The lto setting controls the -C lto flag which controls LLVM’s link time optimizations. LTO can produce better optimized code, using whole-program analysis, at the cost of longer linking time.

The valid options are:

    false: Performs “thin local LTO” which performs “thin” LTO on the local crate only across its codegen units. No LTO is performed if codegen units is 1 or opt-level is 0.
    true or "fat": Performs “fat” LTO which attempts to perform optimizations across all crates within the dependency graph.
    "thin": Performs “thin” LTO. This is similar to “fat”, but takes substantially less time to run while still achieving performance gains similar to “fat”.
    "off": Disables LTO.

Basically, increases performance at cost of compile time. Great for release builds, not so much for debug builds (because most of the time that rustc spends is on linking anyways.)