typeck: check for infer before type impls trait

src/librustc_typeck/check/cast.rs

@@ -387,6 +387,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                             // Check for infer types because cases like `Option<{integer}>` would
                             // panic otherwise.
                             if !expr_ty.has_infer_types()
+                                && !ty.has_infer_types()
                                 && fcx.tcx.type_implements_trait((
                                     from_trait,
                                     ty,

src/test/ui/issues/issue-73886.rs

@@ -0,0 +1,6 @@
+fn main() {
+    let _ = &&[0] as &[_];
+    //~^ ERROR non-primitive cast: `&&[i32; 1]` as `&[_]`
+    let _ = 7u32 as Option<_>;
+    //~^ ERROR non-primitive cast: `u32` as `std::option::Option<_>`
+}

src/test/ui/issues/issue-73886.stderr

@@ -0,0 +1,15 @@
+error[E0605]: non-primitive cast: `&&[i32; 1]` as `&[_]`
+  --> $DIR/issue-73886.rs:2:13
+   |
+LL |     let _ = &&[0] as &[_];
+   |             ^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+
+error[E0605]: non-primitive cast: `u32` as `std::option::Option<_>`
+  --> $DIR/issue-73886.rs:4:13
+   |
+LL |     let _ = 7u32 as Option<_>;
+   |             ^^^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0605`.