cmd/vet: printf checker does not know about fmt.Formatter

[josharian]

The printf checker should allow any verb for a value that implements fmt.Formatter. It does not.

package main

import "fmt"

type T int

func (t T) Format(f fmt.State, c rune) {
    f.Write([]byte{byte(c)})
}

func main() {
    var t T
    fmt.Printf("%z\n", t)
}

$ go run formatter.go 
z
$ go tool vet formatter.go 
formatter.go:13: unrecognized printf verb 'z'

As an aside, it looks like the implementation of fmt.Stringer detection in cmd/vet could be simplified by using go/types-based interface satisfaction analysis. (See the copylock check for an example.) I suspect the printf check was written before go/types had that functionality.

cc @valyala @robpike

[josharian]

When this is fixed, we should revert CL 28959.

[gopherbot]

CL https://golang.org/cl/32851 mentions this issue.

[gopherbot]

CL https://golang.org/cl/33166 mentions this issue.