Refactor CI

CI changes:
- replace actions-rs with dtolnay/rust-toolchain
- deny warnings from cargo check
- add cargo-semver-checks-action
- add cargo-check-external-types

.github/workflows/rust.yml

@@ -1,7 +1,12 @@
-on: [push, pull_request]
-
 name: Continuous integration
 
+on:
+  push:
+  pull_request:
+  merge_group:
+  schedule:
+    - cron: '0 18 * * *'
+
 jobs:
   check:
     name: Check
@@ -13,23 +18,20 @@ jobs:
           - 1.63.0
           - nightly
     steps:
-      - uses: actions/checkout@v2
-      - uses: actions-rs/toolchain@v1
+      - uses: actions/checkout@v4
+      - name: Install ${{ matrix.rust }} toolchain
+        uses: dtolnay/rust-toolchain@master
         with:
-          profile: minimal
           toolchain: ${{ matrix.rust }}
-          override: true
       - name: Cargo update
         run: cargo update
       - name: Cargo update (fix for MSRV)
         run: cargo update -p time --precise 0.3.20
         if: matrix.rust == '1.63.0'
-      - uses: actions-rs/cargo@v1
-        with:
-          command: check
+      - run: RUSTFLAGS="-D warnings" cargo check
 
-  check_features:
-    name: Check features
+  test_features:
+    name: Test suite (with features)
     runs-on: ubuntu-latest
     strategy:
       matrix:
@@ -39,81 +41,72 @@ jobs:
           - --features=bigint,serialize,debug
           - --features=bigint,serialize,trace
     steps:
-      - uses: actions/checkout@v2
-      - uses: actions-rs/toolchain@v1
-        with:
-          profile: minimal
-          toolchain: stable
-          override: true
-      - name: Cargo update
-        run: cargo update
-      - uses: actions-rs/cargo@v1
-        with:
-          command: test
-          args: ${{ matrix.features }}
+      - uses: actions/checkout@v4
+      - name: Install stable toolchain
+        uses: dtolnay/rust-toolchain@stable
+      - run: cargo test ${{ matrix.features }}
 
   no_std:
     name: no-std
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
-      - uses: actions-rs/toolchain@v1
-        with:
-          profile: minimal
-          toolchain: stable
-          override: true
-      - uses: actions-rs/cargo@v1
-        with:
-          command: check
-          args: --no-default-features
+      - uses: actions/checkout@v4
+      - name: Install stable toolchain
+        uses: dtolnay/rust-toolchain@stable
+      - run: RUSTFLAGS="-D warnings" cargo check --no-default-features
 
   fmt:
     name: Rustfmt
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
-      - uses: actions-rs/toolchain@v1
+      - uses: actions/checkout@v4
+      - name: Install stable rustfmt
+        uses: dtolnay/rust-toolchain@stable
         with:
-          profile: minimal
-          toolchain: stable
-          override: true
-      - run: rustup component add rustfmt
-      - uses: actions-rs/cargo@v1
-        with:
-          command: fmt
-          args: --all -- --check
+          components: rustfmt
+      - run: cargo fmt --all -- --check
 
   clippy:
     name: Clippy
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
-      - uses: actions-rs/toolchain@v1
-        with:
-          profile: minimal
-          toolchain: stable
-          override: true
-      - run: rustup component add clippy
-      - uses: actions-rs/cargo@v1
+      - uses: actions/checkout@v4
+      - name: Install nightly clippy
+        uses: dtolnay/rust-toolchain@nightly
         with:
-          command: clippy
-          args: --all-features -- -D warnings
+          components: clippy
+      - run: cargo clippy -- -D warnings
 
   doc:
     name: Build documentation
     runs-on: ubuntu-latest
-    # this can be removed when cargo-doc stops panic'ing in collect_intra_doc_links
-    continue-on-error: true
     env:
       RUSTDOCFLAGS: --cfg docsrs
     steps:
-      - uses: actions/checkout@v2
-      - uses: actions-rs/toolchain@v1
-        with:
-          profile: minimal
-          toolchain: nightly
-          override: true
-      - uses: actions-rs/cargo@v1
+      - uses: actions/checkout@v4
+      - name: Install nightly rust
+        uses: dtolnay/rust-toolchain@nightly
+      - run: cargo doc --workspace --no-deps --all-features
+
+  semver:
+    name: Check semver compatibility
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v4
+      - name: Check semver
+        uses: obi1kenobi/cargo-semver-checks-action@v2
+
+  check-external-types:
+    name: Validate external types appearing in public API
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v4
+      - name: Install rust toolchain
+        uses: dtolnay/rust-toolchain@master
         with:
-          command: doc
-          args: --workspace --no-deps --all-features
+          toolchain: nightly-2024-02-07
+          # ^ sync with https://github.com/awslabs/cargo-check-external-types/blob/main/rust-toolchain.toml
+      - run: cargo install cargo-check-external-types
+      - run: cargo check-external-types