cmd/go: build is slow to report misspelled package name

[adonovan]

The go command's support for versioned modules means that go build takes 12 seconds to report a typo in a package name.

$ pwd
/home/adonovan/got/src/golang.org/x/tools

$ time go build ./cmd/ssadump
real	0m0.185s
user	0m0.332s
sys	0m0.140s

$ time go build ./cmd/ssadum
go: finding golang.org/x/tools/cmd/ssadum latest
go: finding golang.org/x/tools/cmd latest
go: import "golang.org/x/tools/cmd/ssadum": cannot find module providing package golang.org/x/tools/cmd/ssadum

real	0m12.422s
user	0m0.200s
sys	0m0.096s

[iamoryanmoshe]

What has changed that caused it?
Was it shorter before?

[cespare]

@oryanmoshe yes, module semantics have made this slow. On my machine (Go 1.10) it takes about 10ms if I misspell the name.

[iamoryanmoshe]

@cespare
Thanks, I didn’t know what to look for until you said modules.. Didn't get the chance to dive into modules internals yet.

I'll look at it and see if I can spot why it's taking so long.

[bcmills]

That's especially odd since, at least according to my understanding, patterns starting with ./ should only match packages in the current module (see discussion on #26317).

[bcmills]

@alandonovan, are you seeing this slowness for all builds, or only those with modules enabled?

[rsc]

The reason is clear from the output: it tries to do a lookup for a different module that might provide ssadum. We need to avoid that.

[gopherbot]

Change https://golang.org/cl/129061 mentions this issue: cmd/go: do not turn list ./nonexist into a network lookup

[gopherbot]

Change https://golang.org/cl/151800 mentions this issue: cmd/go: emit go list error for local non-existant packages