Rollup merge of #127966 - oli-obk:structured_diag, r=compiler-errors

Use structured suggestions for unconstrained generic parameters on impl blocks

I did not deduplicate with `UnusedGenericParameter`, because in contrast to type declarations, just using a generic parameter in an impl isn't enough, it must be used with the right variance and not just as part of a projection.

compiler/rustc_hir_analysis/src/errors.rs

@@ -1604,6 +1604,20 @@ pub(crate) enum UnusedGenericParameterHelp {
     TyAlias { param_name: Ident },
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_unconstrained_generic_parameter)]
+pub(crate) struct UnconstrainedGenericParameter {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    pub param_name: Symbol,
+    pub param_def_kind: &'static str,
+    #[note(hir_analysis_const_param_note)]
+    pub const_param_note: Option<()>,
+    #[note(hir_analysis_const_param_note2)]
+    pub const_param_note2: Option<()>,
+}
+
 #[derive(Diagnostic)]
 pub enum UnnamedFieldsRepr<'a> {
     #[diag(hir_analysis_unnamed_fields_repr_missing_repr_c)]