We can use `rustc [filename].rs -Z mir-opt-level=0 --emit mir` to view unoptimized MIR.
This requires the nightly toolchain.


**Variable declarations.** If we drill in a bit, we'll see it begins
with a bunch of variable declarations. They look like this:

```mir
let mut _0: ();                      // return place
let mut _1: std::vec::Vec<i32>;      // in scope 0 at src/main.rs:2:9: 2:16
let mut _2: ();
let mut _3: &mut std::vec::Vec<i32>;
let mut _4: ();
let mut _5: &mut std::vec::Vec<i32>;
```

You can see that variables in MIR don't have names, they have indices,
like `_0` or `_1`.  We also intermingle the user's variables (e.g.,
`_1`) with temporary values (e.g., `_2` or `_3`). You can tell apart
user-defined variables because they have debuginfo associated to them (see below).

**User variable debuginfo.** Below the variable declarations, we find the only
hint that `_1` represents a user variable:
```mir
scope 1 {
    debug vec => _1;                 // in scope 1 at src/main.rs:2:9: 2:16
}
```
Each `debug <Name> => <Place>;` annotation describes a named user variable,
and where (i.e. the place) a debugger can find the data of that variable.
Here the mapping is trivial, but optimizations may complicate the place,
or lead to multiple user variables sharing the same place.
Additionally, closure captures are described using the same system, and so
they're complicated even without optimizations, e.g.: `debug x => (*((*_1).0: &T));`.

The "scope" blocks (e.g., `scope 1 { .. }`) describe the lexical structure of
the source program (which names were in scope when), so any part of the program
annotated with `// in scope 0` would be missing `vec`, if you were stepping
through the code in a debugger, for example.

**Basic blocks.** Reading further, we see our first **basic block** (naturally
it may look slightly different when you view it, and I am ignoring some of the
comments):

```mir
bb0: {
    StorageLive(_1);
    _1 = const <std::vec::Vec<T>>::new() -> bb2;
}
```

A basic block is defined by a series of **statements** and a final
**terminator**.  In this case, there is one statement:

```mir
StorageLive(_1);
```

This statement indicates that the variable `_1` is "live", meaning
that it may be used later – this will persist until we encounter a
`StorageDead(_1)` statement, which indicates that the variable `_1` is
done being used. These "storage statements" are used by LLVM to
allocate stack space.

The **terminator** of the block `bb0` is the call to `Vec::new`:

```mir
_1 = const <std::vec::Vec<T>>::new() -> bb2;
```

Terminators are different from statements because they can have more
than one successor – that is, control may flow to different
places. Function calls like the call to `Vec::new` are always
terminators because of the possibility of unwinding, although in the
case of `Vec::new` we are able to see that indeed unwinding is not
possible, and hence we list only one successor block, `bb2`.

If we look ahead to `bb2`, we will see it looks like this:

```mir
bb2: {
    StorageLive(_3);
    _3 = &mut _1;
    _2 = const <std::vec::Vec<T>>::push(move _3, const 1i32) -> [return: bb3, unwind: bb4];
}
```

Here there are two statements: another `StorageLive`, introducing the `_3`
temporary, and then an assignment:

```mir
_3 = &mut _1;
```

Assignments in general have the form:

```text
<Place> = <Rvalue>
```

A place is an expression like `_3`, `_3.f` or `*_3` – it denotes a
location in memory.  An **Rvalue** is an expression that creates a
value: in this case, the rvalue is a mutable borrow expression, which
looks like `&mut <Place>`. So we can kind of define a grammar for
rvalues like so:

```text
<Rvalue>  = & (mut)? <Place>
          | <Operand> + <Operand>
          | <Operand> - <Operand>
          | ...

<Operand> = Constant
          | copy Place
          | move Place
```

As you can see from this grammar, rvalues cannot be nested – they can
only reference places and constants. Moreover, when you use a place,
we indicate whether we are **copying it** (which requires that the