Rollup merge of #120556 - fmease:improve-unused-generic-param-diags, r=oli-obk

Improve the diagnostics for unused generic parameters

* Don't emit two errors (namely E0091 *and* E0392) for unused type parameters on *lazy* type aliases
* Fix the diagnostic help message of E0392 for *lazy* type aliases: Don't talk about the “fields” of lazy type aliases (use the term “body” instead) and don't suggest `PhantomData` for them, it doesn't make much sense
* Consolidate the diagnostics for E0091 (unused type parameters in type aliases) and E0392 (unused generic parameters due to bivariance) and make it translatable
  * Still keep the error codes distinct (for now)
  * Naturally leads to better diagnostics for E0091

r? ```@oli-obk``` (to ballast your review load :P) or compiler

1 files changed, 24 insertions, 0 deletions

diff --git a/compiler/rustc_hir_analysis/src/errors.rs b/compiler/rustc_hir_analysis/src/errors.rs
index e5871276d64..4eba31e327f 100644
--- a/compiler/rustc_hir_analysis/src/errors.rs
+++ b/compiler/rustc_hir_analysis/src/errors.rs
@@ -1511,3 +1511,27 @@ pub struct NotSupportedDelegation<'a> {
     #[label]
     pub callee_span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_unused_generic_parameter)]
+pub(crate) struct UnusedGenericParameter {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    pub param_name: Ident,
+    pub param_def_kind: &'static str,
+    #[subdiagnostic]
+    pub help: UnusedGenericParameterHelp,
+    #[help(hir_analysis_const_param_help)]
+    pub const_param_help: Option<()>,
+}
+
+#[derive(Subdiagnostic)]
+pub(crate) enum UnusedGenericParameterHelp {
+    #[help(hir_analysis_unused_generic_parameter_adt_help)]
+    Adt { param_name: Ident, phantom_data: String },
+    #[help(hir_analysis_unused_generic_parameter_adt_no_phantom_data_help)]
+    AdtNoPhantomData { param_name: Ident },
+    #[help(hir_analysis_unused_generic_parameter_ty_alias_help)]
+    TyAlias { param_name: Ident },
+}