cmd/vet: ensure errors implement Is, As, and Unwrap anonymous interfaces

[carnott-snap]

The interfaces for errors.Is and errors.As seem stable and complete. Unfortunately, it requires reading through the godoc to ensure that your custom error type correctly implements interface{ As(interface{}) bool }, interface{ Is(error) bool }, or interface{ Unwrap() error }. This can lead to implementation bugs that fail pathologically.

Instead of exposing these symbols, as was proposed in #39539, we can add a vet check to ensure that errors with Is, As, or Unwrap methods implement the correct interfaces. This is not a requirement that all errors implement Is, As, and Unwrap, but simply that if the symbol exists, the syntax matches.

Technically, this is not required, as you can use an anonymous type to perform this check at compile time. But this is ungainly, not automatic, and fails to the same typographical issues due to its distributed implementation:

var (
        _ = (interface{ As(interface{}) bool })(myError{})
        _ = (interface{ Is(error) bool })(myError{})
        _ = (interface{ Unwrap() error })(myError{})
        _ = error(myError{})

[carnott-snap]

@ianlancetaylor: can you add this to the proposals project?

[ianlancetaylor]

@carnott-snap Done.

[urandom2]

did this get lost in the incoming stack?

[ianlancetaylor]

The incoming stack has more than 200 issues in it, so it's going to take a while to get through them all.

See https://github.com/golang/go/projects/1 .

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[neild]

I don't object to cmd/vet checking the signature of As/Is/Unwrap methods for types that implement the error interface; it seems unlikely to have significant false positives. However, the best way to ensure an error implements these interfaces correctly is to write a test for the error, because a test will validate not only the method signature but the implementation.

[gopherbot]

Change https://golang.org/cl/312829 mentions this issue: passes/stdmethods: warn when an error has an unexpected Is, As, or Unwrap method.

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

[timothy-king]

Tested golang.org/cl/312829 against a large Go codebase that uses Is, As and Unwrap. No findings so no false positives.