Struct rustc_middle::ty::DebruijnIndex[][src]

pub struct DebruijnIndex {
    pub(crate) 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 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.


private: u32

Auto Trait Implementations

impl RefUnwindSafe for DebruijnIndex

impl Send for DebruijnIndex

impl Sync for DebruijnIndex

impl Unpin for DebruijnIndex

impl UnwindSafe for DebruijnIndex

Blanket Implementations

impl<T> Any for T where
    T: 'static + ?Sized

impl<'tcx, T> ArenaAllocatable<'tcx, ()> for T where
    T: Copy

impl<T> Borrow<T> for T where
    T: ?Sized

impl<T> BorrowMut<T> for T where
    T: ?Sized

impl<'a, T> Captures<'a> for T where
    T: ?Sized

impl<Ctxt, T> DepNodeParams<Ctxt> for T where
    T: HashStable<<Ctxt as DepContext>::StableHashingContext> + Debug,
    Ctxt: DepContext

impl<T> From<T> for T[src]

impl<T, U> Into<U> for T where
    U: From<T>, 

impl<T> MaybeResult<T> for T[src]

type Error = !

impl<T> ToOwned for T where
    T: Clone

type Owned = T

The resulting type after obtaining ownership.

impl<T, U> TryFrom<U> for T where
    U: Into<T>, 

type Error = Infallible

The type returned in the event of a conversion error.

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    U: TryFrom<T>, 

type Error = <U as TryFrom<T>>::Error

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impl<T> WithConstness for T[src]