rebuild_unit_graph_shared in cargo::ops::cargo_compile - Rust
Description: This is used to rebuild the unit graph, sharing host dependencies if possible, and applying other unit adjustments based on the whole graph.

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## rebuild_unit_graph_shared

# Function rebuild_unit_graph_shared

Source

    fn rebuild_unit_graph_shared(
        interner: &UnitInterner,
        unit_graph: UnitGraph,
        roots: &[Unit],
        scrape_units: &[Unit],
        to_host: Option<CompileKind>,
        compile_time_deps_only: bool,
    ) -> (Vec<Unit>, Vec<Unit>, UnitGraph)

Expand description

This is used to rebuild the unit graph, sharing host dependencies if possible, and applying other unit adjustments based on the whole graph.

This will translate any unit’s `CompileKind::Target(host)` to `CompileKind::Host` if `to_host` is not `None` and the kind is equal to `to_host`. This also handles generating the unit `dep_hash`, and merging shared units if possible.

This is necessary because if normal dependencies used `CompileKind::Host`, there would be no way to distinguish those units from build-dependency units or artifact dependency units. This can cause a problem if a shared normal/build/artifact dependency needs to link to another dependency whose features differ based on whether or not it is a normal, build or artifact dependency. If all units used `CompileKind::Host`, then they would end up being identical, causing a collision in the `UnitGraph`, and Cargo would end up randomly choosing one value or the other.

The solution is to keep normal, build and artifact dependencies separate when building the unit graph, and then run this second pass which will try to combine shared dependencies safely. By adding a hash of the dependencies to the `Unit`, this allows the `CompileKind` to be changed back to `Host` and `artifact_target_for_features` to be removed without fear of an unwanted collision for build or artifact dependencies.

This is also responsible for adjusting the `strip` profile option to opportunistically strip if debug is 0 for all dependencies. This helps remove debuginfo added by the standard library.

This is also responsible for adjusting the `debug` setting for host dependencies, turning off debug if the user has not explicitly enabled it, and the unit is not shared with a target unit.

This is also responsible for adjusting whether each unit should be compiled or not regarding `--compile-time-deps` flag.