fix warnings

Status Job ID Name Coverage
  Build
passed #18777
build:linux

00:00:23

failed #18776
build:linux:stable

00:00:13

passed #18778
build:redox

00:00:23

 
  Test
skipped #18780
test:linux
skipped #18779
test:linux:stable
skipped #18781
test:redox
 
Name Stage Failure
failed
build:linux:stable Build
Checking out a1c43eec as remove-warnings...

Skipping Git submodules setup
Checking cache for default...
No URL provided, cache will not be downloaded from shared cache server. Instead a local version of cache will be extracted.
Successfully extracted cache
$ cargo +stable build --verbose
error: toolchain 'stable-x86_64-unknown-linux-gnu' is not installed
ERROR: Job failed: exit code 1