Auto merge of #42735 - arielb1:generic-closure-fn, r=eddyb

collector: apply param substs to closures cast to fn items

Fixes #42718.

r? @eddyb
beta-nominating because serious ICE in newly-stabilized feature.

src/test/run-pass/closure-to-fn-coercion.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use std::mem;
+
 const FOO: fn(u8) -> u8 = |v: u8| { v };
 
 const BAR: [fn(&mut u32); 5] = [
@@ -21,6 +23,10 @@ fn func_specific() -> (fn() -> u32) {
     || return 42
 }
 
+fn generic<T>(_: T) -> fn() -> usize {
+    || mem::size_of::<T>()
+}
+
 fn main() {
     // Items
     assert_eq!(func_specific()(), 42);
@@ -34,4 +40,5 @@ fn main() {
     assert_eq!({ BAR[2](&mut a); a }, 3);
     assert_eq!({ BAR[3](&mut a); a }, 6);
     assert_eq!({ BAR[4](&mut a); a }, 10);
+    assert_eq!(generic(0i8)(), 1);
 }