Use "target()" to parse "when" block

`target()` as used in parsing "let" and "if let" implements parsing
nested tuples and structs. But it does not implement parsing literals.

The functions `match_variant()` and `with_parameters()` as used in
parsing "when" blocks do not implement parsing nested structs, but it
implements parsing literals.

This PR combines `match_variant()` and `with_parameters()` into
`target()`, so that all `{%when%}` support nested structs, too.

1 files changed, 7 insertions, 0 deletions

diff --git a/testing/tests/ui/lit_on_assignment_lhs.stderr b/testing/tests/ui/lit_on_assignment_lhs.stderr
new file mode 100644
index 0000000..fa488cb
--- /dev/null
+++ b/testing/tests/ui/lit_on_assignment_lhs.stderr
@@ -0,0 +1,7 @@
+error: proc-macro derive panicked
+ --> $DIR/lit_on_assignment_lhs.rs:3:10
+  |
+3 | #[derive(Template)]
+  |          ^^^^^^^^
+  |
+  = help: message: Cannot have literals on the left-hand-side of an assignment.