bin/clippy: Add option to skip cleaning cache

Add --use-cache option in bin/clippy to run cargo-clippy without
deleting cache. It is useful when we use bin/clippy for multiple commits
in pre-upload hook.

BUG=chromium:1105466
TEST=bin/clippy w,w/o --use-cache

Change-Id: I386c7e08ad48ea2446a91e99d4b6523673211d6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2297005
Reviewed-by: Zach Reizner <zachr@chromium.org>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Tested-by: Keiichi Watanabe <keiichiw@chromium.org>
Commit-Queue: Keiichi Watanabe <keiichiw@chromium.org>

bin/clippy

@@ -9,6 +9,17 @@
 
 set -euo pipefail
 
+USE_CACHE=false
+CLIPPY_ARGS=( "$@" )
+
+# TODO: When we add more options, use a fancier parsing mechanism such as
+# getopts. Also use the Rust convention of -- separating the arguments for this
+# script itself from the ones for clippy.
+if (( "$#" > 0 )) && [[ "$1" == "--use-cache" ]]; then
+    USE_CACHE=true
+    CLIPPY_ARGS=( "${CLIPPY_ARGS[@]:1}" )
+fi
+
 # Change into directory of script, which is crosvm/bin.
 cd "$(dirname "${BASH_SOURCE[0]}")"
 
@@ -69,7 +80,9 @@ SUPPRESS=(
 )
 
 # Needed or else clippy won't re-run on code that has already compiled.
-cargo clean
+if [[ "${USE_CACHE}" == false ]]; then
+    cargo clean
+fi
 
 # Need to set pass --sysroot for cargo-clippy manually.
 # cf. https://github.com/rust-lang/rust-clippy/issues/3523
@@ -77,5 +90,5 @@ RUST_SYSROOT=$(rustc --print sysroot)
 RUSTFLAGS="${RUSTFLAGS:-}"
 export RUSTFLAGS="$RUSTFLAGS --sysroot=$RUST_SYSROOT"
 
-cargo clippy --all-features --all-targets -- ${SUPPRESS[@]/#/-Aclippy::} "$@" \
-      -D warnings
+cargo clippy --all-features --all-targets -- ${SUPPRESS[@]/#/-Aclippy::} \
+      "${CLIPPY_ARGS[@]}" -D warnings