Deprecate the use of "a=b" for non-temporary assignment in 0.15.

It is still the syntax for temporary assignment; only using it as a standalone
command is deprecated.

NEXT-RELEASE.md

@@ -25,6 +25,12 @@ This is the draft release notes for 0.15.0, scheduled to be released on
 The following deprecated features trigger a warning whenever the code is parsed
 or compiled, even if it is not executed:
 
+-   Using the syntax of temporary assignment (`var=value`) for non-temporary
+    assignment is now deprecated. The syntax is still valid for temporary
+    assignment. For example, using `foo=bar` as a standalone command is
+    deprecated, but using it as part of command, like `foo=bar ls`, is not
+    deprecated.
+
 -   The `chr` command is now deprecated. Use `str:from-codepoints` instead.
 
 -   The `ord` command is now deprecated. Use `str:to-codepoints` instead.

pkg/eval/compile_effect.go

@@ -161,7 +161,9 @@ func (cp *compiler) formOp(n *parse.Form) effectOp {
 	if len(n.Assignments) > 0 {
 		assignmentOps = cp.assignmentOps(n.Assignments)
 		if n.Head == nil {
-			// Permanent assignment.
+			// Using the syntax of temporary assignment for non-temporary
+			// assignment. Still supported as of 0.15 but deprecated.
+			cp.deprecate(n, `using the syntax of temporary assignment for non-temporary assignment is deprecated; use "var" or "set" instead`, 15)
 			return seqOp{assignmentOps}
 		}
 		for _, a := range n.Assignments {

pkg/eval/compile_effect_test.go

@@ -69,6 +69,11 @@ func TestCommand(t *testing.T) {
 	)
 }
 
+func TestCommand_DeprecateUsingTemporaryAssignmentForNonTemporaryAssignment(t *testing.T) {
+	testCompileTimeDeprecation(t, "foo=bar", `using the syntax of temporary assignment for non-temporary assignment is deprecated; use "var" or "set" instead`, 15)
+
+}
+
 func TestCommand_Special(t *testing.T) {
 	Test(t,
 		// Regression test for #1204; ensures that the arguments of special