Try to recover from a => -> = or -> typo in a match arm

Author: FabianWolff

compiler/rustc_parse/src/parser/expr.rs

@@ -2322,7 +2322,21 @@ impl<'a> Parser<'a> {
                 None
             };
             let arrow_span = this.token.span;
-            this.expect(&token::FatArrow)?;
+            if let Err(mut err) = this.expect(&token::FatArrow) {
+                // We might have a `=>` -> `=` or `->` typo (issue #89396).
+                if let token::Eq | token::RArrow = this.token.kind {
+                    err.span_suggestion(
+                        this.token.span,
+                        "try using a fat arrow here",
+                        "=>".to_string(),
+                        Applicability::MaybeIncorrect,
+                    );
+                    err.emit();
+                    this.bump();
+                } else {
+                    return Err(err);
+                }
+            }
             let arm_start_span = this.token.span;
 
             let expr = this.parse_expr_res(Restrictions::STMT_EXPR, None).map_err(|mut err| {

src/test/ui/parser/issue-89396.fixed

@@ -0,0 +1,16 @@
+// Regression test for issue #89396: Try to recover from a
+// `=>` -> `=` or `->` typo in a match arm.
+
+// run-rustfix
+
+fn main() {
+    let opt = Some(42);
+    let _ = match opt {
+        Some(_) => true,
+        //~^ ERROR: expected one of
+        //~| HELP: try using a fat arrow here
+        None => false,
+        //~^ ERROR: expected one of
+        //~| HELP: try using a fat arrow here
+    };
+}

src/test/ui/parser/issue-89396.rs

@@ -0,0 +1,16 @@
+// Regression test for issue #89396: Try to recover from a
+// `=>` -> `=` or `->` typo in a match arm.
+
+// run-rustfix
+
+fn main() {
+    let opt = Some(42);
+    let _ = match opt {
+        Some(_) = true,
+        //~^ ERROR: expected one of
+        //~| HELP: try using a fat arrow here
+        None -> false,
+        //~^ ERROR: expected one of
+        //~| HELP: try using a fat arrow here
+    };
+}

src/test/ui/parser/issue-89396.stderr

@@ -0,0 +1,20 @@
+error: expected one of `=>`, `if`, or `|`, found `=`
+  --> $DIR/issue-89396.rs:9:17
+   |
+LL |         Some(_) = true,
+   |                 ^
+   |                 |
+   |                 expected one of `=>`, `if`, or `|`
+   |                 help: try using a fat arrow here: `=>`
+
+error: expected one of `=>`, `@`, `if`, or `|`, found `->`
+  --> $DIR/issue-89396.rs:12:14
+   |
+LL |         None -> false,
+   |              ^^
+   |              |
+   |              expected one of `=>`, `@`, `if`, or `|`
+   |              help: try using a fat arrow here: `=>`
+
+error: aborting due to 2 previous errors
+