Change a "top level" name (e.g. a package reference) in a go file

toplevelnamechange.go

package main

import (
        "bytes"
        "go/ast"
        "go/parser"
        "go/printer"
        "go/token"
        "io/ioutil"
        "log"
        "os"
)

func main() {
        from := os.Args[1]
        to := os.Args[2]
        file := os.Args[3]

        fs := token.NewFileSet()

        // Parse the file
        fileAST, err := parser.ParseFile(fs, file, nil, parser.ParseComments)
        if err != nil {
                log.Printf("error: %v", file, err)
                return
        }

        count, collision := modifyTopLevelRefs(fileAST, from, to)
        if collision {
                log.Fatalf("%q: colliding use of %q, aborting", file, to)
        }
        if count == 0 {
                log.Printf("no uses of %q", from)
                return
        }

        cfg := &printer.Config{
                Mode:     printer.TabIndent | printer.UseSpaces,
                Tabwidth: 8,
        }

        var outputBuffer bytes.Buffer
        if err := cfg.Fprint(&outputBuffer, fs, fileAST); err != nil {
                log.Fatalf("couldn't format: %v", err)
        }

        if err := ioutil.WriteFile(file, outputBuffer.Bytes(), os.ModePerm); err != nil {
		log.Fatalf("failed to write file %q: %v", file, err)
        }
}

func modifyTopLevelRefs(f *ast.File, from, to string) (count int, collision bool) {
        ast.Walk(visitFn(func(n ast.Node) {
                sel, ok := n.(*ast.SelectorExpr)
                if !ok {
                        return
                }
                id, ok := sel.X.(*ast.Ident)
                if !ok {
                        return
                }
                if id.Obj != nil {
                        return
                }
                if id.Name == to {
                        collision = true
                }
                if id.Name == from {
                        count++
                        id.Name = to
                }
        }), f)

        return
}

type visitFn func(node ast.Node)

func (fn visitFn) Visit(node ast.Node) ast.Visitor {
        fn(node)
        return fn
}

Starting point was https://github.com/KSubedi/gomove and golang.org/x/tools/go/ast/astutil