This option was added to support OSX 10.6, but it's not ideal as it requires that a separate toolchain of Rust is downloaded to compile Rust code for Firefox on OSX. Ideally our support would look something like this:

• The compiler recognizes the "standard method" of specifying that OSX 10.6 is being targeted (not the current version). I believe that this is an environment variable?
• The compiler will then generate an appropriate #[cfg] when this is set to allow the crate to know that it's targeting 10.6 (or whatever the relevant OSX target is)
• The standard library is then compiled against 10.6 always
• The macros in the standard library will then select the "best implementation" based on the context they're built into.

This will incur a perf hit for the standard library itself, but it would allow us to go back to "one distribution works everywhere" which is much nicer than "go build a separate copy of Rust to work on 10.6".

cc @rillian