Enforce QTextDocument::MarkdownFeature compatibility at compile time

We use md4c for parsing markdown.  It provides flags to control the
feature set that will be supported when parsing particular documents.
QTextMarkdownImporter::Feature is a fine-grained set of flags that
exactly match the md4c feature flags that we support in Qt so far.
QTextMarkdownImporter is a private exported class (new in 5.14).
We don't expect the corresponding flags in md4c to change in
incompatible ways in the future: the md4c authors have as much respect
for avoiding compatibility issues as we do, and likely will only add
features, not remove them.

We now enforce QTextMarkdownImporter::Features compatibility with
QTextDocument::MarkdownFeatures by setting them directly.  We check
QTextMarkdownImporter::Features compatibility with md4c's #define'd
feature flags using static asserts, so that any hypothetical
incompatibility would be detected at compile time.
The enum conversion from QTextDocument::MarkdownFeatures to
QTextMarkdownImporter::Features is moved to a new QTextMarkdownImporter
constructor; thus the conversions from QTextDocument::MarkdownFeatures
to QTextMarkdownImporter::Features, and then to unsigned (in
QTextMarkdownImporter::import()) are adjacent in the same private class
implementation.  If incompatibility ever occurred, we would need to
replace one or both of those with another suitable conversion function.

Change-Id: I0bf8a21eb7559df1d38406b948ef657f9060c67b
Reviewed-by: Vitaly Fanaskov <vitaly.fanaskov@qt.io>

4 files changed, 40 insertions, 18 deletions

diff --git a/src/gui/text/qtextdocument.cpp b/src/gui/text/qtextdocument.cpp
index 22c249d604..e94f635651 100644
--- a/src/gui/text/qtextdocument.cpp
+++ b/src/gui/text/qtextdocument.cpp
@@ -3420,7 +3420,7 @@ QString QTextDocument::toMarkdown(QTextDocument::MarkdownFeatures features) cons
 #if QT_CONFIG(textmarkdownreader)
 void QTextDocument::setMarkdown(const QString &markdown, QTextDocument::MarkdownFeatures features)
 {
-    QTextMarkdownImporter(static_cast<QTextMarkdownImporter::Features>(int(features))).import(this, markdown);
+    QTextMarkdownImporter(features).import(this, markdown);
 }
 #endif
 
diff --git a/src/gui/text/qtextdocument.h b/src/gui/text/qtextdocument.h
index 2fadc40cd2..7d46238257 100644
--- a/src/gui/text/qtextdocument.h
+++ b/src/gui/text/qtextdocument.h
@@ -152,7 +152,6 @@ public:
 #endif
 
 #if QT_CONFIG(textmarkdownwriter) || QT_CONFIG(textmarkdownreader)
-    // Must be in sync with QTextMarkdownImporter::Features, should be in sync with #define MD_FLAG_* in md4c
     enum MarkdownFeature {
         MarkdownNoHTML = 0x0020 | 0x0040,
         MarkdownDialectCommonMark = 0,
diff --git a/src/gui/text/qtextmarkdownimporter.cpp b/src/gui/text/qtextmarkdownimporter.cpp
index fe7e422923..87ade1f973 100644
--- a/src/gui/text/qtextmarkdownimporter.cpp
+++ b/src/gui/text/qtextmarkdownimporter.cpp
@@ -60,6 +60,22 @@ static const QChar Space = QLatin1Char(' ');
 // TODO maybe eliminate the margins after all views recognize BlockQuoteLevel, CSS can format it, etc.
 static const int BlockQuoteIndent = 40; // pixels, same as in QTextHtmlParserNode::initializeProperties
 
+Q_STATIC_ASSERT(int(QTextMarkdownImporter::FeatureCollapseWhitespace) == MD_FLAG_COLLAPSEWHITESPACE);
+Q_STATIC_ASSERT(int(QTextMarkdownImporter::FeaturePermissiveATXHeaders) == MD_FLAG_PERMISSIVEATXHEADERS);
+Q_STATIC_ASSERT(int(QTextMarkdownImporter::FeaturePermissiveURLAutoLinks) == MD_FLAG_PERMISSIVEURLAUTOLINKS);
+Q_STATIC_ASSERT(int(QTextMarkdownImporter::FeaturePermissiveMailAutoLinks) == MD_FLAG_PERMISSIVEEMAILAUTOLINKS);
+Q_STATIC_ASSERT(int(QTextMarkdownImporter::FeatureNoIndentedCodeBlocks) == MD_FLAG_NOINDENTEDCODEBLOCKS);
+Q_STATIC_ASSERT(int(QTextMarkdownImporter::FeatureNoHTMLBlocks) == MD_FLAG_NOHTMLBLOCKS);
+Q_STATIC_ASSERT(int(QTextMarkdownImporter::FeatureNoHTMLSpans) == MD_FLAG_NOHTMLSPANS);
+Q_STATIC_ASSERT(int(QTextMarkdownImporter::FeatureTables) == MD_FLAG_TABLES);
+Q_STATIC_ASSERT(int(QTextMarkdownImporter::FeatureStrikeThrough) == MD_FLAG_STRIKETHROUGH);
+Q_STATIC_ASSERT(int(QTextMarkdownImporter::FeaturePermissiveWWWAutoLinks) == MD_FLAG_PERMISSIVEWWWAUTOLINKS);
+Q_STATIC_ASSERT(int(QTextMarkdownImporter::FeaturePermissiveAutoLinks) == MD_FLAG_PERMISSIVEAUTOLINKS);
+Q_STATIC_ASSERT(int(QTextMarkdownImporter::FeatureTasklists) == MD_FLAG_TASKLISTS);
+Q_STATIC_ASSERT(int(QTextMarkdownImporter::FeatureNoHTML) == MD_FLAG_NOHTML);
+Q_STATIC_ASSERT(int(QTextMarkdownImporter::DialectCommonMark) == MD_DIALECT_COMMONMARK);
+Q_STATIC_ASSERT(int(QTextMarkdownImporter::DialectGitHub) == MD_DIALECT_GITHUB);
+
 // --------------------------------------------------------
 // MD4C callback function wrappers
 
@@ -122,6 +138,11 @@ QTextMarkdownImporter::QTextMarkdownImporter(QTextMarkdownImporter::Features fea
 {
 }
 
+QTextMarkdownImporter::QTextMarkdownImporter(QTextDocument::MarkdownFeatures features)
+  : QTextMarkdownImporter(static_cast<QTextMarkdownImporter::Features>(int(features)))
+{
+}
+
 void QTextMarkdownImporter::import(QTextDocument *doc, const QString &markdown)
 {
     MD_PARSER callbacks = {
diff --git a/src/gui/text/qtextmarkdownimporter_p.h b/src/gui/text/qtextmarkdownimporter_p.h
index 35655aff8a..f450da5eb3 100644
--- a/src/gui/text/qtextmarkdownimporter_p.h
+++ b/src/gui/text/qtextmarkdownimporter_p.h
@@ -54,6 +54,7 @@
 #include <QtGui/qfont.h>
 #include <QtGui/qtguiglobal.h>
 #include <QtGui/qpalette.h>
+#include <QtGui/qtextdocument.h>
 #include <QtGui/qtextlist.h>
 #include <QtCore/qstack.h>
 
@@ -67,27 +68,28 @@ class Q_GUI_EXPORT QTextMarkdownImporter
 {
 public:
     enum Feature {
-        // Must be kept in sync with MD_FLAG_* in md4c.h
-        FeatureCollapseWhitespace =       0x0001, // MD_FLAG_COLLAPSEWHITESPACE
-        FeaturePermissiveATXHeaders =     0x0002, // MD_FLAG_PERMISSIVEATXHEADERS
-        FeaturePermissiveURLAutoLinks =   0x0004, // MD_FLAG_PERMISSIVEURLAUTOLINKS
-        FeaturePermissiveMailAutoLinks =  0x0008, // MD_FLAG_PERMISSIVEEMAILAUTOLINKS
-        FeatureNoIndentedCodeBlocks =     0x0010, // MD_FLAG_NOINDENTEDCODEBLOCKS
-        FeatureNoHTMLBlocks =             0x0020, // MD_FLAG_NOHTMLBLOCKS
-        FeatureNoHTMLSpans =              0x0040, // MD_FLAG_NOHTMLSPANS
-        FeatureTables =                   0x0100, // MD_FLAG_TABLES
-        FeatureStrikeThrough =            0x0200, // MD_FLAG_STRIKETHROUGH
-        FeaturePermissiveWWWAutoLinks =   0x0400, // MD_FLAG_PERMISSIVEWWWAUTOLINKS
-        FeatureTasklists =                0x0800, // MD_FLAG_TASKLISTS
+        FeatureCollapseWhitespace =       0x0001,
+        FeaturePermissiveATXHeaders =     0x0002,
+        FeaturePermissiveURLAutoLinks =   0x0004,
+        FeaturePermissiveMailAutoLinks =  0x0008,
+        FeatureNoIndentedCodeBlocks =     0x0010,
+        FeatureNoHTMLBlocks =             0x0020,
+        FeatureNoHTMLSpans =              0x0040,
+        FeatureTables =                   0x0100,
+        FeatureStrikeThrough =            0x0200,
+        FeaturePermissiveWWWAutoLinks =   0x0400,
+        FeatureTasklists =                0x0800,
         // composite flags
-        FeaturePermissiveAutoLinks =  FeaturePermissiveMailAutoLinks | FeaturePermissiveURLAutoLinks | FeaturePermissiveWWWAutoLinks, // MD_FLAG_PERMISSIVEAUTOLINKS
-        FeatureNoHTML = FeatureNoHTMLBlocks | FeatureNoHTMLSpans, // MD_FLAG_NOHTML
-        DialectCommonMark = 0,                   // MD_DIALECT_COMMONMARK
-        DialectGitHub = FeaturePermissiveAutoLinks | FeatureTables | FeatureStrikeThrough | FeatureTasklists // MD_DIALECT_GITHUB
+        FeaturePermissiveAutoLinks = FeaturePermissiveMailAutoLinks
+            | FeaturePermissiveURLAutoLinks | FeaturePermissiveWWWAutoLinks,
+        FeatureNoHTML = QTextDocument::MarkdownNoHTML,
+        DialectCommonMark = QTextDocument::MarkdownDialectCommonMark,
+        DialectGitHub = QTextDocument::MarkdownDialectGitHub
     };
     Q_DECLARE_FLAGS(Features, Feature)
 
     QTextMarkdownImporter(Features features);
+    QTextMarkdownImporter(QTextDocument::MarkdownFeatures features);
 
     void import(QTextDocument *doc, const QString &markdown);