Background topics

This section covers a numbers of common compiler terms that arise in this guide. We try to give the general definition while providing some Rust-specific context.

What is a control-flow graph?

A control-flow graph is a common term from compilers. If you've ever used a flow-chart, then the concept of a control-flow graph will be pretty familiar to you. It's a representation of your program that exposes the underlying control flow in a very clear way.

A control-flow graph is structured as a set of basic blocks connected by edges. The key idea of a basic block is that it is a set of statements that execute "together" – that is, whenever you branch to a basic block, you start at the first statement and then execute all the remainder. Only at the end of the block is there the possibility of branching to more than one place (in MIR, we call that final statement the terminator):

bb0: {
    statement0;
    statement1;
    statement2;
    ...
    terminator;
}

Many expressions that you are used to in Rust compile down to multiple basic blocks. For example, consider an if statement:

a = 1;
if some_variable {
    b = 1;
} else {
    c = 1;
}
d = 1;

This would compile into four basic blocks:

BB0: {
    a = 1;
    if some_variable { goto BB1 } else { goto BB2 }
}

BB1: {
    b = 1;
    goto BB3;
}

BB2: {
    c = 1;
    goto BB3;
}

BB3: {
    d = 1;
    ...;
}

When using a control-flow graph, a loop simply appears as a cycle in the graph, and the break keyword translates into a path out of that cycle.

What is a dataflow analysis?

Static Program Analysis by Anders Møller and Michael I. Schwartzbach is an incredible resource!

Dataflow analysis is a type of static analysis that is common in many compilers. It describes a general technique, rather than a particular analysis.

The basic idea is that we can walk over a CFG and keep track of what some value could be. At the end of the walk, we might have shown that some claim is true or not necessarily true (e.g. "this variable must be initialized"). rustc tends to do dataflow analyses over the MIR, since that is already a CFG.

For example, suppose we want to check that x is initialized before it is used in this snippet:

fn foo() {
    let mut x;

    if some_cond {
        x = 1;
    }

    dbg!(x);
}

A CFG for this code might look like this:

 +------+
 | Init | (A)
 +------+
    |   \
    |   if some_cond
  else    \ +-------+
    |      \| x = 1 | (B)
    |       +-------+
    |      /
 +---------+
 | dbg!(x) | (C)
 +---------+

We can do the dataflow analysis as follows: we will start off with a flag init which indicates if we know x is initialized. As we walk the CFG, we will update the flag. At the end, we can check its value.

So first, in block (A), the variable x is declared but not initialized, so init = false. In block (B), we initialize the value, so we know that x is initialized. So at the end of (B), init = true.

Block (C) is where things get interesting. Notice that there are two incoming edges, one from (A) and one from (B), corresponding to whether some_cond is true or not. But we cannot know that! It could be the case the some_cond is always true, so that x is actually always initialized. It could also be the case that some_cond depends on something random (e.g. the time), so x may not be initialized. In general, we cannot know statically (due to Rice's Theorem). So what should the value of init be in block (C)?

Generally, in dataflow analyses, if a block has multiple parents (like (C) in our example), its dataflow value will be some function of all its parents (and of course, what happens in (C)). Which function we use depends on the analysis we are doing.

In this case, we want to be able to prove definitively that x must be initialized before use. This forces us to be conservative and assume that some_cond might be false sometimes. So our "merging function" is "and". That is, init = true in (C) if init = true in (A) and in (B) (or if x is initialized in (C)). But this is not the case; in particular, init = false in (A), and x is not initialized in (C). Thus, init = false in (C); we can report an error that "x may not be initialized before use".

There is definitely a lot more that can be said about dataflow analyses. There is an extensive body of research literature on the topic, including a lot of theory. We only discussed a forwards analysis, but backwards dataflow analysis is also useful. For example, rather than starting from block (A) and moving forwards, we might have started with the usage of x and moved backwards to try to find its initialization.

What is "universally quantified"? What about "existentially quantified"?

In math, a predicate may be universally quantified or existentially quantified:

  • Universal quantification:
    • the predicate holds if it is true for all possible inputs.
    • Traditional notation: ∀x: P(x). Read as "for all x, P(x) holds".
  • Existential quantification:
    • the predicate holds if there is any input where it is true, i.e., there only has to be a single input.
    • Traditional notation: ∃x: P(x). Read as "there exists x such that P(x) holds".

In Rust, they come up in type checking and trait solving. For example,

fn foo<T>()

This function claims that the function is well-typed for all types T: ∀ T: well_typed(foo).

Another example:

fn foo<'a>(_: &'a usize)

This function claims that for any lifetime 'a (determined by the caller), it is well-typed: ∀ 'a: well_typed(foo).

Another example:

fn foo<F>()
where for<'a> F: Fn(&'a u8)

This function claims that it is well-typed for all types F such that for all lifetimes 'a, F: Fn(&'a u8): ∀ F: ∀ 'a: (F: Fn(&'a u8)) => well_typed(foo).

One more example:

fn foo(_: dyn Debug)

This function claims that there exists some type T that implements Debug such that the function is well-typed: ∃ T: (T: Debug) and well_typed(foo).

What is a DeBruijn Index?

DeBruijn indices are a way of representing which variables are bound in which binders using only integers. They were originally invented for use in lambda calculus evaluation. In rustc, we use a similar idea for the representation of generic types.

Here is a basic example of how DeBruijn indices might be used for closures (we don't actually do this in rustc though):

|x| {
    f(x) // de Bruijn index of `x` is 1 because `x` is bound 1 level up

    |y| {
        g(x, y) // index of `x` is 2 because it is bound 2 levels up
                // index of `y` is 1 because it is bound 1 level up
    }
}

What is co- and contra-variance?

Check out the subtyping chapter from the Rust Nomicon.

See the variance chapter of this guide for more info on how the type checker handles variance.

What is a "free region" or a "free variable"? What about "bound region"?

Let's describe the concepts of free vs bound in terms of program variables, since that's the thing we're most familiar with.

  • Consider this expression, which creates a closure: |a, b| a + b. Here, the a and b in a + b refer to the arguments that the closure will be given when it is called. We say that the a and b there are bound to the closure, and that the closure signature |a, b| is a binder for the names a and b (because any references to a or b within refer to the variables that it introduces).
  • Consider this expression: a + b. In this expression, a and b refer to local variables that are defined outside of the expression. We say that those variables appear free in the expression (i.e., they are free, not bound (tied up)).

So there you have it: a variable "appears free" in some expression/statement/whatever if it refers to something defined outside of that expressions/statement/whatever. Equivalently, we can then refer to the "free variables" of an expression – which is just the set of variables that "appear free".

So what does this have to do with regions? Well, we can apply the analogous concept to type and regions. For example, in the type &'a u32, 'a appears free. But in the type for<'a> fn(&'a u32), it does not.

Further Reading About Compilers

Thanks to mem, scottmcm, and Levi on the official Discord for the recommendations, and to tinaun for posting a link to a twitter thread from Graydon Hoare which had some more recommendations!

Other sources: https://gcc.gnu.org/wiki/ListOfCompilerBooks

If you have other suggestions, please feel free to open an issue or PR.

Books

Courses

Wikis

Misc Papers and Blog Posts