Switch CI back to latest nightly.

The issue that required the pin to the older version has been resolved.

This keeps the x64 backend tests on an older nightly version to support the
`-Z` flags being passed without having to update Cargo.toml to a new feature
resolver version.

The doc task is also kept on the older nightly for the same reason.

.github/workflows/main.yml

@@ -73,14 +73,11 @@ jobs:
     - uses: actions/checkout@v2
       with:
         submodules: true
-    # Note that we use nightly Rust here to get intra-doc links which are a
-    # nightly-only feature right now.
+    # 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:
-        # TODO (rust-lang/rust#79661): We are seeing an internal compiler error when
-        # building with the latest (2020-12-06) nightly; pin on a slightly older
-        # version for now.
-        toolchain: nightly-2020-11-29
+        toolchain: nightly-2020-12-26
     - 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
@@ -168,7 +165,7 @@ jobs:
     # flags to rustc.
     - uses: ./.github/actions/install-rust
       with:
-        toolchain: nightly-2020-11-29
+        toolchain: nightly
     - run: cargo install cargo-fuzz --vers "^0.8"
     - run: cargo fetch
       working-directory: ./fuzz
@@ -225,7 +222,7 @@ jobs:
             rust: beta
           - build: nightly
             os: ubuntu-latest
-            rust: nightly-2020-11-29
+            rust: nightly
           - build: macos
             os: macos-latest
             rust: stable
@@ -321,8 +318,10 @@ jobs:
         RUST_BACKTRACE: 1
 
   # Perform all tests (debug mode) for `wasmtime` with the experimental x64
-  # backend. This runs on the nightly channel of Rust (because of issues with
-  # unifying Cargo features on stable) on Ubuntu.
+  # 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.
   test_x64:
     name: Test x64 new backend
     runs-on: ubuntu-latest
@@ -332,7 +331,7 @@ jobs:
         submodules: true
     - uses: ./.github/actions/install-rust
       with:
-        toolchain: nightly-2020-11-29
+        toolchain: nightly-2020-12-26
     - uses: ./.github/actions/define-llvm-env
 
     # Install wasm32 targets in order to build various tests throughout the
@@ -343,7 +342,7 @@ jobs:
     # Run the x64 CI script.
     - run: ./ci/run-experimental-x64-ci.sh
       env:
-        CARGO_VERSION: "+nightly-2020-11-29"
+        CARGO_VERSION: "+nightly-2020-12-26"
         RUST_BACKTRACE: 1
 
   # Build and test the wasi-nn module.
@@ -356,7 +355,7 @@ jobs:
           submodules: true
       - uses: ./.github/actions/install-rust
         with:
-          toolchain: nightly-2020-11-29
+          toolchain: nightly
       - run: rustup target add wasm32-wasi
       - uses: ./.github/actions/install-openvino
       - run: ./ci/run-wasi-nn-example.sh