go/ast/astutil: add merging pass to AddImport and AddNamedImport.

After inserting an import, merge all import declarations.

Fixes golang/go#14075.

Change-Id: I17fceb60f490deced2ee8eadf78091720580ffa1
Reviewed-on: https://go-review.googlesource.com/19828
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

go/ast/astutil/imports.go

@@ -143,6 +143,30 @@ func AddNamedImport(fset *token.FileSet, f *ast.File, name, ipath string) (added
 	}
 
 	f.Imports = append(f.Imports, newImport)
+
+	if len(f.Decls) <= 1 {
+		return true
+	}
+
+	// Merge all the import declarations into the first one.
+	var first *ast.GenDecl
+	for i, decl := range f.Decls {
+		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.
+		}
+		// 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:]...)
+	}
+
 	return true
 }
 

go/ast/astutil/imports_test.go

@@ -396,6 +396,119 @@ import (
 	"fmt"   // b
 	"log"
 )
+`,
+	},
+	// Issue 14075: Merge import declarations
+	{
+		name: "issue 14075",
+		pkg:  "bufio",
+		in: `package main
+
+import "bytes"
+import "fmt"
+`,
+		out: `package main
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+)
+`,
+	},
+	{
+		name: "issue 14075 update position",
+		pkg:  "bufio",
+		in: `package main
+
+import "bytes"
+import (
+	"fmt"
+)
+`,
+		out: `package main
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+)
+`,
+	},
+	{
+		name: `issue 14075 ignore import "C"`,
+		pkg:  "bufio",
+		in: `package main
+
+// Comment
+import "C"
+
+import "bytes"
+import "fmt"
+`,
+		out: `package main
+
+// Comment
+import "C"
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+)
+`,
+	},
+	{
+		name: `issue 14075 ignore adjacent import "C"`,
+		pkg:  "bufio",
+		in: `package main
+
+// Comment
+import "C"
+import "fmt"
+`,
+		out: `package main
+
+// Comment
+import "C"
+import (
+	"bufio"
+	"fmt"
+)
+`,
+	},
+	{
+		name: `issue 14075 ignore adjacent import "C" (without factored import)`,
+		pkg:  "bufio",
+		in: `package main
+
+// Comment
+import "C"
+import "fmt"
+`,
+		out: `package main
+
+// Comment
+import "C"
+import (
+	"bufio"
+	"fmt"
+)
+`,
+	},
+	{
+		name: `issue 14075 ignore single import "C"`,
+		pkg:  "bufio",
+		in: `package main
+
+// Comment
+import "C"
+`,
+		out: `package main
+
+// Comment
+import "C"
+import "bufio"
 `,
 	},
 }