Suggest removing leading left angle brackets.

This commit adds errors and accompanying suggestions as below:

```
bar::<<<<<T as Foo>::Output>();
     ^^^ help: remove extra angle brackets
```

src/libsyntax/parse/parser.rs

@@ -242,6 +242,12 @@ pub struct Parser<'a> {
     desugar_doc_comments: bool,
     /// Whether we should configure out of line modules as we parse.
     pub cfg_mods: bool,
+    /// This field is used to keep track of how many left angle brackets we have seen. This is
+    /// required in order to detect extra leading left angle brackets (`<` characters) and error
+    /// appropriately.
+    ///
+    /// See the comments in the `parse_path_segment` function for more details.
+    crate unmatched_angle_bracket_count: u32,
 }
 
 
@@ -563,6 +569,7 @@ impl<'a> Parser<'a> {
             },
             desugar_doc_comments,
             cfg_mods: true,
+            unmatched_angle_bracket_count: 0,
         };
 
         let tok = parser.next_tok();
@@ -1027,7 +1034,7 @@ impl<'a> Parser<'a> {
     /// starting token.
     fn eat_lt(&mut self) -> bool {
         self.expected_tokens.push(TokenType::Token(token::Lt));
-        match self.token {
+        let ate = match self.token {
             token::Lt => {
                 self.bump();
                 true
@@ -1038,7 +1045,15 @@ impl<'a> Parser<'a> {
                 true
             }
             _ => false,
+        };
+
+        if ate {
+            // See doc comment for `unmatched_angle_bracket_count`.
+            self.unmatched_angle_bracket_count += 1;
+            debug!("eat_lt: (increment) count={:?}", self.unmatched_angle_bracket_count);
         }
+
+        ate
     }
 
     fn expect_lt(&mut self) -> PResult<'a, ()> {
@@ -1054,24 +1069,35 @@ impl<'a> Parser<'a> {
     /// signal an error.
     fn expect_gt(&mut self) -> PResult<'a, ()> {
         self.expected_tokens.push(TokenType::Token(token::Gt));
-        match self.token {
+        let ate = match self.token {
             token::Gt => {
                 self.bump();
-                Ok(())
+                Some(())
             }
             token::BinOp(token::Shr) => {
                 let span = self.span.with_lo(self.span.lo() + BytePos(1));
-                Ok(self.bump_with(token::Gt, span))
+                Some(self.bump_with(token::Gt, span))
             }
             token::BinOpEq(token::Shr) => {
                 let span = self.span.with_lo(self.span.lo() + BytePos(1));
-                Ok(self.bump_with(token::Ge, span))
+                Some(self.bump_with(token::Ge, span))
             }
             token::Ge => {
                 let span = self.span.with_lo(self.span.lo() + BytePos(1));
-                Ok(self.bump_with(token::Eq, span))
+                Some(self.bump_with(token::Eq, span))
             }
-            _ => self.unexpected()
+            _ => None,
+        };
+
+        match ate {
+            Some(x) => {
+                // See doc comment for `unmatched_angle_bracket_count`.
+                self.unmatched_angle_bracket_count -= 1;
+                debug!("expect_gt: (decrement) count={:?}", self.unmatched_angle_bracket_count);
+
+                Ok(x)
+            },
+            None => self.unexpected(),
         }
     }
 
@@ -2079,7 +2105,11 @@ impl<'a> Parser<'a> {
             path_span = self.span.to(self.span);
         }
 
+        // See doc comment for `unmatched_angle_bracket_count`.
         self.expect(&token::Gt)?;
+        self.unmatched_angle_bracket_count -= 1;
+        debug!("parse_qpath: (decrement) count={:?}", self.unmatched_angle_bracket_count);
+
         self.expect(&token::ModSep)?;
 
         let qself = QSelf { ty, path_span, position: path.segments.len() };
@@ -2182,9 +2212,15 @@ impl<'a> Parser<'a> {
             }
             let lo = self.span;
 
+            // We use `style == PathStyle::Expr` to check if this is in a recursion or not. If
+            // it isn't, then we reset the unmatched angle bracket count as we're about to start
+            // parsing a new path.
+            if style == PathStyle::Expr { self.unmatched_angle_bracket_count = 0; }
+
             let args = if self.eat_lt() {
                 // `<'a, T, A = U>`
-                let (args, bindings) = self.parse_generic_args()?;
+                let (args, bindings) =
+                    self.parse_generic_args_with_leaning_angle_bracket_recovery(style, lo)?;
                 self.expect_gt()?;
                 let span = lo.to(self.prev_span);
                 AngleBracketedArgs { args, bindings, span }.into()
@@ -5319,6 +5355,163 @@ impl<'a> Parser<'a> {
         }
     }
 
+    /// Parse generic args (within a path segment) with recovery for extra leading angle brackets.
+    /// For the purposes of understanding the parsing logic of generic arguments, this function
+    /// can be thought of being the same as just calling `self.parse_generic_args()` if the source
+    /// had the correct amount of leading angle brackets.
+    ///
+    /// ```ignore (diagnostics)
+    /// bar::<<<<T as Foo>::Output>();
+    ///      ^^ help: remove extra angle brackets
+    /// ```
+    fn parse_generic_args_with_leaning_angle_bracket_recovery(
+        &mut self,
+        style: PathStyle,
+        lo: Span,
+    ) -> PResult<'a, (Vec<GenericArg>, Vec<TypeBinding>)> {
+        // We need to detect whether there are extra leading left angle brackets and produce an
+        // appropriate error and suggestion. This cannot be implemented by looking ahead at
+        // upcoming tokens for a matching `>` character - if there are unmatched `<` tokens
+        // then there won't be matching `>` tokens to find.
+        //
+        // To explain how this detection works, consider the following example:
+        //
+        // ```ignore (diagnostics)
+        // bar::<<<<T as Foo>::Output>();
+        //      ^^ help: remove extra angle brackets
+        // ```
+        //
+        // Parsing of the left angle brackets starts in this function. We start by parsing the
+        // `<` token (incrementing the counter of unmatched angle brackets on `Parser` via
+        // `eat_lt`):
+        //
+        // *Upcoming tokens:* `<<<<T as Foo>::Output>;`
+        // *Unmatched count:* 1
+        // *`parse_path_segment` calls deep:* 0
+        //
+        // This has the effect of recursing as this function is called if a `<` character
+        // is found within the expected generic arguments:
+        //
+        // *Upcoming tokens:* `<<<T as Foo>::Output>;`
+        // *Unmatched count:* 2
+        // *`parse_path_segment` calls deep:* 1
+        //
+        // Eventually we will have recursed until having consumed all of the `<` tokens and
+        // this will be reflected in the count:
+        //
+        // *Upcoming tokens:* `T as Foo>::Output>;`
+        // *Unmatched count:* 4
+        // `parse_path_segment` calls deep:* 3
+        //
+        // The parser will continue until reaching the first `>` - this will decrement the
+        // unmatched angle bracket count and return to the parent invocation of this function
+        // having succeeded in parsing:
+        //
+        // *Upcoming tokens:* `::Output>;`
+        // *Unmatched count:* 3
+        // *`parse_path_segment` calls deep:* 2
+        //
+        // This will continue until the next `>` character which will also return successfully
+        // to the parent invocation of this function and decrement the count:
+        //
+        // *Upcoming tokens:* `;`
+        // *Unmatched count:* 2
+        // *`parse_path_segment` calls deep:* 1
+        //
+        // At this point, this function will expect to find another matching `>` character but
+        // won't be able to and will return an error. This will continue all the way up the
+        // call stack until the first invocation:
+        //
+        // *Upcoming tokens:* `;`
+        // *Unmatched count:* 2
+        // *`parse_path_segment` calls deep:* 0
+        //
+        // In doing this, we have managed to work out how many unmatched leading left angle
+        // brackets there are, but we cannot recover as the unmatched angle brackets have
+        // already been consumed. To remedy this, whenever `parse_generic_args` is invoked, we
+        // make a snapshot of the current parser state and invoke it on that and inspect
+        // the result:
+        //
+        // - If success (ie. when it found a matching `>` character) then the snapshot state
+        //   is kept (this is required to propagate the count upwards).
+        //
+        // - If error and in was in a recursive call, then the snapshot state is kept (this is
+        //   required to propagate the count upwards).
+        //
+        // - If error and this was the first invocation (before any recursion had taken place)
+        //   then we choose not to keep the snapshot state - that way we haven't actually
+        //   consumed any of the `<` characters, but can still inspect the count from the
+        //   snapshot to know how many `<` characters to remove. Using this information, we can
+        //   emit an error and consume the extra `<` characters before attempting to parse
+        //   the generic arguments again (this time hopefullt successfully as the unmatched `<`
+        //   characters are gone).
+        //
+        // In practice, the recursion of this function is indirect and there will be other
+        // locations that consume some `<` characters - as long as we update the count when
+        // this happens, it isn't an issue.
+        let mut snapshot = self.clone();
+        debug!("parse_generic_args_with_leading_angle_bracket_recovery: (snapshotting)");
+        match snapshot.parse_generic_args() {
+            Ok(value) => {
+                debug!(
+                    "parse_generic_args_with_leading_angle_bracket_recovery: (snapshot success) \
+                     snapshot.count={:?}",
+                    snapshot.unmatched_angle_bracket_count,
+                );
+                mem::replace(self, snapshot);
+                Ok(value)
+            },
+            Err(mut e) => {
+                debug!(
+                    "parse_generic_args_with_leading_angle_bracket_recovery: (snapshot failure) \
+                     snapshot.count={:?}",
+                    snapshot.unmatched_angle_bracket_count,
+                );
+                if style == PathStyle::Expr && snapshot.unmatched_angle_bracket_count > 0 {
+                    // Cancel error from being unable to find `>`. We know the error
+                    // must have been this due to a non-zero unmatched angle bracket
+                    // count.
+                    e.cancel();
+
+                    // Eat the unmatched angle brackets.
+                    for _ in 0..snapshot.unmatched_angle_bracket_count {
+                        self.eat_lt();
+                    }
+
+                    // Make a span over ${unmatched angle bracket count} characters.
+                    let span = lo.with_hi(
+                        lo.lo() + BytePos(snapshot.unmatched_angle_bracket_count)
+                    );
+                    let plural = snapshot.unmatched_angle_bracket_count > 1;
+                    self.diagnostic()
+                        .struct_span_err(
+                            span,
+                            &format!(
+                                "unmatched angle bracket{}",
+                                if plural { "s" } else { "" }
+                            ),
+                        )
+                        .span_suggestion_with_applicability(
+                            span,
+                            &format!(
+                                "remove extra angle bracket{}",
+                                if plural { "s" } else { "" }
+                            ),
+                            String::new(),
+                            Applicability::MachineApplicable,
+                        )
+                        .emit();
+
+                    // Try again without unmatched angle bracket characters.
+                    self.parse_generic_args()
+                } else {
+                    mem::replace(self, snapshot);
+                    Err(e)
+                }
+            },
+        }
+    }
+
     /// Parses (possibly empty) list of lifetime and type arguments and associated type bindings,
     /// possibly including trailing comma.
     fn parse_generic_args(&mut self) -> PResult<'a, (Vec<GenericArg>, Vec<TypeBinding>)> {

src/test/ui/issues/issue-57819.fixed

@@ -0,0 +1,47 @@
+// run-rustfix
+
+#![allow(warnings)]
+
+// This test checks that the following error is emitted and the suggestion works:
+//
+// ```
+// let _ = vec![1, 2, 3].into_iter().collect::<<<Vec<usize>>();
+//                                            ^^ help: remove extra angle brackets
+// ```
+
+trait Foo {
+    type Output;
+}
+
+fn foo<T: Foo>() {
+    // More complex cases with more than one correct leading `<` character:
+
+    bar::<<T as Foo>::Output>();
+    //~^ ERROR unmatched angle bracket
+
+    bar::<<T as Foo>::Output>();
+    //~^ ERROR unmatched angle bracket
+
+    bar::<<T as Foo>::Output>();
+    //~^ ERROR unmatched angle bracket
+
+    bar::<<T as Foo>::Output>();
+}
+
+fn bar<T>() {}
+
+fn main() {
+    let _ = vec![1, 2, 3].into_iter().collect::<Vec<usize>>();
+    //~^ ERROR unmatched angle bracket
+
+    let _ = vec![1, 2, 3].into_iter().collect::<Vec<usize>>();
+    //~^ ERROR unmatched angle bracket
+
+    let _ = vec![1, 2, 3].into_iter().collect::<Vec<usize>>();
+    //~^ ERROR unmatched angle bracket
+
+    let _ = vec![1, 2, 3].into_iter().collect::<Vec<usize>>();
+    //~^ ERROR unmatched angle bracket
+
+    let _ = vec![1, 2, 3].into_iter().collect::<Vec<usize>>();
+}

src/test/ui/issues/issue-57819.rs

@@ -0,0 +1,47 @@
+// run-rustfix
+
+#![allow(warnings)]
+
+// This test checks that the following error is emitted and the suggestion works:
+//
+// ```
+// let _ = vec![1, 2, 3].into_iter().collect::<<<Vec<usize>>();
+//                                            ^^ help: remove extra angle brackets
+// ```
+
+trait Foo {
+    type Output;
+}
+
+fn foo<T: Foo>() {
+    // More complex cases with more than one correct leading `<` character:
+
+    bar::<<<<<T as Foo>::Output>();
+    //~^ ERROR unmatched angle bracket
+
+    bar::<<<<T as Foo>::Output>();
+    //~^ ERROR unmatched angle bracket
+
+    bar::<<<T as Foo>::Output>();
+    //~^ ERROR unmatched angle bracket
+
+    bar::<<T as Foo>::Output>();
+}
+
+fn bar<T>() {}
+
+fn main() {
+    let _ = vec![1, 2, 3].into_iter().collect::<<<<<Vec<usize>>();
+    //~^ ERROR unmatched angle bracket
+
+    let _ = vec![1, 2, 3].into_iter().collect::<<<<Vec<usize>>();
+    //~^ ERROR unmatched angle bracket
+
+    let _ = vec![1, 2, 3].into_iter().collect::<<<Vec<usize>>();
+    //~^ ERROR unmatched angle bracket
+
+    let _ = vec![1, 2, 3].into_iter().collect::<<Vec<usize>>();
+    //~^ ERROR unmatched angle bracket
+
+    let _ = vec![1, 2, 3].into_iter().collect::<Vec<usize>>();
+}

src/test/ui/issues/issue-57819.stderr

@@ -0,0 +1,44 @@
+error: unmatched angle brackets
+  --> $DIR/issue-57819.rs:19:10
+   |
+LL |     bar::<<<<<T as Foo>::Output>();
+   |          ^^^ help: remove extra angle brackets
+
+error: unmatched angle brackets
+  --> $DIR/issue-57819.rs:22:10
+   |
+LL |     bar::<<<<T as Foo>::Output>();
+   |          ^^ help: remove extra angle brackets
+
+error: unmatched angle bracket
+  --> $DIR/issue-57819.rs:25:10
+   |
+LL |     bar::<<<T as Foo>::Output>();
+   |          ^ help: remove extra angle bracket
+
+error: unmatched angle brackets
+  --> $DIR/issue-57819.rs:34:48
+   |
+LL |     let _ = vec![1, 2, 3].into_iter().collect::<<<<<Vec<usize>>();
+   |                                                ^^^^ help: remove extra angle brackets
+
+error: unmatched angle brackets
+  --> $DIR/issue-57819.rs:37:48
+   |
+LL |     let _ = vec![1, 2, 3].into_iter().collect::<<<<Vec<usize>>();
+   |                                                ^^^ help: remove extra angle brackets
+
+error: unmatched angle brackets
+  --> $DIR/issue-57819.rs:40:48
+   |
+LL |     let _ = vec![1, 2, 3].into_iter().collect::<<<Vec<usize>>();
+   |                                                ^^ help: remove extra angle brackets
+
+error: unmatched angle bracket
+  --> $DIR/issue-57819.rs:43:48
+   |
+LL |     let _ = vec![1, 2, 3].into_iter().collect::<<Vec<usize>>();
+   |                                                ^ help: remove extra angle bracket
+
+error: aborting due to 7 previous errors
+