Update, Oct 7 2020: see #27628 (comment) for most recent proposal in this issue.

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

tip (2e5c325), also Go 1.11

Does this issue reproduce with the latest release?

Yes

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

darwin/amd64

What did you do?

$ go build -toolexec=/usr/bin/time hello.go
# command-line-arguments
        0.01 real         0.00 user         0.00 sys
# command-line-arguments
        0.12 real         0.11 user         0.02 sys
$ go build hello.go 
# command-line-arguments
        0.01 real         0.00 user         0.00 sys
# command-line-arguments
        0.12 real         0.11 user         0.02 sys
$

What did you expect to see?

The second invocation of go build doesn't have -toolexec, so it should not invoke the toolexec command (which I think it doesn't), nor reprint its output.

What did you see instead?

toolexec output is reprinted.

In fact, I think it probably should not cache at all if -toolexec is specified, since the external command that toolexec invokes may do anything, and (intentionally) not reproducible.

cc @dr2chase