Allow using 'static lifetimes in functions

[daxpedda]

Currently #[wasm_bindgen] disallows any kind of lifetime on functions.
This change allows it to accept 'static.

Fixes #1187.

[Liamolucko]

This applies to arguments too, where using 'static isn't okay. It's still sound because the way references work internally is that #[wasm_bindgen] generates a local variable and takes a reference to it, so you just get a borrow checker error, but it's a worse error than you'd have gotten before:

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub fn invalid(_: &'static str) {}

error[E0597]: `*arg0` does not live long enough
 --> src/lib.rs:3:1
  |
3 | #[wasm_bindgen]
  | ^^^^^^^^^^^^^^-
  | |             |
  | |             `*arg0` dropped here while still borrowed
  | borrowed value does not live long enough
  | argument requires that `*arg0` is borrowed for `'static`
  |
  = note: this error originates in the attribute macro `wasm_bindgen` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0597`.
error: could not compile `wasm-bindgen-playground` (lib) due to 1 previous error

LGTM if you're okay with that though.

[Liamolucko]

Also, this fixes #1187 (the thing it was originally opened for anyway, not the struct use case someone brought up later).

[daxpedda]

LGTM if you're okay with that though.

I'm fine with that considering its the same error message you already get when doing:

#[wasm_bindgen]
pub fn invalid(_: &str) {}

What kind of references are actually allowed in exported functions? Otherwise I could go ahead and outright ban them.

[Liamolucko]

I'm fine with that considering its the same error message you already get when doing:

#[wasm_bindgen]
pub fn invalid(_: &str) {}

What do you mean? That example compiles just fine.

What kind of references are actually allowed in exported functions? Otherwise I could go ahead and outright ban them.

Up until now, just the kind above, and I assumed the whole point of this PR was to allow ones like this:

#[wasm_bindgen]
pub fn foo() -> &'static str {
    "hello world"
}

I didn't test it thoroughly enough and that actually still doesn't compile because of an extra error ('cannot return a borrowed ref with #[wasm_bindgen]'), but it now works if you wrap it in something:

#[wasm_bindgen]
pub fn foo() -> Option<&'static str> {
    Some("hello world")
}

That's also the use-case #1187 was asking for (although it should probably be reopened now that I've realised this doesn't fully fix it).

[daxpedda]

I'm fine with that considering its the same error message you already get when doing:

#[wasm_bindgen]
pub fn invalid(_: &str) {}

What do you mean? That example compiles just fine.

Yep, my mistake.

Up until now, just the kind above, and I assumed the whole point of this PR was to allow ones like this:

#[wasm_bindgen]
pub fn foo() -> &'static str {
    "hello world"
}

Sorry, should have probably mentioned this in OP. My problem was just lifetimes in pointers as parameters, e.g. par1: *const FnOnce(&'static u32). I was originally considering removing the whole lifetime check, but I wasn't really sure what other issues this was addressing, so just enabling 'static seemed safe to me.

That's also the use-case #1187 was asking for (although it should probably be reopened now that I've realised this doesn't fully fix it).

I've re-opened it.

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My PR's have been pretty sloppy lately, so thank you for your diligent and careful reviews!