Rollup merge of #108136 - eggyal:unmet_trait_alias_bound_on_generic_impl, r=compiler-errors

Do not ICE on unmet trait alias impl bounds

Fixes #108132

I've also added some documentation to the `impl_def_id` field of `DerivedObligationCause` to try and minimise the risk of such errors in future.

r? `@compiler-errors`

compiler/rustc_middle/src/traits/mod.rs

@@ -474,7 +474,11 @@ pub enum WellFormedLoc {
 #[derive(TypeVisitable, TypeFoldable)]
 pub struct ImplDerivedObligationCause<'tcx> {
     pub derived: DerivedObligationCause<'tcx>,
-    pub impl_def_id: DefId,
+    /// The `DefId` of the `impl` that gave rise to the `derived` obligation.
+    /// If the `derived` obligation arose from a trait alias, which conceptually has a synthetic impl,
+    /// then this will be the `DefId` of that trait alias. Care should therefore be taken to handle
+    /// that exceptional case where appropriate.
+    pub impl_or_alias_def_id: DefId,
     /// The index of the derived predicate in the parent impl's predicates.
     pub impl_def_predicate_index: Option<usize>,
     pub span: Span,