116f4ae171
Currently it is possible to avoid linking the allocator shim when __rust_no_alloc_shim_is_unstable_v2 is defined when linking rlibs directly as some build systems need. However this requires liballoc to be compiled with --cfg no_global_oom_handling, which places huge restrictions on what functions you can call and makes it impossible to use libstd. Or alternatively you have to define __rust_alloc_error_handler and (when using libstd) __rust_alloc_error_handler_should_panic using #[rustc_std_internal_symbol]. With this commit you can either use libstd and define __rust_alloc_error_handler_should_panic or not use libstd and use #[alloc_error_handler] instead. Both options are still unstable though. Eventually the alloc_error_handler may either be removed entirely (though the PR for that has been stale for years now) or we may start using weak symbols for it instead. For the latter case this commit is a prerequisite anyway.
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc dev guide.