chore(ci): add a minimal-versions check (#2015)

This adds a minimal-versions check to the tracing project. Adapted from
`tokio-rs/tokio`. Adding this avoids breaking downstream dependencies
from accidentally under-constraining minimal versions of dependencies
when they depend on tracing.

I've currently just introduced the check. I will try to and do encourage
others to add patches to fix this where possible since it can be a fair
bit of work to chase down a version of all dependencies that passes
minimal-versions and is msrv. I've also seen some really odd
windows-specific issues (which are not being tested for here).

This is currently only testing `tracing`, `tracing-core`, and
`tracing-subscriber`. Packages such as `tracing-futures` are proving to
be a bit harder to deal with due to having features which enable very
old dependencies.

Steps to test the build minimal versions locally:

```sh
cargo install cargo-hack
rustup default nightly
cargo hack --remove-dev-deps --workspace
cargo update -Z minimal-versions
cargo hack check --all-features --ignore-private
```

CC: tokio-rs/tokio#4513

.github/workflows/CI.yml

@@ -222,3 +222,32 @@ jobs:
       with:
         token: ${{ secrets.GITHUB_TOKEN }}
         args: --all --examples --tests --benches -- -D warnings
+
+  minimal-versions:
+    # Check for minimal-versions errors where a dependency is too
+    # underconstrained to build on the minimal supported version of all
+    # dependencies in the dependency graph.
+    name: minimal-versions
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: nightly
+          profile: minimal
+          override: true
+      - 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: "check --all-features -Z minimal-versions"
+        run: |
+          # Remove dev-dependencies from Cargo.toml to prevent the next `cargo update`
+          # from determining minimal versions based on dev-dependencies.
+          cargo hack --remove-dev-deps --workspace
+          # Update Cargo.lock to minimal version dependencies.
+          cargo update -Z minimal-versions
+          cargo hack check \
+            --package tracing \
+            --package tracing-core \
+            --package tracing-subscriber \
+            --all-features --ignore-private

tracing-core/Cargo.toml

@@ -34,7 +34,7 @@ std = ["lazy_static"]
 maintenance = { status = "actively-developed" }
 
 [dependencies]
-lazy_static = { version = "1", optional = true }
+lazy_static = { version = "1.0.2", optional = true }
 
 [target.'cfg(tracing_unstable)'.dependencies]
 valuable = { version = "0.1.0", optional = true, default_features = false }