Struct rustc::middle::ty::DebruijnIndexUnstable
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[src]
pub struct DebruijnIndex {
pub depth: 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 int, &'a int), &'a char) ^ ^ | | | | | | | | | +------------+ 1 | | | | | +--------------------------------+ 2 | | | +------------------------------------------+ 1
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 int that is the first
argument to the inner function. This region 'b is assigned a De
Bruijn index of 1, meaning "the innermost binder" (in this case, a
fn). The region 'a that appears in the second argument type (&'a
int) would then be assigned a De Bruijn index of 2, 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 1, because the innermost binder in that location
is the outer fn.
Fields
depth |