Recover from missing param list in function definitions

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

@@ -2492,6 +2492,16 @@ 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...
+        if self.token.kind != TokenKind::OpenDelim(Delimiter::Parenthesis)
+        // might be typo'd trait impl, handled elsewhere
+        && !self.token.is_keyword(kw::For)
+        {
+            // recover from missing argument list, e.g. `fn main -> () {}`
+            self.sess
+                .emit_err(errors::MissingFnParams { span: self.prev_token.span.shrink_to_hi() });
+            return Ok(ThinVec::new());
+        }
+
         let (mut params, _) = self.parse_paren_comma_seq(|p| {
             p.recover_diff_marker();
             let param = p.parse_param_general(req_name, first_param).or_else(|mut e| {