Rust 1.5 features fine-grained dependency tracking that we need for
proper incremental builds. As a side effect, it also features choosing
between the builtin-to-Rust jemalloc and the system allocator, which
Gecko needs for interoperation. (Gecko needs to make Rust use the
system allocator, which then gets redirected into Gecko's copy of
jemalloc if necessary.) It's not great to require an unstable Rust
target here, but we need to upgrade at some point anyway, so we might as
well make people aware of the upgrade requirement sooner rather than
later.
Rust 1.5 features fine-grained dependency tracking that we need for
proper incremental builds. As a side effect, it also features choosing
between the builtin-to-Rust jemalloc and the system allocator, which
Gecko needs for interoperation. (Gecko needs to make Rust use the
system allocator, which then gets redirected into Gecko's copy of
jemalloc if necessary.) It's not great to require an unstable Rust
target here, but we need to upgrade at some point anyway, so we might as
well make people aware of the upgrade requirement sooner rather than
later.
