Fix ICE when using function pointer as const generic parameter

Add bounds check in prohibit_explicit_late_bound_lifetimes to prevent
index out of bounds panic when args.args is empty. Add regression test
to ensure function pointers in const generics produce proper error
messages instead of ICE.

Author: enthropy7

compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs

@@ -616,6 +616,10 @@ pub(crate) fn prohibit_explicit_late_bound_lifetimes(
     }
 
     if let Some(span_late) = def.has_late_bound_regions {
+        if args.args.is_empty() {
+            return ExplicitLateBound::No;
+        }
+
         let msg = "cannot specify lifetime arguments explicitly \
                        if late bound lifetime parameters are present";
         let note = "the late bound lifetime parameter is introduced here";

tests/crashes/137084.rs

@@ -0,0 +1,16 @@
+//@ check-fail
+//
+// Regression test for https://github.com/rust-lang/rust/issues/137084
+// Previously caused ICE when using function item as const generic argument
+
+#![feature(min_generic_const_args)]
+#![allow(incomplete_features)]
+
+fn a<const b: i32>() {}
+fn d(e: &String) {
+    a::<d>
+    //~^ ERROR mismatched types
+    //~| ERROR the constant `d` is not of type `i32`
+}
+
+fn main() {}

tests/ui/const-generics/fn-item-as-const-arg-137084.rs

@@ -0,0 +1,35 @@
+error[E0308]: mismatched types
+  --> $DIR/fn-item-as-const-arg-137084.rs:11:5
+   |
+LL | fn a<const b: i32>() {}
+   | -------------------- function `a` defined here
+LL | fn d(e: &String) {
+LL |     a::<d>
+   |     ^^^^^^ expected `()`, found fn item
+   |
+   = note: expected unit type `()`
+                found fn item `fn() {a::<d>}`
+help: try adding a return type
+   |
+LL | fn d(e: &String) -> fn() {
+   |                  +++++++
+help: use parentheses to call this function
+   |
+LL |     a::<d>()
+   |           ++
+
+error: the constant `d` is not of type `i32`
+  --> $DIR/fn-item-as-const-arg-137084.rs:11:9
+   |
+LL |     a::<d>
+   |         ^ expected `i32`, found fn item
+   |
+note: required by a const generic parameter in `a`
+  --> $DIR/fn-item-as-const-arg-137084.rs:9:6
+   |
+LL | fn a<const b: i32>() {}
+   |      ^^^^^^^^^^^^ required by this const generic parameter in `a`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/const-generics/fn-item-as-const-arg-137084.stderr

@@ -0,0 +1,18 @@
+//@ check-fail
+//
+// Regression test for https://github.com/rust-lang/rust/issues/151186
+
+#![feature(min_generic_const_args)]
+#![allow(incomplete_features)]
+
+trait Maybe<T> {}
+
+trait MyTrait<const F: fn() -> ()> {}
+//~^ ERROR using function pointers as const generic parameters is forbidden
+
+fn foo<'a>(x: &'a ()) -> &'a () { x }
+
+impl<T> Maybe<T> for T where T: MyTrait<{ foo }> {}
+//~^ ERROR the constant `foo` is not of type `fn()`
+
+fn main() {}

tests/ui/const-generics/ice-151186-fn-ptr-in-where-clause.rs

@@ -0,0 +1,22 @@
+error: using function pointers as const generic parameters is forbidden
+  --> $DIR/ice-151186-fn-ptr-in-where-clause.rs:10:24
+   |
+LL | trait MyTrait<const F: fn() -> ()> {}
+   |                        ^^^^^^^^^^
+   |
+   = note: the only supported types are integers, `bool`, and `char`
+
+error: the constant `foo` is not of type `fn()`
+  --> $DIR/ice-151186-fn-ptr-in-where-clause.rs:15:33
+   |
+LL | impl<T> Maybe<T> for T where T: MyTrait<{ foo }> {}
+   |                                 ^^^^^^^^^^^^^^^^ expected fn pointer, found fn item
+   |
+note: required by a const generic parameter in `MyTrait`
+  --> $DIR/ice-151186-fn-ptr-in-where-clause.rs:10:15
+   |
+LL | trait MyTrait<const F: fn() -> ()> {}
+   |               ^^^^^^^^^^^^^^^^^^^ required by this const generic parameter in `MyTrait`
+
+error: aborting due to 2 previous errors
+