x/tools/go/analysis/passes/fieldalignment: fix reorders declaration but not unkeyed struct literals, changing behavior

[Dinichthys]

Go version

1.27 linux/amd64

Output of go env in your module/workspace:

AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/home/dinichthys/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/dinichthys/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build2612980177=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/dev/null'
GOMODCACHE='/home/dinichthys/Codiruiy/Work/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/dinichthys/Codiruiy/Work/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/dinichthys/Codiruiy/Work/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/dinichthys/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/home/dinichthys/Codiruiy/Work/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.27-devel_770243423a Sat Apr 4 16:59:04 2026 +0300'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

For example, we have structure A with following order

type A struct {
	b byte
	c int
	d byte
}

// And following usage of this structure
tmp := A{'a', 3, 'c'}

We can use fieldalignment with flag -fix to try to pack this structure effective. It will reorder fields in the structure, but won't reorder elements in curly brackets.

What did you see happen?

Code will transform into

type A struct {
	c int   // Order changed. Fields 'c' and 'b' were swapped
	b byte
	d byte
}

// So usage become invalid, but it still could be compiled, so the result of the program unexpected changed.
// It is UB, but also we can get uncompiled code same way.
tmp := A{'a', 3, 'c'}

What did you expect to see?

We should transform AST with all place of usage structure.

Expected result:

type A struct {
	c int   // Order changed. Fields 'c' and 'b' were swapped
	b byte
	d byte
}

tmp := A{3, 'a', 'c'}

[gabyhelp]

Related Issues

• x/tools/go/analysis/passes/fieldalignment: suggested fix producing garbled output #43233 (closed)
• cmd/go: `go fix` embedded literal produces non-compiling program #80197
• x/tools/go/analysis/passes/fieldalignment/cmd/fieldalignment: comments are removed #54333
• x/tools/go/analysis: overlapping suggested fixes result in a single, unhelpful error message #56535 (closed)
• x/tools: fieldalignment -fix can cause comments to be lost #60567 (closed)
• go fix crashes on code #80274
• proposal: cmd/go: flag to run fieldalignment check #50826 (closed)
• x/tools/gopls: fieldalignment analyzer reports confusing "errors" about memory layout #67762 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[noobyogi0010]

Hey, if this issue is not worked upon by someone else, I would like to take it up.
Thanks!

[adonovan]

I think the appropriate remedy it to suppress the diagnostic if there are unkeyed literals of the struct type in the same package. (Using unkeyed struct literals from another package is a mistake: reordering fields is regarded as a permitted compatible change.)

[Dinichthys]

It is a good variant, but we also need to manage using of unsafe pointers, because sometimes it could be used to work with structures

[dr2chase]

People who use unsafe code assume all these risks. The default answer to "what about unsafe code?" is "not our problem". There are some non-default cases, but they are special.

[Dinichthys]

I have an implementation that adds keyes to all unkeyed struct literals. I added it as a mod of fieldalignment, which can be enabled via the flag -constructor. We can also add warning if we see unkeyed literals of the analyzed struct and suppress the diagnostic as Alan suggested. I can implement it and open a CL if it is a good solution.

[adonovan]

I have an implementation that adds keyes to all unkeyed struct literals. I added it as a mod of fieldalignment, which can be enabled via the flag -constructor. We can also add warning if we see unkeyed literals of the analyzed struct and suppress the diagnostic as Alan suggested. I can implement it and open a CL if it is a good solution.

This feature already exists in the composite analyzer.