perf(shirabe): use mimalloc as the global allocator

Profiling `create-project` showed glibc malloc/free accounting for about
41% of samples: the work is dominated by a very large number of small,
short-lived allocations, mostly from decoding repository JSON into
PhpMixed.

`create-project laravel/laravel`, offline against a warmed cache, all
three variants measured in one hyperfine run (8 runs each, identical
composer.lock):

  glibc      3.633 s +- 0.023   (user 3.580 s, sys 2.255 s)
  mimalloc   3.096 s +- 0.054   (user 3.004 s, sys 2.315 s)   -14.8%
  jemalloc   3.177 s +- 0.028   (user 3.136 s, sys 2.285 s)   -12.5%

The gain is entirely in user time; system time is unchanged, so the
extraction and file-writing half of the run is unaffected. Peak RSS drops
slightly, from 267 MB to 257 MB. mimalloc wins over jemalloc and, unlike
jemalloc, needs no C build of its own at every profile.

The allocator is declared in the library rather than in `main.rs` so that
the test and benchmark binaries, which link this crate but not `main.rs`,
run against the allocator the binary ships with.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

4 files changed, 25 insertions, 0 deletions

diff --git a/Cargo.lock b/Cargo.lock
index a4d1cea6..b97733d0 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1343,6 +1343,15 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "68ab91017fe16c622486840e4c83c9a37afeff978bd239b5293d61ece587de66"
 
 [[package]]
+name = "libmimalloc-sys"
+version = "0.1.49"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6a45a52f43e1c16f667ccfe4dd8c85b7f7c204fd5e3bf46c5b0db9a5c3c0b8e9"
+dependencies = [
+ "cc",
+]
+
+[[package]]
 name = "linux-raw-sys"
 version = "0.12.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -1412,6 +1421,15 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c2a86d3146ed3995b5913c414f6664344b9617457320782e64f0bb44afd49d74"
 
 [[package]]
+name = "mimalloc"
+version = "0.1.52"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2d4139bb28d14ad1facf21d5eb8825051b326e172d216b39f6d31df53cc97862"
+dependencies = [
+ "libmimalloc-sys",
+]
+
+[[package]]
 name = "mime"
 version = "0.3.17"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -2326,6 +2344,7 @@ dependencies = [
  "indexmap",
  "jsonschema",
  "md5",
+ "mimalloc",
  "mockall",
  "nix",
  "regex",
diff --git a/Cargo.toml b/Cargo.toml
index 657f7da6..22504795 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -38,6 +38,7 @@ indexmap = { version = "2.14.0", features = ["serde"] }
 jsonschema = { version = "0.46.6", default-features = false }
 md5 = "0.7.0"
 memchr = "2.8.3"
+mimalloc = "0.1.52"
 mockall = "0.14.0"
 nix = { version = "0.31.3", features = ["feature", "fs", "poll", "process", "signal", "user"] }
 regex = "1.12.3"
diff --git a/crates/shirabe/Cargo.toml b/crates/shirabe/Cargo.toml
index 66f2b5fc..653591d4 100644
--- a/crates/shirabe/Cargo.toml
+++ b/crates/shirabe/Cargo.toml
@@ -29,6 +29,7 @@ futures.workspace = true
 indexmap.workspace = true
 jsonschema.workspace = true
 md5.workspace = true
+mimalloc.workspace = true
 nix.workspace = true
 regex.workspace = true
 reqwest.workspace = true
diff --git a/crates/shirabe/src/lib.rs b/crates/shirabe/src/lib.rs
index 9daf2188..3b142e74 100644
--- a/crates/shirabe/src/lib.rs
+++ b/crates/shirabe/src/lib.rs
@@ -25,6 +25,10 @@ pub mod self_update;
 pub mod signal;
 pub mod util;
 
+// Declared here to use this allocator in all binaries including tests and benches.
+#[global_allocator]
+static GLOBAL: mimalloc::MiMalloc = mimalloc::MiMalloc;
+
 // InstalledVersions is intentionally unported to Rust. It is a runtime API for plugins and project
 // code, never read by Composer itself. Its real state is stored in PHP's InstalledVersions class.
 // See also `__shirabe_installed_versions_reload` in crates/shirabe-php-rpc/php/worker.php.