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Almost-final-draft of primitives post
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---
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layout: post
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title: "Rust's primitives are Weird (and cool)"
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title: "Rust's Primitives are Weird (and Cool)"
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description: "but mostly weird."
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category:
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tags: [rust, c, java, python, x86]
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---
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I wrote a really small Rust program a while back that I was 100% convinced couldn't possibly run:
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I wrote a really small Rust program a while back because I was curious. I was 100% convinced it
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couldn't possibly run:
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```rust
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fn main() {
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@ -14,7 +15,7 @@ fn main() {
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}
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```
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And to my complete befuddlement, it compiled, it ran, and it produced a completely sensible output.
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And to my complete befuddlement, it compiled, ran, and produced a completely sensible output.
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The reason I was so surprised has to do with how Rust treats a special category of things
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I'm going to call *primitives*. In the current version of the Rust book, you'll see them
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referred to as [scalars](rust_scalar), and in older versions they'll be called [primitives](rust_primitive).
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@ -23,24 +24,13 @@ why this program is so cool requires talking about a number of other programming
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and keeping a consistent terminology makes things easier.
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**You've been warned:** this is going to be a tedious post about a relatively minor issue that involves
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a quick jaunt all the way through Java, Python, C, and x86 Assembly, but demonstrates a really cool
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way that Rust thinks differently about the world.
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But because I'm not a monster, here's someone else who's just as excited as you are to learn about
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primitives:
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> [Unreasonably excited doggo][excited_doggo]
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Java, Python, C, and x86 Assembly. And also me pretending like I know what I'm talking about with assembly.
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# Defining primitives (Java)
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My day job is in Java. I'm continually amazed by how much of the world runs on Java,
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and somehow manages to continue functioning. Like, it can't be that good, because nothing
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in Computer Science functions that well. And yet, Java is maybe one of the few things
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CS people can high-five and say "you know what, we did a good thing."
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But that's not what this post is about. In Java, there's a special name for
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some specific types of values:
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The reason I'm using the name *primitive* comes from how much of my life is Java right now.
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Spoiler alert: a lot of it. And for the most part I like Java, but I digress. In Java, there's a
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special name for some specific types of values:
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> ```
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bool char byte
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@ -72,12 +62,14 @@ Main.java:5: error: int cannot be dereferenced
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1 error
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```
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The reason for this error is that only things inheriting from
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[`Object`](https://docs.oracle.com/javase/9/docs/api/java/lang/Object.html)
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can have instance methods, and the primitive types do not in fact inherit this.
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Specifically, Java considers [`Object`](https://docs.oracle.com/javase/10/docs/api/java/lang/Object.html)
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and things that inherit from it as pointers, and thus we have to dereference the pointer
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before the fields and methods it defines can be used. In contrast, *primitive types are just values* -
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there's nothing to be dereferenced. In memory, they're just a sequence of bits.
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If we really want, we can turn the `int` into an
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[`Integer`](https://docs.oracle.com/javase/9/docs/api/java/lang/Integer.html) and then
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turn that into a `String` and print it, but that seems like a lot of work:
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[`Integer`](https://docs.oracle.com/javase/10/docs/api/java/lang/Integer.html) and then
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dereference it, but it's a bit wasteful:
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```java
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class Main {
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@ -89,23 +81,15 @@ class Main {
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}
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```
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This allows us to create the variable `y` of type `Integer`, and at run time peek into `y`
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to locate the `toString()` function and call it.
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So why do we have to jump through the extra hoops for this? The reason is partially that Java
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treats the primitive values as just a "bag of bits"; there are no functions to call, no references
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to maintain, it's just a set number of bits to represent a value. If you call a function using
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`int` or `long` as an argument, internally Java will copy the bits across and your original value
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can't be modified.
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And if Rust has a similar "bag of bits" representation for its primitives (spoiler alert: it does),
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that gives us our first question: how does Rust get away with calling the equivalent of instance methods?
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This creates the variable `y` of type `Integer` (which inherits `Object`), and at run time we
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dereference `y` to locate the `toString()` function and call it. Rust obviously handles things a bit
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differently, but we have to look at some low-level details to see how differently it actually is.
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# Low Level Handling of Primitives (C)
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Now, I still want to show off the "bag of bits" representation of primitives in Rust. But to do that,
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we have to expose a bit of how your computer thinks about those values. Let's consider the following
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code in C:
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We first need to build a foundation for reading and understanding the assembly code the
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final answer involves. Let's begin with showing how the `C` language (and your computer)
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thinks about "primitive" values in memory:
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```c
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void my_function(int num) {}
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@ -116,21 +100,26 @@ int main() {
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}
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```
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And to drive the point home (and pretend like I understand assembly), let's take a look at the result
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using the [compiler explorer](https://godbolt.org/z/lgNYcc): <span style="font-size:.6em">whose output has been lightly edited</span>
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The [compiler explorer](https://godbolt.org/z/lgNYcc) gives us an easy way of showing off
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the assembly-level code that's generated: <span style="font-size:.6em">whose output has been lightly edited</span>
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```
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```nasm
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main:
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push rbp
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mov rbp, rsp
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sub rsp, 16
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; We assign the value `8` to `x` here
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mov DWORD PTR [rbp-4], 8
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; And copy the bits making up `x` to a location
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; `my_function` can access
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; `my_function` can access (`edi`)
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mov eax, DWORD PTR [rbp-4]
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mov edi, eax
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; Call `my_function` and give it control
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call my_function
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mov eax, 0
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leave
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ret
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@ -138,17 +127,18 @@ main:
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my_function:
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push rbp
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mov rbp, rsp
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; Copy the bits out of the pre-determined location
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; Copy the bits out of the pre-determined location (`edi`)
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; to somewhere we can use
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mov DWORD PTR [rbp-4], edi
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nop
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pop rbp
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ret
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```
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At a really low level of memory, we're copying bits around; nothing crazy. That's what the `mov` instruction
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is intended to do (use [this][x86_guide] as a reference). But to show how similar Rust is, let's take a look at the equivalent
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Rust code in the [compiler explorer](https://godbolt.org/z/cAlmk0): <span style="font-size:.6em">again, lightly edited</span>
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At a really low level of memory, we're copying bits around using the [`mov`][x86_guide] instruction; nothing crazy.
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But to show how similar Rust is, let's take a look at our program translated from C to Rust:
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```rust
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fn my_function(x: i32) {}
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@ -159,38 +149,48 @@ fn main() {
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}
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```
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```
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And the assembly generated when we stick it in the [compiler explorer](https://godbolt.org/z/cAlmk0):
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<span style="font-size:.6em">again, lightly edited</span>
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```nasm
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example::main:
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push rax
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; Look familiar? We're copying bits to a location for `my_function`
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; The compiler just optimizes out holding `x` in memory
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mov edi, 8
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; Call `my_function` and give it control
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call example::my_function
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pop rax
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ret
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example::my_function:
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sub rsp, 4
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; And copying those bits again, just like in C
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mov dword ptr [rsp], edi
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add rsp, 4
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ret
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```
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The generated Rust looks almost identical to C, and is the same as how Java thinks of primitives: just bits in memory.
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The generated Rust assembly is functionally pretty close to the C assembly (and Java as well):
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*When working with primitives, we're just dealing with bits in memory*.
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And now that we're a bit more familiar with the low-level representation of primitives, it's time to answer:
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how exactly does Rust manage to compile `8.to_string()`?
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In Java we have to dereference a pointer to call its functions; in Rust, there's no pointer to dereference. So what
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exactly is going on with this `.to_string()` function call?
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# impl primitive (and Python)
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Now it's time to reveal my <strike>trap card</strike> <strike>dirty secret</strike> revelation: *Rust has
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Now it's time to <strike>reveal my trap card</strike> show the revelation that tied all this together: *Rust has
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implementations for its primitive types.* That's right, `impl` blocks aren't only for `structs` and `traits`,
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primitives get them too. Don't believe me? Check out [u32](https://doc.rust-lang.org/std/primitive.u32.html),
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[f64](https://doc.rust-lang.org/std/primitive.f64.html) and [char](https://doc.rust-lang.org/std/primitive.char.html)
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as examples.
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But the really interesting bit is how Rust turns the code we started with into assembly. Let's break out the
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But the really interesting bit is how Rust turns those `impl` blocks into assembly. Let's break out the
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[compiler explorer](https://godbolt.org/z/6LBEwq) once again:
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```rust
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@ -199,31 +199,32 @@ pub fn main() {
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}
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```
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And the interesting bits in the assembly:
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And the interesting bits in the assembly: <span style="font-size:.6em">heavily trimmed down</span>
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```
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```nasm
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example::main:
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sub rsp, 24
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mov rdi, rsp
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lea rax, [rip + .Lbyte_str.u]
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mov rsi, rax
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; Bombshell right here
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call <T as alloc::string::ToString>::to_string@PLT
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mov rdi, rsp
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call core::ptr::drop_in_place
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add rsp, 24
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ret
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```
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Now, this assembly is far more complicated, but here's the big revelation: **we're calling
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`to_string()` as a function that isn't bound to the instance of `8`**. Instead of thinking
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of the value 8 as an instance of `u32` and then peeking in to find the location of the function
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we want to call, we have a function that exists outside of the instance and just give
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that function the value `8`.
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Now, this assembly is a bit more complicated, but here's the big revelation: **we're calling
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`to_string()` as a function that exists all on its own, and giving it the instance of `8`**.
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Instead of thinking of the value 8 as an instance of `u32` and then peeking in to find
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the location of the function we want to call (like Java), we have a function that exists
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outside of the instance and just give that function the value `8`.
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This is an incredibly technical detail, but the interesting idea I had was this:
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*if `to_string()` is a static function, can I refer to the unbound function and give
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it an instance?*
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*if `to_string()` is a static function, can I refer to the unbound function and give it an instance?*
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Better explained in code (and a [compiler explorer](https://godbolt.org/z/fJY-gA) link
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because I seriously love this thing):
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@ -242,7 +243,7 @@ impl MyVal {
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pub fn main() {
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let my_val = MyVal { x: 8 };
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// THESE ARE THE SAME
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// THESE ARE TOTALLY EQUIVALENT
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my_val.to_string();
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MyVal::to_string(&my_val);
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}
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@ -252,7 +253,7 @@ Rust is totally fine "binding" the function call to the instance, and also as a
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MIND == BLOWN.
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Python does something equivalent where I can both call functions bound to their instances
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Python does the same thing where I can both call functions bound to their instances
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and also call as an unbound function where I give it the instance:
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```python
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@ -268,9 +269,9 @@ m.my_function()
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MyClass.my_function(m)
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```
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That said, Python still doesn't treat "primitives" as things that can have instance methods:
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And Python tries to make you *think* that primitives can have instance methods...
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```
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```python
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>>> dir(8)
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['__abs__', '__add__', '__and__', '__class__', '__cmp__', '__coerce__',
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'__delattr__', '__div__', '__divmod__', '__doc__', '__float__', '__floordiv__',
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@ -285,27 +286,31 @@ That said, Python still doesn't treat "primitives" as things that can have insta
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8.__str__()
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^
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SyntaxError: invalid syntax
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>>> # It will run if we assign it first though:
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>>> x = 8
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>>> x.__str__()
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'8'
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```
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...but in practice it's a bit complicated.
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So while Python handles binding instance methods in a way similar to Rust, it's still not able
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to run the example we started with.
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# Conclusion
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This was a super-roundabout way of demonstrating it, but the way Rust handles incredibly minor details
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like primitives is one of the reasons I enjoy the language. It's optimized like C in how it lays out
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memory and is efficient ("bag of bits" representation). And it still has a lot of
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the nice features I like in Python that make it easy to work with the language (late/static binding).
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like primitives leads to really cool effects. Primitives are optimized like C in how they have a
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space-efficient memory layout, yet the language still has a lot of features I enjoy in Python
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(like both instance and late binding).
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And even given that, there are still areas where Rust shines that none of the other languages discussed do;
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as a kinda quirky feature of Rust's type system, `8.to_string()` is actually valid code.
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And when you put it together, there are areas where Rust does cool things nobody else can;
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as a quirky feature of Rust's type system, `8.to_string()` is actually valid code.
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There aren't too many grand lessons to be learned from this, the behavior I'm talking about is
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a relatively minor detail in the grand picture. But it's still something I learned where Rust
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just gets the details right, and I love it.
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Now go forth and fool your friends into thinking you know assembly. This is all I've got.
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[x86_guide]: http://www.cs.virginia.edu/~evans/cs216/guides/x86.html
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[excited_doggo]: https://flic.kr/p/2jr8Zp
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[java_primitive]: https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html
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[compiler_explorer]: https://godbolt.org/
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[rust_scalar]: https://doc.rust-lang.org/book/second-edition/ch03-02-data-types.html#scalar-types
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@ -93,7 +93,7 @@ a {
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position: relative;
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display: inline-block;
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padding: 5px 1px;
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padding: 1px 1px;
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transition: color ease 0.3s;
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/* Hover animation effect for all buttons */
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@ -166,10 +166,7 @@ hr {
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pre { overflow: auto; }
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code, pre {
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}
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small {
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color: gray;
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color: gray;
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}
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