X-Git-Url: https://git.ralfj.de/web.git/blobdiff_plain/91178cc37df56eede73de24b55f82d530990e252..0172870f7b724f38ab106202319ffbebd702d7ee:/ralf/_posts/2018-08-07-stacked-borrows.md diff --git a/ralf/_posts/2018-08-07-stacked-borrows.md b/ralf/_posts/2018-08-07-stacked-borrows.md index 10b2e44..94f2bb6 100644 --- a/ralf/_posts/2018-08-07-stacked-borrows.md +++ b/ralf/_posts/2018-08-07-stacked-borrows.md @@ -13,7 +13,7 @@ The model I am proposing here is by far not the first attempt at giving a defini But before I delve into my latest proposal, I want to briefly discuss a key difference between my previous model and this one: -"Types as Contracts" was a fully "validity"-based model, while "Stacked Borrows" is (to some extend) "access"-based. +"Types as Contracts" was a fully "validity"-based model, while "Stacked Borrows" is (to some extent) "access"-based. ## 1 Validity-based vs. Access-based @@ -147,7 +147,7 @@ Shared references with interior mutability do not really have any restrictions i For every location in memory, we keep track of a stack of borrows (`Uniq(_)` or `Raw`), and potentially "top off" this stack by freezing the location. A frozen location is never written to, and no `Uniq` is pushed. -Whenever a mutable reference is created, a matching `Uniq` is pushed onto the stack for every location "covered by" the reference -- i.e., the locations that would be accessed when the reference is used (starting at where it points to, and going on for `mem::size_of::` many bytes). +Whenever a mutable reference is created, a matching `Uniq` is pushed onto the stack for every location "covered by" the reference -- i.e., the locations that would be accessed when the reference is used (starting at where it points to, and going on for `size_of_val` many bytes). Whenever a shared reference is created, if there is no interior mutability, we freeze the locations if they are not already frozen. If there is interior mutability, we just push a `Raw`. Whenever a raw pointer is created from a mutable reference, we push a `Raw`. @@ -169,7 +169,7 @@ In particular, notice that `x` and `y` in the first example have the same addres If we compared them as raw pointers, they would turn out equal. And yet, it makes a huge difference if we use `x` or `y`! -If you read my previous post on [why pointers are complicated](2018-07-24-pointers-and-bytes), this should not come as too much of a surprise. +If you read my previous post on [why pointers are complicated]({% post_url 2018-07-24-pointers-and-bytes %}), this should not come as too much of a surprise. There is more to a pointer, or a reference (I am using these terms mostly interchangeably), than the address in memory that it points to. For the purpose of this model, we assume that a value of reference type consists of two parts: An address in memory, and a tag used to store the time when the reference was created. @@ -239,7 +239,7 @@ It is also needed to explain why we can put the [`noalias` parameter attribute]( Consider the following code: {% highlight rust %} -fn demo5(x: &mut i32, y: usize) -> i32 { +fn demo5(x: &mut i32, y: usize) { *x = 42; foo(y); }