Apr 07 2025

A while back, someone on the internet asked about this syntax in Rust:

*pointer_of_some_kind = blah;

They wanted to know how the compiler understands this code, especially if the pointer wasn’t a reference, but a smart pointer. I wrote them a lengthy reply, but wanted to expand and adapt it into a blog post in case a broader audience may be interested.

Now, I don’t work on the Rust compiler, and haven’t really ever, but what I do know is language semantics. If you’re a language nerd, this post may not be super interesting to you, other than to learn about Rust’s value categories, but if you haven’t spent a lot of time with the finer details of programming languages, I’m hoping that this may be a neat peek into that world.

Programming languages are themselves languages, in the same sense that human languages are. Well, mostly, anyway. The point is, to understand some Rust code like this:

*pointer_of_some_kind = blah;

We can apply similar tools to how we might understand some “English code” like this:

You can't judge a book by its cover.

Oh, you also might ask yourself as you’re reading the next sections, “why so many steps?” The short answer is a classic one: by breaking down the big problem of “what does this mean?” into smaller steps, each step is easier. Doing everything at once is way more difficult than a larger number of smaller steps. I’m going to cover the classical ways compilers work, there’s a ton of variety when you start getting into more modern ways of doing things, and often these steps are blended together, or done out of order, all kinds of other things. Handling errors is a huge topic in and of itself! Consider this a starting point, not an ending one.

Let’s get into it.

Lexical Analysis (aka ‘scanning’ or ‘tokenizing’)

The first thing we want to do is to try and figure out if these words are even valid words at all. With computer languages, this process is called “lexical analysis,” though you’ll also hear the term “tokenizing” to describe it. In other words, we’re not interested in any sort of meaning at all at this stage, we just want to figure out what we’re even talking about.

So if we look at this English sentence:

You can't judge a book by its cover.

We follow a two step process in order to tokenize it: we first “scan” it to produce a sequence of “lexemes.” We do this by following some rules. I’m not going to give you a sample of the rules for English here, as this post is already long enough. But you might end up with something like this:

You
can't
judge
a
book
by
its
cover
.

Note how we do have ' in can't, but . is separate from cover. These are the kinds of rules we’d be following: the ' is because this is a contraction, but the . is not really part of cover, but its own thing.

We then run a second step and evaluate each individual string of characters, turning them into “tokens.” A token is some sort of data type in your compiler, probably, so for example we could do this in Rust:

enum Token {
    Word(String),
    Punctuation(String),
}

And so the output of our tokenizer might be an array of something like

[
    Word("You"),
    Word("can't"),
    Word("judge"),
    Word("a"),
    Word("book"),
    Word("by"),
    Word("its"),
    Word("cover"),
    Punctuation("."),
]

At this point, we know we have something semi-coherent, but we’re still not sure it’s valid yet. On to the next step!

Syntactic Analysis (aka ‘parsing’)

Funny enough, this is an area where human-language linguistics and compilers mean things that are slightly different. With human language, parsing is often combined with our next step, which is semantic analysis. But we (most of the time) try to separate syntax and semantic analysis in compilers.

Again, I’m going to massively simplify with the English. We’re going to use these rules for what a sentence is:

  1. A sentence is a sequence of words.
  2. A sentence has a “subject,” which is the first word, and must be capitalized.
  3. A sentence ends with a period.

Obviously this is a tiny subset of English, but you get the idea. The goal of syntactic analysis is to turn our sequence of tokens into a richer data structure that’s easier to work with. In other words, we’ve figured out that our sentence is made up of valid sequences of characters, but do they fit the grammatical rules of our language? Note that we also don’t need to store everything; for example, maybe our English data structure looks like this:

struct Sentence {
    subject: String,
    words: Vec<String>,
}

Where’s the period? How do we know subject must be capitalized? That’s the job of syntactic analysis. Since every sentence ends with a period, we don’t need to track it in our data structure: the analysis makes sure that it’s true, and then we aren’t doing it again. Likewise, we don’t need to store our subject as a capitalized string if we don’t want to: we determined that the input was, but we can transform it as needed. So our value after syntax analysis might look like this:

Sentence {
    subject: "you",
    words: ["can't", "judge", "a", "book", "by", "its", "cover"],
}

Often, for computer languages, a tree-like structure works well, and so you’d see an “abstract syntax tree,” or “AST” at the end of this stage. But it’s not strictly speaking required, whatever data structure makes sense for you can work.

Now that we have a richer data structure, we’re almost done. Now we have to get into meaning.

Semantic Analysis (aka ‘wtf does this mean’)

Imagine our sentence wasn’t “You can’t judge a book by its cover.” but instead this:

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor
incididunt ut labore et dolore magna aliqua.

This is a famous bit of text that’s incoherent. The words are all Latin words, and it feels like it might be a sentence, but it’s nonsense. We could parse this into a Sentence:

Sentence {
    subject: "Lorem",
    words: ["ipsum", "dolor", "sit", "amet", /* and continued */ ],
}

But it’s not valid. How do we determine that?

Well, in the context of English, “Lorem” isn’t a valid English word. So if we were to check that the subject is a valid word, we’d successfully reject this sentence. In computer languages, we’d do similar things like type checking: 5 + "hello" might lex and parse just fine, but when we try and figure out what it means, we learn it’s nonsense. Except if your language lets you add numbers and strings!

After semantic analysis, we’ve determined that our program is good, aka “well formed.” In a compiler, we’d then go on to generate machine code or byte code to represent our program. But that stuff, while incredibly interesting, isn’t what we’re talking about here: remember our original objective? It was to understand this:

*pointer_of_some_kind = blah;

That’s semantics. So now that we have some background, let’s talk about how to understand this code.

So… how do I understand this code?

Well, to understand the code, we first need to understand how it lexes and parses. In other words, what the grammar of our language is. How our language would lex, tokenize, and then parse our code. In this case, it’s Rust. Rust’s grammar is large and complex, so we’ll only be talking about part of it today. We’re going to focus on statements vs expressions.

You may have heard that Rust is an “expression based language” before. Well, this is what people mean. You see, when it comes to most of the things you say in a program, they’re often one of these two things. Expressions are things that produce some sort of value, and statements are used to sequence the evaluation of expressions. That’s a bit abstract, so let’s get concrete.

Statements

Rust has a few kinds of statements: first, there’s “declaration statements” and “expression statements,” and each have their own sub-kinds as well.

Declaration statements have two kinds: item declarations, and let statements. Item declarations are things like mod or struct or fn: they declare certain things exist. let statements are probably the most famous form of statement in Rust, they look like this:

 OuterAttribute* let PatternNoTopAlt ( : Type )? (= Expression † ( else BlockExpression) ? ) ? ;

That’s… a mouthful. We haven’t talked about * or ? yet, and we don’t really want to cover some of the more exotic parts of Rust right now. So we’re going to talk about this via a simpler grammar first:

let Variable = Expression;

This is how we create new variables in Rust: we say let, and then a name, an =, and then finally some expression. The result of evaluating that expression becomes the value of the variable.

This is leaving out a lot: the name isn’t just a name, it’s a pattern, which is very cool. let else exists in Rust now, and that’s cool. We’re ignoring types here. But you can get the basics with just this simple version.

Expression statements are much simpler:

ExpressionWithoutBlock ; | ExpressionWithBlock ;?

The | there is an or, so we can either have a single expression followed by a ;, or a block (denoted by {}s, which can optionally (the ? means it can exist or not) be followed by a ;.)

So to think like a compiler, you can start to figure out how to combine these rules. For example:

let x = {
    5 + 6
};

Here, we have a let statement, but the expression on the right hand side of the = is an ExpressionWithBlock. Here’s a pop quiz for you: is the ? part of the let expression, or part of the expression on the right hand side?

The answer is, it’s part of the let. The let expression has a mandatory ;, but the block does not, and so:

let x = ExpressionWithBlock;

If we had the semicolon with the block, we’d still need the one for the let, and so we’d have };;. Which the compiler accepts, but it warns about it.

Going back to our original code:

*pointer_of_some_kind = blah;

We don’t have a let, and this isn’t an item declaration: this is an expression statement. We have a ExpressionWithoutBlock, followed by a ;. So now we have to talk about expressions.

Expressions

There are a lot of expression types in Rust. Section 8.2 of the Reference has 19 sub-sections. Whew! In this case, this code is an Operation Expression, and more specifically, an Assignment Expression:

Expression = Expression

Easy enough! So the left hand side of the = is an expression with *pointer_of_some_kind, and the right hand side is blah. Easy enough!

But these two expressions are in some ways, the entire reason that I wrote this post. We just finally got here! You see, the reference has this to say about assignment expressions:

An assignment expression moves a value into a specified place.

What are those?

Places and Values

C, and older versions of C++, called these two things “lvalue” and “rvalue,” for “left” and “right”, like the sides of the =. More recent C++ standards have more categories. Rust splits the difference: it only has two categories, like C, but they map more cleanly to two of C++‘s categories. Rust calls lvalues, the left hand side, a “place,” and rvalues, the right hand side, a “value.” Here’s two more precise definitions, from the Unsafe Code Guidelines:

Both of these have expression forms, so a place expression produces a place when it’s evaluated, and a value expression produces a value. And that’s how = works: we have a place expression on the left, a value expression on the right, and we put that value in that place. Easy enough!

Once again, the code we were trying to figure out:

*pointer_of_some_kind = blah;

So *, the dereference operator, takes the pointer, and evaluates to the place where it points: its address. And blah gives us the value to put there.

Case closed? Not quite!

Deref

Rust has a trait, Deref, that lets us override the * operator. So let’s talk about this example, to make things easier:

use std::ops::{Deref, DerefMut};

struct DerefMutExample<T> {
    value: T
}

impl<T> Deref for DerefMutExample<T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        &self.value
    }
}

impl<T> DerefMut for DerefMutExample<T> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.value
    }
}

fn main () {
    let mut x = DerefMutExample { value: 'a' };
    *x = 'b';
    assert_eq!('b', x.value);
}

You can play with it here.

We didn’t talk about the kind of expression that’s relevant here yet: a path expression.

Path expressions that resolve to local or static variables are place expressions, other paths are value expressions.

We talked about let earlier: in let mut x = DerefMutExample { value: 'a' }; above, the x is a path expression, and since it’s resolving to our new variable, that means it’s a place expression. The DerefMutExample { value: 'a' } is a value expression, because it doesn’t resolve to a variable.

Let’s talk about *x = 'b';, remember our assignment expression:

expression = expression;

And what it does: it moves a value into a place.

To understand how * works, we only need to add one more thing: the Dereference expression. It’s produced by the deference operator, *. It looks like this:

*expression

Its semantics are pretty straightforward:

That’s it. Now we have enough to fully understand *x = 'b':

Whew! That’s that.

Conclusion

Thinking like a compiler can be fun! Once you’ve mastered the idea of grammars and gotten used to substituting things, you can figure out all sorts of interesting stuff. In this specific case, sometimes people wonder why Deref returns a reference if the whole goal is to show where something should point… and if you didn’t know that dereference expressions expanded to something with * in it, it would be confusing! But now you know. And hopefully learned some interesting things about values, places, and how compilers think about code along the way.

Here’s my post about this post on BlueSky:

Steve Klabnik @steveklabnik.com

Thinking like a compiler: places and values in #rustlang steveklabnik.com/writing/thin...

Thinking like a compiler: places and values in Rust

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