Update WriteFile support for VFS

Author: rbuckton

internal/vfs/vfs.go

@@ -3,7 +3,6 @@ package vfs
 import (
 	"bytes"
 	"encoding/binary"
-	"errors"
 	"fmt"
 	"io/fs"
 	"os"
@@ -73,11 +72,23 @@ type RealpathFS interface {
 	Realpath(path string) (string, error)
 }
 
+type WriteFileFS interface {
+	fs.FS
+	WriteFile(path string, data []byte, perm os.FileMode) error
+}
+
+type MkdirAllFS interface {
+	fs.FS
+	MkdirAll(path string, perm os.FileMode) error
+}
+
 // FromIOFS creates a new FS from an [fs.FS].
 //
 // For paths like `c:/foo/bar`, fsys will be used as though it's rooted at `/` and the path is `/c:/foo/bar`.
 //
 // If the provided [fs.FS] implements [RealpathFS], it will be used to implement the Realpath method.
+// If the provided [fs.FS] implements [WriteFileFS], it will be used to implement the WriteFile method.
+// If the provided [fs.FS] implements [MkdirAllFS], it will be used to implement the MkdirAll method.
 //
 // Deprecated: FromIOFS does not actually handle case-insensitivity; ensure the passed in [fs.FS]
 // respects case-insensitive file names if needed. Consider using [vfstest.FromMapFS] for testing.
@@ -101,6 +112,31 @@ func FromIOFS(fsys fs.FS, useCaseSensitiveFileNames bool) FS {
 		}
 	}
 
+	var writeFile func(path string, content string, writeByteOrderMark bool) error
+	if fsys, ok := fsys.(WriteFileFS); ok {
+		writeFile = func(path string, content string, writeByteOrderMark bool) error {
+			if writeByteOrderMark {
+				content = "\uFEFF" + content
+			}
+			return fsys.WriteFile(path, []byte(content), 0o666)
+		}
+	} else {
+		writeFile = func(string, string, bool) error {
+			panic("writeFile not supported")
+		}
+	}
+
+	var mkdirAll func(path string) error
+	if fsys, ok := fsys.(MkdirAllFS); ok {
+		mkdirAll = func(path string) error {
+			return fsys.MkdirAll(path, 0o777)
+		}
+	} else {
+		mkdirAll = func(string) error {
+			panic("mkdirAll not supported")
+		}
+	}
+
 	return &vfs{
 		useCaseSensitiveFileNames: useCaseSensitiveFileNames,
 		rootFor: func(root string) fs.FS {
@@ -115,13 +151,9 @@ func FromIOFS(fsys fs.FS, useCaseSensitiveFileNames bool) FS {
 			}
 			return sub
 		},
-		realpath: realpath,
-		writeFile: func(string, string, bool) error {
-			return errors.ErrUnsupported
-		},
-		mkdirAll: func(string) error {
-			return errors.ErrUnsupported
-		},
+		realpath:  realpath,
+		writeFile: writeFile,
+		mkdirAll:  mkdirAll,
 	}
 }