Uploaded August 2026 | Updated September 2026, 1 week ago
Watch the complete Topcoat series:
youtube.com/watch?v=iP9iL4dkiB0&list=PLXP4ot4ftIyY&index=1
GitHub repo with all the Polonius Alpha examples:
github.com/FrancescoXX/polonius-borrow-checker
Rust’s borrow checker just got more precise.
Polonius Alpha is now enabled by default on Rust Nightly, and in this video we put it to the test with actual Rust code.
The interesting part? We take the SAME safe Rust code and run it with two different borrow checkers.
With the current stable borrow checker: ❌ it doesn’t compile.
With Polonius Alpha: ✅ it compiles.
And no, Rust is not becoming less safe.
Polonius is getting better at understanding when a mutable reference is actually still being used, instead of conservatively rejecting code that is perfectly safe.
We start with a tiny example where the difference is immediately visible, then move to a much more realistic HashMap example involving mutable references, `match`, `Some` and `None`.
We’ll also turn Polonius OFF while still using Rust Nightly to prove that the new borrow checker is really what changes the result.
Along the way, we talk about NLL (Non-Lexical Lifetimes), flow-sensitive borrow checking, why the current Rust borrow checker rejects these patterns, and why Polonius can understand them.
And Polonius Alpha is still Alpha.
There are cases it still cannot compile, and doing more precise analysis also comes with potential compile-time performance regressions. We take a look at the Rust team’s benchmarks across some of the most downloaded crates on crates.io.
If you’ve ever fought with:
“cannot borrow as mutable more than once at a time”
this is a very interesting look at where Rust is heading next.
Official Rust announcement:
blog.rust-lang.org/2026/08/04/enabling-polonius-alpha-on-nightly
Chapters:
0:00 Rust Has a New Borrow Checker! Polonius Alpha
1:57 The Official Polonius Alpha Announcement
3:35 Shocking Example: Code That Didn’t Compile Before Now Compiles
5:52 A Bigger Example: HashMaps and Mutable References
7:35 Polonius Alpha Limitations and Performance
9:37 Conclusions
#rust #rustlang #programming
Watch the complete Topcoat series:
youtube.com/watch?v=iP9iL4dkiB0&list=PLXP4ot4ftIyY&index=1
GitHub repo with all the Polonius Alpha examples:
github.com/FrancescoXX/polonius-borrow-checker
Rust’s borrow checker just got more precise.
Polonius Alpha is now enabled by default on Rust Nightly, and in this video we put it to the test with actual Rust code.
The interesting part? We take the SAME safe Rust code and run it with two different borrow checkers.
With the current stable borrow checker: ❌ it doesn’t compile.
With Polonius Alpha: ✅ it compiles.
And no, Rust is not becoming less safe.
Polonius is getting better at understanding when a mutable reference is actually still being used, instead of conservatively rejecting code that is perfectly safe.
We start with a tiny example where the difference is immediately visible, then move to a much more realistic HashMap example involving mutable references, `match`, `Some` and `None`.
We’ll also turn Polonius OFF while still using Rust Nightly to prove that the new borrow checker is really what changes the result.
Along the way, we talk about NLL (Non-Lexical Lifetimes), flow-sensitive borrow checking, why the current Rust borrow checker rejects these patterns, and why Polonius can understand them.
And Polonius Alpha is still Alpha.
There are cases it still cannot compile, and doing more precise analysis also comes with potential compile-time performance regressions. We take a look at the Rust team’s benchmarks across some of the most downloaded crates on crates.io.
If you’ve ever fought with:
“cannot borrow as mutable more than once at a time”
this is a very interesting look at where Rust is heading next.
Official Rust announcement:
blog.rust-lang.org/2026/08/04/enabling-polonius-alpha-on-nightly
Chapters:
0:00 Rust Has a New Borrow Checker! Polonius Alpha
1:57 The Official Polonius Alpha Announcement
3:35 Shocking Example: Code That Didn’t Compile Before Now Compiles
5:52 A Bigger Example: HashMaps and Mutable References
7:35 Polonius Alpha Limitations and Performance
9:37 Conclusions
#rust #rustlang #programming
