat(GateOr::No)?;
+        let pat = self.parse_top_pat(GateOr::No, RecoverComma::Yes)?;
         self.expect(&token::Eq)?;
         let expr = self.with_res(self.restrictions | Restrictions::NO_STRUCT_LITERAL, |this| {
             this.parse_assoc_expr_with(1 + prec_let_scrutinee_needs_par(), None.into())
@@ -1792,7 +1792,7 @@ impl<'a> Parser<'a> {
             _ => None,
         };
 
-        let pat = self.parse_top_pat(GateOr::Yes)?;
+        let pat = self.parse_top_pat(GateOr::Yes, RecoverComma::Yes)?;
         if !self.eat_keyword(kw::In) {
             self.error_missing_in_for_loop();
         }
@@ -1902,7 +1902,7 @@ impl<'a> Parser<'a> {
     pub(super) fn parse_arm(&mut self) -> PResult<'a, Arm> {
         let attrs = self.parse_outer_attributes()?;
         let lo = self.token.span;
-        let pat = self.parse_top_pat(GateOr::No)?;
+        let pat = self.parse_top_pat(GateOr::No, RecoverComma::Yes)?;
         let guard = if self.eat_keyword(kw::If) {
             let if_span = self.prev_token.span;
             let cond = self.parse_expr()?;

compiler/rustc_parse/src/parser/mod.rs

@@ -12,6 +12,7 @@ mod ty;
 use crate::lexer::UnmatchedBrace;
 pub use diagnostics::AttemptLocalParseRecovery;
 use diagnostics::Error;
+pub use pat::OrPatNonterminalMode;
 pub use path::PathStyle;
 
 use rustc_ast::ptr::P;

compiler/rustc_parse/src/parser/nonterminal.rs

@@ -4,6 +4,7 @@ use rustc_ast_pretty::pprust;
 use rustc_errors::PResult;
 use rustc_span::symbol::{kw, Ident};
 
+use crate::parser::pat::{GateOr, OrPatNonterminalMode, RecoverComma};
 use crate::parser::{FollowedByType, Parser, PathStyle};
 
 impl<'a> Parser<'a> {
@@ -11,7 +12,11 @@ impl<'a> Parser<'a> {
     ///
     /// Returning `false` is a *stability guarantee* that such a matcher will *never* begin with that
     /// token. Be conservative (return true) if not sure.
-    pub fn nonterminal_may_begin_with(kind: NonterminalKind, token: &Token) -> bool {
+    pub fn nonterminal_may_begin_with(
+        kind: NonterminalKind,
+        token: &Token,
+        or_pat_mode: OrPatNonterminalMode,
+    ) -> bool {
         /// Checks whether the non-terminal may contain a single (non-keyword) identifier.
         fn may_be_ident(nt: &token::Nonterminal) -> bool {
             match *nt {
@@ -70,6 +75,8 @@ impl<'a> Parser<'a> {
                 token::ModSep |                     // path
                 token::Lt |                         // path (UFCS constant)
                 token::BinOp(token::Shl) => true,   // path (double UFCS)
+                // leading vert `|` or-pattern
+                token::BinOp(token::Or) =>  matches!(or_pat_mode, OrPatNonterminalMode::TopPat),
                 token::Interpolated(ref nt) => may_be_ident(nt),
                 _ => false,
             },
@@ -86,7 +93,12 @@ impl<'a> Parser<'a> {
         }
     }
 
-    pub fn parse_nonterminal(&mut self, kind: NonterminalKind) -> PResult<'a, Nonterminal> {
+    /// Parse a non-terminal (e.g. MBE `:pat` or `:ident`).
+    pub fn parse_nonterminal(
+        &mut self,
+        kind: NonterminalKind,
+        or_pat_mode: OrPatNonterminalMode,
+    ) -> PResult<'a, Nonterminal> {
         // Any `Nonterminal` which stores its tokens (currently `NtItem` and `NtExpr`)
         // needs to have them force-captured here.
         // A `macro_rules!` invocation may pass a captured item/expr to a proc-macro,
@@ -130,7 +142,12 @@ impl<'a> Parser<'a> {
                 }
             }
             NonterminalKind::Pat => {
-                let (mut pat, tokens) = self.collect_tokens(|this| this.parse_pat(None))?;
+                let (mut pat, tokens) = self.collect_tokens(|this| match or_pat_mode {
+                    OrPatNonterminalMode::TopPat => {
+                        this.parse_top_pat(GateOr::Yes, RecoverComma::No)
+                    }
+                    OrPatNonterminalMode::NoTopAlt => this.parse_pat(None),
+                })?;
                 // We have have eaten an NtPat, which could already have tokens
                 if pat.tokens.is_none() {
                     pat.tokens = tokens;

compiler/rustc_parse/src/parser/pat.rs

@@ -26,11 +26,18 @@ pub(super) enum GateOr {
 
 /// Whether or not to recover a `,` when parsing or-patterns.
 #[derive(PartialEq, Copy, Clone)]
-enum RecoverComma {
+pub(super) enum RecoverComma {
     Yes,
     No,
 }
 
+/// Used when parsing a non-terminal (see `parse_nonterminal`) to determine if `:pat` should match
+/// `top_pat` or `pat<no_top_alt>`. See issue <https://github.com/rust-lang/rust/pull/78935>.
+pub enum OrPatNonterminalMode {
+    TopPat,
+    NoTopAlt,
+}
+
 impl<'a> Parser<'a> {
     /// Parses a pattern.
     ///
@@ -43,13 +50,17 @@ impl<'a> Parser<'a> {
 
     /// Entry point to the main pattern parser.
     /// Corresponds to `top_pat` in RFC 2535 and allows or-pattern at the top level.
-    pub(super) fn parse_top_pat(&mut self, gate_or: GateOr) -> PResult<'a, P<Pat>> {
+    pub(super) fn parse_top_pat(
+        &mut self,
+        gate_or: GateOr,
+        rc: RecoverComma,
+    ) -> PResult<'a, P<Pat>> {
         // Allow a '|' before the pats (RFCs 1925, 2530, and 2535).
         let gated_leading_vert = self.eat_or_separator(None) && gate_or == GateOr::Yes;
         let leading_vert_span = self.prev_token.span;
 
         // Parse the possibly-or-pattern.
-        let pat = self.parse_pat_with_or(None, gate_or, RecoverComma::Yes)?;
+        let pat = self.parse_pat_with_or(None, gate_or, rc)?;
 
         // If we parsed a leading `|` which should be gated,
         // and no other gated or-pattern has been parsed thus far,

compiler/rustc_parse/src/parser/stmt.rs

@@ -1,7 +1,7 @@
 use super::attr::DEFAULT_INNER_ATTR_FORBIDDEN;
 use super::diagnostics::{AttemptLocalParseRecovery, Error};
 use super::expr::LhsExpr;
-use super::pat::GateOr;
+use super::pat::{GateOr, RecoverComma};
 use super::path::PathStyle;
 use super::{BlockMode, Parser, Restrictions, SemiColonMode};
 use crate::maybe_whole;
@@ -185,7 +185,7 @@ impl<'a> Parser<'a> {
     /// Parses a local variable declaration.
     fn parse_local(&mut self, attrs: AttrVec) -> PResult<'a, P<Local>> {
         let lo = self.prev_token.span;
-        let pat = self.parse_top_pat(GateOr::Yes)?;
+        let pat = self.parse_top_pat(GateOr::Yes, RecoverComma::Yes)?;
 
         let (err, ty) = if self.eat(&token::Colon) {
             // Save the state of the parser before parsing type normally, in case there is a `:`