Compiler could be more helpful

[3tilley]

Code

Cargo.toml

[package]
name = "clippy-lint-unwrap"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
fs-err = "2.9.0"
serde = { version = "1.0.189", features = ["derive"] }
serde_json = "1.0.107"

main.rs

use std::fs::File;
use std::path::Path;

fn main() {
    let path = Path::new(file!()).parent().unwrap();

    let f = fs_err::File::open(path);
    let j = serde_json::from_reader(f).unwrap();
    println!("{:?}", f);
}

Current output

error[E0277]: the trait bound `std::result::Result<fs_err::File, std::io::Error>: std::io::Read` is not satisfied
    --> src\main.rs:8:37
     |
8    |     let j = serde_json::from_reader(f).unwrap();
     |             ----------------------- ^ the trait `std::io::Read` is not implemented for `std::result::Result<fs_err::File, std::io::Error>`
     |             |
     |             required by a bound introduced by this call
     |
note: required by a bound in `serde_json::from_reader`
    --> xxxxxxxxxxxxxxxxxxx\serde_json-1.0.107\src\de.rs:2590:8
     |
2588 | pub fn from_reader<R, T>(rdr: R) -> Result<T>
     |        ----------- required by a bound in this function
2589 | where
2590 |     R: crate::io::Read,
     |        ^^^^^^^^^^^^^^^ required by this bound in `from_reader`

Desired output

The error occurs because I forgot to deal with the Result, but I think the compiler could suggest here that .expect could be called here as I've seen in other situations. It seems possible to suggest that if Result<T,V> fails a trait requirement but T passes then this could be an incident of Result forgetfulness.

Rationale and extra context

This would only be checked in case of failure, so I don't think it would add overhead to the compilation time?

Other cases

I checked with std::fs and fs-err just in case, the output was the same

Anything else?

No response

[chenyukang]

For anyone may want to resolve this issue, I made a sample without the crate:

use std::path::Path;

struct File {
    path: String,
}

impl File {
    fn new(path: &Path) -> Self {
        Self { path: path.to_str().unwrap().to_string() }
    }
}

fn open(path: &Path) -> Option<File> {
    Some(File::new(&path))
}

trait FileAPI {
    fn read(self) -> String;
}

impl FileAPI for File {
    fn read(self) -> String {
        std::fs::read_to_string(&self.path).unwrap()
    }
}

fn test_file<T>(f: T) -> String
where
    T: FileAPI,
{
    f.read()
}

fn main() {
    let path = Path::new(file!()).parent().unwrap();
    let file = open(path);
    let _ = test_file(file);
}

[3tilley]

@chenyukang are you working on this or can I claim?

[chenyukang]

@3tilley you can work on it, please go ahead.

[3tilley]

@rustbot claim

[estebank]

Shorter repro:

fn foo(_: impl std::io::Read) {}
fn main() {
    let path = std::path::Path::new(file!()).parent().unwrap();

    let f = std::fs::File::open(path);
    foo(f);
}

error[E0277]: the trait bound `Result<File, std::io::Error>: std::io::Read` is not satisfied
  --> src/main.rs:10:9
   |
10 |     foo(f);
   |     --- ^ the trait `std::io::Read` is not implemented for `Result<File, std::io::Error>`
   |     |
   |     required by a bound introduced by this call
   |
note: required by a bound in `foo`
  --> src/main.rs:4:16
   |
4  | fn foo(_: impl std::io::Read) {
   |                ^^^^^^^^^^^^^ required by this bound in `foo`