Emit error on intrinsic to fn ptr casts

src/test/ui/reify-intrinsic.rs

@@ -0,0 +1,15 @@
+// check-fail
+
+#![feature(intrinsics)]
+
+fn a() {
+    let _: unsafe extern "rust-intrinsic" fn(isize) -> usize = std::mem::transmute;
+    //~^ ERROR cannot coerce
+}
+
+fn b() {
+    let _ = std::mem::transmute as unsafe extern "rust-intrinsic" fn(isize) -> usize;
+    //~^ ERROR casting
+}
+
+fn main() {}

src/test/ui/reify-intrinsic.stderr

@@ -0,0 +1,22 @@
+error[E0308]: cannot coerce intrinsics to function pointers
+  --> $DIR/reify-intrinsic.rs:6:64
+   |
+LL |     let _: unsafe extern "rust-intrinsic" fn(isize) -> usize = std::mem::transmute;
+   |                                                                ^^^^^^^^^^^^^^^^^^^
+   |                                                                |
+   |                                                                cannot coerce intrinsics to function pointers
+   |                                                                help: use parentheses to call this function: `std::mem::transmute(...)`
+   |
+   = note: expected type `unsafe extern "rust-intrinsic" fn(isize) -> usize`
+              found type `unsafe extern "rust-intrinsic" fn(_) -> _ {std::intrinsics::transmute::<_, _>}`
+
+error[E0606]: casting `unsafe extern "rust-intrinsic" fn(_) -> _ {std::intrinsics::transmute::<_, _>}` as `unsafe extern "rust-intrinsic" fn(isize) -> usize` is invalid
+  --> $DIR/reify-intrinsic.rs:11:13
+   |
+LL |     let _ = std::mem::transmute as unsafe extern "rust-intrinsic" fn(isize) -> usize;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0308, E0606.
+For more information about an error, try `rustc --explain E0308`.