chore: update MSRVs from 1.42 to 1.49 (tokio-rs#1913)

This updates all crates' MSRVs to 1.49 if they were not already greater
than that (`tracing-appender` is at 1.53). Rust 1.49+ is required to
update `parking_lot` to v0.12 (see tokio-rs#1878). Also, `futures-task` (which I
believe is only needed as a transitive dep) now needs 1.45+, so this
also fixes our CI build.

Because `tracing-opentelemetry` previously required 1.46.0, it had a
separate CI MSRV job. Since 1.49.0 is greater than 1.46.0, the separate
check for `tracing-opentelemetry` is no longer needed.

In the process, I removed deprecated uses of
`core::atomic::spin_loop_hint`, which is replaced with
`core::hint::spin_loop` in 1.49.

.github/workflows/check_msrv.yml

@@ -0,0 +1,63 @@
+name: Check MSRV
+
+on:
+  push:
+    branches:
+    - master
+    - "v0.1.x"
+  pull_request: {}
+
+env:
+  # Disable incremental compilation.
+  #
+  # Incremental compilation is useful as part of an edit-build-test-edit cycle,
+  # as it lets the compiler avoid recompiling code that hasn't changed. However,
+  # on CI, we're not making small edits; we're almost always building the entire
+  # project from scratch. Thus, incremental compilation on CI actually
+  # introduces *additional* overhead to support making future builds
+  # faster...but no future builds will ever occur in any given CI environment.
+  #
+  # See https://matklad.github.io/2021/09/04/fast-rust-builds.html#ci-workflow
+  # for details.
+  CARGO_INCREMENTAL: 0
+  # Allow more retries for network requests in cargo (downloading crates) and
+  # rustup (installing toolchains). This should help to reduce flaky CI failures
+  # from transient network timeouts or other issues.
+  CARGO_NET_RETRY: 10
+  RUSTUP_MAX_RETRIES: 10
+  # Don't emit giant backtraces in the CI logs.
+  RUST_BACKTRACE: short
+
+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
+      with:
+        toolchain: 1.49.0
+        profile: minimal
+        override: true
+    - name: Check
+      uses: actions-rs/cargo@v1
+      with:
+        command: check
+        args: --all --exclude=tracing-appender
+
+  # 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
+      with:
+        toolchain: 1.53.0
+        profile: minimal
+        override: true
+    - name: Check
+      uses: actions-rs/cargo@v1
+      with:
+        command: check
+        args: --lib=tracing-appender

tracing-opentelemetry/README.md

@@ -50,7 +50,7 @@ The crate provides the following types:
 [`tracing`]: https://crates.io/crates/tracing
 [OpenTelemetry]: https://opentelemetry.io/
 
-*Compiler support: [requires `rustc` 1.42+][msrv]*
+*Compiler support: [requires `rustc` 1.49+][msrv]*
 
 [msrv]: #supported-rust-versions
 

tracing-opentelemetry/src/lib.rs

@@ -9,7 +9,7 @@
 //! [OpenTelemetry]: https://opentelemetry.io
 //! [`tracing`]: https://github.com/tokio-rs/tracing
 //!
-//! *Compiler support: [requires `rustc` 1.42+][msrv]*
+//! *Compiler support: [requires `rustc` 1.49+][msrv]*
 //!
 //! [msrv]: #supported-rust-versions
 //!
@@ -79,7 +79,7 @@
 //! ## Supported Rust Versions
 //!
 //! Tracing is built against the latest stable release. The minimum supported
-//! version is 1.42. The current Tracing version is not guaranteed to build on
+//! version is 1.49. The current Tracing version is not guaranteed to build on
 //! Rust versions earlier than the minimum supported version.
 //!
 //! Tracing follows the same compiler support policies as the rest of the Tokio