universally directly depend on serde_derive instead of accessing it through serde.

[lolbinarycat]

bootstrap does this in order to flatten its dependencies and improve build times, but since other crates (like compiletest and rustdoc) use the derive feature flag, i believe this means serde has to be built twice to use any of these programs.

wouldn't it be best to always use the same set of feature flags to serde?

[lolbinarycat]

this of course would require that the derive feature is never used in the dependency tree, but looking at the output of cargo tree -e features, it doesn't seem to be, at least not for compiletest.

[workingjubilee]

isn't this what cargo's feature unification is for?

[hanna-kruppe]

Feature unification is currently always done w.r.t. the set of build-targets in any given Cargo invocation (modulo host-target distinctions that I'll ignore for simplicity):

• If cargo build -p bootstrap does not build anything that enables serde/derive, then serde will be built without that feature.
• If a subsequent cargo build does build something that enables serde/derive then serde itself will have to be built a second time. Anything that transitively depends on serde also has to be rebuilt as a consequence.
• (Both variants of serde and its reverse dependencies can co-exist in the build cache, so there's no thrashing when you switch back and forth between the two kinds of builds.)

Once rust-lang/cargo#14774 is implemented it'll be possible to change this behavior. It's not clear to me if this would be desirable -- it negates any benefit that bootstrap may derive from directly depending on serde_derive because serde/derive would always be enabled.

All of the above assumes that bootstrap itself and the rest of the repository are a single Cargo workspace or at least share a build cache (target/ directory). I don't know if that's the case today, so serde and other shared dependencies may have to be built twice in any case. However, depending directly on serde_derive may still be useful for faster builds of rustdoc, compiletest, etc. regardless of what bootstrap itself depends on.