Support "local" rustflags only affecting the main crate #215
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The currently available way of setting Rustflags is by setting the `RUSTFLAGS` environment variable, which affects all crates in the dependency graph. This may be undesirable in some circumstances, so a future commit will the option to specify RUSTFLAGS via `cargo rustc`, which only affects the current crate and not dependencies. This commit prepares by refactoring some names to make the distinction easier.
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For some scenarios it is better to not set Rustflags for all crates in the dependency graph and instead only set it for the top-level crate. For example rust-lang/cargo#8716 can be avoided in some scenarios by setting the rustflags via rustc, which allows for faster rebuilds in such cases.
Using local rustflags passed via `cargo rustc` has the advantage, that they are not set for dependencies. This is mostly benenficial in workspaces, where different crates link different C dependencies, but share common rust dependencies. If the RUSTFLAGS environment variable is used this will cause rebuilds of the common rust dependencies, since the RUSTFLAGS differ. When passing rustflags via cargo rustc, this does not happen, since they only affects the main crate.
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For some scenarios it is better to not set Rustflags for all crates
in the dependency graph and instead only set it for the top-level
crate.
For example rust-lang/cargo#8716
can be avoided in some scenarios by setting the rustflags via
rustc, which allows for faster rebuilds in such cases.
Note:
Switching to "cargo rustc" is slightly problematic in cases where a package has multiple crates (e.g. lib and one or more bins), however Corrosion doesn't actually support those cases right now anyway.
A follow-up PR will add support for multiple targets per package.