command: Restore single-file support in "terraform fmt"

This possibility was lost in the rewrite to use HCL2, but it's used by
a number of external utilities and text editor integrations, so we'll
restore it here.

Using the stdin/stdout mode is generally preferable for text editor use
since it allows formatting of the in-memory buffer rather than directly
the file on disk, but for editors that don't have support for that sort of
tooling it can be convenient to just launch a single command and directly
modify the on-disk file.

command/fmt.go

@@ -64,14 +64,14 @@ func (c *FmtCommand) Run(args []string) int {
 		return 1
 	}
 
-	var dirs []string
+	var paths []string
 	if len(args) == 0 {
-		dirs = []string{"."}
+		paths = []string{"."}
 	} else if args[0] == stdinArg {
 		c.list = false
 		c.write = false
 	} else {
-		dirs = []string{args[0]}
+		paths = []string{args[0]}
 	}
 
 	var output io.Writer
@@ -86,7 +86,7 @@ func (c *FmtCommand) Run(args []string) int {
 		output = &cli.UiWriter{Ui: c.Ui}
 	}
 
-	diags := c.fmt(dirs, c.input, output)
+	diags := c.fmt(paths, c.input, output)
 	c.showDiagnostics(diags)
 	if diags.HasErrors() {
 		return 2
@@ -123,8 +123,33 @@ func (c *FmtCommand) fmt(paths []string, stdin io.Reader, stdout io.Writer) tfdi
 
 	for _, path := range paths {
 		path = c.normalizePath(path)
+		info, err := os.Stat(path)
+		if err != nil {
+			diags = diags.Append(fmt.Errorf("No file or directory at %s", path))
+			return diags
+		}
+		if info.IsDir() {
 			dirDiags := c.processDir(path, stdout)
 			diags = diags.Append(dirDiags)
+		} else {
+			switch filepath.Ext(path) {
+			case ".tf", ".tfvars":
+				f, err := os.Open(path)
+				if err != nil {
+					// Open does not produce error messages that are end-user-appropriate,
+					// so we'll need to simplify here.
+					diags = diags.Append(fmt.Errorf("Failed to read file %s", path))
+					continue
+				}
+
+				fileDiags := c.processFile(c.normalizePath(path), f, stdout, false)
+				diags = diags.Append(fileDiags)
+				f.Close()
+			default:
+				diags = diags.Append(fmt.Errorf("Only .tf and .tfvars files can be processed with terraform fmt"))
+				continue
+			}
+		}
 	}
 
 	return diags

command/fmt_test.go

@@ -29,7 +29,7 @@ func TestFmt_nonexist(t *testing.T) {
 		t.Fatalf("wrong exit code. errors: \n%s", ui.ErrorWriter.String())
 	}
 
-	expected := "There is no configuration directory at"
+	expected := "No file or directory at"
 	if actual := ui.ErrorWriter.String(); !strings.Contains(actual, expected) {
 		t.Fatalf("expected:\n%s\n\nto include: %q", actual, expected)
 	}
@@ -148,6 +148,33 @@ func TestFmt_directoryArg(t *testing.T) {
 	}
 }
 
+func TestFmt_fileArg(t *testing.T) {
+	tempDir := fmtFixtureWriteDir(t)
+
+	ui := new(cli.MockUi)
+	c := &FmtCommand{
+		Meta: Meta{
+			testingOverrides: metaOverridesForProvider(testProvider()),
+			Ui:               ui,
+		},
+	}
+
+	args := []string{filepath.Join(tempDir, fmtFixture.filename)}
+	if code := c.Run(args); code != 0 {
+		t.Fatalf("wrong exit code. errors: \n%s", ui.ErrorWriter.String())
+	}
+
+	got, err := filepath.Abs(strings.TrimSpace(ui.OutputWriter.String()))
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := filepath.Join(tempDir, fmtFixture.filename)
+
+	if got != want {
+		t.Fatalf("wrong output\ngot:  %s\nwant: %s", got, want)
+	}
+}
+
 func TestFmt_stdinArg(t *testing.T) {
 	input := new(bytes.Buffer)
 	input.Write(fmtFixture.input)