cmd/go: improve error message when a package does not exist

[perillo]

What did you do?

In a simple Go program I have a misspelled package import.
As an example

package main

import "logz"

What did you expect to see?

The go tool should report an error message with the file, line and column where the error occurred.
As an example:

main.go:3:2: cannot find package "logz" in any of:
	/usr/lib/go/src/logz (from $GOROOT)
	/home/manlio/.local/lib/go/src/logz (from $GOPATH)
	/home/manlio/code/src/go/src/logz

What did you see instead?

build xxx: cannot load logz: cannot find module providing package logz

There is no additional information; just the module where the problem occurred and the error message.

System details

go version go1.12 linux/amd64
GOARCH="amd64"
GOBIN="/home/manlio/.local/bin"
GOCACHE="/home/manlio/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/manlio/.local/lib/go:/home/manlio/code/src/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/lib/go"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/manlio/code/src/python/app/lci/isosmart.it/go.mod"
GOROOT/bin/go version: go version go1.12 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.12
uname -sr: Linux 4.20.13-arch1-1-ARCH
/usr/lib/libc.so.6: GNU C Library (GNU libc) stable release version 2.28.
gdb --version: GNU gdb (GDB) 8.2.1

[perillo]

Of course the expected output should be something like:

build xxx: main.go:3:2: cannot load logz: cannot find module providing package logz

Thanks.

[bcmills]

See also #30661 and #27102.

(CC @jayconrod)

[manishmshiva]

Yeah. This looks far better than the original log. I can try and fix this if @jayconrod is busy.

[jayconrod]

@manishmshiva You're welcome to take a look. I'm not sure if I'll get to this for a few days.

This error is reported by the build list loading code in src/cmd/go/internal/modload/load.go. Some additional plumbing will be required to store the import positions.

[bcmills]

@jayconrod, was this fixed by your import-stack changes?

[jayconrod]

I'm not sure when this was fixed, but the output in go1.13beta1 looks good:

$ go build ./a
b/b.go:3:8: unknown import path "example.com/logz": cannot find module providing package example.com/logz

It's debatable whether cannot find module providing package communicates enough information, but #28459 seems more specific to that. The problem in this issue was that we didn't print the location where the error occurred.