Normalize the type Self resolves to in an impl

This is required at the very least in order to evaluate associated
constants for arrays (see rust-lang#58212).

src/librustc_typeck/astconv.rs

@@ -1700,7 +1700,8 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx> + 'o {
                 // `Self` in impl (we know the concrete type).
                 assert_eq!(opt_self_ty, None);
                 self.prohibit_generics(&path.segments);
-                tcx.at(span).type_of(def_id)
+                // Try to evaluate any array length constants
+                self.normalize_ty(span, tcx.at(span).type_of(def_id))
             }
             Def::SelfTy(Some(_), None) => {
                 // `Self` in trait.

src/test/run-pass/issues/issue-58212.rs

@@ -0,0 +1,13 @@
+trait FromUnchecked {
+    unsafe fn from_unchecked();
+}
+
+impl FromUnchecked for [u8; 1] {
+    unsafe fn from_unchecked() {
+        let mut array: Self = std::mem::uninitialized();
+        let _ptr = &mut array as *mut [u8] as *mut u8;
+    }
+}
+
+fn main() {
+}