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 +4049,18 @@ 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;
 

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()
     {

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()
 {

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();

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
 {

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
 {

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
 {