Crosspath: add IsRoot(path) bool function

Add a function that can determine whether the given path is considered
a "root path". Considers `/`, `X:`, `X:\` and `X:/` root paths, where `X`
is any upper or lower case Latin letters.

pkg/crosspath/crosspath.go

@@ -75,3 +75,32 @@ func ToNative(path string) string {
 		panic(fmt.Sprintf("this platform has an unknown path separator: %q", filepath.Separator))
 	}
 }
+
+func validDriveLetter(r rune) bool {
+	return ('A' <= r && r <= 'Z') || ('a' <= r && r <= 'z')
+}
+
+// IsRoot returns whether the given path is a root path or not.
+// Paths "/", "C:", "C:\", and "C:/" are considered root, for all latin drive
+// letters A-Z.
+//
+// NOTE: this does NOT resolve symlinks or `..` entries.
+func IsRoot(path string) bool {
+	switch {
+	case path == "":
+		return false
+	case path == "/":
+		return true
+	// From here on, it can only be a DOS root, so must have a drive letter and a colon.
+	case len(path) < 2, len(path) > 3:
+		return false
+	case path[1] != ':':
+		return false
+	// C:\ and C:/ are both considered roots.
+	case len(path) == 3 && path[2] != '/' && path[2] != '\\':
+		return false
+	}
+
+	runes := []rune(path)
+	return validDriveLetter(runes[0])
+}

pkg/crosspath/crosspath_test.go

@@ -169,3 +169,34 @@ func TestToNative_unsupported(t *testing.T) {
 	t.Fatalf("ToNative not supported on this platform %q with path separator %q",
 		runtime.GOOS, filepath.Separator)
 }
+
+func TestIsRoot(t *testing.T) {
+	tests := []struct {
+		name string
+		path string
+		want bool
+	}{
+		{"empty", "", false},
+
+		{"UNIX", "/", true},
+		{"Drive only", "C:", true},
+		{"Drive and slash", "C:/", true},
+		{"Drive and backslash", `C:\`, true},
+
+		{"backslash", `\`, false},
+		{"just letters", "just letters", false},
+		{"subdir of root", "/subdir", false},
+		{"subdir of drive", "C:\\subdir", false},
+
+		{"indirectly root", "/subdir/..", false},
+		{"UNC notation", `\\NAS\Share\`, false},
+		{"Slashed UNC notation", `//NAS/Share/`, false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := IsRoot(tt.path); got != tt.want {
+				t.Errorf("IsRoot() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}