RFC: Rust Has Provenance

https://github.com/rust-lang/rfcs/pull/3559

by killcoder • 2 years ago

55 18 2 years ago

github.com

vaylian• 2 years ago

The pointer's "provenance" says where in memory the pointer is allowed to access when.

Is that the definition of provenance or is there a wider definition that we should know?

DougBTX• 2 years ago

See the doc for a definition: https://github.com/RalfJung/rfcs/blob/provenance/text/0000-r...

aredox2• 2 years ago

"where in memory the pointer is allowed to access when." - ... when what?

Sorry, this sentence is difficult to understand.

_nalply• 2 years ago

Probably just a mistake. I just would assume that the word "when" is superfluous and respond in a way that allows the asker to correct their mistake but not invest too much in an answer because the assumption might be wrong.

I am picky in language as well, so I understand where you are coming from. But I discovered that LLMs often answered anyway when my question was incomplete or contained mistakes and they got it correct not too rarely.

It's a useful trick I discovered late in my life (I am in my fifties). I started to use it with my children and my wife. Their utterances are often not mathematically and linguistically perfect, but very human and lovely. When I got stumped I just thought about what they might have asked and answered that. More often than not they were happy with my answer.

asplake• 2 years ago

‘when’ here meaning “and under what circumstances”. An ‘and’ might help

Buttons8403• 2 years ago

Isn't this a definition of lifetimes too?

A lifetime is some set of memory a reference may refer to. Consider:

fn foo<'a, 'b>(x: &'a str, y: &'b str) -> &'a str

Aren't 'a and 'b sets of memory a reference can refer to? As far as this function call is concerned, both 'a and 'b will live throughout, so it's not about life and death, it's about what memory the references may refer too.

Is a lifetime and a "provenance" the same thing?

Georgelemental2• 2 years ago

Aren't 'a and 'b sets of memory a reference can refer to?

No, the set of memory the reference refers to is encoded by the reference's bits. Two references with the same lifetime can refer to different memory, and vice-versa. Lifetimes determine when the reference is valid.

Rusky2• 2 years ago

Provenance is a dynamic property of pointer values. The actual underlying rules that a program must follow, even when using raw pointers and `unsafe`, are written in terms of provenance. Miri (https://github.com/rust-lang/miri) represents provenance as an actual value stored alongside each pointer's address, so it can check for violations of these rules.

Lifetimes are a static approximation of provenance. They are erased after being validated by the borrow checker, and do not exist in Miri or have any impact on what transformations the optimizer may perform. In other words, the provenance rules allow a superset of what the borrow checker allows.

quelsolaar• 2 years ago

Provenance In C and Rust, essentially mean that each object, lives in its own memory space, and you cant travel between them. You cant use a pointer to one object, add an offset and get a valid pointer to another object.

This is key to a lot of compiler optimizations. It is de facto in C compilers, and therefor in a lot of compiler infrastructure used by other languages. There is an attempt to make it more clearly defined in the C standard:

https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3005.pdf

ralfj• 2 years ago

It's not just per-object. When you consider things like `restrict` pointers in C, and the aliasing model in Rust (e.g. Stacked Borrows), you have provenance distinctions even within a single allocated object.

RFC: Rust Has Provenance

Related Stories