Fix Evergreen task for MSRV compilation with async-std (#672)

Author: kmahar

.evergreen/MSRV-Cargo.lock

@@ -6,9 +6,25 @@ set -o errexit
 rustup update $RUST_VERSION
 
 if [ "$ASYNC_RUNTIME" = "tokio" ]; then
-    .evergreen/compile-only-tokio.sh 
+    FEATURE_FLAGS=snappy-compression,zlib-compression
+
+    # Zstd requires Rust version 1.54
+    if [[ $RUST_VERSION == "nightly" ]]; then
+        FEATURE_FLAGS=$FEATURE_FLAGS,zstd-compression
+    fi
+
+    rustup run $RUST_VERSION cargo build --features $FEATURE_FLAGS
+    rustup run $RUST_VERSION cargo build --features tokio-sync,$FEATURE_FLAGS
+
 elif [ "$ASYNC_RUNTIME" = "async-std" ]; then
-    .evergreen/compile-only-async-std.sh
+    # v2.1 of async-global-executor bumped its MSRV to 1.59, so we need a Cargo.lock
+    # pinning to v2.0 to build with our MSRV.
+    if [  "$MSRV" = "true" ]; then
+        cp .evergreen/MSRV-Cargo.lock Cargo.lock
+    fi
+
+    rustup run $RUST_VERSION cargo build --no-default-features --features async-std-runtime
+    rustup run $RUST_VERSION cargo build --no-default-features --features sync
 else
     echo "invalid async runtime: ${ASYNC_RUNTIME}" >&2
     exit 1

.evergreen/compile-only-async-std.sh

@@ -535,7 +535,7 @@ functions:
         working_dir: "src"
         script: |
           ${PREPARE_SHELL}
-          ASYNC_RUNTIME=${ASYNC_RUNTIME} RUST_VERSION=${RUST_VERSION} .evergreen/compile-only.sh
+          ASYNC_RUNTIME=${ASYNC_RUNTIME} RUST_VERSION=${RUST_VERSION} MSRV=${MSRV} .evergreen/compile-only.sh
 
   "check rustfmt":
     - command: shell.exec
@@ -1292,6 +1292,7 @@ axes:
         display_name: "1.51 (minimum supported version)"
         variables:
           RUST_VERSION: "1.51.0"
+          MSRV: true
       - id: "nightly"
         display_name: "nightly"
         variables: