Revisit Orphan Rules

language design
15

This is, as I understand it, the core of the proposal.

Right. As I said, that is global analysis, which is highly uncharacteristic of Rust, a language that prides itself in having the signature of a function indicate all that is necessary to decide where it can be used. (which is a useful property that allows for decent error messages. Contrast with C++...)

fn func_1(vec: Vec<f64>) -> Vec<f64> { vec }
fn func_2(vec: Vec<f64>) -> Vec<f64> { vec.into_iter().collect() }

type VecFn = fn(Vec<f64>) -> Vec<f64>;

fn foo() {
    let funcs: Vec<VecFn> = vec![func_1 as _, func_2 as _];

    // hand off to some crate that accepts `&[fn(Vec<f64>) -> Vec<f64>]`
    some_crate::have_some_funcs(&funcs)
}

Now suppose some_crate::have_some_funcs does what I previously mentioned, supplying a Vec<f64>:my::Hash and expecting a Vec<f64>:bob::Hash. Where is the error message now?

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