chore(ci): don't require nightly to pass (#89)

This changes the nightly CI build to not be required. This is because
nightly Rust may contain regressions that incorrectly break our build.
If such a regression occurs, this is a Rustc bug rather than an issue
with a PR, and we should still be able to merge the PR. The purpose of
the nightly CI build is to warn us if something is broken on nightly so
that we can open issues against Rust upstream for nightly regressions,
rather than as a gatekeeper for merging PRs.

.github/workflows/ci.yml

@@ -12,34 +12,29 @@ env:
 
 jobs:
   build:
-    name: Build ((${{ matrix.rust }}, ${{ matrix.target }})
+    name: Build (stable, ${{ matrix.target }})
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        rust:
-          - stable
-          - nightly
         target:
           - x86_64-unknown-linux-gnu
-        include:
-          - rust: stable
-            target: i686-unknown-linux-musl
+          - i686-unknown-linux-musl
     steps:
       - uses: actions/checkout@master
       - name: Install toolchain
         uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: ${{ matrix.rust }}
+          toolchain: stable
           target: ${{ matrix.target }}
           override: true
-      - name: cargo +${{ matrix.rust }} build --target ${{ matrix.target }}
+      - name: cargo build --target ${{ matrix.target }}
         uses: actions-rs/cargo@v1
         with:
           command: build
           args: --all-targets --target ${{ matrix.target }}
 
-  msrv:
+  build-msrv:
     name: Build (MSRV)
     runs-on: ubuntu-latest
     steps:
@@ -57,8 +52,27 @@ jobs:
         env:
           RUSTFLAGS: "" # remove -Dwarnings
 
+  build-nightly:
+    name: Build (nightly)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@master
+      - name: Install toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: nightly
+          override: true
+      - name: cargo +nightly build
+        uses: actions-rs/cargo@v1
+        with:
+          command: build
+        env:
+          RUSTFLAGS: "" # remove -Dwarnings
+
   test:
     name: Tests (stable)
+    needs: build
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@master
@@ -73,6 +87,7 @@ jobs:
 
   test-loom:
     name: Loom tests (stable)
+    needs: build
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@master
@@ -125,7 +140,11 @@ jobs:
     name: "all systems go!"
     needs:
       - build
-      - msrv
+      - build-msrv
+      # Note: we explicitly *don't* require the `build-nightly` job to pass,
+      # since the nightly Rust compiler is unstable. We don't want nightly
+      # regressions to break our build --- this CI job is intended for
+      # informational reasons rather than as a gatekeeper for merging PRs.
       - test
       - test-loom
       - clippy