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# MIR borrow check
The borrow check is Rust's "secret sauce" – it is tasked with
enforcing a number of properties:
- That all variables are initialized before they are used.
- That you can't move the same value twice.
- That you can't move a value while it is borrowed.
- That you can't access a place while it is mutably borrowed (except through
the reference).
- That you can't mutate a place while it is immutably borrowed.
- etc
The borrow checker operates on the MIR. An older implementation operated on the
HIR. Doing borrow checking on MIR has several advantages:
- The MIR is *far* less complex than the HIR; the radical desugaring
helps prevent bugs in the borrow checker. (If you're curious, you
can see
[a list of bugs that the MIR-based borrow checker fixes here][47366].)
- Even more importantly, using the MIR enables ["non-lexical lifetimes"][nll],
which are regions derived from the control-flow graph.
[47366]: https://github.com/rust-lang/rust/issues/47366
[nll]: https://rust-lang.github.io/rfcs/2094-nll.html
### Major phases of the borrow checker
The borrow checker source is found in
[the `rustc_borrow_ck` crate][b_c]. The main entry point is
the [`mir_borrowck`] query.
[b_c]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/index.html
[`mir_borrowck`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/fn.mir_borrowck.html
- We first create a **local copy** of the MIR. In the coming steps,
we will modify this copy in place to modify the types and things to
include references to the new regions that we are computing.
- We then invoke [`replace_regions_in_mir`] to modify our local MIR.
Among other things, this function will replace all of the [regions](./appendix/glossary.md#region)
in the MIR with fresh [inference variables](./appendix/glossary.md#inf-var).
- Next, we perform a number of
[dataflow analyses](./appendix/background.md#dataflow) that
compute what data is moved and when.
- We then do a [second type check](borrow_check/type_check.md) across the MIR:
the purpose of this type check is to determine all of the constraints between
different regions.
- Next, we do [region inference](borrow_check/region_inference.md), which computes
the values of each region — basically, the points in the control-flow graph where
each lifetime must be valid according to the constraints we collected.
- At this point, we can compute the "borrows in scope" at each point.
- Finally, we do a second walk over the MIR, looking at the actions it
does and reporting errors. For example, if we see a statement like
`*a + 1`, then we would check that the variable `a` is initialized
and that it is not mutably borrowed, as either of those would
require an error to be reported. Doing this check requires the results of all
the previous analyses.
[`replace_regions_in_mir`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/nll/fn.replace_regions_in_mir.html
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