extra_files is just a nice way to to tell certain IDEs (and those reading the file) that this file is also a dependency. Meson will use the .d file generated by the compiler to figure out what the target actually depends on.
43b0d5fa04 meson: fix formatting and add extra_files to i965
src/mesa/drivers/dri/i965/meson.build | 22 ++++++++++++----------
1 file changed, 12 insertions(+), 10 deletions(-)