The Algebra of Loans in Rust

I don't know 100% for sure. It's a bit confusing...

1. Always keep the language reference with you. It's absolutely not a replacement for a good introductory textbook. But it's an unusually effective resource for anybody who has crossed that milestone. It's very effective in spontaneously uncovering new language features and in refining your understanding of the language semantics.

What we need to do with it is to refer it occasionally for even constructs that you're familiar with - for loops, for example. I wish that it was available as auto popups in code editors.

2. Use clippy, the linter. I don't have much to add here. Your code will work without it. But for some reason, clippy is an impeccable tutor into idiomatic Rust coding. And you get the advantage of the fact that it stays in sync with the latest language features. So it's yet another way to keep yourself automatically updated with the language features.

Python at least is very clear about this: lists, class instances, dicts, and tuples are all passed by reference (but tuples are immutable so it doesn't matter).

I assume I'm the one who taught you this, and for the edification of others, you can do labeled break not only in Rust, but also C#, Java, and JavaScript. An even more powerful version of function-local labels and goto is available in Go (yes, in Go!), as well as, naturally, C and C++.

The point being, the existence of obscure features does not a large or baroque language make, unless you're willing to call Go a large language.

Python for example has a ton of stuff that can be done with classes using sunder methods and other magic. I'm aware of it, but in all the years I've been writing Python I've never actually needed it. The only time I've had to directly interact with it was when trying to figure out how the fuck OpenAPI generates FastAPI server code. Which fairly deep into a framework and code generation tool.

Many "other languages", particularly ones that compile to native code in traditional way have fairly explicit ways of specifying how said parameters to be treated

The correct solution for the semantics of function parameters is the one described in the "DoD requirements for high order computer programming languages: IRONMAN" (January 1977, revised in July 1977), which have been implemented in the language Ada and in a few other languages inspired by Ada.

According to" IRONMAN", the formal parameters of a function must be designated as belonging to one of 3 classes, input parameters, output parameters and input-output parameters.

This completely defines the behavior of the parameters, without constraining in any way the implementation, i.e. any kind of parameters may be passed by value or by reference, whichever the compiler chooses for each individual case. (An input-output parameter where the compiler chooses to pass it by value will be copied twice, which can still be better than passing it by reference, e.g. when the parameter is passed in a register.)

When a programming language of the 21st century still requires for the programmer to specify whether it is passed by value or by reference, that is a serious defect for the language, because in general the programmer does not have the information needed to make a correct choice and this is an implementation detail with which the programmer should not be burdened.

The fact that C++ lacks this tripartite classification of the formal parameters of a function has lead to the ugliest complications of C++, which have been invented as bad workarounds for this defect, i.e. the fact that constructors are not normal functions, the fact that there exist several kinds of superfluous constructors which would not have been needed otherwise (e.g. the copy constuctor), the fact that C++ 2011 had to add some features like the "move" semantics to fix performance problems of the original C++.

For 3, some objects only need to be pinned under certain circumstances, e.g. futures only need to be pinned after they're polled for the first time, but not before. So it's convenient to separate the pinnability property to allow them to be moved freely beforehand.

I don't quite understand the usecase you have in mind for 4.

This is a very insightful observation, and Niko Matsakis (leading influence of Rust's borrow checker) would likely agree with you that this is an instance where Rust's default borrowing rules are probably too permissive, in the sense that being more restrictive by default regarding the "swappability" of &mut could lead to Rust being able to provide more interesting static guarantees. See his blog post here: https://smallcultfollowing.com/babysteps/blog/2024/09/26/ove...

> Why is pinning a weird sticky property of a reference? Shouldn’t non-movability of an object be a property of the object’s type?

See this blog post from withoutboats: https://without.boats/blog/pinned-places/ for arguments as to why pinning is properly modeled as a property of a place rather than a type (particularly the section "Comparison to immovable types"), as well as this post from Niko that ties this point in with the above point regarding swappability: https://smallcultfollowing.com/babysteps/blog/2024/10/14/ove...

A type that might require stable pointers, like async{}, might want to be movable prior to use, so you don't want the type to require the value be pinned immediately. Or if you do, you need a construction like pinned-init that offers `&pin out T` - a pinned place that can be written to on initialisation of the type.

&own, &pin, and &uninit are proposals for additional pointer types. They don't actually exist in the type system right now, but other parts of the compiler do have to care about them. Another blog post that floated around here about a month ago called these "inconceivable types"[0]; adding them to the type system would allow formally extending these behaviors across function boundaries.

Like, right now, implementers of Drop can't actually move anything out of the value that's about to be destroyed. The Drop trait gets a &mut, but what we really want is to say "destroy this value over there". Rust's type system cannot understand that you own the value but not the place it lives in. What you need is an "owned reference" - i.e. &own, where the borrow checker knows that you can safely move out of it because it's going to get destroyed anyway.

Rust also can't support constructors, for the same reason. What we really have are factory functions: you call them, they return a value, you put it somewhere. This is good enough that Rust users just treat factory functions as if they were constructors, but we can't do "placement new" type construction with them, or partial initialization. At least not in a way the type system can actually check.

&pin is a first-class version of Pin<T>. Rust was originally designed under the assumption that any type can be memcpy'd at any time; but it turns out not being able to move types is actually super useful. Fortunately, it also turned out you could use smart pointers to pin types, which was 'good enough' for what it was being used for - async code.

Actually, the blog post that coined "inconceivable types" was specifically talking about writing async functions without async. It turns out Future impls encode a lot of details Rust's type system can't handle - notably, self-borrows. If a value is borrowed across an await, what's the type of the variable that got borrowed from? It's really a negative type: borrowing T to get &T also turns T into !'a T that you can't access until 'a ends. Each borrowed reference is paired to a debt that needs to be paid back, and to do that you need lifetime variables and syntax to explicitly say "pay back this debt by ending this borrow's lifetime".

How much of this complexity is actually needed is another question. There's a problem that each and every one of these reference types (or, anti-types) is intended to solve. Obviously if we added all of them, we'd overcomplicate the type system. But at the same time, the current Rust type system is already known to be oversimplified, to the point where we had to hack in pinning for async. And it's already kind of ridiculous to say "Well, async is all special compiler magic" because it prevents even reasonable-sounding tweaks to the system[1].

[0] https://blog.polybdenum.com/2024/06/07/the-inconceivable-typ...

[1] For example, async does not currently have a way to represent "ambient context" - i.e. things we want the function to be able to access but NOT hold onto across yields. That would require a new Future trait with a different poll method signature, which the current Rust compiler doesn't know how to fill or desugar to. So you have to use the core Future trait and signature which doesn't support this kind of context borrow.

To work around this limiation involves a lot of unnecessarily verbose code to drop and regain context between awaits, i.e. https://github.com/ruffle-rs/ruffle/blob/b5732b9783dce5d2311...