cmd/go: poor error message for nonmatching go-import tags

[robpike]

What version of Go are you using (go version)?

$ go version
go version devel +1e6ad65b43 Tue Sep 8 18:52:38 2020 +0000 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

Catalina, MacOS, amd64.

What did you do?

I typed by mistake the wrong import string; I forgot the cmd, and got 1472 characters of this:

% go get robpike.io/tab
package robpike.io/tab: unrecognized import path "robpike.io/tab": parse https://robpike.io/tab?go-get=1: no go-import meta tags (meta tag robpike.io/cmd/doc did not match import path robpike.io/tab, meta tag robpike.io/ivy did not match import path robpike.io/tab, meta tag robpike.io/cmd/freq did not match import path robpike.io/tab, meta tag robpike.io/cmd/hira did not match import path robpike.io/tab, meta tag robpike.io/cmd/kana did not match import path robpike.io/tab, meta tag robpike.io/cmd/kata did not match import path robpike.io/tab, meta tag robpike.io/cmd/now did not match import path robpike.io/tab, meta tag robpike.io/cmd/strings did not match import path robpike.io/tab, meta tag robpike.io/cmd/translate did not match import path robpike.io/tab, meta tag robpike.io/cmd/typo did not match import path robpike.io/tab, meta tag robpike.io/cmd/unicode did not match import path robpike.io/tab, meta tag robpike.io/filter did not match import path robpike.io/tab, meta tag robpike.io/toy did not match import path robpike.io/tab, meta tag robpike.io/nihongo did not match import path robpike.io/tab, meta tag robpike.io/cmd/scrub did not match import path robpike.io/tab, meta tag robpike.io/cmd/tab did not match import path robpike.io/tab, meta tag robpike.io/cmd/sid did not match import path robpike.io/tab, meta tag robpike.io/cmd/script did not match import path robpike.io/tab, meta tag robpike.io/lisp did not match import path robpike.io/tab)
% go get robpike.io/cmd/tab
%

Reflowed using fmt, that's:

package robpike.io/tab: unrecognized import path "robpike.io/tab":
parse https://robpike.io/tab?go-get=1: no go-import meta tags (meta
tag robpike.io/cmd/doc did not match import path robpike.io/tab,
meta tag robpike.io/ivy did not match import path robpike.io/tab,
meta tag robpike.io/cmd/freq did not match import path robpike.io/tab,
meta tag robpike.io/cmd/hira did not match import path robpike.io/tab,
meta tag robpike.io/cmd/kana did not match import path robpike.io/tab,
meta tag robpike.io/cmd/kata did not match import path robpike.io/tab,
meta tag robpike.io/cmd/now did not match import path robpike.io/tab,
meta tag robpike.io/cmd/strings did not match import path robpike.io/tab,
meta tag robpike.io/cmd/translate did not match import path
robpike.io/tab, meta tag robpike.io/cmd/typo did not match import
path robpike.io/tab, meta tag robpike.io/cmd/unicode did not match
import path robpike.io/tab, meta tag robpike.io/filter did not match
import path robpike.io/tab, meta tag robpike.io/toy did not match
import path robpike.io/tab, meta tag robpike.io/nihongo did not
match import path robpike.io/tab, meta tag robpike.io/cmd/scrub did
not match import path robpike.io/tab, meta tag robpike.io/cmd/tab
did not match import path robpike.io/tab, meta tag robpike.io/cmd/sid
did not match import path robpike.io/tab, meta tag robpike.io/cmd/script
did not match import path robpike.io/tab, meta tag robpike.io/lisp
did not match import path robpike.io/tab)

What did you expect to see?

A shorter, better-formatted error message. This one doesn't even have newlines.

% go get robpike.io/tab
package robpike.io/tab: unrecognized import path "robpike.io/tab": parse https://robpike.io/tab?go-get=1: no go-import m

What did you see instead?

A long, poorly-formatted error mess.

[bcmills]

CC @matloob @jayconrod

[bcmills]

The error text in question comes from here:

go/src/cmd/go/internal/vcs/vcs.go

Lines 975 to 981 in 9a702fd

	func (m ImportMismatchError) Error() string {
	formattedStrings := make([]string, len(m.mismatches))
	for i, pre := range m.mismatches {
	formattedStrings[i] = fmt.Sprintf("meta tag %s did not match import path %s", pre, m.importPath)
	}
	return strings.Join(formattedStrings, ", ")
	}

(The verbose error message was added in CL 25121, released in Go 1.8.)

[bcmills]

There are some other redundancies in that error string that we should eliminate too.

(This is another instance of the over-wrapping problem described in my experience report.)

[gopherbot]

Change https://golang.org/cl/261777 mentions this issue: cmd/go/internal/vcs: improve error message for mismatching go-import tags