Suggests turbofish in patterns

compiler/rustc_parse/src/errors.rs

@@ -2731,3 +2731,17 @@ pub(crate) struct WhereClauseBeforeConstBodySugg {
     #[suggestion_part(code = "")]
     pub right: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(parse_generic_args_in_pat_require_turbofish_syntax)]
+pub(crate) struct GenericArgsInPatRequireTurbofishSyntax {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(
+        parse_sugg_turbofish_syntax,
+        style = "verbose",
+        code = "::",
+        applicability = "maybe-incorrect"
+    )]
+    pub suggest_turbofish: Span,
+}

compiler/rustc_parse/src/parser/pat.rs

@@ -805,6 +805,7 @@ impl<'a> Parser<'a> {
             | token::DotDotDot | token::DotDotEq | token::DotDot // A range pattern.
             | token::ModSep // A tuple / struct variant pattern.
             | token::Not)) // A macro expanding to a pattern.
+        && !(self.look_ahead(1, |t| t.kind == token::Lt) && self.look_ahead(2, |t| t.can_begin_type())) // May suggest the turbofish syntax for generics, only valid for recoveries.
     }
 
     /// Parses `ident` or `ident @ pat`.

compiler/rustc_parse/src/parser/path.rs

@@ -1,6 +1,6 @@
 use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
 use super::{Parser, Restrictions, TokenType};
-use crate::errors::PathSingleColon;
+use crate::errors::{GenericArgsInPatRequireTurbofishSyntax, PathSingleColon};
 use crate::{errors, maybe_whole};
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, Token, TokenKind};
@@ -382,6 +382,14 @@ impl<'a> Parser<'a> {
                 };
 
                 PathSegment { ident, args: Some(args), id: ast::DUMMY_NODE_ID }
+            } else if style == PathStyle::Pat
+                && self.check_noexpect(&token::Lt)
+                && self.look_ahead(1, |t| t.can_begin_type())
+            {
+                return Err(self.sess.create_err(GenericArgsInPatRequireTurbofishSyntax {
+                    span: self.token.span,
+                    suggest_turbofish: self.token.span.shrink_to_lo(),
+                }));
             } else {
                 // Generic arguments are not found.
                 PathSegment::from_ident(ident)