Struct rustc_middle::ty::sty::DebruijnIndex

pub struct DebruijnIndex {
    private: u32,
}

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 arrays 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: u32

Implementations

impl DebruijnIndex

pub const MAX_AS_U32: u32

pub const MAX: Self

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

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

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

pub const fn index(self) -> usize

Extracts the value of this index as an integer.

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

#[must_use]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.

#[must_use]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) -> Self

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).

Trait Implementations

impl Add<usize> for DebruijnIndex

type Output = Self

The resulting type after applying the + operator.

pub fn add(self, other: usize) -> Self

Performs the + operation.

impl Clone for DebruijnIndex

pub fn clone(&self) -> Self

Returns a copy of the value.

pub fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for DebruijnIndex

impl Debug for DebruijnIndex

pub fn fmt(&self, fmt: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<D: Decoder> Decodable<D> for DebruijnIndex

pub fn decode(d: &mut D) -> Result<Self, D::Error>

impl<E: Encoder> Encodable<E> for DebruijnIndex

pub fn encode(&self, e: &mut E) -> Result<(), E::Error>

impl Eq for DebruijnIndex

impl From<DebruijnIndex> for u32

pub fn from(v: DebruijnIndex) -> u32

Performs the conversion.

impl From<DebruijnIndex> for usize

pub fn from(v: DebruijnIndex) -> usize

Performs the conversion.

impl From<u32> for DebruijnIndex

pub fn from(value: u32) -> Self

Performs the conversion.

impl From<usize> for DebruijnIndex

pub fn from(value: usize) -> Self

Performs the conversion.

impl Hash for DebruijnIndex

pub fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

pub fn hash_slice<H>(data: &[Self], state: &mut H) where
    H: Hasher, 

Feeds a slice of this type into the given Hasher.

impl<'__ctx> HashStable<StableHashingContext<'__ctx>> for DebruijnIndex

pub fn hash_stable(
    &self,
    __hcx: &mut StableHashingContext<'__ctx>,
    __hasher: &mut StableHasher
)

impl Idx for DebruijnIndex

pub fn new(value: usize) -> Self

pub fn index(self) -> usize

pub fn increment_by(&mut self, amount: usize)

pub fn plus(self, amount: usize) -> Self

impl Ord for DebruijnIndex

pub fn cmp(&self, other: &DebruijnIndex) -> Ordering

This method returns an Ordering between self and other.

#[must_use]pub fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

#[must_use]pub fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

#[must_use]pub fn clamp(self, min: Self, max: Self) -> Self

🔬 This is a nightly-only experimental API. (clamp)

Restrict a value to a certain interval.

impl PartialEq<DebruijnIndex> for DebruijnIndex

pub fn eq(&self, other: &DebruijnIndex) -> bool

This method tests for self and other values to be equal, and is used by ==.

pub fn ne(&self, other: &DebruijnIndex) -> bool

This method tests for !=.

impl PartialOrd<DebruijnIndex> for DebruijnIndex

pub fn partial_cmp(&self, other: &DebruijnIndex) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

pub fn lt(&self, other: &DebruijnIndex) -> bool

This method tests less than (for self and other) and is used by the < operator.

pub fn le(&self, other: &DebruijnIndex) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

pub fn gt(&self, other: &DebruijnIndex) -> bool

This method tests greater than (for self and other) and is used by the > operator.

pub fn ge(&self, other: &DebruijnIndex) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl Step for DebruijnIndex

pub fn steps_between(start: &Self, end: &Self) -> Option<usize>

🔬 This is a nightly-only experimental API. (step_trait)

recently redesigned

Returns the number of successor steps required to get from start to end.

pub fn forward_checked(start: Self, u: usize) -> Option<Self>

🔬 This is a nightly-only experimental API. (step_trait_ext)

recently added

Returns the value that would be obtained by taking the successor of self count times.

pub fn backward_checked(start: Self, u: usize) -> Option<Self>

🔬 This is a nightly-only experimental API. (step_trait_ext)

recently added

Returns the value that would be obtained by taking the successor of self count times.

pub fn forward(start: Self, count: usize) -> Self

🔬 This is a nightly-only experimental API. (step_trait_ext)

recently added

Returns the value that would be obtained by taking the successor of self count times.

pub unsafe fn forward_unchecked(start: Self, count: usize) -> Self

🔬 This is a nightly-only experimental API. (unchecked_math)

niche optimization path

Returns the value that would be obtained by taking the successor of self count times.

pub fn backward(start: Self, count: usize) -> Self

🔬 This is a nightly-only experimental API. (step_trait_ext)

recently added

Returns the value that would be obtained by taking the predecessor of self count times.

pub unsafe fn backward_unchecked(start: Self, count: usize) -> Self

🔬 This is a nightly-only experimental API. (unchecked_math)

niche optimization path

Returns the value that would be obtained by taking the predecessor of self count times.

impl StructuralEq for DebruijnIndex

impl StructuralPartialEq for DebruijnIndex