Make to_shmem Gecko-only

[SimonSapin]

As suggested in #26713 (comment)

[highfive]

Heads up! This PR modifies the following files:

• @emilio: components/style/values/generics/flex.rs, components/style/stylesheets/style_rule.rs, components/style/gecko/url.rs, components/style/gecko_string_cache/mod.rs, components/style/stylesheets/origin.rs and 85 more

[highfive]

Warning

• These commits modify style code, but no tests are modified. Please consider adding a test!

[jdm]

I don't think we need to add feature checks to any of the style/gecko/ or style/gecko_string_cache files.

[SimonSapin]

Good point. Amended.

[jdm]

@bors-servo r+

[bors-servo]

📌 Commit caf4cc6 has been approved by jdm

[emilio]

@bors-servo r-

Can we discuss this for a second? This is going to make syncs even more painful and if I understand correctly this affects llvm IR but not generated code, right?

[bors-servo]

☔ The latest upstream changes (presumably #26775) made this pull request unmergeable. Please resolve the merge conflicts.

[SimonSapin]

Correct, this is about reducing compile times. I would expect the linker to remove functions that are never called. (Maybe even without LTO? IIRC with --gc-sections is the default in rustc.)

I hadn’t considered the indirect impact on sync as you explained it on Matrix (mozilla-central contributors accidentally breaking the Servo configuration more often as it diverges more).

to_shmem_slice_ptr does show up in #26713 (comment) but with only 0.4% of IR lines in libscript. This is probably not worth the downside, so closing.