Describe dep-info files a little better.

[cargo.git] / src / doc / src / guide / build-cache.md

## Build cache

Cargo stores the output of a build into the "target" directory. By default,
this is the directory named `target` in the root of your workspace. To change
the location, you can set the `CARGO_TARGET_DIR` [environment variable], the
[`build.target-dir`] config value, or the `--target-dir` command-line flag.

The directory layout depends on whether or not you are cross-compiling for a
different platform with the `--target` flag. When not cross-compiling, the
output goes into the root of the target directory, separated based on whether
or not it is a release build:

Directory | Description
----------|------------
<code style="white-space: nowrap">target/debug/</code> | Contains debug build output.
<code style="white-space: nowrap">target/release/</code> | Contains release build output (with `--release` flag).

When building for another target, the output is placed in a directory with the
name of the target:

Directory | Example
----------|--------
<code style="white-space: nowrap">target/&lt;triple&gt;/debug/</code> | <code style="white-space: nowrap">target/thumbv7em-none-eabihf/debug/</code>
<code style="white-space: nowrap">target/&lt;triple&gt;/release/</code> | <code style="white-space: nowrap">target/thumbv7em-none-eabihf/release/</code>

Within the profile directory (`debug` or `release`), artifacts are placed into
the following directories:

Directory | Description
----------|------------
<code style="white-space: nowrap">target/debug/</code> | Contains the output of the package being built (the `[[bin]]` executables and `[lib]` library targets).
<code style="white-space: nowrap">target/debug/examples/</code> | Contains examples (`[[example]]` targets).

Some commands place their output in dedicated directories in the top level of
the `target` directory:

Directory | Description
----------|------------
<code style="white-space: nowrap">target/doc/</code> | Contains rustdoc documentation ([`cargo doc`]).
<code style="white-space: nowrap">target/package/</code> | Contains the output of the [`cargo package`] and [`cargo publish`] commands.

Cargo also creates several other directories and files needed for the build
process. Their layout is considered internal to Cargo, and is subject to
change. Some of these directories are:

Directory | Description
----------|------------
<code style="white-space: nowrap">target/debug/deps/</code> |  Dependencies and other artifacts.
<code style="white-space: nowrap">target/debug/incremental/</code> |  `rustc` [incremental output], a cache used to speed up subsequent builds.
<code style="white-space: nowrap">target/debug/build/</code> |  Output from [build scripts].

### Dep-info files

Next to each compiled artifact is a file called a "dep info" file with a `.d`
suffix. This file is a Makefile-like syntax that indicates all of the file
dependencies required to rebuild the artifact. These are intended to be used
with external build systems so that they can detect if Cargo needs to be
re-executed. The paths in the file are absolute by default. See the
[`build.dep-info-basedir`] config option to use relative paths.

```Makefile
# Example dep-info file found in target/debug/foo.d
/path/to/myproj/target/debug/foo: /path/to/myproj/src/lib.rs /path/to/myproj/src/main.rs
```

### Shared cache

A third party tool, [sccache], can be used to share built dependencies across
different workspaces.

To setup `sccache`, install it with `cargo install sccache` and set
`RUSTC_WRAPPER` environmental variable to `sccache` before invoking Cargo. If
you use bash, it makes sense to add `export RUSTC_WRAPPER=sccache` to
`.bashrc`. Alternatively, you can set [`build.rustc-wrapper`] in the [Cargo
configuration][config]. Refer to sccache documentation for more details.

[`build.dep-info-basedir`]: ../reference/config.md#builddep-info-basedir
[`build.target-dir`]: ../reference/config.md#buildtarget-dir
[`cargo doc`]: ../commands/cargo-doc.md
[`cargo package`]: ../commands/cargo-package.md
[`cargo publish`]: ../commands/cargo-publish.md
[build scripts]: ../reference/build-scripts.md
[config]: ../reference/config.md
[environment variable]: ../reference/environment-variables.md
[incremental output]: ../reference/profiles.md#incremental
[sccache]: https://github.com/mozilla/sccache