[modular_aot] Fix simplification of string interpolation

Issue: https://github.com/dart-lang/sdk/issues/61635
Change-Id: I6404c761f9849a50ee25c0387557c20c7b4d632c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/508700
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

pkg/cfg/lib/passes/simplification.dart

@@ -256,6 +256,10 @@ final class Simplification extends Pass
   Instruction visitStringInterpolation(StringInterpolation instr) {
     final buf = _StringInterpolationBuffer(constantFolding);
     buf.addStringInterpolation(instr);
+    assert(buf.consumed.isEmpty || buf.optimized);
+    for (final i in buf.consumed) {
+      i.removeFromGraph();
+    }
     if (buf.inputs.length == 1) {
       final input = buf.inputs.single;
       if (input is String) {
@@ -633,6 +637,9 @@ class _StringInterpolationBuffer {
   // Contains either String or Definition.
   final List<Object> inputs = [];
 
+  // Consumed StringInterpolation instructions.
+  final List<StringInterpolation> consumed = [];
+
   bool optimized = false;
 
   _StringInterpolationBuffer(this.constantFolding);
@@ -669,7 +676,11 @@ class _StringInterpolationBuffer {
           break;
         case StringInterpolation() when input.singleUser == instr:
           addStringInterpolation(input);
-          input.removeFromGraph();
+          // Do not remove the consumed instruction from the graph immediately,
+          // as removal may change "has single user" property.
+          // As a result, instruction which is used multiple times would be both added
+          // as input and removed, leaving the graph in the inconsistent state.
+          consumed.add(input);
           optimized = true;
           break;
         default:

pkg/cfg/testcases/simplification.dart

@@ -119,6 +119,17 @@ void stringInterpolation(int x) {
   print('enclosing start... ${'some nested $s, x=$x'} ...end');
 }
 
+void stringInterpolation2(List<String> x) {
+  var path = '${x[0]}${x[1]}';
+  var right = 'prefix1:$path';
+  var wrong = 'prefix2:$path';
+
+  print(
+    'A very very looooooooooooooooooooooooooooong prefix: '
+    "'$right', not '$wrong'.",
+  );
+}
+
 void closureCall() {
   final x = (int arg) {
     print(arg);

pkg/cfg/testcases/simplification.dart.expect

@@ -219,6 +219,27 @@ B0 = EntryBlock()
   DirectCall print(v43)
   Return(v12)
 
+--- stringInterpolation2
+B0 = EntryBlock()
+  v3 = Constant(0)
+  v6 = Constant(1)
+  Constant("prefix1:")
+  Constant("prefix2:")
+  Constant("A very very looooooooooooooooooooooooooooong prefix: ")
+  Constant("'")
+  Constant("', not '")
+  v23 = Constant("'.")
+  v26 = Constant(null)
+  v29 = Constant("A very very looooooooooooooooooooooooooooong prefix: 'prefix1:")
+  v30 = Constant("', not 'prefix2:")
+  v1 = Parameter(x)
+  v4 = InterfaceCall List.[](v1, v3)
+  v7 = InterfaceCall List.[](v1, v6)
+  v8 = StringInterpolation(v4, v7)
+  v28 = StringInterpolation(v29, v8, v30, v8, v23)
+  DirectCall print(v28)
+  Return(v26)
+
 --- closureCall
 B0 = EntryBlock()
   v4 = Constant(42)