[majewsky]

I would like a language that has knobs (e.g. file-level pragmas) for strictness, which you could turn all to one side (to get something as strict as Rust) or all the way to the other side (to get something like Ruby) or somewhere in between.

For example, a REPL would be pretty non-strict: When you define a function, you don't have to annotate types on the arguments, and everything gets passed around as generic objects. However, the standard library would be pretty strict [1], so in your unstrict REPL code, types are checked (and implicit clones are made to satisfy the borrow checker) as soon as you call into the strict code.

[1] In fact, the package repository for this hypothetical language (the analog to crates.io etc.) should only accept strict code, or alternatively put HUGE warning signs on libraries that contain unstrict code.

[kqr]

For the record, in PL jargon, "strictness" commonly refers to how soon the arguments in a function call are to be scheduled for evaluation. Using it to talk about the extent of static correctness checks can be slightly confusing!

[stmw]

So Rust has a lot of that, both in turning off some checks and in ability to write whole blocks of unsafe code.