Make SHA-256 gadget a nightly feature

This was effectively the meaning of the prior `unstable` feature flag.

.github/workflows/ci.yml

@@ -28,7 +28,7 @@ jobs:
         uses: actions-rs/cargo@v1
         with:
           command: test
-          args: --verbose --release --workspace --features batch,dev-graph,gadget-traces,test-dependencies,unstable ${{ matrix.extra_flags }}
+          args: --verbose --release --workspace --features batch,dev-graph,gadget-traces,test-dependencies ${{ matrix.extra_flags }}
 
   build:
     name: Build target ${{ matrix.target }}
@@ -51,7 +51,7 @@ jobs:
         uses: actions-rs/cargo@v1
         with:
           command: build
-          args: --features batch,dev-graph,gadget-traces,nightly,unstable --target ${{ matrix.target }}
+          args: --features batch,dev-graph,gadget-traces,nightly --target ${{ matrix.target }}
 
   bitrot:
     name: Bitrot check
@@ -112,7 +112,7 @@ jobs:
         uses: actions-rs/cargo@v1
         with:
           command: tarpaulin
-          args: --features batch,dev-graph,gadget-traces,test-dependencies,nightly,unstable --timeout 600 --out Xml
+          args: --features batch,dev-graph,gadget-traces,test-dependencies,nightly --timeout 600 --out Xml
       - name: Upload coverage to Codecov
         uses: codecov/codecov-action@v3.1.1
 

book/src/dev/features.md

@@ -49,3 +49,4 @@ Two meta-flags are provided to enable all features at a particular stabilisation
 
 | Feature flag | Stage | Notes |
 | --- | --- | --- |
+| `unstable-sha256-gadget` | `nightly` | The SHA-256 gadget and chip.

halo2_gadgets/Cargo.toml

@@ -55,7 +55,6 @@ test-dev-graph = [
     "plotters/bitmap_encoder",
 ]
 test-dependencies = ["proptest"]
-unstable = []
 
 # In-development features
 # See https://zcash.github.io/halo2/dev/features.html
@@ -65,7 +64,9 @@ beta = [
 nightly = [
     "beta",
     "halo2_proofs/nightly",
+    "unstable-sha256-gadget",
 ]
+unstable-sha256-gadget = []
 # Add flags for in-development features above this line.
 
 [[bench]]
@@ -79,4 +80,4 @@ harness = false
 [[bench]]
 name = "sha256"
 harness = false
-required-features = ["unstable"]
+required-features = ["unstable-sha256-gadget"]

halo2_gadgets/src/lib.rs

@@ -23,8 +23,8 @@
 
 pub mod ecc;
 pub mod poseidon;
-#[cfg(feature = "unstable")]
-#[cfg_attr(docsrs, doc(cfg(feature = "unstable")))]
+#[cfg(feature = "unstable-sha256-gadget")]
+#[cfg_attr(docsrs, doc(cfg(feature = "unstable-sha256-gadget")))]
 pub mod sha256;
 pub mod sinsemilla;
 pub mod utilities;