Function remapped_flags 

pub fn remapped_flags(
    encoded: Option<OsString>,
    plain: Option<OsString>,
    remaps: &[(PathBuf, String)],
) -> String

The value of ENCODED for a build with remaps: the flags already in encoded (that variable), else those in plain (RUSTFLAGS, split on white space), then a --remap-path-prefix for each remap.