Out of source builds (external build directory) with Cargo?

Question

Having used CMake, I\'ve become used to out-of-source builds, which are encouraged with CMake. How can out-of-source builds be done with Cargo?

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Using in-source-bui

Answer 1

You can specify the directory of the target/ folder either via configuration file (key build.target-dir) or environment variable (CARGO_TARGET_DIR). Here is an example using a configuration file:

Suppose you want to have a directory ~/work/ in which you want to save the Cargo project (~/work/foo/) and next to it the target directory (~/work/my-target/).

$ cd ~/work
$ cargo new --bin foo
$ mkdir .cargo
$ $EDITOR .cargo/config

Then insert the following into the configuration file:

[build]
target-dir = "./my-target"

If you then build in your normal Cargo project directory:

$ cd foo
$ cargo build

You will notice that there is no target/ dir, but everything is in ~/work/my-target/.

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However, the Cargo.lock is still saved inside the Cargo project directory, but that kinda makes sense. For executables, you should check the Cargo.lock file into your git! For libraries, you shouldn't. I guess having to ignore one file is better than having to ignore an entire folder.

Lastly, there are a few caveats to changing the target-dir, which are listed in the PR which introduced the feature.

Answer 2

While useful manually setting this up isn't all that convenient, I wanted to be able to build multiple crates within a source tree, having all of them out-of-source, something that ../target-dir configuration option wouldn't achieve.

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Helper utility for convenient out-of-source builds

Using the environment variable I've written a small utility to wrap cargo, so it automatically builds out-of-source, supporting crates both at the top-level, on in a subdirectory of the source tree.

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Thanks to Lukas for pointing out CARGO_TARGET_DIR and target-dir configuration option.