Idea: Borrowck Transparent Function Calls

josh:

we could support writing something like &mut{_} self to get a compiler error with a suggestion

I was kinda hoping we could write nothing, actually, and have it get filled in.

Like today (example inspired from the other thread) if you have https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=b3fc0ce0ebc6c302bd2befd4f35b8522

fn foo(a: &i32, b: &i32, c: &i32) -> &i32 {
    if *c > 0 { a } else { b }
}

Then the error you get is

  = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `a`, `b`, or `c`
help: consider introducing a named lifetime parameter
  |
1 | fn foo<'a>(a: &'a i32, b: &'a i32, c: &'a i32) -> &'a i32 {
  |       ++++     ++          ++          ++          ++

but ideally we could use the MIR borrow-check results to suggest

fn foo<'a>(a: &'a i32, b: &'a i32, c: &i32) -> &'a i32 {

with a note that that's what the body is currently doing.

Though having typed all that I guess that's largely irrelevant to the "view type" part, which yeah, might need a way to say "intentionally wrong; please fix".

That might just be an empty list, though. &mut{} self is probably not actually going to compile, and the error message can tell you which things to put in there that the current implementation needs, with the structured suggestion.