cmd/cover: support -mod and other build flags

[dtelyukh]

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

$ go version
go version go1.13.4 linux/amd64

Does this issue reproduce with the latest release?

Yes, it does

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/dionysius/.cache/go-build"
GOENV="/home/dionysius/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/dionysius/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build322712138=/tmp/go-build -gno-record-gcc-switches"

What did you do?

GO_PATH/~/go/pkg/mod/ dir is empty

$go tool cover -html=../coverage.cov -o ../coverage.html

What did you expect to see?

After running the cover command GO_PATH/~/go/pkg/mod/ dir is still empty

What did you see instead?

After running the cover command GO_PATH/~/go/pkg/mod/ dir is not empty

[dmitshur]

/cc @jayconrod @bcmills This may be related to cmd/go or cmd/cover, but I'm not sure who owns the cover cool.

I don't see cmd/cover mentioned in #24661. We should add it there so it's easier to know whether it supports module mode.

[jayconrod]

This is similar to #35200, but this is for cmd/cover instead of cmd/doc.

Both commands invoke the go list internally. Neither command provides a way to pass through build flags. We should at least support -mod.

@dtelyukh You can work around this in Go 1.13 by adding -mod=vendor to the GOFLAGS environment variable when invoking go tool cover. For example GOFLAGS=-mod=vendor go tool cover -html=../coverage.cov -o ../coverage.html.