> Why isn’t there a variant of &mut that doesn’t allow swapping the value?

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...

Yes I'm especially interested in what OP thinks about the overlap (or not?) between the ideas in this post and the ideas in this part of boats' post:

> One could imagine an alternative design in which instead of places being unpinned by default and opting into pinning, places are pinned (or perhaps “immovable”) by default, and have to opt into supporting the ability to move out of them. This would make it so that by default places have the least power (can only access via shared reference) and they gain a monotonically increasing set of powers (can assign to them, can move out of them).

> In addition to places having to opt into moving, there would be three reference types instead of two: immutable, mutable, and movable references.