Incremental builds

[MGlolenstine]

Is there a way to prebuild certain libraries, so that next builds are a lot faster?

[Alexhuszagh]

I believe this is only an issue if you switch between using cross and cargo on the host. As it is, this should already be supported, incremental builds are used by default.

A simple test is:

# Ensure we're using the latest cross
 cargo install cross --git https://github.com/cross-rs/cross

# Build a sample project with numerous dependencies.
git clone https://github.com/hyperium/hyper
cd hyper
git checkout v0.14.16

# Build once
cross build --target sparc64-unknown-linux-gnu

# Apply a patch and rebuild
git apply << EOF 
diff --git a/src/lib.rs b/src/lib.rs
index 41a0b375..1d1ef726 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -6,6 +6,8 @@
 #![cfg_attr(all(test, feature = "nightly"), feature(test))]
 #![cfg_attr(docsrs, feature(doc_cfg))]
 
+// Modified file
+
 //! # hyper
 //!
 //! hyper is a **fast** and **correct** HTTP implementation written in and for Rust.
EOF
cross build --target sparc64-unknown-linux-gnu --verbose

In this example, only hyper is rebuilt, and does a similar incremental build to cargo on the host. Adding the --verbose flag shows only lib.rs is being rebuilt.

If you cargo on the host and use cross afterwards, however, yes, the entire dependency tree is rebuilt.

[Emilgardis]

it shouldn't invalidate incremental data, as the directory for incremental data will be in target/{target}/{profile}, exactly what happens when you use --target in cargo natively (since that's what we actually do). I can switch between cross and cargo between diffs and keep incremental data, I did one time observe it being invalidated, but haven't been able to replicate it more.

[Alexhuszagh]

I believe this is linked to #280.

[Alexhuszagh]

If this reproduces outside of a situation where you switch between cargo and cross or update toolchain versions for the same target, I'll re-open this. Otherwise, it's a duplicate.