Faster shared parts

[davidmorgan]

Using shared parts (edit: the the source_gen feature) currently slows large builds by about a factor of x2; see the benchmark added for #3800 for the data.

[bwilkerson]

By "shared parts" do you mean a part file that two libraries both include?

If so, is there a requirement for supporting this? The enhanced parts feature removes the ability to share parts in this way, so it's important to know if that isn't going to work. And if not, then we might want to let users know that moving off this pattern sooner will make builds faster, but not actually spend the time to improve performance.

[davidmorgan]

By "shared parts" do you mean a part file that two libraries both include?

Sorry, I should have been clearer, I meant the source_gen feature which combines the output of multiple generators. So nothing to do with language/tooling, very much a build_runner thing :)

[tylandercasper]

loosely linked to #language-4241
automatic "part " inclusions would make it a lot easier for every generator to have it's own part file

[bwilkerson]

Even without automatic part inclusions, enhanced parts would make it possible for each generator to generate a separate part file and then have build_runner generate a single part file that includes all of the generated parts. Users would only be required to add a single part directive for the top-level generated part.

[jakemac53]

Good point about enhanced parts, that would be a nice way to do this in the future