cmd/compile: what exactly should -L flag do?

[jkassis]

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

[I] xxxxx@Jeremys-MBP ~/C/ledgie> go version
go version go1.13.5 darwin/amd64

Does this issue reproduce with the latest release?

yes

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

macos 10.15.2

go env Output

[I] jkassis@Jeremys-MBP ~/C/ledgie> go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/jkassis/Library/Caches/go-build"
GOENV="/Users/jkassis/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/jkassis/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.13.5/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.13.5/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/jkassis/Code/ledgie/go.mod"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/mm/7l9_l1sn39n2kc_jpb8t0tnh0000gn/T/go-build372119512=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

[I] xxxxxx@Jeremys-MBP ~/C/ledgie> go build -gcflags="-L"  dockie/main.go
# github.com/jkassis/xxxxx.net/dockie/lib
dockie/lib/schema.go:112:1: missing return at end of function
dockie/lib/service.go:419:35: serviceDoc.V.String undefined (type *ServiceDocV has no field or method String)

What did you expect to see?

full paths in the errors list

What did you see instead?

partial paths

[jkassis]

thanks for looking at this... it's significant because IDEs like VSCode cannot click through to the filename listed in the terminal if it is not rooted in the workspace.

[jkassis]

should be an easy one i would think.

[odeke-em]

Thank you for the report @jkassis and welcome to the Go project!
Sorry that we didn’t look at this for Go1.15, but I shall put it on the backlog and assign to myself for perhaps Go1.16.

[aclements]

I tried this back to 1.10 and as far as I can tell, -L has never worked. This was introduced (in its current incarnation) in CL 77090 by @griesemer . @griesemer , can you comment?

[griesemer]

@aclements Will investigate.

[griesemer]

Note that CL 77090 explicitly states that -L prints absolute positions for positions following line directives. Given

package main

var _ = int

//line foo.go:42
var x = y

func /*line bar.go:7:1*/ init(int){}

(full path $HOME/tmp/main.go) I get the following outputs when invoking the compiler in a variety of combinations:

$ go tool compile main.go
main.go:3:9: type int is not an expression
foo.go:42: undefined: y
bar.go:7:2: func init must have no arguments and no return values

$ go tool compile -L main.go
main.go:3:9: type int is not an expression
foo.go:42[main.go:6:9]: undefined: y
bar.go:7:2[main.go:8:26]: func init must have no arguments and no return values

$ go build main.go
# command-line-arguments
./main.go:3:9: type int is not an expression
foo.go:42: undefined: y
bar.go:7:2: func init must have no arguments and no return values

$ go build -gcflags=-L main.go
# command-line-arguments
./main.go:3:9: type int is not an expression
foo.go:42[/Users/gri/tmp/main.go:6:9]: undefined: y
bar.go:7:2[/Users/gri/tmp/main.go:8:26]: func init must have no arguments and no return values

Thus, -L does work as defined, but perhaps not as expected.

It shouldn't be difficult to make -L work for all positions if that is what this issue is asking for.

[griesemer]

Since -L "doesn't work" as expected, it is unlikely that there is widespread use, so we have a chance to correct the current behavior.

We have a variety of options for reporting error positions: file paths can be relative (to cwd) or absolute. And positions can be relative to the most recent //line directive (or start of file), or absolute (pointing to actual file containing the source that's being compiled).

We can abbreviate these 4 possible combinations with 4 letters: r and a stand for relative vs absolute positions using relative paths, and R and A stand for the respective positions r and a but using absolute paths. Note though that it's not clear what R means: The filename is simply taken from the //line directive, and making it "absolute" (say, if it doesn't start with a /) is not obviously correct. So we really only have 3 meaningful formats: r, a, and A.

Currently, without the -L flag, errors use the r format:

r: message

With the -L flag, errors use one if these formats:

r: message       // example: main.go:3:9: type int is not an expression
r[A]: message    // example: foo.go:42[/Users/gri/tmp/main.go:6:9]: undefined: y

where the r[A] version is only used if r != a, that is, if the position is after a //line directive.

There are many possible combinations (r, r[a], r[A], a, a[r], a[A], etc.), not all of which are meaningful.

I suspect (from the original comment in this issue) that the expectation for -L is that it simply switches from relative to absolute filenames. Furthermore, if we have a relative position, we probably should always provide the absolute position in []. Thus, the suggestion is to change the behavior as follows:

1. If relative and absolute position are the same (r == a), i.e., if the file position is not dependent on a //line directive, the format is:

a: message

if there's no -L flag; or

A: message

if -L is given.

2. If relative and absolute positions are different, i.e., if the file position is dependent on a //line directive, the format is:

r[a]: message

if there's no -L flag; or

r[A]

if -L is given.

Comments?

[gopherbot]

This issue is currently labeled as early-in-cycle for Go 1.17.
That time is now, so a friendly reminder to look at it again.

[ianlancetaylor]

Pushing to Go1.18 milestone.

[cagedmantis]

This issue is currently labeled as early-in-cycle for Go 1.18.
That time is now, so this is a friendly ping so the issue is looked at again.

[griesemer]

Not going to happen for 1.18; we have more urgent issues.