Until now, rustc implicitly linked the jemalloc memory allocator to every Rust program (not
cdylib libraries) by default on some platforms (including Linux and macOS). jemalloc is often more performant than the system’s allocator, but not always, and it adds a non-trivial amount to the size of the executable.
In Rust 1.28, we stabilized the
#[global_allocator] attribute that allows a program to opt into another memory allocator, and the
std::alloc::System type to represent the system’s default allocator.
PR #55238 has landed today, removing jemalloc from the Rust standard library. So starting with tomorrow’s Rust Nightly, Rust programs will use the system allocator by default. You can use the jemallocator crate together with
#[global_allocator] if you want to keep using jemalloc.
Changing the default took a long time because it was tricky to figure out a way to do it while still using jemalloc in rustc and LLVM-in-rustc, because of the unusual way rustc is linked. See issues #36963 and #51038 for more background. jemalloc is a significant performance improvement in almost all cases for rustc’s workloads.