fix Issue 20150 - -dip1000 return attribute must be explicit in pure functions

Author: WalterBright

src/dmd/escape.d

@@ -1219,15 +1219,18 @@ private bool checkReturnEscapeImpl(Scope* sc, Expression e, bool refs, bool gag)
 
         Dsymbol p = v.toParent2();
 
-        if ((v.isScope() || (v.storage_class & STC.maybescope)) &&
-            !(v.storage_class & STC.return_) &&
+        bool needsReturn = false;
+        if (!(v.storage_class & STC.return_) &&
             v.isParameter() &&
-            !v.doNotInferReturn &&
-            sc.func.flags & FUNCFLAG.returnInprocess &&
             p == sc.func)
         {
-            inferReturn(sc.func, v);        // infer addition of 'return'
-            continue;
+            if ((v.isScope() || (v.storage_class & STC.maybescope)) &&
+                sc.func.flags & FUNCFLAG.returnInprocess)
+            {
+                inferReturn(sc.func, v);        // infer addition of 'return'
+                continue;
+            }
+            needsReturn = true;                 // needs 'return' annotation on parameter
         }
 
         if (v.isScope())
@@ -1275,6 +1278,17 @@ private bool checkReturnEscapeImpl(Scope* sc, Expression e, bool refs, bool gag)
                 result = false;
             }
         }
+        else if (needsReturn && sc.func.type.isTypeFunction().purity != PURE.impure && global.params.vsafe &&
+                 sc._module && sc._module.isRoot())
+        {
+            /* Pure functions assume their parameters are `scope`, but they don't assume `return`.
+             * Hence, the annotation needs to be explicit.
+             */
+            if (!gag)
+                error(e.loc, "`pure` function `%s` returns parameter `%s`, annotate with `return`",
+                    sc.func.ident.toChars(), v.toChars());
+            result = true;
+        }
         else
         {
             //printf("no infer for %s in %s, %d\n", v.toChars(), sc.func.ident.toChars(), __LINE__);

test/fail_compilation/retscope6.d

@@ -170,6 +170,22 @@ T9 testfred()
     return fred(&i);   // error
 }
 
+
+/* TEST_OUTPUT:
+---
+fail_compilation/retscope6.d(9505): Error: `pure` function `escape` returns parameter `r`, annotate with `return`
+---
+*/
+
+#line 9500
+
+// https://issues.dlang.org/show_bug.cgi?id=20150
+
+int* escape(int* r) @safe pure
+{
+    return r;
+}
+
 /* TEST_OUTPUT:
 ---
 fail_compilation/retscope6.d(10003): Error: scope variable `values` assigned to non-scope parameter `values` calling retscope6.escape