config to switch linker to mold for linux target (#51)

* config to switch linker to mold for linux target

PROBLEM:
dev hot-rebuilds currently take ~10s on a 4c8t intel laptop
from circa 2017. (rustc 1.61.0)

SOLUTION:
After switching to mold, this time drops to ~3.5s.

FURTHER INFORMATION:
Profiling via rustc --self-profile reveals that most of the time
for a hot reload comes from linking. mold is a faster linker than
lld or ld.

POSSIBLE IMPROVEMENTS:
Switching to rust nightly (currently 1.63.0) yield further 20-30%
improvements.

* add mold to github actions config

PROBLEM: unit tests and integration tests error out because
	mold not installed on gh actions machines

SOLUTION: Use Rui Ueyama's first party github action
	https://github.com/rui314/setup-mold

* change mold path to /usr/local/bin/mold

PROBLEM:
Ueyama's gh action installs mold to /usr/local/bin/mold
whereas cargo configured to look for it at /usr/bin/mold

SOLUTION: change cargo path to /usr/local/bin/mold

POSSIBLE IMPROVEMENTS:
I really wish it was possible to have the config file include
macros somehow. (so something like /usr/bin/env mold, or which
mold), but that doesn't seem feasible without having a config
generator which seems excessive for just this, but might be
good if there's more dynamic configuration in the future.

* change linker flag from --ld-path to -fuse-ld

PROBLEM: gh-actions clang is old and does not support --ld-path

SOLUTION: switch to deprecated -fuse-ld

NOTES:
I suspect that the rust project will switch to mold as default
linker for linux before we have to worry about changing this
path anyhow.

Co-authored-by: Abhishek Cherath <[email protected]>

Author: pretentious7

.cargo/config.toml

@@ -0,0 +1,3 @@
+[target.x86_64-unknown-linux-gnu]
+linker = "/usr/bin/clang"
+rustflags = ["-C", "link-arg=-fuse-ld=/usr/local/bin/mold"]

.github/workflows/integration-tests.yml

@@ -15,6 +15,7 @@ jobs:
 
     steps:
     - uses: actions/checkout@v3
+    - uses: rui314/setup-mold@v1
     - uses: Swatinem/rust-cache@v1
       with:
         cache-on-failure: true

.github/workflows/unit-tests.yml

@@ -15,6 +15,7 @@ jobs:
 
     steps:
     - uses: actions/checkout@v3
+    - uses: rui314/setup-mold@v1
     - uses: Swatinem/rust-cache@v1
       with:
         cache-on-failure: true