A brief introduction to the big ideas behind the Rust programming language.

1. Nicholas Matsakis!
Mozilla Research
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2. So you want to start a project
Bugs!
C++?
Control!
Dangling pointers Double frees
Segmentation faults Data races
…
2

3. What about GC?
No control.
Requires a runtime.
Insufficient to prevent related problems:
resource management, data races.
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4. Rust’s Solution
Type system enforces ownership and
borrowing:
!
1. All memory has a clear owner.
2. Others can borrow from the owner.
3. Owner cannot free or mutate the
memory while it is borrowed.
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5. Ownership/Borrowing
Memory
safety
Data-race
freedom
No need for
a runtime
GCC++
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6. Credit where it is due
Rust has an active, amazing community.
❤
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7. Ownership!
!
n. The act, state, or right of possessing something.
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8. Ownership
8

9. vec
data
length
capacity
vec
data
length
capacity
1
2
fn give() {
let mut vec = Vec::new();
vec.push(1);
vec.push(2);
take(vec);
…
}
fn take(vec: Vec<i32>) {
// …
}
!
!
!
Take ownership
of a Vec<i32>
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10. fn give() {
let mut vec = Vec::new();
vec.push(1);
vec.push(2);
take(vec);
…
}
vec.push(2);
Compiler enforces moves
fn take(vec: Vec<i32>) {
// …
}
!
!
!Error: vec has been moved
Prevents:
- use after free
- double moves
- …
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11. Borrow!
!
v. To receive something with the promise of returning it.
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12. * Actually: mutation only in controlled circumstances
*
Shared borrow (&T)
Sharing Mutation

13. Mutable borrow (&mut T)
13
Sharing Mutation

14. fn example() {
let mut names = Vec::new();
names.push(..);
names.push(..);
let name = &names[1];
names.push(..);
print(name);
}
names
data
length
capacity
“brson”
“pcwalton”
name
“brson”
“pcwalton”
“acrichto”
Sharing: more than
one pointer to same
memory.
Dangling pointer: pointer
to freed memory.
Mutating the vector
freed old contents.
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15. fn example() {
let mut names = Vec::new();
names.push(..);
names.push(..);
let name = &names[1];
names.push(..);
print(name);
}
Borrow “locks”
`names` until `name`
goes out of scopeError: cannot mutate
`names` while borrowed
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16. fn example() {
let mut names = Vec::new();
names.push(..);
names.push(..);
while something-or-other {
let name = &names[1];
…
names.push(..);
print(name);
}
names.push(..);
}
Outside of borrow scope — OK.
Scope of borrow
in this case covers
only the loop body.
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17. Parallel!
!
adj. occurring or existing at the same time
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18. Data race
Two unsynchronized threads
accessing same data!
where at least one writes.
✎
✎
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19. Sharing
Mutation
No ordering
Data race
Sound familiar?
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20. Double buffering
A B
A B
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21. fn process(a: &mut Vec<i32>, b: &mut Vec<i32>) {
while !done {
computation(a, b);
computation(b, a);
}
}
!
fn computation(input: &Vec<i32>,
output: &mut Vec<i32>) {
…
}
Double buffering
Temporarily
immutable
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22. Double buffering in parallel
A B
A B
B
A
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23. fn par_computation(input: &Vec<i32>,
output: &mut Vec<i32>) {
let len = output.len();
let (l, r) = output.split_at_mut(len/2);
let left_thread = scoped(|| computation(input, l));
computation(input, r);
left_thread.join();
}
Create two disjoint views
Start a “scoped” thread
to process `l`Process `r` in the
main thread
Block until `left_thread`
completes (implicit)
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24. What about data races?
fn par_computation(input: &Vec<i32>,
output: &mut Vec<i32>) {
let len = output.len();
let (l, r) = output.split_at_mut(len/2);
let left_thread = scoped(|| computation(input, l));
computation(input, l);
left_thread.join();
}
Process `l` in
both threads?
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25. fn par_computation(input: &Vec<i32>,
output: &mut Vec<i32>) {
let len = output.len();
let (l, r) = output.split_at_mut(len/2);
let left_thread = scoped(|| computation(input, l));
computation(input, l);
left_thread.join();
}
Shared borrow of
`input` — OK
Shared borrow of `input`
Mutable borrow of `l`
Error: `l` is mutably
borrowed, cannot access
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26. And beyond…
Parallelism is an area of active development.
!
Either already have or have plans for:
- Atomic primitives
- Non-blocking queues
- Concurrent hashtables
- Lightweight thread pools
- Futures
- CILK-style fork-join concurrency
- etc.
All “just libraries”. And all ensure data-race freedom.
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27. Unsafe!
!
adj. not safe; hazardous
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28. Safe abstractions
unsafe {
…
}
• Useful for:
• Uninitialized memory
• Interfacing with C code
• Building parallel abstractions
• Ownership/borrowing permit creating
safe abstraction boundaries.
Trust me.
fn something_safe(…) {
!
!
!
!
}
Validates input, etc.
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29. Conclusions
• Rust combines high-level features with low-level
control.
• Rust gives stronger safety guarantees than GC:
• No null pointers
• Data race freedom
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