Handle , to ; substitution in arg params

compiler/rustc_parse/src/parser/path.rs

@@ -478,6 +478,23 @@ impl<'a> Parser<'a> {
         while let Some(arg) = self.parse_angle_arg(ty_generics)? {
             args.push(arg);
             if !self.eat(&token::Comma) {
+                if self.token.kind == token::Semi
+                    && self.look_ahead(1, |t| t.is_ident() || t.is_lifetime())
+                {
+                    // Add `>` to the list of expected tokens.
+                    self.check(&token::Gt);
+                    // Handle `,` to `;` substitution
+                    let mut err = self.unexpected::<()>().unwrap_err();
+                    self.bump();
+                    err.span_suggestion_verbose(
+                        self.prev_token.span.until(self.token.span),
+                        "use a comma to separate type parameters",
+                        ", ".to_string(),
+                        Applicability::MachineApplicable,
+                    );
+                    err.emit();
+                    continue;
+                }
                 if !self.token.kind.should_end_const_arg() {
                     if self.handle_ambiguous_unbraced_const_arg(&mut args)? {
                         // We've managed to (partially) recover, so continue trying to parse

src/test/ui/parser/lifetime-semicolon.fixed

@@ -0,0 +1,10 @@
+// run-rustfix
+#![allow(unused)]
+struct Foo<'a, 'b> {
+    a: &'a &'b i32
+}
+
+fn foo<'a, 'b>(_x: &mut Foo<'a, 'b>) {}
+//~^ ERROR expected one of `,`, `:`, `=`, or `>`, found `;`
+
+fn main() {}

src/test/ui/parser/lifetime-semicolon.rs

@@ -1,8 +1,10 @@
+// run-rustfix
+#![allow(unused)]
 struct Foo<'a, 'b> {
     a: &'a &'b i32
 }
 
-fn foo<'a, 'b>(x: &mut Foo<'a; 'b>) {}
+fn foo<'a, 'b>(_x: &mut Foo<'a; 'b>) {}
 //~^ ERROR expected one of `,`, `:`, `=`, or `>`, found `;`
 
 fn main() {}

src/test/ui/parser/lifetime-semicolon.stderr

@@ -1,13 +1,13 @@
 error: expected one of `,`, `:`, `=`, or `>`, found `;`
-  --> $DIR/lifetime-semicolon.rs:5:30
+  --> $DIR/lifetime-semicolon.rs:7:31
    |
-LL | fn foo<'a, 'b>(x: &mut Foo<'a; 'b>) {}
+LL | fn foo<'a, 'b>(_x: &mut Foo<'a; 'b>) {}
    |                               ^ expected one of `,`, `:`, `=`, or `>`
    |
-help: you might have meant to end the type parameters here
+help: use a comma to separate type parameters
    |
-LL | fn foo<'a, 'b>(x: &mut Foo<'a>; 'b>) {}
-   |                              +
+LL | fn foo<'a, 'b>(_x: &mut Foo<'a, 'b>) {}
+   |                               ~
 
 error: aborting due to previous error