Prevent comptime fns from being called at runtime

Author: oli-obk

compiler/rustc_hir_typeck/src/callee.rs

@@ -904,6 +904,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         callee_did: DefId,
         callee_args: GenericArgsRef<'tcx>,
     ) {
+        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");
+                }
+            }
+        }
+
         // FIXME(const_trait_impl): We should be enforcing these effects unconditionally.
         // This can be done as soon as we convert the standard library back to
         // using const traits, since if we were to enforce these conditions now,
@@ -917,7 +928,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return;
         }
 
-        let host = match self.tcx.hir_body_const_context(self.body_id) {
+        let host = match const_context {
             Some(hir::ConstContext::Const { .. } | hir::ConstContext::Static(_)) => {
                 ty::BoundConstness::Const
             }

tests/ui/comptime/not_callable.rs

@@ -1,25 +1,22 @@
-#![feature(rustc_attrs)]
-
-//TODO: should fail
-//@check-pass
+#![feature(rustc_attrs, const_trait_impl)]
 
 #[rustc_comptime]
 fn foo() {}
 
 fn main() {
     // Ok
     const { foo() };
-    // TODO: Not ok
-    foo();
+    // Not Ok
+    foo(); //~ ERROR: comptime fns can only be called at compile time
 }
 
 const fn bar() {
-    // TODO: Not ok
-    foo();
+    // Not Ok
+    foo(); //~ ERROR: comptime fns can only be called at compile time
 }
 
 #[rustc_comptime]
 fn baz() {
-    // Should be allowed
-    foo();
+    // TODO: 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:15:5
+   |
+LL |     foo();
+   |     ^^^^^
+
+error: comptime fns can only be called at compile time
+  --> $DIR/not_callable.rs:21:5
+   |
+LL |     foo();
+   |     ^^^^^
+
+error: aborting due to 3 previous errors
+