struct HasEscapingVarsVisitor {
    outer_index: DebruijnIndex,
Expand description

An “escaping var” is a bound var whose binder is not part of t. A bound var can be a bound region or a bound type.

So, for example, consider a type like the following, which has two binders:

for<’a> fn(x: for<’b> fn(&’a isize, &’b isize)) ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ outer scope ^~~~~~~~~~~~~~~~~~~~~~~~~~~~ inner scope

This type has bound regions ('a, 'b), but it does not have escaping regions, because the binders of both 'a and 'b are part of the type itself. However, if we consider the inner fn type, that type has an escaping region: 'a.

Note that what I’m calling an “escaping var” is often just called a “free var”. However, we already use the term “free var”. It refers to the regions or types that we use to represent bound regions or type params on a fn definition while we are type checking its body.

To clarify, conceptually there is no particular difference between an “escaping” var and a “free” var. However, there is a big difference in practice. Basically, when “entering” a binding level, one is generally required to do some sort of processing to a bound var, such as replacing it with a fresh/placeholder var, or making an entry in the environment to represent the scope to which it is attached, etc. An escaping var represents a bound var for which this processing has not yet been done.


outer_index: DebruijnIndex

Anything bound by outer_index or “above” is escaping.

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