Improve error message when a field is accessed in an opaque struct

[nilehmann]

The error message could explain why the field of an opaque struct cannot be accessed and suggest using #[flux::trusted].

Example of someone confused about it

Relevant pointers:

• https://github.com/flux-rs/flux/blob/199a8ce4324cc86ae2a1fc7d1fa90a51e5817c7d/crates/flux-refineck/src/checker.rs#L1797-L1803
• https://github.com/flux-rs/flux/blob/199a8ce4324cc86ae2a1fc7d1fa90a51e5817c7d/crates/flux-refineck/locales/en-US.ftl#L40-L41

[vrindisbacher]

@nilehmann, I can take this if you'd like?

[nilehmann]

@vrindisbacher that'd be great! One extra pointer. We use rustc's infrastructure for reporting errors which is documented here https://rustc-dev-guide.rust-lang.org/diagnostics/diagnostic-structs.html

[vrindisbacher]

@nilehmann, how does this look?

File:

#![allow(unused)]
#![allow(dead_code)]

#[flux_rs::opaque]
struct MyStruct {
    inner: u32,
}

impl MyStruct {
    pub fn inner(&self) -> u32 {
        self.inner
    }
}

Output:

error[E0999]: cannot access fields of opaque struct `MyStruct`.
  --> ../opaque_struct.rs:13:9
   |
13 |         self.inner
   |         ^^^^^^^^^^
-Ztrack-diagnostics: created at crates/flux-refineck/src/lib.rs:108:78
   |
help: If you'd like to use fields of `MyStruct` in methods, use `flux::trusted` instead of `flux::opaque`.
  --> ../opaque_struct.rs:7:1
   |
7  | struct MyStruct {
   | ^^^^^^^^^^^^^^^

error: aborting due to 1 previous error

[nilehmann]

@vrindisbacher, this wasn't clear in the issue description, but the way to solve a cannot access fields of opaque struct is to trust the function accessing the field. In your example above you'd need to trust fn inner:

#[flux_rs::opaque]
struct MyStruct {
    inner: u32,
}

impl MyStruct {
   #[flux::trusted]
    pub fn inner(&self) -> u32 {
        self.inner
    }
}

If you mark a struct as opaque, the only thing you can do in untrusted code is "pass it around". The expectation is that the module defining the opaque struct will define a trusted API, and clients will use it without accessing the fields and only through the API.

Not sure what's the best way to convey this in the error message.

[vrindisbacher]

Ah I see. Perhaps, the best way to do this is to have a help span over the function suggesting they add trusted and then point them to some documentation (maybe in the flux book) about use of opaque?

[nilehmann]

Pointing to documentation would be cool, we would have to write it though XD

So ideally, we could have something like

error[E0999]: cannot access fields of opaque struct `MyStruct`.
  --> ../opaque_struct.rs:13:9
   |
13 |         self.inner
   |         ^^^^^^^^^^
-Ztrack-diagnostics: created at crates/flux-refineck/src/lib.rs:108:78
   |
help: try annotating this method with `#[flux::trusted]`
  --> ../opaque_struct.rs:7:1
   |
12 | pub fn inner(&self) -> u32 {
   |        ^^^^^
 note: fields of opaque structs can only be accessed inside trusted code
 note: to know more about opaque http://flux-rs.github.io/....

[vrindisbacher]

Ya that's exactly what I was thinking. I'm happy to write the documentation as part of this if you want.