Deprecate the legacy lambda syntax from 0.17.x.

This addresses #664.

0.17.0-release-notes.md

@@ -16,8 +16,12 @@ or compiled, even if it is not executed:
 -   The `dir-history` command is deprecated. Use `store:dirs` instead.
 
 -   The legacy assignment form is deprecated. Depending on whether the left-hand
-    variable already exists or not, use `var` or `set` instead. There is a
-    dedicated program to help migrate scripts; see https://go.elv.sh/u0.17.
+    variable already exists or not, use `var` or `set` instead. Use the
+    [upgrader](https://go.elv.sh/u0.17) to migrate scripts.
+
+-   The lambda syntax that declares arguments and options within `[]` before `{`
+    has been deprecated. The new syntax now declares arguments and options
+    within a pair of `|`, after `{`.
 
 # Notable new features
 

pkg/eval/compile_value.go

@@ -367,6 +367,9 @@ func (op outputCaptureOp) exec(fm *Frame) ([]interface{}, Exception) {
 }
 
 func (cp *compiler) lambda(n *parse.Primary) valuesOp {
+	if n.LegacyLambda {
+		cp.deprecate(n, "legacy lambda syntax is deprecated; migrate scripts with https://go.elv.sh/u0.17", 17)
+	}
 	// Parse signature.
 	var (
 		argNames      []string

pkg/eval/compile_value_test.go

@@ -234,3 +234,7 @@ func TestClosure(t *testing.T) {
 		That("[@a @b]{ }").DoesNotCompile(),
 	)
 }
+
+func TestClosure_LegacySyntaxIsDeprecated(t *testing.T) {
+	testCompileTimeDeprecation(t, "a = []{ }", "legacy lambda syntax is deprecated", 17)
+}