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cue/load: add support for loading from io/fs.FS #4222

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cue/load: add support for loading from io/fs.FS #4222

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47 changes: 41 additions & 6 deletions cue/load/config.go
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Original file line number Diff line number Diff line change
Expand Up @@ -20,6 +20,7 @@ import (
"io"
"io/fs"
"os"
"path"
"path/filepath"
"runtime"

Expand Down Expand Up @@ -164,6 +165,28 @@ type Config struct {
// in the _ package.
Package string

// FS, if non-nil, provides the filesystem used by the loader for discovering
// packages, resolving modules, and reading files.
//
// When FS is nil, the loader uses the host operating system filesystem
// (os.Stat, os.ReadDir, os.Open), which is the default and preserves the
// existing behavior.
//
// When FS is set, all filesystem operations performed by the loader are
// routed through FS. In this mode, paths are interpreted as slash-separated
// paths within FS, and Config.Dir is treated as a directory inside FS rather
// than a host filesystem path.
//
// The loader does not require FS to support symbolic links. When using FS,
// any operations that would rely on os.Lstat semantics on the host filesystem
// are performed using fs.Stat instead, consistent with the capabilities of
// io/fs.
//
// FS enables loading CUE packages and modules from virtual or embedded
// filesystems (for example, embed.FS or fstest.MapFS) without accessing
// the host filesystem.
FS fs.FS

// Dir is the base directory for import path resolution.
// For example, it is used to determine the main module,
// and rooted import paths starting with "./" are relative to it.
Expand Down Expand Up @@ -279,7 +302,7 @@ type Config struct {
// An application may supply a custom implementation of ParseFile to change
// the effective file contents or the behavior of the parser, or to modify
// the syntax tree.
ParseFile func(name string, src interface{}, cfg parser.Config) (*ast.File, error)
ParseFile func(name string, src any, cfg parser.Config) (*ast.File, error)

// Overlay provides a mapping of absolute file paths to file contents,
// which are overlaid on top of the host operating system when loading files.
Expand All @@ -289,6 +312,9 @@ type Config struct {
// If an overlaid file does not exist in the host filesystem,
// the loader behaves as if the overlaid file exists with its contents,
// and that that all of its parent directories exist too.
//
// When FS is set, overlay paths must be absolute loadFS paths
// (that is, rooted at loadFsRoot).
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@rogpeppe rogpeppe Jan 28, 2026

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loadFsRoot is a private constant, so this wouldn't make much sense to the reader of the API docs. Also, in general, I'm not sure that it's right that the @fs prefix is exposed to the user. For example, if there are errors evaluating the CUE, the @fs/ prefix will be exposed in the filenames in the errors, so if some tooling wants to do something with those files, it will have to first strip the prefix before it can use them in the filesystem.

Also, I wonder if Overlay is actually useful/required if we've got FS. Perhaps we should forbid supplying both at once.

Overlay map[string]Source

// Stdin defines an alternative for os.Stdin for the file "-". When used,
Expand Down Expand Up @@ -353,13 +379,22 @@ func addImportQualifier(pkg importPath, name string) (importPath, error) {
func (c Config) complete() (cfg *Config, err error) {
// Ensure [Config.Dir] is a clean and absolute path,
// necessary for matching directory prefixes later.
if c.Dir == "" {
c.Dir, err = os.Getwd()
if err != nil {
if c.FS == nil {
if c.Dir == "" {
c.Dir, err = os.Getwd()
if err != nil {
return nil, err
}
} else if c.Dir, err = filepath.Abs(c.Dir); err != nil {
return nil, err
}
} else if c.Dir, err = filepath.Abs(c.Dir); err != nil {
return nil, err
} else {
if c.Dir == "" {
c.Dir = "."
}
if !isLoaderAbs(c.Dir) {
c.Dir = path.Clean(path.Join(loadFSRoot, c.Dir))
}
}

// TODO: we could populate this already with absolute file paths,
Expand Down
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