undef' values are things that do not have specified contents. These are used for a variety of purposes, including global variable initializers and operands to instructions. 'undef' values can occur with any first-class type. Undef values aren't exactly constants; if they have multiple uses, they can appear to have different bit patterns at each use. See LangRef.html::undefvalues for details.

This blog post explores an alternative formulation of Rust’s type system that eschews lifetimes in favor of places. The TL;DR is that instead of having 'a represent a lifetime in the code, it can represent a set of loans, like shared(a.b.c) or mut(x). If this sounds familiar, it should, it’s the basis for polonius, but reformulated as a type system instead of a static analysis. This blog post is just going to give the high-level ideas. In follow-up posts I’ll dig into how we can use this to support interior references and other advanced borrowing patterns. In terms of implementation, I’ve mocked this up a bit, but I intend to start extending a-mir-formality to include this analysis.