If Part I was learning to hear Rust,
and Part II was understanding harmony,
then Part III is about time.
Because music does not exist without rhythm.
And software does not exist without concurrency.
Time Is the Hardest Problem
Most programming bugs are not logic errors.
They are timing errors.
- A value changes too soon
- A resource is accessed too late
- Two things happen at once that were never meant to
Concurrency is where most languages panic, hand-wave, or give up.
Rust does something different.
Rust composes time.
Concurrency Is an Ensemble Problem
Single-threaded code is a solo performance.
Concurrency is an ensemble.
The question is never:
“Can this code run in parallel?”
The question is:
“Can these performers stay in time without colliding?”
Rust answers this before the concert begins.
Threads: Independent Sections
std::thread::spawn(|| {
play_note();
});
A thread is a separate section of the orchestra.
It runs independently.
It does not share sheet music unless explicitly allowed.
Rust enforces this with ownership:
- Data moved into a thread must be owned
- Shared data must be synchronized
- Nothing is implicitly shared
This is not paranoia.
This is rehearsal discipline.
Send and Sync: Can This Travel? Can This Be Shared?
Two traits quietly govern all concurrency in Rust:
Send— can this move between threads?Sync— can this be safely referenced from multiple threads?
You almost never write them.
You feel them.
When the compiler says:
“This type cannot be sent between threads safely”
What it means is:
“This instrument cannot survive being passed mid-performance.”
Rust refuses unsafe handoffs.
Mutexes: One Performer at a Time
use std::sync::Mutex;
let tempo = Mutex::new(120);
A Mutex is not a lock.
It is a conductor’s baton.
Only one performer may hold it at a time.
let mut t = tempo.lock().unwrap();
*t += 10;
This reads as:
“I am stepping forward, adjusting the tempo, and stepping back.”
Rust forces you to acknowledge the critical section.
No silent contention.
No accidental overlap.
Arc: Shared Sheet Music
use std::sync::Arc;
let score = Arc::new(Score::new());
An Arc is shared ownership — reference-counted sheet music.
Everyone may read it.
No one may destroy it prematurely.
When you see Arc<Mutex<T>>, hear:
“Shared state, carefully synchronized.”
Yes, it’s verbose.
So is a symphony score.
Deadlocks: When the Music Freezes
Rust cannot prevent all deadlocks.
But it makes them harder to write accidentally.
Why?
Because:
- Lock acquisition is explicit
- Scope determines release
- Ownership controls lifetime
Rust reduces deadlocks the way good rhythm reduces train wrecks.
Async Is Not Threads
This is crucial.
Async is not parallelism.
Async is interleaving.
async fn play_phrase() {
await_note().await;
}
Async code is one performer playing multiple melodies by switching rapidly.
It is polyrhythm, not multiple orchestras.
await: A Rest, Not a Block
let note = await_note().await;
This does not mean:
“Stop the world.”
It means:
“Pause this phrase and let others continue.”
await is a rest in the score.
The function yields control politely.
Rust’s async model is cooperative, not preemptive.
Futures: Promises of Sound
An async fn returns a Future.
A future is not a value.
It is a promise that music will happen later.
Nothing plays until the executor conducts.
This is intentional.
Executors: The Conductor
Tokio. async-std. smol.
They are not magic.
They are conductors deciding:
- who plays next
- when control shifts
- how time is sliced
Rust separates:
- what should happen (your async code)
- when it happens (the executor)
This separation is power.
Why Rust Async Feels “Hard”
Because Rust refuses to lie about time.
Many languages pretend async is just syntax sugar.
Rust says:
“Time matters. Control flow matters. You must hear it.”
Once you do, async becomes elegant instead of mysterious.
Data Races Are Rhythm Violations
A data race is two performers playing different tempos on the same beat.
Rust eliminates data races by construction.
If code compiles, the rhythm is sound.
This is not a runtime guarantee.
It is a score guarantee.
Structured Concurrency: Staying Together
Modern Rust patterns emphasize:
- tasks that start together
- tasks that end together
- errors that propagate honestly
No orphaned performers.
No forgotten rhythms.
The ensemble enters together.
The ensemble exits together.
Why This Matters (Deeply)
Games.
Engines.
Servers.
Simulations.
Real-time systems.
All of them live or die by timing.
Rust is one of the only languages that treats time as a first-class musical concern.
What Mastery Feels Like
When this clicks:
- You stop fearing concurrency
- You stop guessing about safety
- You stop debugging heisenbugs at 3am
You don’t fight the music.
You conduct it.
Closing: Rust Is Written in Time
Rust is not about control.
It is about coordination.
Not about speed.
About precision.
Not about cleverness.
About listening.
Rust as Music is complete only when you hear the rhythm.
Next: Part IV: Dynamics and Expression
Performance, optimization, zero-cost abstractions, and playing softly… or fortissimo.
Because sometimes the difference between good code and great code
is not correctness —
it’s how loudly it plays.
Play in time.
Respect the rests.
Trust the conductor.