Improve error message for non-promotion due to assignment.

This is clearer because:

- The use of "could not be promoted" is consistent with the other "why
  not promoted" messages.

- We avoid speculating about whether the variable might be assigned a
  null value; this should help avoid confusion in cases where the user
  can see that assigning a null value is impossible, but promotion
  still fails.

- We avoid the phrase "intervening write", which is a little too
  erudite.

Bug: https://github.com/dart-lang/sdk/issues/44898
Change-Id: I0b8a2132dc99dd06769677f98e6257e9b55ad9d6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/193820
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Bob Nystrom <rnystrom@google.com>

pkg/_fe_analyzer_shared/lib/src/messages/codes_generated.dart

@@ -9707,14 +9707,17 @@ const MessageCode messageVarReturnType = const MessageCode("VarReturnType",
         r"""Try removing the keyword 'var', or replacing it with the name of the return type.""");
 
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
-const Template<Message Function(String name, String string)>
-    templateVariableCouldBeNullDueToWrite =
-    const Template<Message Function(String name, String string)>(
-        messageTemplate:
-            r"""Variable '#name' could be null due to an intervening write.""",
-        tipTemplate:
-            r"""Try null checking the variable after the write.  See #string""",
-        withArguments: _withArgumentsVariableCouldBeNullDueToWrite);
+const Template<
+    Message Function(
+        String name,
+        String
+            string)> templateVariableCouldBeNullDueToWrite = const Template<
+        Message Function(String name, String string)>(
+    messageTemplate:
+        r"""Variable '#name' could not be promoted due to an assignment.""",
+    tipTemplate:
+        r"""Try null checking the variable after the assignment.  See #string""",
+    withArguments: _withArgumentsVariableCouldBeNullDueToWrite);
 
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 const Code<Message Function(String name, String string)>
@@ -9731,8 +9734,9 @@ Message _withArgumentsVariableCouldBeNullDueToWrite(
   if (string.isEmpty) throw 'No string provided';
   return new Message(codeVariableCouldBeNullDueToWrite,
       message:
-          """Variable '${name}' could be null due to an intervening write.""",
-      tip: """Try null checking the variable after the write.  See ${string}""",
+          """Variable '${name}' could not be promoted due to an assignment.""",
+      tip:
+          """Try null checking the variable after the assignment.  See ${string}""",
       arguments: {'name': name, 'string': string});
 }
 

pkg/analyzer/lib/src/generated/resolver.dart

@@ -3555,8 +3555,8 @@ class _WhyNotPromotedVisitor
       Expression writeExpression, NonPromotionReason reason) {
     return DiagnosticMessageImpl(
         filePath: source.fullName,
-        message: "Variable '$variableName' could be null due to an intervening "
-            "write.  See ${reason.documentationLink}",
+        message: "Variable '$variableName' could not be promoted due to an "
+            "assignment.  See ${reason.documentationLink}",
         offset: writeExpression.offset,
         length: writeExpression.length);
   }

pkg/front_end/messages.yaml

@@ -4655,8 +4655,8 @@ MultipleVarianceModifiers:
   analyzerCode: ParserErrorCode.MULTIPLE_VARIANCE_MODIFIERS
 
 VariableCouldBeNullDueToWrite:
-  template: "Variable '#name' could be null due to an intervening write."
-  tip: "Try null checking the variable after the write.  See #string"
+  template: "Variable '#name' could not be promoted due to an assignment."
+  tip: "Try null checking the variable after the assignment.  See #string"
 
 FieldNotPromoted:
   template: "'#name' refers to a property so it couldn't be promoted."

tests/language/why_not_promoted/assignment_error_test.dart

@@ -11,7 +11,7 @@ direct_assignment(int? i, int? j) {
   if (i == null) return;
   i = j;
 //^
-// [context 1] Variable 'i' could be null due to an intervening write.
+// [context 1] Variable 'i' could not be promoted due to an assignment.
   i.isEven;
 //  ^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.UNCHECKED_USE_OF_NULLABLE_VALUE
@@ -23,7 +23,7 @@ compound_assignment(C? c, int i) {
   if (c == null) return;
   c += i;
 //^
-// [context 2] Variable 'c' could be null due to an intervening write.
+// [context 2] Variable 'c' could not be promoted due to an assignment.
   c.cProperty;
 //  ^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.UNCHECKED_USE_OF_NULLABLE_VALUE
@@ -35,7 +35,7 @@ via_postfix_op(C? c) {
   if (c == null) return;
   c++;
 //^
-// [context 3] Variable 'c' could be null due to an intervening write.
+// [context 3] Variable 'c' could not be promoted due to an assignment.
   c.cProperty;
 //  ^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.UNCHECKED_USE_OF_NULLABLE_VALUE
@@ -47,7 +47,7 @@ via_prefix_op(C? c) {
   if (c == null) return;
   ++c;
   //^
-  // [context 4] Variable 'c' could be null due to an intervening write.
+  // [context 4] Variable 'c' could not be promoted due to an assignment.
   c.cProperty;
 //  ^^^^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.UNCHECKED_USE_OF_NULLABLE_VALUE
@@ -59,7 +59,7 @@ via_for_each_statement(int? i, List<int?> list) {
   if (i == null) return;
   for (i in list) {
   //   ^
-  // [context 5] Variable 'i' could be null due to an intervening write.
+  // [context 5] Variable 'i' could not be promoted due to an assignment.
     i.isEven;
 //    ^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.UNCHECKED_USE_OF_NULLABLE_VALUE
@@ -72,7 +72,7 @@ via_for_each_list_element(int? i, List<int?> list) {
   if (i == null) return;
   [for (i in list) i.isEven];
   //    ^
-  // [context 6] Variable 'i' could be null due to an intervening write.
+  // [context 6] Variable 'i' could not be promoted due to an assignment.
   //                 ^^^^^^
   // [analyzer] COMPILE_TIME_ERROR.UNCHECKED_USE_OF_NULLABLE_VALUE
   //                 ^
@@ -83,7 +83,7 @@ via_for_each_set_element(int? i, List<int?> list) {
   if (i == null) return;
   ({for (i in list) i.isEven});
   //     ^
-  // [context 7] Variable 'i' could be null due to an intervening write.
+  // [context 7] Variable 'i' could not be promoted due to an assignment.
   //                  ^^^^^^
   // [analyzer] COMPILE_TIME_ERROR.UNCHECKED_USE_OF_NULLABLE_VALUE
   //                  ^
@@ -94,7 +94,7 @@ via_for_each_map_key(int? i, List<int?> list) {
   if (i == null) return;
   ({for (i in list) i.isEven: null});
   //     ^
-  // [context 8] Variable 'i' could be null due to an intervening write.
+  // [context 8] Variable 'i' could not be promoted due to an assignment.
   //                  ^^^^^^
   // [analyzer] COMPILE_TIME_ERROR.UNCHECKED_USE_OF_NULLABLE_VALUE
   //                  ^
@@ -105,7 +105,7 @@ via_for_each_map_value(int? i, List<int?> list) {
   if (i == null) return;
   ({for (i in list) null: i.isEven});
   //     ^
-  // [context 9] Variable 'i' could be null due to an intervening write.
+  // [context 9] Variable 'i' could not be promoted due to an assignment.
   //                        ^^^^^^
   // [analyzer] COMPILE_TIME_ERROR.UNCHECKED_USE_OF_NULLABLE_VALUE
   //                        ^

tests/language/why_not_promoted/nullable_method_call_error_test.dart

@@ -32,7 +32,7 @@ property_get_of_variable(int? i, int? j) {
   if (i == null) return;
   i = j;
 //^
-// [context 1] Variable 'i' could be null due to an intervening write.
+// [context 1] Variable 'i' could not be promoted due to an assignment.
   i.isEven;
 //  ^^^^^^
 // [analyzer] COMPILE_TIME_ERROR.UNCHECKED_USE_OF_NULLABLE_VALUE
@@ -44,7 +44,7 @@ extension_property_get_of_variable(int? i, int? j) {
   if (i == null) return;
   i = j;
 //^
-// [context 2] Variable 'i' could be null due to an intervening write.
+// [context 2] Variable 'i' could not be promoted due to an assignment.
   i.propertyOnNullableInt;
   i.propertyOnNonNullInt;
 //  ^^^^^^^^^^^^^^^^^^^^