Fix issue 3827 - Deprecate implicit string concatenation

changelog.dd

@@ -18,6 +18,8 @@ $(BUGSTITLE Language Changes,
     )
 
     $(LI $(RELATIVE_LINK2 align_by_ctfe, Align attribute can be used with CTFEable expression.))
+
+    $(LI $(RELATIVE_LINK2 deprecated_implicit_cat, Implicit string concatenation is deprecated.))
 )
 
 $(BUGSTITLE Compiler Changes,
@@ -202,6 +204,22 @@ $(BUGSTITLE Compiler Changes,
         ---
         $(P The number after `spec:` is the nesting of the speculative compiles.)
     )
+
+    $(LI $(LNAME2 deprecated_implicit_cat, Implicit string concatenation is deprecated.)
+
+    $(P Implicit concatenation of string literal is a very early feature that is now supplanted
+        by the concatenation operator ('~'), the later being more explicit.)
+
+    $(P It could result in hard to spot bug, where one missed a coma in an array expression:)
+
+        ---
+        void foo ()
+        {
+          string[] arr = [ "Hello", "buggy" "World" ];
+          assert(arr.length = 3); // Fail, the length of the array is 2 and the content is [ "Hello", "buggyWorld" ]
+        }
+        ---
+    )
 )
 
 Macros:

src/parse.d

@@ -7192,6 +7192,7 @@ final class Parser : Lexer
                 auto postfix = token.postfix;
                 while (1)
                 {
+                    const prev = token;
                     nextToken();
                     if (token.value == TOKstring || token.value == TOKxstring)
                     {
@@ -7202,6 +7203,9 @@ final class Parser : Lexer
                             postfix = token.postfix;
                         }
 
+                        deprecation("Implicit string concatenation is deprecated, use %s ~ %s instead",
+                                    prev.toChars(), token.toChars());
+
                         const len1 = len;
                         const len2 = token.len;
                         len = len1 + len2;

test/d_do_test.d

@@ -17,25 +17,25 @@ import core.sys.posix.sys.wait;
 void usage()
 {
     write("d_do_test <input_dir> <test_name> <test_extension>\n"
-          "\n"
-          "   input_dir: one of: compilable, fail_compilation, runnable\n"
-          "   test_name: basename of test case to run\n"
-          "   test_extension: one of: d, html, or sh\n"
-          "\n"
-          "   example: d_do_test runnable pi d\n"
-          "\n"
-          "   relevant environment variables:\n"
-          "      ARGS:          set to execute all combinations of\n"
-          "      REQUIRED_ARGS: arguments always passed to the compiler\n"
-          "      DMD:           compiler to use, ex: ../src/dmd\n"
-          "      CC:            C++ compiler to use, ex: dmc, g++\n"
-          "      OS:            win32, win64, linux, freebsd, osx\n"
-          "      RESULTS_DIR:   base directory for test results\n"
-          "   windows vs non-windows portability env vars:\n"
-          "      DSEP:          \\\\ or /\n"
-          "      SEP:           \\ or /\n"
-          "      OBJ:          .obj or .o\n"
-          "      EXE:          .exe or <null>\n");
+          ~ "\n"
+          ~ "   input_dir: one of: compilable, fail_compilation, runnable\n"
+          ~ "   test_name: basename of test case to run\n"
+          ~ "   test_extension: one of: d, html, or sh\n"
+          ~ "\n"
+          ~ "   example: d_do_test runnable pi d\n"
+          ~ "\n"
+          ~ "   relevant environment variables:\n"
+          ~ "      ARGS:          set to execute all combinations of\n"
+          ~ "      REQUIRED_ARGS: arguments always passed to the compiler\n"
+          ~ "      DMD:           compiler to use, ex: ../src/dmd\n"
+          ~ "      CC:            C++ compiler to use, ex: dmc, g++\n"
+          ~ "      OS:            win32, win64, linux, freebsd, osx\n"
+          ~ "      RESULTS_DIR:   base directory for test results\n"
+          ~ "   windows vs non-windows portability env vars:\n"
+          ~ "      DSEP:          \\\\ or /\n"
+          ~ "      SEP:           \\ or /\n"
+          ~ "      OBJ:          .obj or .o\n"
+          ~ "      EXE:          .exe or <null>\n");
 }
 
 enum TestMode

test/fail_compilation/diag10805.d

@@ -1,9 +1,10 @@
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/diag10805.d(10): Error: delimited string must end in FOO"
-fail_compilation/diag10805.d(12): Error: unterminated string constant starting at fail_compilation/diag10805.d(12)
-fail_compilation/diag10805.d(13): Error: semicolon expected following auto declaration, not 'EOF'
+fail_compilation/diag10805.d(11): Error: delimited string must end in FOO"
+fail_compilation/diag10805.d(13): Error: unterminated string constant starting at fail_compilation/diag10805.d(13)
+fail_compilation/diag10805.d(13): Deprecation: Implicit string concatenation is deprecated, use "" ~ "" instead
+fail_compilation/diag10805.d(14): Error: semicolon expected following auto declaration, not 'EOF'
 ---
 */
 

test/fail_compilation/issue3827.d

@@ -0,0 +1,14 @@
+// REQUIRED_ARGS: -de
+/*
+TEST_OUTPUT:
+---
+fail_compilation/issue3827.d(12): Deprecation: Implicit string concatenation is deprecated, use "Hello" ~ "World" instead
+fail_compilation/issue3827.d(13): Deprecation: Implicit string concatenation is deprecated, use "A" ~ "B" instead
+---
+*/
+
+void main ()
+{
+    string[] arr = [ "Hello" "World" ];
+    auto foo = "A" "B";
+}