mod/module: add qualifier in ImportPath when necessary

Currently the `ImportPath.ExplicitQualifier` field is treated as gospel:
if it's false, the qualifier will never be added.
However, it's useful to be able to manipulate the
`ImportPath` fields (for example to change the package
path) and then invoke its `String` method to produce
a well-formed import path.

So we change `ImportPath.String` to include a package qualifier
if needed, even if `ExplicitQualifier` is false.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: Iaa7484f5506e10cf2bb9d399e987c69021644c99
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194666
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>

Author: rogpeppe

mod/module/module_test.go

@@ -304,3 +304,12 @@ func TestParseImportPath(t *testing.T) {
 		})
 	}
 }
+
+func TestImportPathStringAddsQualifier(t *testing.T) {
+	ip := ImportPath{
+		Path:      "foo.com/bar",
+		Version:   "v0",
+		Qualifier: "baz",
+	}
+	qt.Assert(t, qt.Equals(ip.String(), "foo.com/bar@v0:baz"))
+}

mod/module/path.go

@@ -411,8 +411,9 @@ type ImportPath struct {
 	// This is not guaranteed to be a valid CUE identifier.
 	Qualifier string
 
-	// ExplicitQualifier holds whether the qualifier was explicitly
-	// present in the import path.
+	// ExplicitQualifier holds whether the qualifier will
+	// always be added regardless of whether it matches
+	// the final path element.
 	ExplicitQualifier bool
 }
 
@@ -435,6 +436,13 @@ func (parts ImportPath) Unqualified() ImportPath {
 }
 
 func (parts ImportPath) String() string {
+	needQualifier := parts.ExplicitQualifier
+	if !needQualifier && parts.Qualifier != "" {
+		_, last, _ := cutLast(parts.Path, "/")
+		if last != "" && last != parts.Qualifier {
+			needQualifier = true
+		}
+	}
 	if parts.Version == "" && !parts.ExplicitQualifier {
 		// Fast path.
 		return parts.Path
@@ -445,7 +453,7 @@ func (parts ImportPath) String() string {
 		buf.WriteByte('@')
 		buf.WriteString(parts.Version)
 	}
-	if parts.ExplicitQualifier {
+	if needQualifier {
 		buf.WriteByte(':')
 		buf.WriteString(parts.Qualifier)
 	}
@@ -487,3 +495,10 @@ func CheckPathMajor(v, pathMajor string) error {
 	}
 	return nil
 }
+
+func cutLast(s, sep string) (before, after string, found bool) {
+	if i := strings.LastIndex(s, sep); i >= 0 {
+		return s[:i], s[i+len(sep):], true
+	}
+	return "", s, false
+}