Make HTTP/3 feature more unstable (#1780)

.github/workflows/ci.yml

@@ -17,6 +17,7 @@ jobs:
     needs:
       - style
       - test
+      - unstable
       - nightly
       - msrv
       - android
@@ -182,6 +183,22 @@ jobs:
       - name: Test
         run: cargo test ${{ matrix.features }} ${{ matrix.test-features }} -- --test-threads=1
 
+  unstable:
+    name: "unstable features"
+    needs: [style]
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Install rust
+        uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: 'stable'
+
+      - name: Check
+        run: RUSTFLAGS="--cfg reqwest_unstable" cargo check --features http3
+
   docs:
     name: Docs
     runs-on: ubuntu-latest
@@ -195,7 +212,7 @@ jobs:
 
       - name: Check documentation
         env:
-          RUSTDOCFLAGS: -D warnings
+          RUSTDOCFLAGS: --cfg reqwest_unstable -D warnings
         run: cargo doc --no-deps --document-private-items --all-features
 
   # Separate build job for nightly because of the missing feature for allowed failures at
@@ -222,6 +239,8 @@ jobs:
       #   run: cargo test --features __internal_proxy_sys_no_cache -- --test-threads=1
 
       - name: Check minimal versions
+        env:
+          RUSTFLAGS: --cfg reqwest_unstable
         run: |
           cargo clean
           cargo update -Z minimal-versions

src/lib.rs

@@ -191,6 +191,22 @@
 //! - **trust-dns**: Enables a trust-dns async resolver instead of default
 //!   threadpool using `getaddrinfo`.
 //!
+//! ## Unstable Features
+//!
+//! Some feature flags require additional opt-in by the application, by setting
+//! a `reqwest_unstable` flag.
+//!
+//! - **http3** *(unstable)*: Enables support for sending HTTP/3 requests.
+//!
+//! These features are unstable, and experimental. Details about them may be
+//! changed in patch releases.
+//!
+//! You can pass such a flag to the compiler via `.cargo/config`, or
+//! environment variables, such as:
+//!
+//! ```notrust
+//! RUSTFLAGS="--cfg reqwest_unstable" cargo build
+//! ```
 //!
 //! [hyper]: http://hyper.rs
 //! [blocking]: ./blocking/index.html
@@ -203,6 +219,14 @@
 //! [Proxy]: ./struct.Proxy.html
 //! [cargo-features]: https://doc.rust-lang.org/stable/cargo/reference/manifest.html#the-features-section
 
+#[cfg(all(feature = "http3", not(reqwest_unstable)))]
+compile_error!(
+    "\
+    The `http3` feature is unstable, and requires the \
+    `RUSTFLAGS='--cfg reqwest_unstable'` environment variable to be set.\
+"
+);
+
 macro_rules! if_wasm {
     ($($item:item)*) => {$(
         #[cfg(target_arch = "wasm32")]