internal/imports: merge import declarations

When an import is added to the ast, the import declarations are merged
together into the first import declaration. Since this is a part of
the formatting functionality of goimports, do this during formatting
regardless.

The merging pass was added to astutil.AddNamedImport in order to address
issue golang/go#14075. This joined imports from other blocks into the first
import declaration, so that a single block of imports isn't split across
multiple blocks.

This functionality is more of a formatting change than a fix imports
change, in line with sorting the imports, which occurs even when
FormatOnly. The formatting was only applied when an import was added
(not renamed or deleted). This change makes formatting by goimports
more consistent across runs and is not dependent on the exact fixes
that need to be applied.

Change-Id: Icb90bf694ff35e2d6405a3d477cf82fcd3e697e0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/189941
Run-TryBot: Suzy Mueller <suzmue@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

internal/imports/fix_test.go

@@ -372,7 +372,37 @@ func foo() {
 }
 `,
 	},
+	// Merge import blocks, even when no additions are required.
+	{
+		name: "merge_import_blocks_no_fix",
+		in: `package foo
 
+import (
+	"fmt"
+)
+import "os"
+
+import (
+	"rsc.io/p"
+)
+
+var _, _ = os.Args, fmt.Println
+var _, _ = snappy.ErrCorrupt, p.P
+`,
+		out: `package foo
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/golang/snappy"
+	"rsc.io/p"
+)
+
+var _, _ = os.Args, fmt.Println
+var _, _ = snappy.ErrCorrupt, p.P
+`,
+	},
 	// Delete existing empty import block
 	{
 		name: "delete_empty_import_block",

internal/imports/imports.go

@@ -136,6 +136,7 @@ func initialize(filename string, src []byte, opt *Options) ([]byte, *Options, er
 }
 
 func formatFile(fileSet *token.FileSet, file *ast.File, src []byte, adjust func(orig []byte, src []byte) []byte, opt *Options) ([]byte, error) {
+	mergeImports(opt.Env, fileSet, file)
 	sortImports(opt.Env, fileSet, file)
 	imps := astutil.Imports(fileSet, file)
 	var spacesBefore []string // import paths we need spaces before

internal/imports/sortimports.go

@@ -58,6 +58,53 @@ func sortImports(env *ProcessEnv, fset *token.FileSet, f *ast.File) {
 	}
 }
 
+// mergeImports merges all the import declarations into the first one.
+// Taken from golang.org/x/tools/ast/astutil.
+func mergeImports(env *ProcessEnv, fset *token.FileSet, f *ast.File) {
+	if len(f.Decls) <= 1 {
+		return
+	}
+
+	// Merge all the import declarations into the first one.
+	var first *ast.GenDecl
+	for i := 0; i < len(f.Decls); i++ {
+		decl := f.Decls[i]
+		gen, ok := decl.(*ast.GenDecl)
+		if !ok || gen.Tok != token.IMPORT || declImports(gen, "C") {
+			continue
+		}
+		if first == nil {
+			first = gen
+			continue // Don't touch the first one.
+		}
+		// We now know there is more than one package in this import
+		// declaration. Ensure that it ends up parenthesized.
+		first.Lparen = first.Pos()
+		// Move the imports of the other import declaration to the first one.
+		for _, spec := range gen.Specs {
+			spec.(*ast.ImportSpec).Path.ValuePos = first.Pos()
+			first.Specs = append(first.Specs, spec)
+		}
+		f.Decls = append(f.Decls[:i], f.Decls[i+1:]...)
+		i--
+	}
+}
+
+// declImports reports whether gen contains an import of path.
+// Taken from golang.org/x/tools/ast/astutil.
+func declImports(gen *ast.GenDecl, path string) bool {
+	if gen.Tok != token.IMPORT {
+		return false
+	}
+	for _, spec := range gen.Specs {
+		impspec := spec.(*ast.ImportSpec)
+		if importPath(impspec) == path {
+			return true
+		}
+	}
+	return false
+}
+
 func importPath(s ast.Spec) string {
 	t, err := strconv.Unquote(s.(*ast.ImportSpec).Path.Value)
 	if err == nil {