The Real Story Behind Polonius: Rust’s Next Borrow Checker
When Rust first introduced the borrow checker, it changed how developers think about memory forever. No garbage collector. No segfaults. Just pure compile-time guarantees.
But the deeper people went, the more edges they found. Lifetimes that refused to compile. Borrow errors that looked nonsensical. You’d stare at the compiler’s angry red messages, muttering, “But I’m not even using that variable anymore!”
That frustration gave birth to something quietly revolutionary inside the Rust team — a project called Polonius. Its mission? To make Rust’s borrow checker smarter, fairer, and finally correct in every corner case.
This is the real story behind it.
The Problem: The Borrow Checker Isn’t Always Right (Yet)
Let’s start with what the borrow checker does.
It enforces one simple rule:
You can either have one mutable reference, or many immutable ones — but not both. This is what prevents data races and dangling pointers. But in reality, the current borrow checker — often called “the lexical borrow checker” — makes approximations.
Take this classic Rust example:
fn main() {
let mut data = vec![1, 2, 3];
let x = &data[0];
data.push(4);
println!("{}", x);
}
This doesn’t compile. Rust complains: “Cannot borrow data as mutable because it is also borrowed as immutable.”
Makes sense — you can’t mutate data while you have a reference to it.
But what about this?
fn main() {
let mut data = vec![1, 2, 3];
let x = &data[0];
println!("{}", x);
data.push(4);
}
You might think, “Hey, x isn’t used after the print. The borrow is over!” But the compiler still refuses. Why?
Because the current borrow checker doesn’t analyze control flow deeply — it just tracks lifetimes lexically (based on scope boundaries). It assumes the borrow lasts until the end of the scope, even if logically it’s done earlier.
That’s the pain Polonius wants to fix.
The Architecture: How Polonius Changes the Game
Polonius is essentially a new borrow checker engine — not a rewrite of Rust, but a smarter layer that plugs into the compiler.
It’s built around three principles:
- Precise flow-sensitive analysis (understands when borrows actually end).
- Non-lexical lifetimes (already partially implemented in Rust 2018).
- Declarative logic using Datalog — yes, an actual logic programming language.
Here’s a conceptual diagram:
+--------------------------------+
| Rust Compiler (rustc) |
| ├─ Parser |
| ├─ Type Checker |
| ├─ Borrow Checker (old) |
| └─ → Polonius Engine (new) |
+--------------------------------+
│
▼
+------------------+
| Datalog Rules |
| + borrow facts |
| + subset facts |
| + loan invalids |
+------------------+
│
▼
+------------------+
| Results → rustc |
+------------------+
Polonius works by turning the program’s ownership relationships into facts and rules. Then it runs those through a Datalog engine — a declarative logic solver — to compute whether all references are valid.
What the Borrow Checker Sees (Before Polonius)
In the old borrow checker (before non-lexical lifetimes), the compiler sees borrows like this:
{
let x = &data; // borrow starts here
println!("{:?}", x);
data.push(1); // ❌ error: still borrowed
} // borrow ends here
It assumes the borrow lasts from where it begins to the end of the block — lexically.
What Polonius Sees Instead
Polonius doesn’t think in text blocks — it thinks in data flow graphs.
┌─────────────┐
│ borrow x │
└─────┬───────┘
│
▼
┌─────────────┐
│ use x │
└─────┬───────┘
│
▼
┌─────────────┐
│ drop x │ ← borrow ends here
└─────────────┘
So when you’re done using x, Polonius knows it’s safe to mutate data again.
In other words, Polonius brings temporal precision to lifetimes.
Example: The Borrow That Should Have Worked
Let’s look at this piece of code that should compile logically, but doesn’t in the old borrow checker:
fn main() {
let mut v = vec![10, 20, 30];
let x = &v[0];
println!("{}", x);
v.push(40);
}
Current Rust: ❌ Compile error. Polonius-powered Rust: ✅ Compiles.
Why? Because Polonius correctly infers that x is no longer used after the println!.
Here’s a simplified version of what Polonius computes internally:
borrow(x, v) use(x) drop(x) mutate(v) → allowed
That’s the magic: it knows when borrows actually end in time, not just in code structure.
How It Works: Datalog Rules and Facts
At its core, Polonius turns borrow checking into a logic problem.
Example of simplified logic rules (in pseudo-Datalog):
subset(a, b) :- outlives(a, b).
invalidates(borrow, location) :-
borrowed_at(borrow, origin),
killed_at(location, origin).
In English:
- “If a outlives b, then all borrows from a are valid in b.”
- “If a borrow is killed at a location, it’s no longer valid afterward.”
Polonius evaluates these rules like a database query over facts extracted from your code.
That makes it modular, testable, and future-proof.
Why Rust Needed This
The old borrow checker works, but it was written as a giant web of imperative logic inside rustc. Over the years, it became fragile and hard to extend — a black box of ownership rules.
Polonius is the Rust team’s way of:
- Making the logic declarative (in Datalog, not Rust code).
- Making ownership analysis explainable.
- Preparing for more advanced features like generators, async, and GATs (Generic Associated Types).
So while Polonius won’t suddenly make all borrow errors go away, it lays the groundwork for Rust’s next decade.
Architecture Diagram: How Polonius Fits into rustc
Here’s a simplified internal architecture of Rust’s compiler with Polonius integrated:
+--------------------------+
| rustc front-end |
| (parsing, macro expand) |
+-------------+------------+
|
▼
+--------------------------+
| Type + Lifetime Check |
+-------------+------------+
|
▼
+--------------------------+
| Borrow Checker (NLL) |
+-------------+------------+
|
▼
+--------------------------+
| Polonius Facts Emitter |
| (extracts borrows, uses) |
+-------------+------------+
|
▼
+--------------------------+
| Polonius Engine (Datalog)|
| computes validity graph |
+-------------+------------+
|
▼
+--------------------------+
| Diagnostics + Errors |
+--------------------------+
In practice, Polonius runs as an optional stage in the compiler pipeline right after borrow checking, validating or refining the results.
The Real Reason It’s Taking So Long
If Polonius sounds so perfect, why isn’t it the default yet?
Because it’s heavy. Running a Datalog solver for every function in every crate adds serious compile-time cost. In fact, early tests showed that Polonius could slow compilation by 3–5× on large projects.
The Rust team has been working on optimizing it — pruning facts, caching results, and building incremental solvers — but it’s still a major engineering challenge.
As of 2025, Polonius is still experimental, available behind compiler flags like:
RUSTC_BORROWCK=mir Polonius cargo check
But its logic is gradually influencing how the borrow checker evolves.
The Human Side of It
If you hang around Rust forums, you’ll see Polonius mentioned like an old friend that never quite arrived.
People joke, “Polonius will fix this someday,” when they hit a borrow checker wall. But the truth is — the project is alive, just quiet. It’s one of those deep, foundational efforts that moves the language forward invisibly.
The name itself — Polonius — comes from Shakespeare’s Hamlet:
“Neither a borrower nor a lender be.”
That’s not a joke. The name literally represents the heart of Rust’s philosophy — owning, borrowing, and returning safely.
Key Takeaways
- Polonius is Rust’s next-gen borrow checker, using Datalog logic for precise lifetime analysis.
- It eliminates false positives from lexical lifetimes and brings true flow-sensitive borrow checking.
- It’s still experimental due to performance overhead, but it’s shaping future compiler design.
- The goal isn’t to make Rust more permissive — it’s to make it more correct.
Final Thought
Polonius isn’t a quick compiler feature — it’s a rethink of ownership analysis itself. It’s Rust trying to mathematically define what safety means, instead of approximating it.
Someday, when you write code that should compile and finally does, you’ll have Polonius — the ghost in the borrow checker — to thank.