internal/mod/modpkgload: a package with no files is an error

Currently if there are no files in a package, `modpkgload.LoadPackages`
is silent, but this should really be an error.

For #3155.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I8fb77ef69509f5e0a99300723c80a14763af3975
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1195281
Reviewed-by: Chief Cueckoo <chief.cueckoo@gmail.com>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

cmd/cue/cmd/testdata/script/pkg_resolution_multiple_packages_no_matching_path_element.txtar

@@ -3,10 +3,6 @@
 # the final element of the import path.
 
 ! exec cue eval root.cue
-# TODO: the error for this isn't quite right: it says:
-#	root.cue: package is root, want x
-# but the evaluation should not care about the package in the root directory
-# because we are not trying to evaluate that package.
 cmp stderr import_stderr.golden
 ! exec cue eval mod.com/x
 cmp stderr absolute_stderr.golden
@@ -32,13 +28,7 @@ package z
 
 z: 5
 -- import_stderr.golden --
-import failed: build constraints exclude all CUE files in mod.com/x:
-    root.cue: package is root, want x
-    x/y.cue: package is y, want x
-    x/z.cue: package is z, want x:
+import failed: cannot find package "mod.com/x": no files in package directory with package name "x":
     ./root.cue:3:8
 -- absolute_stderr.golden --
-build constraints exclude all CUE files in mod.com/x:
-    root.cue: package is root, want x
-    x/y.cue: package is y, want x
-    x/z.cue: package is z, want x
+cannot find package "mod.com/x": no files in package directory with package name "x"

cmd/cue/cmd/testdata/script/pkg_resolution_path_element_invalid_ident.txtar

@@ -45,3 +45,4 @@ import failed: cannot determine package name for "mod.com/1x"; set it explicitly
     ./test2/root.cue:3:8
 -- test2-abs-stderr.golden --
 cannot determine package name for "mod.com/1x"; set it explicitly with ':'
+cannot find package "mod.com/1x": cannot determine package name from import path "mod.com/1x"

cmd/cue/cmd/testdata/script/pkg_resolution_single_package_not_matching_path_element.txtar

@@ -3,10 +3,6 @@
 # that does not match the final element of the import path.
 
 ! exec cue eval root.cue
-# TODO: the error for this isn't quite right: it says:
-#	root.cue: package is root, want x
-# but the evaluation should not care about the package in the root directory
-# because we are not trying to evaluate that package.
 cmp stderr import_stderr.golden
 ! exec cue eval mod.com/x
 cmp stderr absolute_stderr.golden
@@ -28,11 +24,7 @@ package y
 
 y: 5
 -- import_stderr.golden --
-import failed: build constraints exclude all CUE files in mod.com/x:
-    root.cue: package is root, want x
-    x/y.cue: package is y, want x:
+import failed: cannot find package "mod.com/x": no files in package directory with package name "x":
     ./root.cue:3:8
 -- absolute_stderr.golden --
-build constraints exclude all CUE files in mod.com/x:
-    root.cue: package is root, want x
-    x/y.cue: package is y, want x
+cannot find package "mod.com/x": no files in package directory with package name "x"

cue/load/loader_test.go

@@ -182,15 +182,13 @@ imports:
 		name: "NoPackageName",
 		cfg:  dirCfg,
 		args: []string{"mod.test/test/hello:nonexist"},
-		want: `err:    build constraints exclude all CUE files in mod.test/test/hello:nonexist:
-    anon.cue: no package name
-    test.cue: package is test, want nonexist
-    hello/test.cue: package is test, want nonexist
+		want: `err:    cannot find package "mod.test/test/hello": no files in package directory with package name "nonexist"
 path:   mod.test/test/hello:nonexist
 module: mod.test/test@v0
 root:   $CWD/testdata/testmod
-dir:    $CWD/testdata/testmod/hello
-display:mod.test/test/hello:nonexist`}, {
+dir:    ""
+display:mod.test/test/hello:nonexist`,
+	}, {
 		name: "ExplicitNonPackageFiles",
 		cfg:  dirCfg,
 		args: []string{"./anon.cue", "./other/anon.cue"},

internal/mod/modpkgload/pkgload.go

@@ -245,9 +245,21 @@ func (pkgs *Packages) load(ctx context.Context, pkg *Package) {
 		pkgs.applyPkgFlags(ctx, pkg, PkgInAll)
 	}
 	pkgQual := module.ParseImportPath(pkg.path).Qualifier
+	if pkgQual == "" {
+		pkg.err = fmt.Errorf("cannot determine package name from import path %q", pkg.path)
+		return
+	}
 	importsMap := make(map[string]bool)
+	foundPackageFile := false
 	for _, loc := range pkg.locs {
-		imports, err := modimports.AllImports(modimports.PackageFiles(loc.FS, loc.Dir, pkgQual))
+		// TODO(mvdan): using the PackageFiles iterator twice below
+		// causes twice as many io/fs operations on loc.FS.
+		pkgFileIter := modimports.PackageFiles(loc.FS, loc.Dir, pkgQual)
+		pkgFileIter(func(_ modimports.ModuleFile, err error) bool {
+			foundPackageFile = err == nil
+			return false
+		})
+		imports, err := modimports.AllImports(pkgFileIter)
 		if err != nil {
 			pkg.err = fmt.Errorf("cannot get imports: %v", err)
 			return
@@ -256,6 +268,10 @@ func (pkgs *Packages) load(ctx context.Context, pkg *Package) {
 			importsMap[imp] = true
 		}
 	}
+	if !foundPackageFile {
+		pkg.err = fmt.Errorf("no files in package directory with package name %q", pkgQual)
+		return
+	}
 	imports := make([]string, 0, len(importsMap))
 	for imp := range importsMap {
 		imports = append(imports, imp)

internal/mod/modpkgload/testdata/nocuefiles.txtar

@@ -12,9 +12,9 @@ main.test@v0:main
 	imports:
 		example.com/blah@v0
 example.com/blah@v0
-	flags: inAll,isRoot,fromRoot,importsLoaded
-	mod: example.com@v0.0.1
-	location: _registry/example.com_v0.0.1/blah
+	flags: inAll,isRoot,fromRoot
+	error: no files in package directory with package name "blah"
+	missing: false
 -- main.cue --
 package main
 import "example.com/blah@v0"