Interpolation with only string literals must not be frozen

Basically a redo of ruby@a1403fb
but respecting the frozen string literal magic comment

Fixes [Bug #21187]

Author: Earlopain

prism/prism.c

@@ -5280,6 +5280,12 @@ pm_interpolated_string_node_append(pm_interpolated_string_node_t *node, pm_node_
 
     switch (PM_NODE_TYPE(part)) {
         case PM_STRING_NODE:
+            // If inner string is not frozen, it stops being a static literal. We should *not* clear other flags,
+            // because concatenating two frozen strings (`'foo' 'bar'`) is still frozen. This holds true for
+            // as long as this interpolation only consists of other string literals.
+            if (!PM_NODE_FLAG_P(part, PM_STRING_FLAGS_FROZEN)) {
+                pm_node_flag_unset((pm_node_t *) node, PM_NODE_FLAG_STATIC_LITERAL);
+            }
             part->flags = (pm_node_flags_t) ((part->flags | PM_NODE_FLAG_STATIC_LITERAL | PM_STRING_FLAGS_FROZEN) & ~PM_STRING_FLAGS_MUTABLE);
             break;
         case PM_INTERPOLATED_STRING_NODE:

prism_compile.c

@@ -558,7 +558,7 @@ parse_regexp_concat(rb_iseq_t *iseq, const pm_scope_node_t *scope_node, const pm
 static void pm_compile_node(rb_iseq_t *iseq, const pm_node_t *node, LINK_ANCHOR *const ret, bool popped, pm_scope_node_t *scope_node);
 
 static int
-pm_interpolated_node_compile(rb_iseq_t *iseq, const pm_node_list_t *parts, const pm_node_location_t *node_location, LINK_ANCHOR *const ret, bool popped, pm_scope_node_t *scope_node, rb_encoding *implicit_regexp_encoding, rb_encoding *explicit_regexp_encoding, bool mutable_result)
+pm_interpolated_node_compile(rb_iseq_t *iseq, const pm_node_list_t *parts, const pm_node_location_t *node_location, LINK_ANCHOR *const ret, bool popped, pm_scope_node_t *scope_node, rb_encoding *implicit_regexp_encoding, rb_encoding *explicit_regexp_encoding, bool mutable_result, bool frozen_result)
 {
     int stack_size = 0;
     size_t parts_size = parts->size;
@@ -668,10 +668,15 @@ pm_interpolated_node_compile(rb_iseq_t *iseq, const pm_node_list_t *parts, const
         if (RTEST(current_string)) {
             current_string = rb_fstring(current_string);
 
-            if (stack_size == 0 && (interpolated || mutable_result)) {
-                PUSH_INSN1(ret, current_location, putstring, current_string);
-            }
-            else {
+            if (stack_size == 0) {
+                if (frozen_result) {
+                    PUSH_INSN1(ret, current_location, putobject, current_string);
+                } else if (mutable_result || interpolated) {
+                    PUSH_INSN1(ret, current_location, putstring, current_string);
+                } else {
+                    PUSH_INSN1(ret, current_location, putchilledstring, current_string);
+                }
+            } else {
                 PUSH_INSN1(ret, current_location, putobject, current_string);
             }
 
@@ -692,7 +697,7 @@ pm_compile_regexp_dynamic(rb_iseq_t *iseq, const pm_node_t *node, const pm_node_
     rb_encoding *explicit_regexp_encoding = parse_regexp_encoding(scope_node, node);
     rb_encoding *implicit_regexp_encoding = explicit_regexp_encoding != NULL ? explicit_regexp_encoding : scope_node->encoding;
 
-    int length = pm_interpolated_node_compile(iseq, parts, node_location, ret, popped, scope_node, implicit_regexp_encoding, explicit_regexp_encoding, false);
+    int length = pm_interpolated_node_compile(iseq, parts, node_location, ret, popped, scope_node, implicit_regexp_encoding, explicit_regexp_encoding, false, false);
     PUSH_INSN2(ret, *node_location, toregexp, INT2FIX(parse_regexp_flags(node) & 0xFF), INT2FIX(length));
 }
 
@@ -9571,7 +9576,7 @@ pm_compile_node(rb_iseq_t *iseq, const pm_node_t *node, LINK_ANCHOR *const ret,
         }
         else {
             const pm_interpolated_string_node_t *cast = (const pm_interpolated_string_node_t *) node;
-            int length = pm_interpolated_node_compile(iseq, &cast->parts, &location, ret, popped, scope_node, NULL, NULL, !PM_NODE_FLAG_P(cast, PM_INTERPOLATED_STRING_NODE_FLAGS_FROZEN));
+            int length = pm_interpolated_node_compile(iseq, &cast->parts, &location, ret, popped, scope_node, NULL, NULL, PM_NODE_FLAG_P(cast, PM_INTERPOLATED_STRING_NODE_FLAGS_MUTABLE), PM_NODE_FLAG_P(cast, PM_INTERPOLATED_STRING_NODE_FLAGS_FROZEN));
             if (length > 1) PUSH_INSN1(ret, location, concatstrings, INT2FIX(length));
             if (popped) PUSH_INSN(ret, location, pop);
         }
@@ -9582,7 +9587,7 @@ pm_compile_node(rb_iseq_t *iseq, const pm_node_t *node, LINK_ANCHOR *const ret,
         // :"foo #{bar}"
         // ^^^^^^^^^^^^^
         const pm_interpolated_symbol_node_t *cast = (const pm_interpolated_symbol_node_t *) node;
-        int length = pm_interpolated_node_compile(iseq, &cast->parts, &location, ret, popped, scope_node, NULL, NULL, false);
+        int length = pm_interpolated_node_compile(iseq, &cast->parts, &location, ret, popped, scope_node, NULL, NULL, false, false);
 
         if (length > 1) {
             PUSH_INSN1(ret, location, concatstrings, INT2FIX(length));
@@ -9604,7 +9609,7 @@ pm_compile_node(rb_iseq_t *iseq, const pm_node_t *node, LINK_ANCHOR *const ret,
 
         PUSH_INSN(ret, location, putself);
 
-        int length = pm_interpolated_node_compile(iseq, &cast->parts, &location, ret, false, scope_node, NULL, NULL, false);
+        int length = pm_interpolated_node_compile(iseq, &cast->parts, &location, ret, false, scope_node, NULL, NULL, false, false);
         if (length > 1) PUSH_INSN1(ret, location, concatstrings, INT2FIX(length));
 
         PUSH_SEND_WITH_FLAG(ret, location, idBackquote, INT2NUM(1), INT2FIX(VM_CALL_FCALL | VM_CALL_ARGS_SIMPLE));

spec/ruby/core/string/chilled_string_spec.rb

@@ -47,6 +47,14 @@
           input.should == "chilled-mutated"
         end
 
+        it "emits a warning for concatenated strings" do
+          input = "still" "+chilled"
+          -> {
+            input << "-mutated"
+          }.should complain(/literal string will be frozen in the future/)
+          input.should == "still+chilled-mutated"
+        end
+
         it "emits a warning on singleton_class creation" do
           -> {
             "chilled".singleton_class