cargo check is a new subcommand should speed up the development workflow in many cases.

What does it do? Let's take a step back and talk about how rustc compiles your code. Compilation has many "passes", that is, there are many distinct steps that the compiler takes on the road from your source code to producing the final binary. However, you can think of this process in two big steps: first, rustc does all of its safety checks, makes sure your syntax is correct, all that stuff. Second, once it's satisfied that everything is in order, it produces the actual binary code that you end up executing.

It turns out that that second step takes a lot of time. And most of the time, it's not neccesary. That is, when you're working on some Rust code, many developers will get into a workflow like this:

1. Write some code.
2. Run cargo build to make sure it compiles.
3. Repeat 1-2 as needed.
4. Run cargo test to make sure your tests pass.
5. Try the binary yourself
6. GOTO 1.

In step two, you never actually run your code. You're looking for feedback from the compiler, not to actually run the binary. cargo check supports exactly this use-case: it runs all of the compiler's checks, but doesn't produce the final binary. To use it:

$ cargo check

where you may normally cargo build. The workflow now looks like:

1. Write some code.
2. Run cargo check to make sure it compiles.
3. Repeat 1-2 as needed.
4. Run cargo test to make sure your tests pass.
5. Run cargo build to build a binary and try it yourself
6. GOTO 1.

So how much speedup do you actually get? Like most performance related questions, the answer is "it depends." Here are some very un-scientific benchmarks at the time of writing.