disallow explicit underscore import path qualifiers

It is currently possible (and has been since pre CUE v0.4.3) to use an
explicit `_` import path qualifier to import packageless CUE files (as in
`foo.com/bar:_`), but this is unintentional. This change disallows it.

Some users have been using this as a "trick" to allow them to
load JSON files as CUE. The up-coming "embed" implementation (see
https://cuelang.org/issue/2031) should provide a sufficient alternative
solution.

Fixes #3167

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: Idfdec457961d4b144dfdd2890ea2e9887fc5a4dc
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1196721
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cmd/cue/cmd/testdata/script/load_underscore.txtar

@@ -1,18 +1,17 @@
 # Test whether an import with a _ qualifer is allowed or not.
 
-# TODO(rog): this should fail; see https://cuelang.org/issue/3167
-exec cue export ./foo
-cmp stdout want-stdout
+! exec cue export ./foo
+cmp stderr want-stderr
 
 # Check that it also fails with an explicit _ qualifier
-# specified on the command line.
-# TODO(rog): this should fail similarly.
-exec cue export other.example/m:_
-cmp stdout want-stdout
--- want-stdout --
-{
-    "x": 20
-}
+# specified on the command line
+! exec cue export other.example/m:_
+cmp stderr want-stderr-2
+-- want-stderr --
+test.example/foo/foo@v0: import failed: cannot find package "other.example/m": _ is not a valid import path qualifier in "other.example/m:_":
+    ./foo/foo.cue:8:8
+-- want-stderr-2 --
+invalid import path qualifier _ in "other.example/m:_"
 -- cue.mod/gen/other.example/m/m.cue --
 package _
 x: 20

cue/load/config.go

@@ -316,6 +316,9 @@ func addImportQualifier(pkg importPath, name string) (importPath, error) {
 		return pkg, nil
 	}
 	ip := module.ParseImportPath(string(pkg))
+	if ip.Qualifier == "_" {
+		return "", fmt.Errorf("invalid import qualifier _ in %q", pkg)
+	}
 	if ip.ExplicitQualifier && ip.Qualifier != name {
 		return "", fmt.Errorf("non-matching package names (%s != %s)", ip.Qualifier, name)
 	}

cue/load/import.go

@@ -374,6 +374,8 @@ func (l *loader) newInstance(pos token.Pos, p importPath) *build.Instance {
 	i.PkgName = parts.Qualifier
 	if i.PkgName == "" {
 		i.Err = errors.Append(i.Err, l.errPkgf([]token.Pos{pos}, "cannot determine package name for %q; set it explicitly with ':'", p))
+	} else if i.PkgName == "" {
+		i.Err = errors.Append(i.Err, l.errPkgf([]token.Pos{pos}, "_ is not a valid import path qualifier in %q", p))
 	}
 	i.DisplayPath = string(p)
 	i.ImportPath = string(p)

cue/load/search.go

@@ -304,6 +304,9 @@ func appendExpandedPackageArg(c *Config, pkgPaths []resolvedPackageArg, p string
 	p = filepath.ToSlash(p)
 
 	ip := module.ParseImportPath(p)
+	if ip.Qualifier == "_" {
+		return nil, fmt.Errorf("invalid import path qualifier _ in %q", origp)
+	}
 
 	isRel := strings.HasPrefix(ip.Path, "./")
 	// Put argument in canonical form.

internal/mod/modpkgload/pkgload.go

@@ -252,6 +252,10 @@ func (pkgs *Packages) load(ctx context.Context, pkg *Package) {
 		pkg.err = fmt.Errorf("cannot determine package name from import path %q", pkg.path)
 		return
 	}
+	if pkgQual == "_" {
+		pkg.err = fmt.Errorf("_ is not a valid import path qualifier in %q", pkg.path)
+		return
+	}
 	importsMap := make(map[string]bool)
 	foundPackageFile := false
 	for _, loc := range pkg.locs {

mod/module/path.go

@@ -482,7 +482,7 @@ func ParseImportPath(p string) ImportPath {
 		} else {
 			parts.Qualifier = parts.Path
 		}
-		if !ast.IsValidIdent(parts.Qualifier) || strings.HasPrefix(parts.Qualifier, "#") {
+		if !ast.IsValidIdent(parts.Qualifier) || strings.HasPrefix(parts.Qualifier, "#") || parts.Qualifier == "_" {
 			parts.Qualifier = ""
 		}
 	}