chore(ci): run MSRV checks with minimal versions (#2171)

In many cases, new releases of a dependency can break compatibility with
`tracing`'s minimum supported Rust version (MSRV). It shouldn't be
necessary for a `tracing` crate 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 explicitly and avoid bumping. Instead, we should
probably just run our MSRV checks with minimal dependency versions. This
way, we don't need to bump our MSRV when the latest version of a
dependency does, unless we actually *need* to pick up that new version.

This branch changes the `check_msrv` CI jobs to do that. I also did some
minor dependency editing to actually make tracing build with
`-Zminimal-versions`.

Note that `tracing-futures` is currently excluded from the MSRV build
because it depends on a really ancient version of Tokio that pulls in
broken deps. We should probably drop support for Tokio 0.1 and release a
new version of that crate, but for now, we have to skip it from the CI
job temporarily.

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

.github/workflows/check_msrv.yml

@@ -27,37 +27,76 @@ env:
   RUSTUP_MAX_RETRIES: 10
   # Don't emit giant backtraces in the CI logs.
   RUST_BACKTRACE: short
+  MSRV: 1.49.0
+  # TODO: remove this once tracing's MSRV is bumped.
+  APPENDER_MSRV: 1.53.0
 
 jobs:
   check-msrv:
     # Run `cargo check` on our minimum supported Rust version (1.49.0).
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@main
-    - uses: actions-rs/toolchain@v1
+    - uses: actions/checkout@master
+    - name: "install Rust ${{ env.MSRV }}"
+      uses: actions-rs/toolchain@v1
       with:
-        toolchain: 1.49.0
+        toolchain: ${{ env.MSRV }}
         profile: minimal
-        override: true
+    - 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 --exclude=tracing-appender
+        # skip the following crates:
+        # - tracing-appender, as it has its own MSRV.
+        #   TODO(eliza): remove this when appender is on the same MSRV as
+        #   everything else
+        # - the examples, as they are not published & we don't care about
+        #   MSRV support for them.
+        # - tracing-futures, as it depends on ancient tokio versions.
+        #   TODO(eliza): remove this when the ancient tokio deps are dropped
+        args: >-
+          --workspace --all-features --locked
+          --exclude=tracing-appender
+          --exclude=tracing-examples
+          --exclude=tracing-futures
+        toolchain: ${{ env.MSRV }}
 
   # TODO: remove this once tracing's MSRV is bumped.
   check-msrv-appender:
     # Run `cargo check` on our minimum supported Rust version (1.53.0).
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@main
-    - uses: actions-rs/toolchain@v1
+    - uses: actions/checkout@master
+    - name: "install Rust ${{ env.APPENDER_MSRV }}"
+      uses: actions-rs/toolchain@v1
       with:
-        toolchain: 1.53.0
+        toolchain: ${{ env.APPENDER_MSRV }}
         profile: minimal
-        override: true
+    - 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: --lib=tracing-appender
+        args: --all-features --locked -p tracing-appender
+        toolchain: ${{ env.APPENDER_MSRV }}