rustc_trait_selection::infer::canonical::ir

Struct DebruijnIndex

pub struct DebruijnIndex {
    pub(crate) private_use_as_methods_instead: u32,
}

Expand description

A De Bruijn index is a standard means of representing regions (and perhaps later types) in a higher-ranked setting. In particular, imagine a type like this:

   for<'a> fn(for<'b> fn(&'b isize, &'a isize), &'a char)
// ^          ^            |          |           |
// |          |            |          |           |
// |          +------------+ 0        |           |
// |                                  |           |
// +----------------------------------+ 1         |
// |                                              |
// +----------------------------------------------+ 0

In this type, there are two binders (the outer fn and the inner fn). We need to be able to determine, for any given region, which fn type it is bound by, the inner or the outer one. There are various ways you can do this, but a De Bruijn index is one of the more convenient and has some nice properties. The basic idea is to count the number of binders, inside out. Some examples should help clarify what I mean.

Let’s start with the reference type &'b isize that is the first argument to the inner function. This region 'b is assigned a De Bruijn index of 0, meaning “the innermost binder” (in this case, a fn). The region 'a that appears in the second argument type (&'a isize) would then be assigned a De Bruijn index of 1, meaning “the second-innermost binder”. (These indices are written on the arrows in the diagram).

What is interesting is that De Bruijn index attached to a particular variable will vary depending on where it appears. For example, the final type &'a char also refers to the region 'a declared on the outermost fn. But this time, this reference is not nested within any other binders (i.e., it is not an argument to the inner fn, but rather the outer one). Therefore, in this case, it is assigned a De Bruijn index of 0, because the innermost binder in that location is the outer fn.

Fields§

§private_use_as_methods_instead: u32

Implementations§

§

impl DebruijnIndex

pub const MAX_AS_U32: u32 = 4_294_967_040u32

Maximum value the index can take, as a u32.

pub const MAX: DebruijnIndex = _

Maximum value the index can take.

pub const ZERO: DebruijnIndex = _

Zero value of the index.

pub const fn from_usize(value: usize) -> DebruijnIndex

Creates a new index from a given usize.

§Panics

Will panic if value exceeds MAX.

pub const fn from_u32(value: u32) -> DebruijnIndex

Creates a new index from a given u32.

§Panics

Will panic if value exceeds MAX.

pub const fn from_u16(value: u16) -> DebruijnIndex

Creates a new index from a given u16.

§Panics

Will panic if value exceeds MAX.

pub const unsafe fn from_u32_unchecked(value: u32) -> DebruijnIndex

Creates a new index from a given u32.

§Safety

The provided value must be less than or equal to the maximum value for the newtype. Providing a value outside this range is undefined due to layout restrictions.

Prefer using from_u32.

pub const fn index(self) -> usize

Extracts the value of this index as a usize.

pub const fn as_u32(self) -> u32

Extracts the value of this index as a u32.

pub const fn as_usize(self) -> usize

Extracts the value of this index as a usize.

§

impl DebruijnIndex

pub fn shifted_in(self, amount: u32) -> DebruijnIndex

Returns the resulting index when this value is moved into amount number of new binders. So, e.g., if you had

for<’a> fn(&’a x)

and you wanted to change it to

for<’a> fn(for<’b> fn(&’a x))

you would need to shift the index for 'a into a new binder.

pub fn shift_in(&mut self, amount: u32)

Update this index in place by shifting it “in” through amount number of binders.

pub fn shifted_out(self, amount: u32) -> DebruijnIndex

Returns the resulting index when this value is moved out from amount number of new binders.

pub fn shift_out(&mut self, amount: u32)

Update in place by shifting out from amount binders.

pub fn shifted_out_to_binder(self, to_binder: DebruijnIndex) -> DebruijnIndex

Adjusts any De Bruijn indices so as to make to_binder the innermost binder. That is, if we have something bound at to_binder, it will now be bound at INNERMOST. This is an appropriate thing to do when moving a region out from inside binders:

            for<'a>   fn(for<'b>   for<'c>   fn(&'a u32), _)
// Binder:  D3           D2        D1            ^^

Here, the region 'a would have the De Bruijn index D3, because it is the bound 3 binders out. However, if we wanted to refer to that region 'a in the second argument (the _), those two binders would not be in scope. In that case, we might invoke shift_out_to_binder(D3). This would adjust the De Bruijn index of 'a to D1 (the innermost binder).

If we invoke shift_out_to_binder and the region is in fact bound by one of the binders we are shifting out of, that is an error (and should fail an assertion failure).