Use stable Rust on CI to test the x64 backend (#2766)

* Use stable Rust on CI to test the x64 backend

This commit leverages the newly-released 1.51.0 compiler to test the
new backend on Windows and Linux with a stable compiler instead of a
nightly compiler. This isolates the nightly build to just the nightly
documentation generation and fuzzing, both of which rely on nightly for
the best results right now.

* Use updated stable in book build job

* Run rustfmt for new stable

* Silence new warnings for wasi-nn

* Allow some dead code in the x64 backend

Looks like new rustc is better about emitting some dead-code warnings

* Update rust in peepmatic job

* Fix a test in the pooling allocator

* Remove `package.metdata.docs.rs` temporarily

Needs resolution of https://github.com/rust-lang/cargo/pull/9300 first

* Fix a warning in a wasi-nn example

.github/workflows/main.yml

@@ -48,6 +48,7 @@ jobs:
     - uses: actions/checkout@v2
       with:
         submodules: true
+    - run: rustup update stable && rustup default stable
     - run: |
         set -e
         curl -L https://github.com/rust-lang-nursery/mdBook/releases/download/v0.4.4/mdbook-v0.4.4-x86_64-unknown-linux-gnu.tar.gz | tar xzf -
@@ -73,11 +74,9 @@ jobs:
     - uses: actions/checkout@v2
       with:
         submodules: true
-    # Note that we use nightly Rust for the doc_cfg feature (enabled via `nightlydoc` above)
-    # This version is an older nightly for the new x64 backend (see below)
     - uses: ./.github/actions/install-rust
       with:
-        toolchain: nightly-2020-12-26
+        toolchain: nightly
     - run: cargo doc --no-deps --all --exclude wasmtime-cli --exclude test-programs --exclude cranelift-codegen-meta
     - run: cargo doc --package cranelift-codegen-meta --document-private-items
     - uses: actions/upload-artifact@v1
@@ -178,6 +177,7 @@ jobs:
     - uses: actions/checkout@v2
       with:
         submodules: true
+    - run: rustup update stable && rustup default stable
     - name: Test `peepmatic`
       run: |
         cargo test \
@@ -321,10 +321,7 @@ jobs:
         RUST_BACKTRACE: 1
 
   # Perform all tests (debug mode) for `wasmtime` with the experimental x64
-  # backend. This runs on an older nightly of Rust (because of issues with
-  # unifying Cargo features on stable) on Ubuntu such that it's new enough
-  # to build Wasmtime, but old enough where the -Z options being used
-  # haven't been stabilized yet.
+  # backend.
   test_x64:
     name: Test x64 new backend
     runs-on: ubuntu-latest
@@ -334,7 +331,7 @@ jobs:
         submodules: true
     - uses: ./.github/actions/install-rust
       with:
-        toolchain: nightly-2020-12-26
+        toolchain: stable
     - uses: ./.github/actions/define-llvm-env
 
     # Install wasm32 targets in order to build various tests throughout the
@@ -345,7 +342,6 @@ jobs:
     # Run the x64 CI script.
     - run: ./ci/run-experimental-x64-ci.sh
       env:
-        CARGO_VERSION: "+nightly-2020-12-26"
         RUST_BACKTRACE: 1
 
   # Perform tests on the new x64 backend on Windows as well.
@@ -357,8 +353,6 @@ jobs:
       with:
         submodules: true
     - uses: ./.github/actions/install-rust
-      with:
-        toolchain: nightly-2020-11-29
     - uses: ./.github/actions/define-llvm-env
 
     - name: Install libclang
@@ -378,7 +372,6 @@ jobs:
     # Run the x64 CI script.
     - run: ./ci/run-experimental-x64-ci.sh
       env:
-        CARGO_VERSION: "+nightly-2020-11-29"
         RUST_BACKTRACE: 1
 
   # Build and test the wasi-nn module.
@@ -390,8 +383,6 @@ jobs:
         with:
           submodules: true
       - uses: ./.github/actions/install-rust
-        with:
-          toolchain: nightly
       - run: rustup target add wasm32-wasi
       - uses: ./.github/actions/install-openvino
       - run: ./ci/run-wasi-nn-example.sh