Add deprecation message for extern functions with default safety

This message is needed to avoid silently introducing safety violations
to existing code when the default is changed from @System to @safe. It
should become an error when @safe is made the default.

Author: pbackus

src/dmd/dsymbolsem.d

@@ -4049,6 +4049,22 @@ private extern(C++) final class DsymbolSemanticVisitor : Visitor
         if (funcdecl.canInferAttributes(sc))
             funcdecl.initInferAttributes();
 
+        /* Warn about external function declarations that may become incorrect
+         * when @safe is made the default.
+         *
+         * extern(D) virtual functions are ok because the compiler will check
+         * them when they are overridden.
+         */
+        if (funcdecl.isExternal()
+            && funcdecl.type.toTypeFunction().trust == TRUST.default_
+            && !(funcdecl.isVirtual()
+                && (funcdecl.linkage == LINK.d || funcdecl.linkage == LINK.default_)))
+        {
+            deprecation(funcdecl.loc,
+                "`extern` function `%s` should be marked explicitly as `@safe`, `@system`, or `@trusted`",
+                funcdecl.toPrettyChars);
+        }
+
         Module.dprogress++;
         funcdecl.semanticRun = PASS.semanticdone;
 

src/dmd/func.d

@@ -1260,6 +1260,13 @@ extern (C++) class FuncDeclaration : Declaration
         return false;
     }
 
+    final bool isExternal()
+    {
+        if (fbody || storage_class & STC.disable)
+            return false;
+        return (storage_class & STC.extern_) || (linkage != LINK.default_);
+    }
+
     /**********************************
      * Decide if attributes for this function can be inferred from examining
      * the function body.

test/fail_compilation/dep_extern_safety.d

@@ -0,0 +1,11 @@
+/*
+REQUIRED_ARGS: -de
+TEST_OUTPUT:
+---
+fail_compilation/dep_extern_safety.d(10): Deprecation: `extern` function `dep_extern_safety.cfun` should be marked explicitly as `@safe`, `@system`, or `@trusted`
+fail_compilation/dep_extern_safety.d(11): Deprecation: `extern` function `dep_extern_safety.dfun` should be marked explicitly as `@safe`, `@system`, or `@trusted`
+---
+*/
+
+extern extern(C) void cfun();
+extern extern(D) void dfun();