rust-lang/rust — pull request #19089 by nikomatsakis (2014-11-18 22:45:11+00:00)
Title: Infer argument types for unboxed closures by inspecting the expected type

This also lays the groundwork for a simple version of inference to decide on `FnOnce` vs `FnMut` etc.

Fixes #17907

r? @pcwalton