Check for intrinsic to fn ptr casts in unified coercions

compiler/rustc_hir_typeck/src/coercion.rs

@@ -1291,14 +1291,26 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 ty::Closure(..) => {
                     Adjust::Pointer(PointerCoercion::ClosureFnPointer(a_sig.safety()))
                 }
-                ty::FnDef(..) => Adjust::Pointer(PointerCoercion::ReifyFnPointer),
+                ty::FnDef(def_id, ..) => {
+                    // Intrinsics are not coercible to function pointers
+                    if self.tcx.intrinsic(def_id).is_some() {
+                        return Err(TypeError::IntrinsicCast);
+                    }
+                    Adjust::Pointer(PointerCoercion::ReifyFnPointer)
+                }
                 _ => span_bug!(cause.span, "should not try to coerce a {prev_ty} to a fn pointer"),
             };
             let next_adjustment = match new_ty.kind() {
                 ty::Closure(..) => {
                     Adjust::Pointer(PointerCoercion::ClosureFnPointer(b_sig.safety()))
                 }
-                ty::FnDef(..) => Adjust::Pointer(PointerCoercion::ReifyFnPointer),
+                ty::FnDef(def_id, ..) => {
+                    // Intrinsics are not coercible to function pointers
+                    if self.tcx.intrinsic(def_id).is_some() {
+                        return Err(TypeError::IntrinsicCast);
+                    }
+                    Adjust::Pointer(PointerCoercion::ReifyFnPointer)
+                }
                 _ => span_bug!(new.span, "should not try to coerce a {new_ty} to a fn pointer"),
             };
             for expr in exprs.iter().map(|e| e.as_coercion_site()) {

tests/ui/coercion/intrinsic-in-unifying-coercion-149143.rs

@@ -0,0 +1,25 @@
+// Regression test for #149143.
+// The compiler did not check for a coercion from intrinsics
+// to fn ptrs in all possible code paths that could lead to such a coercion.
+// This caused an ICE during a later sanity check.
+
+use std::mem::transmute;
+
+fn main() {
+    unsafe {
+        let f = if true { transmute } else { safe_transmute };
+        //~^ ERROR `if` and `else` have incompatible type
+
+        let _: i64 = f(5i64);
+    }
+    unsafe {
+        let f = if true { safe_transmute } else { transmute };
+        //~^ ERROR `if` and `else` have incompatible type
+
+        let _: i64 = f(5i64);
+    }
+}
+
+unsafe fn safe_transmute<A, B>(x: A) -> B {
+    panic!()
+}

tests/ui/coercion/intrinsic-in-unifying-coercion-149143.stderr

@@ -0,0 +1,27 @@
+error[E0308]: `if` and `else` have incompatible types
+  --> $DIR/intrinsic-in-unifying-coercion-149143.rs:10:46
+   |
+LL |         let f = if true { transmute } else { safe_transmute };
+   |                           ---------          ^^^^^^^^^^^^^^ cannot coerce intrinsics to function pointers
+   |                           |
+   |                           expected because of this
+   |
+   = note: expected fn item `unsafe fn(_) -> _ {std::intrinsics::transmute::<_, _>}`
+              found fn item `unsafe fn(_) -> _ {safe_transmute::<_, _>}`
+   = note: different fn items have unique types, even if their signatures are the same
+
+error[E0308]: `if` and `else` have incompatible types
+  --> $DIR/intrinsic-in-unifying-coercion-149143.rs:16:51
+   |
+LL |         let f = if true { safe_transmute } else { transmute };
+   |                           --------------          ^^^^^^^^^ cannot coerce intrinsics to function pointers
+   |                           |
+   |                           expected because of this
+   |
+   = note: expected fn item `unsafe fn(_) -> _ {safe_transmute::<_, _>}`
+              found fn item `unsafe fn(_) -> _ {std::intrinsics::transmute::<_, _>}`
+   = note: different fn items have unique types, even if their signatures are the same
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.