ADD - migrate InvalidTraitItem and AltInvalidTraitItem errors

Thought of doing this by having a struct and an enum with Default and Alt cases, but not sure if we wanted to have the text in code instead of having “demangling()” and “demangling-alt()” in the ftl file.

Don’t like the current way of having structs representing the same-ish and using long names to distinguish their expectations, instead of putting this in an enum and handling the different cases inside the type.

I am fine with whichever option the team prefers; also understand having them as separate structs keeps it simple.

compiler/rustc_symbol_mangling/src/errors.rs

@@ -10,3 +10,19 @@ pub struct InvalidSymbolName<'a> {
     pub span: Span,
     pub mangled_formatted: &'a str,
 }
+
+#[derive(SessionDiagnostic)]
+#[error(symbol_mangling::invalid_trait_item)]
+pub struct InvalidTraitItem<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub demangling_formatted: &'a str,
+}
+
+#[derive(SessionDiagnostic)]
+#[error(symbol_mangling::alt_invalid_trait_item)]
+pub struct AltInvalidTraitItem<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub alt_demangling_formatted: &'a str,
+}