Recover from missing param list in function definitions

compiler/rustc_parse/messages.ftl

@@ -523,6 +523,9 @@ parse_missing_fn_for_function_definition = missing `fn` for function definition
 parse_missing_fn_for_method_definition = missing `fn` for method definition
     .suggestion = add `fn` here to parse `{$ident}` as a public method
 
+parse_missing_fn_params = missing parameters for function definition
+    .suggestion = add a parameter list
+
 parse_missing_for_in_trait_impl = missing `for` in a trait impl
     .suggestion = add `for` here
 

compiler/rustc_parse/src/errors.rs

@@ -1543,6 +1543,14 @@ pub(crate) enum AmbiguousMissingKwForItemSub {
     HelpMacro,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_missing_fn_params)]
+pub(crate) struct MissingFnParams {
+    #[primary_span]
+    #[suggestion(code = "()", applicability = "machine-applicable", style = "short")]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_missing_trait_in_trait_impl)]
 pub(crate) struct MissingTraitInTraitImpl {

compiler/rustc_parse/src/parser/item.rs

@@ -2492,7 +2492,7 @@ impl<'a> Parser<'a> {
     pub(super) fn parse_fn_params(&mut self, req_name: ReqName) -> PResult<'a, ThinVec<Param>> {
         let mut first_param = true;
         // Parse the arguments, starting out with `self` being allowed...
-        let (mut params, _) = self.parse_paren_comma_seq(|p| {
+        let parsed = self.parse_paren_comma_seq(|p| {
             p.recover_diff_marker();
             let param = p.parse_param_general(req_name, first_param).or_else(|mut e| {
                 e.emit();
@@ -2505,7 +2505,29 @@ impl<'a> Parser<'a> {
             // ...now that we've parsed the first argument, `self` is no longer allowed.
             first_param = false;
             param
-        })?;
+        });
+        let mut params = match parsed {
+            Ok((p, _)) => p,
+            Err(e) => {
+                // some error while parsing params
+                if self.token.kind == TokenKind::OpenDelim(Delimiter::Parenthesis) ||
+                    // removed syntax, handled elsewhere
+                    self.token.kind == TokenKind::Tilde ||
+                    // might be typo'd trait impl, handled elsewhere
+                    self.token.is_keyword(kw::For)
+                {
+                    return Err(e);
+                } else {
+                    // recover from missing argument list, e.g. `fn main -> () {}`
+                    e.cancel();
+                    self.sess.emit_err(errors::MissingFnParams {
+                        span: self.prev_token.span.shrink_to_hi(),
+                    });
+                    return Ok(ThinVec::new());
+                }
+            }
+        };
+
         // Replace duplicated recovered params with `_` pattern to avoid unnecessary errors.
         self.deduplicate_recovered_params_names(&mut params);
         Ok(params)

tests/ui/mismatched_types/recovered-block.rs

@@ -12,10 +12,4 @@ pub fn foo() -> Foo {
 }
 //~^^ ERROR missing `struct` for struct definition
 
-pub fn bar() -> Foo {
-    fn
-    Foo { text: "".to_string() }
-}
-//~^^ ERROR expected one of `(` or `<`, found `{`
-
 fn main() {}

tests/ui/mismatched_types/recovered-block.stderr

@@ -9,11 +9,5 @@ help: add `struct` here to parse `Foo` as a public struct
 LL |     pub struct Foo { text }
    |         ++++++
 
-error: expected one of `(` or `<`, found `{`
-  --> $DIR/recovered-block.rs:17:9
-   |
-LL |     Foo { text: "".to_string() }
-   |         ^ expected one of `(` or `<`
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 

tests/ui/parser/issues/issue-108109-fn-missing-params.fixed

@@ -0,0 +1,9 @@
+// run-rustfix
+
+pub fn missing() -> () {}
+//~^ ERROR missing parameters for function definition
+
+pub fn missing2() {}
+//~^ ERROR missing parameters for function definition
+
+fn main() {}

tests/ui/parser/issues/issue-108109-fn-missing-params.rs

@@ -0,0 +1,9 @@
+// run-rustfix
+
+pub fn missing -> () {}
+//~^ ERROR missing parameters for function definition
+
+pub fn missing2 {}
+//~^ ERROR missing parameters for function definition
+
+fn main() {}

tests/ui/parser/issues/issue-108109-fn-missing-params.stderr

@@ -0,0 +1,14 @@
+error: missing parameters for function definition
+  --> $DIR/issue-108109-fn-missing-params.rs:3:15
+   |
+LL | pub fn missing -> () {}
+   |               ^ help: add a parameter list
+
+error: missing parameters for function definition
+  --> $DIR/issue-108109-fn-missing-params.rs:6:16
+   |
+LL | pub fn missing2 {}
+   |                ^ help: add a parameter list
+
+error: aborting due to 2 previous errors
+