Diagnostics: suggest alternatives when new constructor is not available

[yoshuawuyts]

Failing Code

Consider the following code:

use std::net::TcpStream;

fn main() {
   let stream = TcpStream::new();
}

playground

This fails with an error that simply states that new is not available:

Compiling playground v0.0.1 (/playground)
error[E0599]: no function or associated item named `new` found for type `std::net::TcpStream` in the current scope
 --> src/main.rs:4:28
  |
4 |    let stream = TcpStream::new();
  |                            ^^^ function or associated item not found in `std::net::TcpStream`

error: aborting due to previous error

Working Code

The intent is fairly clear: we want to construct a new TcpStream using a method that follows the Rust naming conventions for constructors, but that method is not available. Instead the solution is likely to be either TcpStream::connect or
TcpStream::connect_timeout:

use std::net::TcpStream;

fn main() {
   let stream = TcpStream::connect("localhost:8080");
}

Diagnostics Suggestions

When new is not available it'd be ideal if the compiler could suggest
alternatives. A first heuristic for which methods to suggest be methods on
the same struct that don't take any self params, have Self as their
return type, and aren't implemented through any trait.

For TcpStream this would include TcpStream::connect and
TcpStream::connect_timeout. But not TcpStream::try_clone,
TcpStream::from_raw_fd, TcpStream::from_raw_socket and
std::net::Incoming::next.

I'm not too sure how to structure the help message exactly, but I could
imagine something along these lines might work:

Compiling playground v0.0.1 (/playground)
error[E0599]: no function or associated item named `new` found for type `std::net::TcpStream` in the current scope
 --> src/main.rs:4:28
  |
4 |    let stream = TcpStream::new();
  |                            ^^^ function or associated item not found in `std::net::TcpStream`
  = help: Did you mean to use `std::net::TcpStream::connect` or `std::net::TcpStream::connect_timeout`?

error: aborting due to previous error

cc/ @estebank

[Centril]

Seems like #[doc(alias(...))]es should be added to the standard library to fix this.

[varkor]

This seems to be the same as #51398.

[Centril]

Closing in favor of that.

[yoshuawuyts]

Seems like #[doc(alias(...))]es should be added to the standard library to fix this.

@Centril For the example I brought up, how would an #[doc(alias(...))] work? The new method does not exist on TcpStream. So it shouldn't qualify as an alias? Additionally this is intended as an automated mechanism to improve the quality of diagnostics, rather than a manual mechanism (like attributes) which would need to be opted into.

While both cases do touch on improving diagnostics for missing methods, I don't think these are equivalent.

[Centril]

No but TcpStream::connect does exist, so e.g., #[doc(alias(TcpStream::new))] on it could add the information necessary (see e.g. #[doc(alias = "memcpy")] on copy_nonoverlapping) for the compiler to provide the diagnostic.

But I suppose the heuristic you've suggested might work without attributes, although I suspect it would suggest too many false positives, but let's reopen I guess.

[estebank]

@yoshuawuyts what do you think of the following?

error[E0599]: no function or associated item named `new` found for struct `TcpStream` in the current scope
   --> tests/ui/resolve/fn-new-doesnt-exist.rs:4:28
    |
4   |    let stream = TcpStream::new(); //~ ERROR no function or associated item named `new` found
    |                            ^^^ function or associated item not found in `TcpStream`
    |
note: if you're trying to build a new `TcpStream` consider using one of the following associated functions:
      TcpStream::connect
      TcpStream::connect_timeout
   --> /home/gh-estebank/rust/library/std/src/net/tcp.rs:156:5
    |
156 |     pub fn connect<A: ToSocketAddrs>(addr: A) -> io::Result<TcpStream> {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
172 |     pub fn connect_timeout(addr: &SocketAddr, timeout: Duration) -> io::Result<TcpStream> {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[yoshuawuyts]

@estebank oh I love it!