x/tools/gopls: No warning "loop variable xx captured by func literal".

[tttoad]

gopls version

golang.org/x/tools/gopls v0.11.0
golang.org/x/tools/gopls@v0.11.0 h1:/nvKHdTtePQmrv9XN3gIUN9MOdUrKzO/dcqgbG6x8EY=
github.com/BurntSushi/toml@v1.2.1 h1:9F2/+DoOYIOksmaJFPw1tGFy1eDnIJXg+UHjuD8lTak=
github.com/google/go-cmp@v0.5.9 h1:O2Tfq5qg4qc4AmwVlvv0oLiVAGB7enBSJ2x2DqQFi38=
github.com/sergi/go-diff@v1.1.0 h1:we8PVUC3FE2uYfodKH/nBHMSetSfHDR6scGdBi+erh0=
golang.org/x/exp@v0.0.0-20221031165847-c99f073a8326 h1:QfTh0HpN6hlw6D3vu8DAwC8pBIwikq0AI1evdm+FksE=
golang.org/x/exp/typeparams@v0.0.0-20221031165847-c99f073a8326 h1:fl8k2zg28yA23264d82M4dp+YlJ3ngDcpuB1bewkQi4=
golang.org/x/mod@v0.7.0 h1:LapD9S96VoQRhi/GrNTqeBJFrUjs5UHCAtTlgwA5oZA=
golang.org/x/sync@v0.1.0 h1:wsuoTGHzEhffawBOhz5CYhcrV4IdKZbEyZjBMuTp12o=
golang.org/x/sys@v0.2.0 h1:ljd4t30dBnAvMZaQCevtY0xLLD0A+bRZXbgLMLU1F/A=
golang.org/x/text@v0.4.0 h1:BrVqGRd7+k1DiOgtnFvAkoQEWQvBc25ouMJM6429SFg=
golang.org/x/tools@v0.3.1-0.20221213193459-ca17b2c27ca8 h1:7/HkGkN/2ktghBCSRRgp31wAww4syfsW52tj7yirjWk=
golang.org/x/vuln@v0.0.0-20221109205719-3af8368ee4fe h1:qptQiQwEpETwDiz85LKtChqif9xhVkAm8Nhxs0xnTww=
honnef.co/go/tools@v0.3.3 h1:oDx7VAwstgpYpb3wv0oxiZlxY+foCpRAwY7Vk6XpAgA=
mvdan.cc/gofumpt@v0.4.0 h1:JVf4NN1mIpHogBj7ABpgOyZc65/UUOkKQFkoURsz4MM=
mvdan.cc/xurls/v2@v2.4.0 h1:tzxjVAj+wSBmDcF6zBB7/myTy3gX9xvi8Tyr28AuQgc=
go: go1.19

go env

GO111MODULE=""
GOARCH="arm64"
GOBIN=""
GOCACHE="/Users/toad/Library/Caches/go-build"
GOENV="/Users/toad/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/toad/go/go1.19/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/toad/go/go1.19"
GOPRIVATE=""
GOPROXY="https://goproxy.cn,direct"
GOROOT="/opt/homebrew/Cellar/go/1.19/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/opt/homebrew/Cellar/go/1.19/libexec/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.19"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/toad/work/gopls-demo/go.mod"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/f
olders/g1/tgmnlrdn3vxgv08kdgh9vpkw0000gn/T/go-build537849748=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

package main

import (
	"fmt"
)

func main() {
	al := make([]string, 0)
	al = append(al, "123", "456")

	for _, a := range al {
		go func() {
			fmt.Println(a)
		}()

		go func(a string) {
			fmt.Println(a)
		}(a)
	}
}

What did you expect to see?

in line 13: warning, loop variable a captured by func literal.

What did you see instead?

2023-05-06.10.18.06.mov

[findleyr]

This is working as intended. The loopclosure analyzer only catches references in go statements that are the last statement in a loop body. Otherwise, the goroutine could be synchronized by a number of means, causing the analyzer to produce false positives.

Knowing whether or not the loop value escapes the loop iteration is a surprisingly tricky problem.

Note that there is discussion about changing loop variable semantics in #56010.

CC @adonovan @timothy-king

[timothy-king]

This specific example can be caught by an alternative definition of "last statements". Here is an example WiP that IIRC can detect this specific example: https://go-review.git.corp.google.com/c/tools/+/455195 . The 'lastness' is modified 'statement must be able to reach the end of the loop'. These definitions are unfortunately a bit hard to predict. It still does not take much to stop such analyzers. Any expression that may panic or function call stops going backwards.

func foo() {
	...
	for _, a := range al {
		go func() {
			fmt.Println(a)
		}()
		extraCall() // extra function call that can panic or potentially synchronize
		go func(a string) {
			fmt.Println(a)
		}(a)
	}
}

Neither syntactic definition ends up entirely satisfying.

Efforts in this direction may end up somewhat moot due to #56010.