Fix a span error when parsing a wrong param of function.

compiler/rustc_parse/src/parser/item.rs

@@ -2773,7 +2773,14 @@ impl<'a> Parser<'a> {
             let snapshot = p.create_snapshot_for_diagnostic();
             let param = p.parse_param_general(req_name, first_param).or_else(|e| {
                 let guar = e.emit();
-                let lo = p.prev_token.span;
+                // When parsing a param failed, we should check to make the span of the param
+                // not contain '(' before it.
+                // For example when parsing `*mut Self` in function `fn oof(*mut Self)`.
+                let lo = if let TokenKind::OpenDelim(Delimiter::Parenthesis) = p.prev_token.kind {
+                    p.prev_token.span.shrink_to_hi()
+                } else {
+                    p.prev_token.span
+                };
                 p.restore_snapshot(snapshot);
                 // Skip every token until next possible arg or end.
                 p.eat_to_tokens(&[&token::Comma, &token::CloseDelim(Delimiter::Parenthesis)]);

tests/ui/suggestions/suggest-add-self-issue-128042.rs

@@ -0,0 +1,12 @@
+struct Thing {
+    state: u8,
+}
+
+impl Thing {
+    fn oof(*mut Self) { //~ ERROR expected parameter name, found `*`
+        self.state = 1;
+        //~^ ERROR expected value, found module `self`
+    }
+}
+
+fn main() {}

tests/ui/suggestions/suggest-add-self-issue-128042.stderr

@@ -0,0 +1,22 @@
+error: expected parameter name, found `*`
+  --> $DIR/suggest-add-self-issue-128042.rs:6:12
+   |
+LL |     fn oof(*mut Self) {
+   |            ^ expected parameter name
+
+error[E0424]: expected value, found module `self`
+  --> $DIR/suggest-add-self-issue-128042.rs:7:9
+   |
+LL |     fn oof(*mut Self) {
+   |        --- this function doesn't have a `self` parameter
+LL |         self.state = 1;
+   |         ^^^^ `self` value is a keyword only available in methods with a `self` parameter
+   |
+help: add a `self` receiver parameter to make the associated `fn` a method
+   |
+LL |     fn oof(&self, *mut Self) {
+   |            ++++++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0424`.