Feature request: force a single closure capture to be by-move

[newpavlov]

Relevant idea:

Idea: "strict" closure syntax

Implicit environment capture by closures is often a confusing feature for beginners, especially with the move keyword added to the mix. And even for proficient users it results in various papercuts. For example: let a: Arc<T1> = ...; let b: T2 = ...; let c: T3 = ...; // Annoying cloning let a_clone = a.clone(); // I don't want to move b, but I have to use move on closure... let b_ref = &b; // Was c moved or copied? Dunno... Need to check T3 docs. // Also use of c could be buried deep in the clo…