fix: Support generics in extract_function assist

This change attempts to resolve issue #7637: Extract into Function does not
create a generic function with constraints when extracting generic code.

In `FunctionBody::analyze_container`, we now traverse the `ancestors` in search
of `AnyHasGenericParams`, and attach any `GenericParamList`s and `WhereClause`s
we find to the `ContainerInfo`.

Later, in `format_function`, we collect all the `GenericParam`s and
`WherePred`s from the container, and filter them to keep only types matching
`TypeParam`s used within the newly extracted function body or param list. We
can then include the new `GenericParamList` and `WhereClause` in the new
function definition.

This change only impacts `TypeParam`s. `LifetimeParam`s and `ConstParam`s are
out of scope for this change.

Author: DorianListens

crates/hir/src/lib.rs

@@ -3307,6 +3307,15 @@ impl Type {
         let tys = hir_ty::replace_errors_with_variables(&(self.ty.clone(), to.ty.clone()));
         hir_ty::could_coerce(db, self.env.clone(), &tys)
     }
+
+    pub fn as_type_param(&self, db: &dyn HirDatabase) -> Option<TypeParam> {
+        match self.ty.kind(Interner) {
+            TyKind::Placeholder(p) => Some(TypeParam {
+                id: TypeParamId::from_unchecked(hir_ty::from_placeholder_idx(db, *p)),
+            }),
+            _ => None,
+        }
+    }
 }
 
 #[derive(Debug)]