Rollup merge of #64887 - Centril:recover-trailing-vert, r=estebank

syntax: recover trailing `|` in or-patterns

Fixes #64879.

For example (this also shows that we are sensitive to the typo `||`):

```
error: a trailing `|` is not allowed in an or-pattern
  --> $DIR/remove-leading-vert.rs:33:11
   |
LL |         A || => {}
   |         - ^^ help: remove the `||`
   |         |
   |         while parsing this or-pattern starting here
   |
   = note: alternatives in or-patterns are separated with `|`, not `||`
```

r? @estebank

src/libsyntax/parse/parser/pat.rs

@@ -18,6 +18,8 @@ type Expected = Option<&'static str>;
 /// `Expected` for function and lambda parameter patterns.
 pub(super) const PARAM_EXPECTED: Expected = Some("parameter name");
 
+const WHILE_PARSING_OR_MSG: &str = "while parsing this or-pattern starting here";
+
 /// Whether or not an or-pattern should be gated when occurring in the current context.
 #[derive(PartialEq)]
 pub enum GateOr { Yes, No }
@@ -40,7 +42,7 @@ impl<'a> Parser<'a> {
     /// 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>> {
         // Allow a '|' before the pats (RFCs 1925, 2530, and 2535).
-        let gated_leading_vert = self.eat_or_separator() && gate_or == GateOr::Yes;
+        let gated_leading_vert = self.eat_or_separator(None) && gate_or == GateOr::Yes;
         let leading_vert_span = self.prev_span;
 
         // Parse the possibly-or-pattern.
@@ -63,7 +65,7 @@ impl<'a> Parser<'a> {
     /// Parse the pattern for a function or function pointer parameter.
     /// Special recovery is provided for or-patterns and leading `|`.
     pub(super) fn parse_fn_param_pat(&mut self) -> PResult<'a, P<Pat>> {
-        self.recover_leading_vert("not allowed in a parameter pattern");
+        self.recover_leading_vert(None, "not allowed in a parameter pattern");
         let pat = self.parse_pat_with_or(PARAM_EXPECTED, GateOr::No, RecoverComma::No)?;
 
         if let PatKind::Or(..) = &pat.kind {
@@ -90,7 +92,7 @@ impl<'a> Parser<'a> {
         gate_or: GateOr,
         rc: RecoverComma,
     ) -> PResult<'a, P<Pat>> {
-        // Parse the first pattern.
+        // Parse the first pattern (`p_0`).
         let first_pat = self.parse_pat(expected)?;
         self.maybe_recover_unexpected_comma(first_pat.span, rc)?;
 
@@ -100,11 +102,12 @@ impl<'a> Parser<'a> {
             return Ok(first_pat)
         }
 
+        // Parse the patterns `p_1 | ... | p_n` where `n > 0`.
         let lo = first_pat.span;
         let mut pats = vec![first_pat];
-        while self.eat_or_separator() {
+        while self.eat_or_separator(Some(lo)) {
             let pat = self.parse_pat(expected).map_err(|mut err| {
-                err.span_label(lo, "while parsing this or-pattern starting here");
+                err.span_label(lo, WHILE_PARSING_OR_MSG);
                 err
             })?;
             self.maybe_recover_unexpected_comma(pat.span, rc)?;
@@ -122,28 +125,65 @@ impl<'a> Parser<'a> {
 
     /// Eat the or-pattern `|` separator.
     /// If instead a `||` token is encountered, recover and pretend we parsed `|`.
-    fn eat_or_separator(&mut self) -> bool {
+    fn eat_or_separator(&mut self, lo: Option<Span>) -> bool {
+        if self.recover_trailing_vert(lo) {
+            return false;
+        }
+
         match self.token.kind {
             token::OrOr => {
                 // Found `||`; Recover and pretend we parsed `|`.
-                self.ban_unexpected_or_or();
+                self.ban_unexpected_or_or(lo);
                 self.bump();
                 true
             }
             _ => self.eat(&token::BinOp(token::Or)),
         }
     }
 
+    /// Recover if `|` or `||` is the current token and we have one of the
+    /// tokens `=>`, `if`, `=`, `:`, `;`, `,`, `]`, `)`, or `}` ahead of us.
+    ///
+    /// These tokens all indicate that we reached the end of the or-pattern
+    /// list and can now reliably say that the `|` was an illegal trailing vert.
+    /// Note that there are more tokens such as `@` for which we know that the `|`
+    /// is an illegal parse. However, the user's intent is less clear in that case.
+    fn recover_trailing_vert(&mut self, lo: Option<Span>) -> bool {
+        let is_end_ahead = self.look_ahead(1, |token| match &token.kind {
+            token::FatArrow // e.g. `a | => 0,`.
+            | token::Ident(kw::If, false) // e.g. `a | if expr`.
+            | token::Eq // e.g. `let a | = 0`.
+            | token::Semi // e.g. `let a |;`.
+            | token::Colon // e.g. `let a | :`.
+            | token::Comma // e.g. `let (a |,)`.
+            | token::CloseDelim(token::Bracket) // e.g. `let [a | ]`.
+            | token::CloseDelim(token::Paren) // e.g. `let (a | )`.
+            | token::CloseDelim(token::Brace) => true, // e.g. `let A { f: a | }`.
+            _ => false,
+        });
+        match (is_end_ahead, &self.token.kind) {
+            (true, token::BinOp(token::Or)) | (true, token::OrOr) => {
+                self.ban_illegal_vert(lo, "trailing", "not allowed in an or-pattern");
+                self.bump();
+                true
+            }
+            _ => false,
+        }
+    }
+
     /// We have parsed `||` instead of `|`. Error and suggest `|` instead.
-    fn ban_unexpected_or_or(&mut self) {
-        self.struct_span_err(self.token.span, "unexpected token `||` after pattern")
-            .span_suggestion(
-                self.token.span,
-                "use a single `|` to separate multiple alternative patterns",
-                "|".to_owned(),
-                Applicability::MachineApplicable
-            )
-            .emit();
+    fn ban_unexpected_or_or(&mut self, lo: Option<Span>) {
+        let mut err = self.struct_span_err(self.token.span, "unexpected token `||` after pattern");
+        err.span_suggestion(
+            self.token.span,
+            "use a single `|` to separate multiple alternative patterns",
+            "|".to_owned(),
+            Applicability::MachineApplicable
+        );
+        if let Some(lo) = lo {
+            err.span_label(lo, WHILE_PARSING_OR_MSG);
+        }
+        err.emit();
     }
 
     /// Some special error handling for the "top-level" patterns in a match arm,
@@ -198,25 +238,38 @@ impl<'a> Parser<'a> {
     /// Recursive possibly-or-pattern parser with recovery for an erroneous leading `|`.
     /// See `parse_pat_with_or` for details on parsing or-patterns.
     fn parse_pat_with_or_inner(&mut self) -> PResult<'a, P<Pat>> {
-        self.recover_leading_vert("only allowed in a top-level pattern");
+        self.recover_leading_vert(None, "only allowed in a top-level pattern");
         self.parse_pat_with_or(None, GateOr::Yes, RecoverComma::No)
     }
 
     /// Recover if `|` or `||` is here.
     /// The user is thinking that a leading `|` is allowed in this position.
-    fn recover_leading_vert(&mut self, ctx: &str) {
+    fn recover_leading_vert(&mut self, lo: Option<Span>, ctx: &str) {
         if let token::BinOp(token::Or) | token::OrOr = self.token.kind {
-            let span = self.token.span;
-            let rm_msg = format!("remove the `{}`", pprust::token_to_string(&self.token));
-
-            self.struct_span_err(span, &format!("a leading `|` is {}", ctx))
-                .span_suggestion(span, &rm_msg, String::new(), Applicability::MachineApplicable)
-                .emit();
-
+            self.ban_illegal_vert(lo, "leading", ctx);
             self.bump();
         }
     }
 
+    /// A `|` or possibly `||` token shouldn't be here. Ban it.
+    fn ban_illegal_vert(&mut self, lo: Option<Span>, pos: &str, ctx: &str) {
+        let span = self.token.span;
+        let mut err = self.struct_span_err(span, &format!("a {} `|` is {}", pos, ctx));
+        err.span_suggestion(
+            span,
+            &format!("remove the `{}`", pprust::token_to_string(&self.token)),
+            String::new(),
+            Applicability::MachineApplicable,
+        );
+        if let Some(lo) = lo {
+            err.span_label(lo, WHILE_PARSING_OR_MSG);
+        }
+        if let token::OrOr = self.token.kind {
+            err.note("alternatives in or-patterns are separated with `|`, not `||`");
+        }
+        err.emit();
+    }
+
     /// Parses a pattern, with a setting whether modern range patterns (e.g., `a..=b`, `a..b` are
     /// allowed).
     fn parse_pat_with_range_pat(
@@ -259,7 +312,7 @@ impl<'a> Parser<'a> {
                 self.bump();
                 self.parse_pat_range_to(RangeEnd::Included(RangeSyntax::DotDotDot), "...")?
             }
-            // At this point, token != &, &&, (, [
+            // At this point, token != `&`, `&&`, `(`, `[`, `..`, `..=`, or `...`.
             _ => if self.eat_keyword(kw::Underscore) {
                 // Parse _
                 PatKind::Wild

src/test/ui/or-patterns/issue-64879-trailing-before-guard.rs

@@ -0,0 +1,15 @@
+// In this regression test we check that a trailing `|` in an or-pattern just
+// before the `if` token of a `match` guard will receive parser recovery with
+// an appropriate error message.
+
+enum E { A, B }
+
+fn main() {
+    match E::A {
+        E::A |
+        E::B | //~ ERROR a trailing `|` is not allowed in an or-pattern
+        if true => {
+            let recovery_witness: bool = 0; //~ ERROR mismatched types
+        }
+    }
+}

src/test/ui/or-patterns/issue-64879-trailing-before-guard.stderr

@@ -0,0 +1,20 @@
+error: a trailing `|` is not allowed in an or-pattern
+  --> $DIR/issue-64879-trailing-before-guard.rs:10:14
+   |
+LL |         E::A |
+   |         ---- while parsing this or-pattern starting here
+LL |         E::B |
+   |              ^ help: remove the `|`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-64879-trailing-before-guard.rs:12:42
+   |
+LL |             let recovery_witness: bool = 0;
+   |                                          ^ expected bool, found integer
+   |
+   = note: expected type `bool`
+              found type `{integer}`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/or-patterns/multiple-pattern-typo.stderr

@@ -2,37 +2,49 @@ error: unexpected token `||` after pattern
   --> $DIR/multiple-pattern-typo.rs:8:15
    |
 LL |         1 | 2 || 3 => (),
-   |               ^^ help: use a single `|` to separate multiple alternative patterns: `|`
+   |         -     ^^ help: use a single `|` to separate multiple alternative patterns: `|`
+   |         |
+   |         while parsing this or-pattern starting here
 
 error: unexpected token `||` after pattern
   --> $DIR/multiple-pattern-typo.rs:13:16
    |
 LL |         (1 | 2 || 3) => (),
-   |                ^^ help: use a single `|` to separate multiple alternative patterns: `|`
+   |          -     ^^ help: use a single `|` to separate multiple alternative patterns: `|`
+   |          |
+   |          while parsing this or-pattern starting here
 
 error: unexpected token `||` after pattern
   --> $DIR/multiple-pattern-typo.rs:18:16
    |
 LL |         (1 | 2 || 3,) => (),
-   |                ^^ help: use a single `|` to separate multiple alternative patterns: `|`
+   |          -     ^^ help: use a single `|` to separate multiple alternative patterns: `|`
+   |          |
+   |          while parsing this or-pattern starting here
 
 error: unexpected token `||` after pattern
   --> $DIR/multiple-pattern-typo.rs:25:18
    |
 LL |         TS(1 | 2 || 3) => (),
-   |                  ^^ help: use a single `|` to separate multiple alternative patterns: `|`
+   |            -     ^^ help: use a single `|` to separate multiple alternative patterns: `|`
+   |            |
+   |            while parsing this or-pattern starting here
 
 error: unexpected token `||` after pattern
   --> $DIR/multiple-pattern-typo.rs:32:23
    |
 LL |         NS { f: 1 | 2 || 3 } => (),
-   |                       ^^ help: use a single `|` to separate multiple alternative patterns: `|`
+   |                 -     ^^ help: use a single `|` to separate multiple alternative patterns: `|`
+   |                 |
+   |                 while parsing this or-pattern starting here
 
 error: unexpected token `||` after pattern
   --> $DIR/multiple-pattern-typo.rs:37:16
    |
 LL |         [1 | 2 || 3] => (),
-   |                ^^ help: use a single `|` to separate multiple alternative patterns: `|`
+   |          -     ^^ help: use a single `|` to separate multiple alternative patterns: `|`
+   |          |
+   |          while parsing this or-pattern starting here
 
 error: unexpected token `||` after pattern
   --> $DIR/multiple-pattern-typo.rs:42:9

src/test/ui/or-patterns/or-patterns-syntactic-fail.stderr

@@ -51,24 +51,32 @@ error: a leading `|` is only allowed in a top-level pattern
    |
 LL |     let ( || A | B) = E::A;
    |           ^^ help: remove the `||`
+   |
+   = note: alternatives in or-patterns are separated with `|`, not `||`
 
 error: a leading `|` is only allowed in a top-level pattern
   --> $DIR/or-patterns-syntactic-fail.rs:48:11
    |
 LL |     let [ || A | B ] = [E::A];
    |           ^^ help: remove the `||`
+   |
+   = note: alternatives in or-patterns are separated with `|`, not `||`
 
 error: a leading `|` is only allowed in a top-level pattern
   --> $DIR/or-patterns-syntactic-fail.rs:49:13
    |
 LL |     let TS( || A | B );
    |             ^^ help: remove the `||`
+   |
+   = note: alternatives in or-patterns are separated with `|`, not `||`
 
 error: a leading `|` is only allowed in a top-level pattern
   --> $DIR/or-patterns-syntactic-fail.rs:50:17
    |
 LL |     let NS { f: || A | B };
    |                 ^^ help: remove the `||`
+   |
+   = note: alternatives in or-patterns are separated with `|`, not `||`
 
 error: no rules expected the token `|`
   --> $DIR/or-patterns-syntactic-fail.rs:14:15

src/test/ui/or-patterns/remove-leading-vert.fixed

@@ -1,4 +1,4 @@
-// Test the suggestion to remove a leading `|`.
+// Test the suggestion to remove a leading, or trailing `|`.
 
 // run-rustfix
 
@@ -8,7 +8,7 @@
 fn main() {}
 
 #[cfg(FALSE)]
-fn leading_vert() {
+fn leading() {
     fn fun1(  A: E) {} //~ ERROR a leading `|` is not allowed in a parameter pattern
     fn fun2(  A: E) {} //~ ERROR a leading `|` is not allowed in a parameter pattern
     let (  A): E; //~ ERROR a leading `|` is only allowed in a top-level pattern
@@ -21,3 +21,26 @@ fn leading_vert() {
     let NS { f:  A }: NS; //~ ERROR a leading `|` is only allowed in a top-level pattern
     let NS { f:  A }: NS; //~ ERROR a leading `|` is only allowed in a top-level pattern
 }
+
+#[cfg(FALSE)]
+fn trailing() {
+    let ( A  ): E; //~ ERROR a trailing `|` is not allowed in an or-pattern
+    let (a ,): (E,); //~ ERROR a trailing `|` is not allowed in an or-pattern
+    let ( A | B  ): E; //~ ERROR a trailing `|` is not allowed in an or-pattern
+    let [ A | B  ]: [E; 1]; //~ ERROR a trailing `|` is not allowed in an or-pattern
+    let S { f: B  }; //~ ERROR a trailing `|` is not allowed in an or-pattern
+    let ( A | B  ): E; //~ ERROR unexpected token `||` after pattern
+    //~^ ERROR a trailing `|` is not allowed in an or-pattern
+    match A {
+        A  => {} //~ ERROR a trailing `|` is not allowed in an or-pattern
+        A  => {} //~ ERROR a trailing `|` is not allowed in an or-pattern
+        A | B  => {} //~ ERROR unexpected token `||` after pattern
+        //~^ ERROR a trailing `|` is not allowed in an or-pattern
+        | A | B  => {}
+        //~^ ERROR a trailing `|` is not allowed in an or-pattern
+    }
+
+    let a  : u8 = 0; //~ ERROR a trailing `|` is not allowed in an or-pattern
+    let a  = 0; //~ ERROR a trailing `|` is not allowed in an or-pattern
+    let a  ; //~ ERROR a trailing `|` is not allowed in an or-pattern
+}

src/test/ui/or-patterns/remove-leading-vert.rs

@@ -1,4 +1,4 @@
-// Test the suggestion to remove a leading `|`.
+// Test the suggestion to remove a leading, or trailing `|`.
 
 // run-rustfix
 
@@ -8,7 +8,7 @@
 fn main() {}
 
 #[cfg(FALSE)]
-fn leading_vert() {
+fn leading() {
     fn fun1( | A: E) {} //~ ERROR a leading `|` is not allowed in a parameter pattern
     fn fun2( || A: E) {} //~ ERROR a leading `|` is not allowed in a parameter pattern
     let ( | A): E; //~ ERROR a leading `|` is only allowed in a top-level pattern
@@ -21,3 +21,26 @@ fn leading_vert() {
     let NS { f: | A }: NS; //~ ERROR a leading `|` is only allowed in a top-level pattern
     let NS { f: || A }: NS; //~ ERROR a leading `|` is only allowed in a top-level pattern
 }
+
+#[cfg(FALSE)]
+fn trailing() {
+    let ( A | ): E; //~ ERROR a trailing `|` is not allowed in an or-pattern
+    let (a |,): (E,); //~ ERROR a trailing `|` is not allowed in an or-pattern
+    let ( A | B | ): E; //~ ERROR a trailing `|` is not allowed in an or-pattern
+    let [ A | B | ]: [E; 1]; //~ ERROR a trailing `|` is not allowed in an or-pattern
+    let S { f: B | }; //~ ERROR a trailing `|` is not allowed in an or-pattern
+    let ( A || B | ): E; //~ ERROR unexpected token `||` after pattern
+    //~^ ERROR a trailing `|` is not allowed in an or-pattern
+    match A {
+        A | => {} //~ ERROR a trailing `|` is not allowed in an or-pattern
+        A || => {} //~ ERROR a trailing `|` is not allowed in an or-pattern
+        A || B | => {} //~ ERROR unexpected token `||` after pattern
+        //~^ ERROR a trailing `|` is not allowed in an or-pattern
+        | A | B | => {}
+        //~^ ERROR a trailing `|` is not allowed in an or-pattern
+    }
+
+    let a | : u8 = 0; //~ ERROR a trailing `|` is not allowed in an or-pattern
+    let a | = 0; //~ ERROR a trailing `|` is not allowed in an or-pattern
+    let a | ; //~ ERROR a trailing `|` is not allowed in an or-pattern
+}