Rollup merge of #90022 - hkmatsumoto:self-upper-as-generic-parameter, r=jackh726

Explain why `Self` is invalid in generic parameters

Close #89985.

r? `@estebank`

compiler/rustc_parse/src/parser/generics.rs

@@ -92,6 +92,19 @@ impl<'a> Parser<'a> {
             let attrs = self.parse_outer_attributes()?;
             let param =
                 self.collect_tokens_trailing_token(attrs, ForceCollect::No, |this, attrs| {
+                    if this.eat_keyword_noexpect(kw::SelfUpper) {
+                        // `Self` as a generic param is invalid. Here we emit the diagnostic and continue parsing
+                        // as if `Self` never existed.
+                        this.struct_span_err(
+                            this.prev_token.span,
+                            "unexpected keyword `Self` in generic parameters",
+                        )
+                        .note("you cannot use `Self` as a generic parameter because it is reserved for associated items")
+                        .emit();
+
+                        this.eat(&token::Comma);
+                    }
+
                     let param = if this.check_lifetime() {
                         let lifetime = this.expect_lifetime();
                         // Parse lifetime parameter.