lib: Add SpanLinkTag::Unresolved variant

keep the tag for unresolved links, and allow distinguishing between
`[tag][tag with empty url]` and `[tag][non-existent tag]`.

closes #26

src/html.rs

@@ -29,6 +29,7 @@ use crate::LinkType;
 use crate::ListKind;
 use crate::OrderedListNumbering::*;
 use crate::Render;
+use crate::SpanLinkType;
 
 pub struct Renderer;
 
@@ -163,7 +164,7 @@ impl<'s, I: Iterator<Item = Event<'s>>, W: std::fmt::Write> Writer<'s, I, W> {
                         Container::CodeBlock { .. } => self.out.write_str("<pre")?,
                         Container::Span | Container::Math { .. } => self.out.write_str("<span")?,
                         Container::Link(dst, ty) => {
-                            if dst.is_empty() {
+                            if matches!(ty, LinkType::Span(SpanLinkType::Unresolved)) {
                                 self.out.write_str("<a")?;
                             } else {
                                 self.out.write_str(r#"<a href=""#)?;

src/lib.rs

@@ -213,9 +213,10 @@ pub enum Container<'s> {
     CodeBlock { lang: Option<&'s str> },
     /// An inline divider element.
     Span,
-    /// An inline link with a destination URL.
+    /// An inline link, the first field is either a destination URL or an unresolved tag.
     Link(CowStr<'s>, LinkType),
-    /// An inline image with a source URL. Inner Str objects compose the alternative text.
+    /// An inline image, the first field is either a destination URL or an unresolved tag. Inner
+    /// Str objects compose the alternative text.
     Image(CowStr<'s>, SpanLinkType),
     /// An inline verbatim string.
     Verbatim,
@@ -333,6 +334,8 @@ pub enum SpanLinkType {
     Inline,
     /// In the form `[text][tag]` or `[tag][]`.
     Reference,
+    /// Like reference, but the tag is unresolved.
+    Unresolved,
 }
 
 /// The type of an inline link.
@@ -729,19 +732,20 @@ impl<'s> Parser<'s> {
                             let link_def =
                                 self.pre_pass.link_definitions.get(tag.as_ref()).cloned();
 
-                            let url = if let Some((url, attrs_def)) = link_def {
+                            let (url_or_tag, ty) = if let Some((url, attrs_def)) = link_def {
                                 attributes.union(attrs_def);
-                                url
+                                (url, SpanLinkType::Reference)
                             } else {
-                                self.pre_pass
-                                    .heading_id_by_tag(tag.as_ref())
-                                    .map_or_else(|| "".into(), |id| format!("#{}", id).into())
+                                self.pre_pass.heading_id_by_tag(tag.as_ref()).map_or_else(
+                                    || (tag, SpanLinkType::Unresolved),
+                                    |id| (format!("#{}", id).into(), SpanLinkType::Reference),
+                                )
                             };
 
                             if matches!(c, inline::Container::ReferenceLink) {
-                                Container::Link(url, LinkType::Span(SpanLinkType::Reference))
+                                Container::Link(url_or_tag, LinkType::Span(ty))
                             } else {
-                                Container::Image(url, SpanLinkType::Reference)
+                                Container::Image(url_or_tag, ty)
                             }
                         }
                         inline::Container::Autolink => {
@@ -1298,6 +1302,32 @@ mod test {
         );
     }
 
+    #[test]
+    fn link_reference_unresolved() {
+        test_parse!(
+            "[text][tag]",
+            Start(Paragraph, Attributes::new()),
+            Start(
+                Link("tag".into(), LinkType::Span(SpanLinkType::Unresolved)),
+                Attributes::new()
+            ),
+            Str("text".into()),
+            End(Link("tag".into(), LinkType::Span(SpanLinkType::Unresolved))),
+            End(Paragraph),
+        );
+        test_parse!(
+            "![text][tag]",
+            Start(Paragraph, Attributes::new()),
+            Start(
+                Image("tag".into(), SpanLinkType::Unresolved),
+                Attributes::new()
+            ),
+            Str("text".into()),
+            End(Image("tag".into(), SpanLinkType::Unresolved)),
+            End(Paragraph),
+        );
+    }
+
     #[test]
     fn link_reference_multiline() {
         test_parse!(

tests/bench/skip

@@ -1,3 +1,3 @@
 block_list_flat:large list marker number
-inline_links_flat:escaped attributes, empty hrefs
+inline_links_flat:space before img, img attrs order
 inline_links_nested:empty link text

tests/suite/skip

@@ -10,7 +10,6 @@ e1f5b5e:untrimmed whitespace before linebreak
 07888f3:div close within raw block
 8423412:heading id conflict with existing id
 00a46ed:clear inline formatting from link tags
-a8e17c3:empty href
 c0a3dec:escape in url
 e66af00:url container precedence
 61876cf:roman alpha ambiguity