From 4e3e27d8f6f8e16c90863322f6ed6109e77df574 Mon Sep 17 00:00:00 2001
From: Paul Backus <snarwin@gmail.com>
Date: Fri, 22 May 2020 11:55:33 -0400
Subject: [PATCH] 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.
---
 src/dmd/dsymbolsem.d                      | 12 ++++++++++++
 test/compilable/compile1.d                |  2 +-
 test/compilable/test16031.d               |  2 +-
 test/fail_compilation/dep_extern_safety.d |  9 +++++++++
 test/fail_compilation/fail20771.d         |  2 +-
 test/fail_compilation/fail20772.d         |  2 +-
 test/fail_compilation/fail20775.d         |  2 +-
 7 files changed, 26 insertions(+), 5 deletions(-)
 create mode 100644 test/fail_compilation/dep_extern_safety.d

diff --git a/src/dmd/dsymbolsem.d b/src/dmd/dsymbolsem.d
index 67fb1d79bbdd..9016b25eff6c 100644
--- a/src/dmd/dsymbolsem.d
+++ b/src/dmd/dsymbolsem.d
@@ -4049,6 +4049,18 @@ 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.
+         */
+        if (!funcdecl.fbody
+            && funcdecl.storage_class == STC.extern_
+            && funcdecl.type.toTypeFunction().trust == TRUST.default_)
+        {
+            deprecation(funcdecl.loc,
+                "`extern` function `%s` should be marked explicitly as `@safe`, `@system`, or `@trusted`",
+                funcdecl.toPrettyChars);
+        }
+
         Module.dprogress++;
         funcdecl.semanticRun = PASS.semanticdone;
 
diff --git a/test/compilable/compile1.d b/test/compilable/compile1.d
index 1b6d1f4eb036..9c41274738c5 100644
--- a/test/compilable/compile1.d
+++ b/test/compilable/compile1.d
@@ -476,7 +476,7 @@ template test8163(T...)
     }
 
     enum N = 2; // N>=2 triggers the bug
-    extern Point[N] bar();
+    @system extern Point[N] bar();
 
     void foo()
     {
diff --git a/test/compilable/test16031.d b/test/compilable/test16031.d
index 7239522e1072..3cc18561112d 100644
--- a/test/compilable/test16031.d
+++ b/test/compilable/test16031.d
@@ -1,7 +1,7 @@
 // REQUIRED_ARGS: -fPIC -lib
 // PERMUTE_ARGS:
 // DISABLED: win32 win64
-extern void throwing();
+@system extern void throwing();
 
 void foo()
 {
diff --git a/test/fail_compilation/dep_extern_safety.d b/test/fail_compilation/dep_extern_safety.d
new file mode 100644
index 000000000000..83830d2b5856
--- /dev/null
+++ b/test/fail_compilation/dep_extern_safety.d
@@ -0,0 +1,9 @@
+/*
+REQUIRED_ARGS: -de
+TEST_OUTPUT:
+---
+fail_compilation/dep_extern_safety.d(9): Deprecation: `extern` function `dep_extern_safety.fun` should be marked explicitly as `@safe`, `@system`, or `@trusted`
+---
+*/
+
+extern extern(C) void fun();
diff --git a/test/fail_compilation/fail20771.d b/test/fail_compilation/fail20771.d
index 18e1c87980da..28d94c2cc0fd 100644
--- a/test/fail_compilation/fail20771.d
+++ b/test/fail_compilation/fail20771.d
@@ -5,7 +5,7 @@ fail_compilation/fail20771.d(19): Error: cannot pass types with postblits or cop
 fail_compilation/fail20771.d(20): Error: cannot pass types with postblits or copy constructors as variadic arguments
 ---
 */
-extern void variadic(...);
+@system extern void variadic(...);
 
 struct S20771
 {
diff --git a/test/fail_compilation/fail20772.d b/test/fail_compilation/fail20772.d
index a2fdac687987..632561810876 100644
--- a/test/fail_compilation/fail20772.d
+++ b/test/fail_compilation/fail20772.d
@@ -5,7 +5,7 @@ fail_compilation/fail20772.d(20): Error: cannot pass types with postblits or cop
 fail_compilation/fail20772.d(21): Error: cannot pass types with postblits or copy constructors as variadic arguments
 ---
 */
-extern void variadic(...);
+@system extern void variadic(...);
 
 struct S20772
 {
diff --git a/test/fail_compilation/fail20775.d b/test/fail_compilation/fail20775.d
index 3e050960a186..f3ab7467ecd7 100644
--- a/test/fail_compilation/fail20775.d
+++ b/test/fail_compilation/fail20775.d
@@ -5,7 +5,7 @@ fail_compilation/fail20775.d(19): Error: cannot pass types that need destruction
 fail_compilation/fail20775.d(20): Error: cannot pass types that need destruction as variadic arguments
 ---
 */
-extern void variadic(...);
+@system extern void variadic(...);
 
 struct S20775
 {