Prevent comptime fns from being called at runtime

Author: oli-obk

compiler/rustc_hir_typeck/src/callee.rs

@@ -917,7 +917,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return;
         }
 
-        let host = match self.tcx.hir_body_const_context(self.body_id) {
+        let const_context = self.tcx.hir_body_const_context(self.body_id);
+
+        if let hir::Constness::Comptime = self.tcx.constness(callee_did) {
+            match const_context {
+                Some(hir::ConstContext::Const { .. } | hir::ConstContext::Static(_)) => {}
+                Some(hir::ConstContext::ConstFn) | None => {
+                    self.dcx().span_err(span, "comptime fns can only be called at compile time");
+                }
+            }
+        }
+
+        let host = match const_context {
             Some(hir::ConstContext::Const { .. } | hir::ConstContext::Static(_)) => {
                 ty::BoundConstness::Const
             }

tests/ui/comptime/not_callable.rs

@@ -1,7 +1,4 @@
-#![feature(rustc_attrs)]
-
-// TODO: this test should fail
-//@check-pass
+#![feature(rustc_attrs, const_trait_impl)]
 
 #[rustc_comptime]
 fn foo() {}
@@ -11,4 +8,18 @@ fn main() {
     const { foo() };
     // Not ok
     foo();
+    //~^ ERROR: comptime fns can only be called at compile time
+}
+
+const fn bar() {
+    // Not ok
+    foo();
+    //~^ ERROR: comptime fns can only be called at compile time
+}
+
+#[rustc_comptime]
+fn baz() {
+    // Should be allowed
+    foo();
+    //~^ ERROR: comptime fns can only be called at compile time
 }

tests/ui/comptime/not_callable.stderr

@@ -0,0 +1,20 @@
+error: comptime fns can only be called at compile time
+  --> $DIR/not_callable.rs:10:5
+   |
+LL |     foo();
+   |     ^^^^^
+
+error: comptime fns can only be called at compile time
+  --> $DIR/not_callable.rs:16:5
+   |
+LL |     foo();
+   |     ^^^^^
+
+error: comptime fns can only be called at compile time
+  --> $DIR/not_callable.rs:23:5
+   |
+LL |     foo();
+   |     ^^^^^
+
+error: aborting due to 3 previous errors
+