Fix ICE in const param defaults with lifetime-dependent types

compiler/rustc_hir_analysis/src/collect.rs

@@ -1561,11 +1561,26 @@ fn const_param_default<'tcx>(
             "`const_param_default` expected a generic parameter with a constant"
         ),
     };
+
+    let param_def_id = def_id.to_def_id();
+    let parent_def_id = tcx.parent(param_def_id);
+    let parent_generics = tcx.generics_of(parent_def_id);
+    let parent_identity_args = ty::GenericArgs::identity_for_item(tcx, parent_def_id);
+
+    let lookup_index = parent_generics.param_def_id_to_index.get(&param_def_id).copied();
+    let param_index = lookup_index.unwrap_or_else(|| {
+        span_bug!(
+            tcx.def_span(def_id),
+            "missing const param index for `{param_def_id:?}` in `{parent_def_id:?}`"
+        )
+    }) as usize;
+    let slice_index = parent_identity_args.len().min(param_index);
+    let params_in_scope = parent_identity_args.split_at(slice_index).0;
+
     let icx = ItemCtxt::new(tcx, def_id);
-    let identity_args = ty::GenericArgs::identity_for_item(tcx, def_id);
     let ct = icx
         .lowerer()
-        .lower_const_arg(default_ct, FeedConstTy::Param(def_id.to_def_id(), identity_args));
+        .lower_const_arg(default_ct, FeedConstTy::Param(param_def_id, params_in_scope));
     ty::EarlyBinder::bind(ct)
 }
 

tests/ui/const-generics/generic_const_parameter_types/ice-142913-const-defaults-with-lifetimes.rs

@@ -0,0 +1,10 @@
+#![allow(incomplete_features)]
+#![feature(generic_const_parameter_types)]
+struct Variant;
+
+fn foo<'a, const N: &'a Variant = {}>() {}
+//~^ ERROR: defaults for generic parameters are not allowed here
+//~| ERROR: anonymous constants with lifetimes in their type are not yet supported
+//~| ERROR: `&'a Variant` is forbidden as the type of a const generic parameter
+
+fn main() {}

tests/ui/const-generics/generic_const_parameter_types/ice-142913-const-defaults-with-lifetimes.stderr

@@ -0,0 +1,30 @@
+error: defaults for generic parameters are not allowed here
+  --> $DIR/ice-142913-const-defaults-with-lifetimes.rs:5:12
+   |
+LL | fn foo<'a, const N: &'a Variant = {}>() {}
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: anonymous constants with lifetimes in their type are not yet supported
+  --> $DIR/ice-142913-const-defaults-with-lifetimes.rs:5:35
+   |
+LL | fn foo<'a, const N: &'a Variant = {}>() {}
+   |                                   ^^
+
+error: `&'a Variant` is forbidden as the type of a const generic parameter
+  --> $DIR/ice-142913-const-defaults-with-lifetimes.rs:5:21
+   |
+LL | fn foo<'a, const N: &'a Variant = {}>() {}
+   |                     ^^^^^^^^^^^
+   |
+   = note: the only supported types are integers, `bool`, and `char`
+help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
+   |
+LL + #![feature(adt_const_params)]
+   |
+help: add `#![feature(unsized_const_params)]` to the crate attributes to enable references to implement the `ConstParamTy` trait
+   |
+LL + #![feature(unsized_const_params)]
+   |
+
+error: aborting due to 3 previous errors
+