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format: deep compare nodes without go/scanner #349

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Merged
mvdan merged 1 commit into from
Dec 15, 2025
Merged

format: deep compare nodes without go/scanner #349

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34 changes: 8 additions & 26 deletions format/format.go
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Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,6 @@ import (
"fmt"
"go/ast"
"go/parser"
"go/scanner"
"go/token"
goversion "go/version"
"os"
Expand Down Expand Up @@ -1093,42 +1092,25 @@ func (f *fumpter) shouldMergeAdjacentFields(f1, f2 *ast.Field) bool {

// Only merge if the types that the syntax nodes represent are equal,
// e.g. two *ast.Ident nodes "int" are equal, but the two *ast.Ident nodes
// "string" and "bool" are not.
// "string" and "bool" are not. We use reflection to quickly discard most cases.
//
// We use an empty [token.FileSet] so that positions are ignored when printing,
// and two syntax nodes with different uses of newlines end up the same.
//
// Note that we could in theory use go/types here, but in practice gofumpt
// needs to be fast, hence it shouldn't rely on expensive typechecking.
if reflect.TypeOf(f1.Type) != reflect.TypeOf(f2.Type) {
return false
}
emptyFset := token.NewFileSet()
var b1, b2 bytes.Buffer
if err := format.Node(&b1, f.fset, f1.Type); err != nil {
if err := format.Node(&b1, emptyFset, f1.Type); err != nil {
return false
}
if err := format.Node(&b2, f.fset, f2.Type); err != nil {
if err := format.Node(&b2, emptyFset, f2.Type); err != nil {
return false
}
return equalTokens(b1.Bytes(), b2.Bytes())
}

func equalTokens(a, b []byte) bool {
if bytes.Equal(a, b) {
return true
}

var s1, s2 scanner.Scanner
fset := token.NewFileSet()
s1.Init(fset.AddFile("", fset.Base(), len(a)), a, nil, 0)
s2.Init(fset.AddFile("", fset.Base(), len(b)), b, nil, 0)
for {
_, tok1, lit1 := s1.Scan()
_, tok2, lit2 := s2.Scan()
if tok1 != tok2 || lit1 != lit2 {
return false
}
if tok1 == token.EOF {
return true
}
}
return bytes.Equal(b1.Bytes(), b2.Bytes())
}

var posType = reflect.TypeOf(token.NoPos)
Expand Down