ci: run MSRV checks with minimal dep versions (#670)

In many cases, new releases of a dependency can break compatibility with
Tower's minimum supported Rust version (MSRV). It shouldn't be necessary
for Tower to bump its MSRV when a dependency does, as users on older
Rust versions should be able to depend on older versions of that crate.
Instead, we should probably just run our MSRV checks with minimal
dependency versions.

This branch changes Tower's CI jobs to do that. It was also necessary to 
make some changes to the `Cargo.toml` to actually fix the build with
minimal dependency versions.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

.github/workflows/CI.yml

@@ -6,21 +6,18 @@ on:
     - master
   pull_request: {}
 
+env:
+  MSRV: 1.49.0
+
 jobs:
-  check:
+  check-stable:
+    # Run `cargo check` first to ensure that the pushed code at least compiles.
     runs-on: ubuntu-latest
-    strategy:
-      # Disable fail-fast. If the test run for a particular Rust version fails,
-      # don't cancel the other test runs, so that we can determine whether a
-      # failure only occurs on a particular version.
-      fail-fast: false
-      matrix:
-        rust: [stable, 1.49.0]
     steps:
     - uses: actions/checkout@master
     - uses: actions-rs/toolchain@v1
       with:
-        toolchain: ${{ matrix.rust }}
+        toolchain: stable
         profile: minimal
         override: true
     - name: Check
@@ -43,6 +40,33 @@ jobs:
         RUSTDOCFLAGS: "-D rustdoc::broken_intra_doc_links"
       run: cargo doc --all-features --no-deps
 
+  check-msrv:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: "install Rust ${{ env.MSRV }}"
+      uses: actions-rs/toolchain@v1
+      with:
+        toolchain: ${{ env.MSRV }}
+        profile: minimal
+    - name: "install Rust nightly"
+      uses: actions-rs/toolchain@v1
+      with:
+        toolchain: nightly
+        profile: minimal
+    - name: Select minimal versions
+      uses: actions-rs/cargo@v1
+      with:
+        command: update
+        args: -Z minimal-versions
+        toolchain: nightly
+    - name: Check
+      uses: actions-rs/cargo@v1
+      with:
+        command: check
+        args: --all --all-targets --all-features --locked
+        toolchain: ${{ env.MSRV }}
+
   cargo-hack:
     runs-on: ubuntu-latest
     steps:
@@ -51,16 +75,15 @@ jobs:
       with:
         toolchain: stable
         profile: minimal
-    - name: Install cargo-hack
-      run: |
-        curl -LsSf https://github.com/taiki-e/cargo-hack/releases/latest/download/cargo-hack-x86_64-unknown-linux-gnu.tar.gz | tar xzf - -C ~/.cargo/bin
+    - name: install cargo-hack
+      uses: taiki-e/install-action@cargo-hack
     - name: cargo hack check
       working-directory: ${{ matrix.subcrate }}
       run: cargo hack check --each-feature --no-dev-deps --workspace
 
   test-versions:
     # Test against the stable, beta, and nightly Rust toolchains on ubuntu-latest.
-    needs: check
+    needs: check-stable
     runs-on: ubuntu-latest
     strategy:
       # Disable fail-fast. If the test run for a particular Rust version fails,
@@ -68,10 +91,11 @@ jobs:
       # failure only occurs on a particular version.
       fail-fast: false
       matrix:
-        rust: [stable, beta, nightly, 1.49.0]
+        rust: [stable, beta, nightly]
     steps:
     - uses: actions/checkout@master
-    - uses: actions-rs/toolchain@v1
+    - name: "install Rust ${{ matrix.rust }}"
+      uses: actions-rs/toolchain@v1
       with:
         toolchain: ${{ matrix.rust }}
         profile: minimal
@@ -82,8 +106,36 @@ jobs:
         command: test
         args: --workspace --all-features
 
+  test-msrv:
+    needs: check-msrv
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: "install Rust ${{ env.MSRV }}"
+      uses: actions-rs/toolchain@v1
+      with:
+        toolchain: ${{ env.MSRV }}
+        profile: minimal
+    - name: "install Rust nightly"
+      uses: actions-rs/toolchain@v1
+      with:
+        toolchain: nightly
+        profile: minimal
+    - name: Select minimal versions
+      uses: actions-rs/cargo@v1
+      with:
+        command: update
+        args: -Z minimal-versions
+        toolchain: nightly
+    - name: test
+      uses: actions-rs/cargo@v1
+      with:
+        command: check
+        args: --workspace --all-features --locked
+        toolchain: ${{ env.MSRV }}
+
   style:
-    needs: check
+    needs: check-stable
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@master

tower/Cargo.toml

@@ -71,7 +71,7 @@ tower-service = { version = "0.3.1", path = "../tower-service" }
 
 futures-core = { version = "0.3", optional = true }
 futures-util = { version = "0.3", default-features = false, features = ["alloc"], optional = true }
-hdrhistogram = { version = "7.0", optional = true }
+hdrhistogram = { version = "7.0", optional = true, default-features = false }
 indexmap = { version = "1.0.2", optional = true }
 rand = { version = "0.8", features = ["small_rng"], optional = true }
 slab = { version = "0.4", optional = true }
@@ -84,14 +84,15 @@ pin-project-lite = { version = "0.2.7", optional = true }
 
 [dev-dependencies]
 futures = "0.3"
-hdrhistogram = "7.0"
+hdrhistogram = { version = "7.0", default-features = false }
 pin-project-lite = "0.2.7"
-tokio = { version = "1.6", features = ["macros", "sync", "test-util", "rt-multi-thread"] }
+tokio = { version = "1.6.2", features = ["macros", "sync", "test-util", "rt-multi-thread"] }
 tokio-stream = "0.1"
 tokio-test = "0.4"
 tower-test = { version = "0.4", path = "../tower-test" }
-tracing-subscriber = "0.2.14"
+tracing-subscriber = { version = "0.3", default-features = false, features = ["fmt", "ansi"] }
 http = "0.2"
+lazy_static = "1.4.0"
 
 [package.metadata.docs.rs]
 all-features = true