Extend trailing > detection for paths.

This commit extends the trailing `>` detection to also work for paths
such as `Foo::<Bar>>:Baz`.

This involves making the existing check take the token that is expected
to follow the path being checked as a parameter.

Care is taken to ensure that this only happens on the construction of a
whole path segment and not a partial path segment (during recursion).

Through this enhancement, it was also observed that the ordering of
right shift token and greater than tokens was overfitted to the examples
being tested.

In practice, given a sequence of `>` characters: `>>>>>>>>>`
..then they will be split into `>>` eagerly: `>> >> >> >> >`.
..but when a `<` is prepended, then the first `>>` is split:
`<T> > >> >> >> >`
..and then when another `<` is prepended, a right shift is first again:
`Vec<<T>> >> >> >> >`

In the previous commits, a example that had two `<<` characters was
always used and therefore it was incorrectly assumed that `>>` would
always be first - but when there is a single `<`, this is not the case.

Author: davidtwco

src/libsyntax/parse/parser.rs

@@ -2149,7 +2149,27 @@ impl<'a> Parser<'a> {
                            enable_warning: bool)
                            -> PResult<'a, ()> {
         loop {
-            segments.push(self.parse_path_segment(style, enable_warning)?);
+            let segment = self.parse_path_segment(style, enable_warning)?;
+            if style == PathStyle::Expr {
+                // In order to check for trailing angle brackets, we must have finished
+                // recursing (`parse_path_segment` can indirectly call this function),
+                // that is, the next token must be the highlighted part of the below example:
+                //
+                // `Foo::<Bar as Baz<T>>::Qux`
+                //                      ^ here
+                //
+                // As opposed to the below highlight (if we had only finished the first
+                // recursion):
+                //
+                // `Foo::<Bar as Baz<T>>::Qux`
+                //                     ^ here
+                //
+                // `PathStyle::Expr` is only provided at the root invocation and never in
+                // `parse_path_segment` to recurse and therefore can be checked to maintain
+                // this invariant.
+                self.check_trailing_angle_brackets(&segment, token::ModSep);
+            }
+            segments.push(segment);
 
             if self.is_import_coupler() || !self.eat(&token::ModSep) {
                 return Ok(());
@@ -2757,7 +2777,7 @@ impl<'a> Parser<'a> {
     // Assuming we have just parsed `.`, continue parsing into an expression.
     fn parse_dot_suffix(&mut self, self_arg: P<Expr>, lo: Span) -> PResult<'a, P<Expr>> {
         let segment = self.parse_path_segment(PathStyle::Expr, true)?;
-        self.check_trailing_angle_brackets(&segment);
+        self.check_trailing_angle_brackets(&segment, token::OpenDelim(token::Paren));
 
         Ok(match self.token {
             token::OpenDelim(token::Paren) => {
@@ -2793,15 +2813,19 @@ impl<'a> Parser<'a> {
     /// let _ = vec![1, 2, 3].into_iter().collect::<Vec<usize>>>>();
     ///                                                        ^^ help: remove extra angle brackets
     /// ```
-    fn check_trailing_angle_brackets(&mut self, segment: &PathSegment) {
-        // This function is intended to be invoked from `parse_dot_suffix` where there are two
+    fn check_trailing_angle_brackets(&mut self, segment: &PathSegment, end: token::Token) {
+        // This function is intended to be invoked after parsing a path segment where there are two
         // cases:
         //
-        // - A field access (eg. `x.foo`)
-        // - A method call (eg. `x.foo()`)
+        // 1. A specific token is expected after the path segment.
+        //    eg. `x.foo(`, `x.foo::<u32>(` (parenthesis - method call),
+        //        `Foo::`, or `Foo::<Bar>::` (mod sep - continued path).
+        // 2. No specific token is expected after the path segment.
+        //    eg. `x.foo` (field access)
         //
-        // This function is called after parsing `.foo` and before parsing any parenthesis (if
-        // present). This includes any angle bracket arguments, such as `.foo::<u32>`.
+        // This function is called after parsing `.foo` and before parsing the token `end` (if
+        // present). This includes any angle bracket arguments, such as `.foo::<u32>` or
+        // `Foo::<Bar>`.
 
         // We only care about trailing angle brackets if we previously parsed angle bracket
         // arguments. This helps stop us incorrectly suggesting that extra angle brackets be
@@ -2836,43 +2860,47 @@ impl<'a> Parser<'a> {
         // actual operators and not trailing characters - ie `x.foo >> 3`).
         let mut position = 0;
 
-        // The first tokens we will encounter are shift right tokens (`>>`) since pairs of `>`
-        // characters will have been grouped together by the tokenizer.
+        // We can encounter `>` or `>>` tokens in any order, so we need to keep track of how
+        // many of each (so we can correctly pluralize our error messages) and continue to
+        // advance.
         let mut number_of_shr = 0;
-        while self.look_ahead(position, |t| *t == token::BinOp(token::BinOpToken::Shr)) {
-            number_of_shr += 1;
-            position += 1;
-        }
-
-        // Afterwards, there will be at most one `>` character remaining (more than one and it'd
-        // have shown up as a `>>`).
-        let encountered_gt = self.look_ahead(position, |t| *t == token::Gt);
-        if encountered_gt {
+        let mut number_of_gt = 0;
+        while self.look_ahead(position, |t| {
+            trace!("check_trailing_angle_brackets: t={:?}", t);
+            if *t == token::BinOp(token::BinOpToken::Shr) {
+                number_of_shr += 1;
+                true
+            } else if *t == token::Gt {
+                number_of_gt += 1;
+                true
+            } else {
+                false
+            }
+        }) {
             position += 1;
         }
 
-        // If we didn't find any trailing `>>` characters or a trailing `>`, then we have
-        // nothing to error about.
+        // If we didn't find any trailing `>` characters, then we have nothing to error about.
         debug!(
-            "check_trailing_angle_brackets: encountered_gt={:?} number_of_shr={:?}",
-            encountered_gt, number_of_shr,
+            "check_trailing_angle_brackets: number_of_gt={:?} number_of_shr={:?}",
+            number_of_gt, number_of_shr,
         );
-        if !encountered_gt && number_of_shr < 1 {
+        if number_of_gt < 1 && number_of_shr < 1 {
             return;
         }
 
-        // Finally, double check that we have a left parenthesis next as otherwise this is the
-        // field case.
-        if self.look_ahead(position, |t| *t == token::OpenDelim(token::Paren)) {
-            // Eat from where we started until the left parenthesis so that parsing can continue
+        // Finally, double check that we have our end token as otherwise this is the
+        // second case.
+        if self.look_ahead(position, |t| {
+            trace!("check_trailing_angle_brackets: t={:?}", t);
+            *t == end
+        }) {
+            // Eat from where we started until the end token so that parsing can continue
             // as if we didn't have those extra angle brackets.
-            self.eat_to_tokens(&[&token::OpenDelim(token::Paren)]);
+            self.eat_to_tokens(&[&end]);
             let span = lo.until(self.span);
 
-            // We needn't check `encountered_gt` to determine if we should pluralize "bracket".
-            // `encountered_gt` can only represent a single `>` character, if `number_of_shr >= 1`
-            // then there is either `>>` or `>>>` - in either case a plural is warranted.
-            let plural = number_of_shr >= 1;
+            let plural = number_of_gt > 1 || number_of_shr >= 1;
             self.diagnostic()
                 .struct_span_err(
                     span,

src/test/ui/issues/issue-54521-2.fixed

@@ -0,0 +1,22 @@
+// run-rustfix
+
+// This test checks that the following error is emitted and the suggestion works:
+//
+// ```
+// let _ = Vec::<usize>>>::new();
+//                     ^^ help: remove extra angle brackets
+// ```
+
+fn main() {
+    let _ = Vec::<usize>::new();
+    //~^ ERROR unmatched angle bracket
+
+    let _ = Vec::<usize>::new();
+    //~^ ERROR unmatched angle bracket
+
+    let _ = Vec::<usize>::new();
+    //~^ ERROR unmatched angle bracket
+
+    let _ = Vec::<usize>::new();
+    //~^ ERROR unmatched angle bracket
+}

src/test/ui/issues/issue-54521-2.rs

@@ -0,0 +1,22 @@
+// run-rustfix
+
+// This test checks that the following error is emitted and the suggestion works:
+//
+// ```
+// let _ = Vec::<usize>>>::new();
+//                     ^^ help: remove extra angle brackets
+// ```
+
+fn main() {
+    let _ = Vec::<usize>>>>>::new();
+    //~^ ERROR unmatched angle bracket
+
+    let _ = Vec::<usize>>>>::new();
+    //~^ ERROR unmatched angle bracket
+
+    let _ = Vec::<usize>>>::new();
+    //~^ ERROR unmatched angle bracket
+
+    let _ = Vec::<usize>>::new();
+    //~^ ERROR unmatched angle bracket
+}

src/test/ui/issues/issue-54521-2.stderr

@@ -0,0 +1,26 @@
+error: unmatched angle brackets
+  --> $DIR/issue-54521-2.rs:11:25
+   |
+LL |     let _ = Vec::<usize>>>>>::new();
+   |                         ^^^^ help: remove extra angle brackets
+
+error: unmatched angle brackets
+  --> $DIR/issue-54521-2.rs:14:25
+   |
+LL |     let _ = Vec::<usize>>>>::new();
+   |                         ^^^ help: remove extra angle brackets
+
+error: unmatched angle brackets
+  --> $DIR/issue-54521-2.rs:17:25
+   |
+LL |     let _ = Vec::<usize>>>::new();
+   |                         ^^ help: remove extra angle brackets
+
+error: unmatched angle bracket
+  --> $DIR/issue-54521-2.rs:20:25
+   |
+LL |     let _ = Vec::<usize>>::new();
+   |                         ^ help: remove extra angle bracket
+
+error: aborting due to 4 previous errors
+