CI install rustdoc nightly with stable date version

.github/workflows/ci.yml

@@ -134,19 +134,25 @@ jobs:
       run: ./.github/workflows/disable-bench-deps.sh
     - name: Install dependencies
       run: sudo bash ./.github/workflows/install-deps.sh
-    - name: Install Rust nightly
-      run: rustup toolchain install nightly
+    - name: Install Rust nightly (stable date version)
+      run: |
+        STABLE_DATE=$(rustdoc --version | perl -ne 'print $& if /\b\d{4}-\d{2}-\d{2}\b/')
+        echo "STABLE_DATE=$STABLE_DATE" >> $GITHUB_ENV
+
+        rustup toolchain install nightly-$STABLE_DATE
+
+        echo "Stable version: $(rustdoc --version)"
+        echo "Nightly version: $(rustdoc +nightly-$STABLE_DATE --version)"
     - name: Restore cargo caches
       uses: Swatinem/rust-cache@v1
-    - name: Run rustdoc
+    - name: Run rustdoc nightly-${{ env.STABLE_DATE }}
       run: |
         cd spdlog
-        # The reason for allowing lint `unexpected_cfgs` not in `build.rs` or `Cargo.toml` but only in CI is that we need to keep MSRV
-        cargo +nightly rustdoc --verbose -- -D warnings -A unexpected_cfgs -A rustdoc::broken-intra-doc-links
-        cargo +nightly rustdoc --all-features --verbose -- -D warnings -A unexpected_cfgs
+        cargo +nightly-${{ env.STABLE_DATE }} rustdoc --verbose -- -D warnings -A rustdoc::broken-intra-doc-links
+        cargo +nightly-${{ env.STABLE_DATE }} rustdoc --all-features --verbose -- -D warnings
         cd ../spdlog-macros
-        cargo +nightly rustdoc --verbose -- -D warnings -A unexpected_cfgs -A rustdoc::broken-intra-doc-links
-        cargo +nightly rustdoc --all-features --verbose -- -D warnings -A unexpected_cfgs
+        cargo +nightly-${{ env.STABLE_DATE }} rustdoc --verbose -- -D warnings -A rustdoc::broken-intra-doc-links
+        cargo +nightly-${{ env.STABLE_DATE }} rustdoc --all-features --verbose -- -D warnings
 
   check-msrv:
     strategy: