feat(derive): Start unstable-v4 feature flag

This is split out of clap-rs#2688 as several changes I'm working on need it.

.github/workflows/ci.yml

@@ -112,6 +112,10 @@ jobs:
   ui:
     name: UI Tests
     runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        features: [default, next]
     steps:
     - name: Checkout repository
       uses: actions/checkout@v3
@@ -123,7 +127,7 @@ jobs:
         override: true
     - uses: Swatinem/rust-cache@v1
     - name: UI Tests
-      run: make test-ui
+      run: make test-ui-${{ matrix.features }}
   docs:
     name: Docs
     runs-on: ubuntu-latest

Cargo.toml

@@ -82,7 +82,8 @@ unicode = ["textwrap/unicode-width", "unicase"]  # Support for unicode character
 unstable-replace = []
 unstable-multicall = []
 unstable-grouped = []
-unstable-v4 = []
+# note: this will always enable clap_derive, change this to `clap_derive?/unstable-v4` when MSRV is bigger than 1.60
+unstable-v4 = ["clap_derive/unstable-v4"]
 
 [lib]
 bench = false

Makefile

@@ -33,5 +33,5 @@ test-%:
 clippy-%:
 	cargo clippy ${_FEATURES_${@:clippy-%=%}} ${ARGS} --all-targets -- -D warnings -A deprecated
 
-test-ui:
-	cargo +${MSRV} test --test derive_ui --features derive
+test-ui-%:
+	cargo +${MSRV} test --test derive_ui --features derive ${_FEATURES_${@:test-ui-%=%}}

clap_derive/Cargo.toml

@@ -44,6 +44,7 @@ proc-macro-error = "1"
 [features]
 default = []
 debug = []
+unstable-v4 = []
 
 [package.metadata.docs.rs]
 targets = ["x86_64-unknown-linux-gnu"]