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<!DOCTYPE html>
<html>
<head>
<title>What makes Rust so special?</title>
<meta charset="utf-8">
<link rel="stylesheet" href="style/style.css"></link>
<style>
</style>
</head>
<body>
<textarea id="source">
class: center, middle
# What makes Rust so special?
.title[.center[]]
.left[
Based on: [Rust ISP 2019](https://github.com/newpavlov/rust-isp-2019) slides
Alexandru Radovici, ilustrations by [Mieuneli](http://miau.laura.ro)
]
---
# Items to talk
An introduction to [Rust](https://www.rust-lang.org/) things.
- Ownership and Borrows
- Enums with payload
- No NULL references
- Traits
- Lifetime annotations
---
# Ownership
- A variable binding _takes ownership_ of its data.
- A piece of data can only have one owner at a time.
- When a binding goes out of scope, the bound data is released automatically. (`Drop` trait)
- For heap-allocated data, this means de-allocation.
- Data _must be guaranteed_ to outlive its references.
```rust
fn foo() {
// Creates a String
// Gives ownership of the String object to s
let s = String::from ("This text is owned by s");
// At the end of the scope, s goes out of scope.
// s still owns the String object, so it can be cleaned up.
}
```
---
## Ownership
So here are the basics.
- When you introduce a variable binding, it takes ownership of its data. And a
piece of data can only have one owner at a time.
- When a variable binding goes out of scope, nothing has access to the data
anymore, so it can be released. Which means, if it's on the heap, it can be
de-allocated.
- And data must be guaranteed to outlive its references. Or, all references are
guaranteed to be valid.
---
## Move Semantics
```rust
let s1 = String::from ("A string");
// Ownership of the String object moves to s2.
let s2 = s1;
println!("{}", s1); // error: use of moved value `s1`
```
- `let s2 = s1;`
- We don't want to copy the data, since that's expensive.
- The data cannot have multiple owners.
- Solution: move the String's ownership into `s2`, and declare `s1` invalid.
- `println!("{}", s1);`
- We know that `s1` is no longer a valid variable binding, so this is an error.
- Rust can reason about this at compile time, so it throws a compiler error.
---
## Move Semantics
- Moving ownership copies data. BUT:
- Often moves are optimized out by compiler.
- When we move ownership of a heap allocated data (e.g. `String`), we do
not touch data on heap, just few bytes allocated on stack are copied
(pointer to heap, length and capacity; 24 bytes on 64-bit machine)
- Moves are automatic (via assignments); no need to use something like C++'s
`std::move`.
- However, there are functions like `std::mem::replace` in Rust to provide
advanced ownership management.
- For more finer-grained control see standrard library functions:
`std::mem::replace`, `std::mem::swap` and others.
---
## Ownership
- Ownership does not always have to be moved.
- What would happen if it did? Rust would get very tedious to write:
```rust
fn string_length(s: String) -> String {
// Do whatever here,
// then return ownership of `s` back to the caller
}
```
- You could imagine that this does not scale well either.
- The more variables you had to hand back, the longer your return type would be!
- Imagine having to pass ownership around for 5+ variables at a time :(
```rust
fn string_length(s1: String, s2: String, ...) -> (String, String, ...) {
// Do whatever here,
// then return ownership of `s1`, `s2`, ... back to the caller
}
```
---
## Borrowing
- Instead of transferring ownership, we can _borrow_ data.
- A variable's data can be borrowed by taking a reference to the variable;
ownership doesn't change.
- When a reference goes out of scope, the borrow is over.
- The original variable retains ownership throughout.
```rust
let s = String::from ("string");
// s_ref is a reference to s.
let s_ref = &s;
// use s_ref to access the data in the String s.
assert_eq!(s.chars().nth(1), s_ref.chars().nth(1));
```
Rust does automatic dereferencing.
---
## Borrowing
- Caveat: this adds restrictions to the original variable.
- Ownership cannot be transferred from a variable while references to it exist.
- That would invalidate the reference.
```rust
let s = String::from("for borrow");
// s_ref is a reference to s.
let s_ref = &s;
// Moving ownership to s_new would invalidate s_ref.
// error: cannot move out of `s` because it is borrowed
let s_new = s;
println! ("{}", s_ref);
```
Try it without the `println!`
---
## Borrowing
```rust
/// `length` only needs `String` temporarily, so it is borrowed.
fn length(str_ref: &String) -> usize {
// vec_ref is auto-dereferenced when you call methods on it.
str_ref.len()
// you can also explicitly dereference.
// (*str_ref).len()
}
fn main() {
let s = String::new ();
length(&s);
println!("{:?}", s); // this is fine
}
```
- Note the type of `length`: `str_ref` is passed by reference, so it's now an `&String`.
- References, like bindings, are *immutable* by default.
- The borrow is over after the reference goes out of scope (at the end of `length`).
---
## Borrowing
```rust
/// `push` needs to modify `string` so it is borrowed mutably.
fn push(str_ref: &mut String, x: &str) {
str_ref.push_str (x);
}
fn main() {
let mut s: String = String::from ("");
let string_ref: &mut String = &mut s;
push(string_ref, "str");
assert_eq!(string_ref, "str");
}
```
- Variables can be borrowed by _mutable_ reference: `&mut string_ref`.
- `string_ref` is a reference to a mutable `String`.
- The type is `&mut String`, not `&String`.
- Different from a reference which is variable.
---
## Borrowing
```rust
/// `push` needs to modify `string` so it is borrowed mutably.
fn push2(str_ref: &mut String, x: &str) {
// error: cannot move out of borrowed content.
let string = *str_ref;
string.push_str(x);
}
fn main() {
let mut vector = String::from ("");
push2(&mut string, "a new str");
}
```
- Error! You can't dereference `string_ref` into a variable binding because that
would change the ownership of the data.
---
## Borrowing
- Rust will auto-dereference variables...
- When making method calls on a reference.
- When passing a reference as a function argument.
```rust
/// `length` only needs `s` temporarily, so it is borrowed.
fn length(s_ref: &&String) -> usize {
// s_ref is auto-dereferenced when you call methods on it.
s_ref.len()
}
fn main() {
let s = String::from("");
length(&&&&&&&&&&&&s);
}
```
---
## Borrowing
- You will have to dereference variables...
- When writing into them.
- And other times that usage may be ambiguous.
```rust
let mut a = 5;
let ref_a = &mut a;
*ref_a = 4;
println!("{}", *ref_a + 4);
// ==> 8
```
---
## `Copy` Types
- Rust defines a trait¹ named `Copy` that signifies that a type may be
copied instead whenever it would be moved.
- Most primitive types are `Copy` (`i32`, `f64`, `char`, `bool`, etc.)
- Types that contain references may not be `Copy` (e.g. `Vec`, `String`).
```rust
let x: i32 = 12;
let y = x; // `i32` is `Copy`, so it's not moved :D
println!("x still works: {}, and so does y: {}", x, y);
```
¹ Like a Java/Go interface or Haskell typeclass
---
## Borrowing Rules
##### _The Holy Grail of Rust_
Learn these rules, and they will serve you well.
- You can't keep borrowing something after it stops existing.
- One object may have many immutable references to it (`&T`).
- **OR** _exactly one_ mutable reference (`&mut T`) (not both).
- That's it!
.card[.small[.center[]]]
---
### Borrowing Prevents...
- Iterator invalidation due to mutating a collection you're iterating over.
- This pattern can be written in C, C++, Java, Python, Javascript...
- But may result in, e.g, `ConcurrentModificationException` (at runtime!)
```rust
let mut vs = [1,2,3,4];
for v in &vs {
vs[1] = 3;
println! ("{}", v);
// ERROR: cannot borrow `vs` as mutable because
// it is also borrowed as immutable
}
```
- `pop` needs to borrow `vs` as mutable in order to modify the data.
- But `vs` is being borrowed as immutable by the loop!
---
### Borrowing Prevents...
- Use-after-free
- Valid in C, C++...
```rust
let y: &i32;
{
let x = 5;
y = &x; // error: `x` does not live long enough
}
println!("{}", *y);
```
- The full error message:
```
error: `x` does not live long enough
note: reference must be valid for the block suffix following statement
0 at 1:16
...but borrowed value is only valid for the block suffix
following statement 0 at 4:18
```
- This eliminates a _huge_ number of memory safety bugs _at compile time_.
- As a side note, this technique of creating a block to limit the scope of a
variable (in this case x) is pretty useful.
---
### Borrowing Prevents...
- Data races in multithreaded environments.
- It checks at compile time if it's safe to share or send a given piece of data.
- Compiler ensures that programm uses necessary synchronization using various primitives: mutexes, atomics, channels, etc.
- NB: data races != race condition.
- Check out TRPL section of ["Fearless Concurrency"](https://doc.rust-lang.org/book/ch16-00-concurrency.html)
---
## Methods parameters
- The first argument to a method, named `self`, determines what kind of ownership the method
requires.
- `&self`: the method *borrows* the value.
- Use this unless you need a different ownership model.
- `&mut self`: the method *mutably borrows* the value.
- The function needs to modify the struct it's called on.
- `self`: the method takes ownership.
- The function consumes the value and may return something else.
---
# `match`
- `switch` on steroids.
```rust
let x = 3;
match x {
1 => println!("one fish"), // <- comma required
2 => {
println!("two fish");
println!("two fish");
}, // <- comma optional when using braces
// we can match several patterns in one arm
3 | 4 => println!("three or four, dunno"),
_ => println!("no fish for you"), // "otherwise" case
}
```
- `match` takes an expression (`x`) and branches on a list of `value => expression` statements.
- The entire match evaluates to one expression.
- Like `if`, all arms must evaluate to the same type.
- `_` is commonly used as a catch-all (cf. Haskell, OCaml)
---
## `match` pattern
```rust
let x = 3;
let y = -3;
let q = 10;
let s = match (x, y) {
(1, 1) => "one".to_string(),
(2, j) => format!("two, {}", j),
(_, 3) => "three".to_string(),
(i, j) if i > 5 && j < 0 => "On guard!".to_string(),
// NB: note that we do not test x == 10 here!
(q, k) => format!("???: {} {}", q, k),
};
println!("{}", s);
```
- The matched expression can be any expression (l-value), including tuples and function calls.
- Matches can bind variables. `_` is a throw-away variable name.
- You _must_ write an exhaustive match in order to compile.
- Use `if`-guards to constrain a match to certain conditions.
- Patterns can get very complex.
---
# Enum
- An enum, or "sum type", is a way to express some data that may be one of several things.
- Similar to the union type in C
- Much more powerful than in Java, C, C++, C#...
- Each enum variant can have optional payloads:
- no data (unit variant)
- named data (struct variant)
- unnamed ordered data (tuple variant)
```rust
enum Resultish {
Ok,
Warning { code: i32, message: String },
Err(String)
}
```
.card[.small[.center[]]]
---
class: split-70
# Enum `match`-ing
- Enum variants are namespaced by their enum type: `Resultish::Ok`.
- You can import all variants with `use Resultish::*`.
- Enums, much as you'd expect, can be matched on like any other data type.
.column[
```rust
match make_request() {
Resultish::Ok =>
println!("Success!"),
Resultish::Warning { code, message } =>
println!("Warning: {}!", message),
Resultish::Err(s) =>
println!("Failed with error: {}", s),
}
```
]
.column[
.card[.small_vertical[.center[]]]
]
---
class: split-70
# `Option` .top_image[]
- Rust has no NULL type
- a variable of a type has to store a value of that actual type
- a reference has to exist and point to a valid memory
Use `Option` enum
.column[
```rust
enum Option<T> {
Some(T)
None,
}
```
T is any valid type
]
.column[
.card[.small_vertical[.center[]]]
]
---
## `Option` example
Rust automatically imports `Option::*`
```rust
fn integer_division (a:isize, b: isize) -> Option<isize> {
if b == 0 {
None
} else {
Some(a / b)
}
}
fn main () {
let x = 120;
let y = 7;
match integer_division (x, y) {
Some(d) => println! ("{}:{} = {}", x, y, d),
None => println! ("division by 0")
};
}
```
---
# Traits
- Similar to interfaces in Java
- Methods that types have to implement
.column[
```rust
struct Professor {
firstname: String,
lastname: String,
age: usize,
// add subjects
}
trait Person {
fn get_name (&self) -> String;
fn get_job (&self) -> String;
}
impl Person for Professor {
fn get_name (&self) -> String {
// ...
}
fn get_job (&self) -> String {
// ...
}
}
```
]
.column[
.card[.small_vertical[.center[]]]
]
---
## Generics with Trait Bounds
- Multiple trait bounds are specified like `T: Clone + Ord`.
- There's no way (yet) to specify [negative trait bounds](https://internals.rust-lang.org/t/pre-rfc-mutually-exclusive-traits/2126).
- e.g. you can't stipulate that a `T` must not be `Clone`.
```rust
fn digital_write<P: Configure + Output>(p: P, value: usize) {
p.make_output ();
if value == 1 {
p.set ();
} else {
p.clear ();
}
}
```
---
## Generic Types With Trait Bounds
- You can also define structs with generic types and trait bounds.
- Be sure to declare all of your generic types in the struct header _and_ the
impl block header.
- Only the impl block header needs to specify trait bounds.
- This is useful if you want to have multiple impls for a struct each with
different trait bounds
```rust
struct MyDriver<P: Pin> {
pin1: &'static P,
pin2: &'static P,
// ...
}
```
And without trait bounds
```rust
struct MyDriver<P> {
pin1: &'static P,
pin2: &'static P,
// ...
}
```
---
## Generic Types With Trait Bounds
```rust
impl<P: Pin> Driver for MyDriver<P> {
// ...
}
```
---
## Inheritance (kinda)
- Some traits may require other traits to be implemented first.
- e.g., `Eq` requires that `PartialEq` be implemented, and `Copy` requires `Clone`.
- Implementing the `Child` trait below requires you to also implement `Parent`.
```rust
trait Parent {
fn foo(&self) {
// ...
}
}
trait Child: Parent {
fn bar(&self) {
self.foo();
// ...
}
}
```
---
# Lifetimes Annotations
- Lifetimes generally have a pretty steep learning curve.
- Don't worry if you don't understand these right away.
.title[.center[]]
---
### Question .top_image[]
Q1: How do you use free?
```c
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
char * without_first_word (char *s);
int main ()
{
char * s = strdup ("ana has apples");
char *wfw = without_first_word (s);
// free (s); <-- before printf
printf ("%s\n", wfw);
// free (wfw); <-- after printf
}
```
---
### Question .top_image[]
Q2: How do you use free?
```c
// program: bear-salamander-anteater
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
char * without_first_word (char *s) {
int pos = 0;
for (unsigned int i=0; i < strlen (s); i++) {
if (s[i] != ' ') pos = pos + 1;
else break;
}
return &s[pos];
}
int main ()
{
char * s = strdup ("ana has apples");
char *wfw = without_first_word (s);
// free (s); <-- before printf
printf ("%s\n", wfw);
// free (wfw); <-- after printf
}
```
---
### Question .top_image[]
Q3: How do you use free?
```c
// program: oyster-guanaco-dinosaur
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
char * without_first_word (char *s) {
int pos = 0;
for (unsigned int i=0; i < strlen (s); i++) {
if (s[i] != ' ') pos = pos + 1;
else break;
}
return strdup (&s[pos]);
}
int main ()
{
char * s = strdup ("ana has apples");
char *wfw = without_first_word (s);
// free (s); <-- before printf
printf ("%s\n", wfw);
// free (wfw); <-- before printf
}
```
---
### Lifetime annotations
From a birds eye view, how should Rust free `s` and `wfw`?
```rust
fn without_first_word<'a> (s: &'a str) -> &'a str;
fn main() {
let s = String::from("ana has apples");
let wfw = without_first_word (&s);
// drop (s)
println! ("{}", wfw);
// drop (s)
}
```
---
### This is why lifetimes are useful.
```rust
fn without_first_word<'a> (s: &'a str) -> &'a str {
let mut pos = 0;
for a in s.chars() {
if a != ' ' { pos = pos + 1; }
else { break; }
}
&s[pos..]
}
fn main() {
let s = String::from("ana has apples");
let wfw = without_first_word (&s);
// drop (s)
println! ("{}", wfw);
}
```
---
### Efective Lifetimes
```rust
fn without_first_word<'a> (s: &'a str) -> &'a str {
let mut pos = 0;
for a in s.chars() {
if a != ' ' { pos = pos + 1; }
else { break; }
}
&s[pos..]
}
fn main() {
let s = String::from("ana has apples"); // <--- s lifetime ('ls) starts here
let wfw = without_first_word (&s); // <--- wfw lifetime ('lwfw) starts here
// drop (s) // <--- s lifetime ('ls) ends here, wfw lifetime ('wfw) ends here (due to 'ls)
// t lifetime ('ts) starts here
println! ("{}", wfw); // <--- wfw lifetime ('lwfw) actually ends here
} // <--- t lifetime ('ts) ends here
```
---
### Lifetime annotations
```rust
fn without_first_word<'a> (s: &'a str) -> &'a str;
fn main() {
let s = String::from("ana has apples"); // <--- s lifetime ('ls) starts here
let wfw = without_first_word (&s); // <--- wfw lifetime ('lwfw) starts here
// drop (s) // <--- s lifetime ('ls) ends here, wfw lifetime ('wfw) ends here (due to 'ls)
// t lifetime ('ts) starts here
println! ("{}", wfw); // <--- wfw lifetime ('lwfw) actually ends here
} // <--- t lifetime ('ts) ends here
```
---
### Question .top_image[]
Can we use free for `s1` and `s2` before `printf`?
```c
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
char * smaller (char *s1, char *s2);
int main () {
char *s1 = strdup ("ip");
char *s2 = strdup ("workshop");
char *small = smaller (s1, s2);
// free (s1);
// free (s2);
printf ("%s\n", small);
}
```
---
### Multiple Lifetime Parameters
```c
// program: wolverine-mallard-gull
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
char * smaller (char *s1, char *s2) {
if (strlen(s1) > strlen(s2)) return s2;
else return s1;
}
int main () {
char *s1 = strdup ("ip");
char *s2 = strdup ("workshop");
char *small = smaller (s1, s2);
// free (s1);
// free (s2);
printf ("%s\n", small);
}
```
---
### Multiple Lifetime Parameters
```rust
fn smaller <'a> (s1: &'a str, s2: &'a str) -> &'a str {}
fn main() {
let s1 = String::from("ip");
let s2 = String::from("workshop");
let small = smaller (&s1, &s2);
// drop (s1)
// drop (s2)
println! ("{}", small);
}
```
---
### Question .top_image[]
Why can't we free s2?
```rust
fn append <'a> (s: &'a mut String, n: &'a str) -> &'a str {
s.push_str (n);
s
}
fn main() {
let mut s1 = String::from("ip");
let s2 = String::from(" workshop");
let title = append (&mut s1, &s2);
// let t1 = s1; // equivalent of free (s1)
let t2 = s2; // equivalent of free (s2)
println! ("{}", title);
}
```
---
### Multiple Lifetime Parameters
We now can free s2.
```rust
fn append <'a, 'b> (s: &'a mut String, n: &'b str) -> &'a str {
s.push_str (n);
s
}
fn main() {
let mut s1 = String::from("ip");
let s2 = String::from(" workshop");
let title = append (&mut s1, &s2);
// drop (s1)
drop (s2)
println! ("{}", title);
}
```
---
# Coding, Snacks & Drinks
Solve the homework before the workshops.
https://classroom.github.com/a/VYBsE08w
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