Rollup merge of rust-lang#70982 - ldm0:fncoerce, r=eddyb

Normalize function signature in function casting check procedure

Fixes rust-lang#54094
```rust
trait Zoo {
    type X;
}

impl Zoo for u16 {
    type X = usize;
}

fn foo(abc: <u16 as Zoo>::X) {}

fn main() {
    let x: *const u8 = foo as _;
}
```

Currently a `FnDef` need to be checked if it's able to cast to `FnPtr` before it is actually casted. But the signature of `FnPtr` target's associated types are not normalized:

https://github.com/rust-lang/rust/blob/96d77f0e5f103612d62b85938aacfb33f5768433/src/librustc_typeck/check/cast.rs#L536-L553
However, during the coercion check, the signature of `FnPtr` target's associated types are normalized (The `<u16 as Zoo>::X` turns into `usize`).

https://github.com/rust-lang/rust/blob/96d77f0e5f103612d62b85938aacfb33f5768433/src/librustc_typeck/check/coercion.rs#L687-L729

This inconsistency leads to the error:`Err(Sorts(ExpectedFound { expected: <u16 as Zoo>::X, found: usize }))`.

src/librustc_typeck/check/cast.rs

@@ -536,7 +536,10 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                 match self.expr_ty.kind {
                     ty::FnDef(..) => {
                         // Attempt a coercion to a fn pointer type.
-                        let f = self.expr_ty.fn_sig(fcx.tcx);
+                        let f = fcx.normalize_associated_types_in(
+                            self.expr.span,
+                            &self.expr_ty.fn_sig(fcx.tcx),
+                        );
                         let res = fcx.try_coerce(
                             self.expr,
                             self.expr_ty,

src/test/ui/issues/issue-54094.rs

@@ -0,0 +1,14 @@
+// check-pass
+trait Zoo {
+    type X;
+}
+
+impl Zoo for u16 {
+    type X = usize;
+}
+
+fn foo(abc: <u16 as Zoo>::X) {}
+
+fn main() {
+    let x: *const u8 = foo as _;
+}