In most places in the compiler temporary files are written to the output directory (using tempdir_in). E.g.:

The one exception is compiler/rustc_codegen_ssa/src/back/link.rs which writes to the OS temp directory (note the use of tempdir instead of tempdir_in):

I don't think there is a good reason to maintain this inconsistency?

Note that this affects -C save-temps where you have to go hunting for the relevant temp file (or redirect the platform-specific temp directory environment variable).