The [patch] section of your Cargo.toml can be used when you want to override certain parts of your dependency graph.

Cargo has a [replace] feature that is similar; while we don't intend to deprecate or remove [replace], you should prefer [patch] in all circumstances.

So what’s it look like? Let’s say we have a Cargo.toml that looks like this:

[dependencies]
foo = "1.2.3"

In addition, our foo package depends on a bar crate, and we find a bug in bar. To test this out, we’d download the source code for bar, and then update our Cargo.toml:

[dependencies]
foo = "1.2.3"

[patch.crates-io]
bar = { path = '/path/to/bar' }

Now, when you cargo build, it will use the local version of bar, rather than the one from crates.io that foo depends on. You can then try out your changes, and fix that bug!

For more details, see the documentation for patch.