cmd/gofmt: rewrite Allman brace style, even if it doesn't compile?

[adamtabrams]

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

$ go version
go version go1.13.1 darwin/amd64

Does this issue reproduce with the latest release?

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/aabrams7/Library/Caches/go-build"
GOENV="/Users/aabrams7/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/aabrams7/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.13.1/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.13.1/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/bt/hgxc0qbd6s3d8ccbfpdv3hqng_1znh/T/go-build684397577=/tmp/go-build -gno-record-gcc-switches -fno-common"
GOROOT/bin/go version: go version go1.13.1 darwin/amd64
GOROOT/bin/go tool compile -V: compile version go1.13.1
uname -v: Darwin Kernel Version 18.7.0: Tue Aug 20 16:57:14 PDT 2019; root:xnu-4903.271.2~2/RELEASE_X86_64
ProductName:	Mac OS X
ProductVersion:	10.14.6
BuildVersion:	18G95
lldb --version: lldb-1001.0.13.3
  Swift-5.0

What did you do?

I have a simple hello world program like this:

package main

import "fmt"

func main()
{
    fmt.Println("Hello World")
}

To fix the formatting I ran this:

 gofmt -w ./hello.go

What did you expect to see?

I expected the file to change to this:

package main

import "fmt"

func main() {
    fmt.Println("Hello World")
}

What did you see instead?

hello.go:6:1: expected declaration, found '{'

Also, the file has not been changed at all.

[ianlancetaylor]

Note that

func main()

is valid Go to indicate that the function main is declared in an assembler file. See https://golang.org/ref/spec#Function_declarations.

We could add a special case in gofmt for a function declaration followed by a block enclosed in curly braces, but it wouldn't be all that simple.

[agnivade]

This program doesn't compile. AFAIR, gofmt fails to run on files which throw compiler errors.

[bradfitz]

To @griesemer for decision.

[griesemer]

As has been already stated,

func main()
{ ... }

which is the same as func main(); { ... } is not valid.

The parser could possibly recognize the case of a stand-alone block (at the package level) preceded by a body-less function header, report an error, but assume the writer meant the "correct" program. We do this in other places in the parser, for better error recovery. This would lead to a better experience for the programmer, which is what we generally strive for.

But the program would still be incorrect (the parser must report an error), and gofmt won't format an erroneous program. (One could imagine the parser to report a form of "soft" error, i.e., an actual syntax error that didn't result in an invalid AST. If a program only consists of "soft" errors, one might allow gofmt to go ahead. But this seems a bit overkill in this case.)

I filed #34946 to look into improved parsing.

But even with that, gofmt won't and can't reasonably go ahead with an incorrect program.