closure modifies `x` and increments it by some value.  Because of this mutation, the compiler, which
started off assigning `x` as an immutable reference type, has to adjust it as a mutable reference.
Likewise in the third example, the closure drops the vector and therefore this requires the variable
`x` to be moved into the closure. Depending on the borrow kind, the closure has to implement the
appropriate trait: `Fn` trait for immutable borrow, `FnMut` for mutable borrow,
and `FnOnce` for move semantics.

Most of the code related to the closure is in the
[`compiler/rustc_hir_typeck/src/upvar.rs`][upvar] file and the data structures are
declared in the file [`compiler/rustc_middle/src/ty/mod.rs`][ty].


Before we go any further, let's discuss how we can examine the flow of control through the rustc
codebase. For closures specifically, set the `RUSTC_LOG` env variable as below and collect the
output in a file:

```console
> RUSTC_LOG=rustc_hir_typeck::upvar rustc +stage1 -Z dump-mir=all \
    <.rs file to compile> 2> <file where the output will be dumped>
```

This uses the stage1 compiler and enables `debug!` logging for the
`rustc_hir_typeck::upvar` module.

The other option is to step through the code using lldb or gdb.

1. `rust-lldb build/host/stage1/bin/rustc test.rs`
2. In lldb:
    1. `b upvar.rs:134`  // Setting the breakpoint on a certain line in the upvar.rs file
    2. `r`  // Run the program until it hits the breakpoint

Let's start with [`upvar.rs`][upvar]. This file has something called
the [`euv::ExprUseVisitor`] which walks the source of the closure and
invokes a callback for each upvar that is borrowed, mutated, or moved.


```rust
fn main() {
    let mut x = vec![21];
    let _cl = || {
        let y = x[0];  // 1.
        x[0] += 1;  // 2.
    };
}
```

In the above example, our visitor will be called twice, for the lines marked 1 and 2, once for a
shared borrow and another one for a mutable borrow. It will also tell us what was borrowed.

The callbacks are defined by implementing the [`Delegate`] trait. The
[`InferBorrowKind`][ibk] type implements `Delegate` and keeps a map that
records for each upvar which mode of capture was required. The modes of capture
can be `ByValue` (moved) or `ByRef` (borrowed). For `ByRef` borrows, the possible
[`BorrowKind`]s are `ImmBorrow`, `UniqueImmBorrow`, `MutBorrow` as defined in the
[`compiler/rustc_middle/src/ty/mod.rs`][middle_ty].


`Delegate` defines a few different methods (the different callbacks):
**consume** for *move* of a variable, **borrow** for a *borrow* of some kind
(shared or mutable), and **mutate** when we see an *assignment* of something.

All of these callbacks have a common argument *cmt* which stands for Category,
Mutability and Type and is defined in
[`compiler/rustc_hir_typeck/src/expr_use_visitor.rs`][cmt]. Borrowing from the code
comments, "`cmt` is a complete categorization of a value indicating where it
originated and how it is located, as well as the mutability of the memory in
which the value is stored". Based on the callback (consume, borrow etc.), we
will call the relevant `adjust_upvar_borrow_kind_for_<something>` and pass the
`cmt` along. Once the borrow type is adjusted, we store it in the table, which
basically says what borrows were made for each closure.

```rust,ignore
self.tables
    .borrow_mut()
    .upvar_capture_map
    .extend(delegate.adjust_upvar_captures);
```