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cmd/go: confusing compiler errors because of _gomod_.go #28569

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Dirbaio opened this issue Nov 2, 2018 · 6 comments

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@Dirbaio
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commented Nov 2, 2018

When compiling the following inside a Go module project (containing go.mod in a parent directory):

package main

var string = "variable named 'string'"

func main() {}

you get the following very confusing error message:

dirbaio@jupiter:~/reby/backend/nexus$ go build weirderror/main.go 
# command-line-arguments
/tmp/go-build698262353/b001/_gomod_.go:7:22: string is not a type

It seems this is because there is an autogenerated file trying to declare a variable of type string.

dirbaio@jupiter:~/reby/backend/nexus$ go build -x weirderror/main.go 
WORK=/tmp/go-build835159733
mkdir -p $WORK/b001/
cat >$WORK/b001/_gomod_.go << 'EOF' # internal

		package main
		import _ "unsafe"
		//go:linkname __debug_modinfo__ runtime/debug.modinfo
		var __debug_modinfo__ string
		func init() {
			__debug_modinfo__ = "0w\xaf\f\x92t\b\x02A\xe1\xc1\a\xe6\xd6\x18\xe6path\tgithub.com/rebythecode/backend/nexus/weirderror\nmod\tgithub.com/rebythecode/backend\t(devel)\t\n\xf92C1\x86\x18 r\x00\x82B\x10A\x16\xd8\xf2"
		}
	EOF
cat >$WORK/b001/importcfg << 'EOF' # internal
# import config
packagefile runtime=/usr/lib/go/pkg/linux_amd64/runtime.a
EOF
cd /home/dirbaio/reby/backend/nexus/weirderror
/usr/lib/go/pkg/tool/linux_amd64/compile -o $WORK/b001/_pkg_.a -trimpath $WORK/b001 -p main -complete -buildid _XG4Y2LGaLdfLVCNTPfu/_XG4Y2LGaLdfLVCNTPfu -goversion go1.11.1 -D _/home/dirbaio/reby/backend/nexus/weirderror -importcfg $WORK/b001/importcfg -pack -c=4 ./main.go $WORK/b001/_gomod_.go
# command-line-arguments
/tmp/go-build835159733/b001/_gomod_.go:7:22: string is not a type

This does not happen when trying to build in a non-modules project.

Go version and env:

dirbaio@jupiter:~$ go version
go version go1.11.1 linux/amd64
dirbaio@jupiter:~$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/dirbaio/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/dirbaio/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=""
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-build429522809=/tmp/go-build -gno-record-gcc-switches"
@mvdan

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commented Nov 2, 2018

/cc @rsc @bcmills @hyangah, the three people who have touched the code generation in question.

I presume there could be other ways to break it, such as declaring __debug_modinfo__, or assigning to it in one's init function. I'm not sure what a good fix here could be.

@mvdan mvdan changed the title "string is not a type" confusing error message cmd/go: confusing compiler errors because of _gomod_.go Nov 2, 2018

@hyangah

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commented Nov 5, 2018

@rsc I think it's better to add the info in a way similar to link's -X flag or gc's buildid recording.

@bcmills

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commented Nov 5, 2018

Can you still reproduce this with a go command built from source?

https://golang.org/cl/134915 should have fixed the string is not a type error. (See #27584.)

@bcmills bcmills added this to the Go1.12 milestone Nov 5, 2018

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commented Nov 5, 2018

You're right @bcmills, the particular error seems like a duplicate. I wonder if we should still be worried about the __debug_modinfo__ name, although that seems extremely unlikely to cause issues.

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commented Nov 5, 2018

It's theoretically a problem, but empirically much less likely than shadowing string.

@Dirbaio

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commented Nov 6, 2018

I just checked. The error is not present in current master.

@Dirbaio Dirbaio closed this Nov 6, 2018

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