Address issues with link references with empty labels

Fixes GitHub issue #421.

specs/regression.txt

@@ -730,3 +730,20 @@ An unordered list before the footnotes:
     <p>Cool.</p>
 </div>
 ````````````````````````````````
+
+ISSUE 421
+
+```````````````````````````````` example
+[][a]
+
+[a]: b
+
+# assimp-rs [![][crates-badge]][crates]
+
+[crates]: https://crates.io/crates/assimp
+[crates-badge]: http://meritbadge.herokuapp.com/assimp
+.
+<p><a href="b"></a></p>
+
+<h1>assimp-rs <a href="https://crates.io/crates/assimp"><img alt="" src="http://meritbadge.herokuapp.com/assimp"></a></h1>
+````````````````````````````````

src/lib.rs

@@ -70,5 +70,7 @@ mod tree;
 #[cfg(all(target_arch = "x86_64", feature = "simd"))]
 mod simd;
 
-pub use crate::parse::{Alignment, CodeBlockKind, Event, LinkType, OffsetIter, Options, Parser, Tag};
+pub use crate::parse::{
+    Alignment, CodeBlockKind, Event, LinkType, OffsetIter, Options, Parser, Tag,
+};
 pub use crate::strings::{CowStr, InlineStr};

src/parse.rs

@@ -1038,9 +1038,7 @@ impl<'a> FirstPass<'a> {
         self.tree.append(Item {
             start: start_ix,
             end: 0, // will get set later
-            body: ItemBody::FencedCodeBlock(
-                self.allocs.allocate_cow(info_string),
-            ),
+            body: ItemBody::FencedCodeBlock(self.allocs.allocate_cow(info_string)),
         });
         self.tree.push();
         loop {
@@ -2147,7 +2145,6 @@ impl<'a> Parser<'a> {
                             // ok, so its not an inline link. maybe it is a reference
                             // to a defined link?
                             let scan_result = scan_reference(&self.tree, block_text, next);
-                            let label_node = self.tree[tos.node].next;
                             let node_after_link = match scan_result {
                                 RefScan::LinkLabel(_, next_node) => next_node,
                                 RefScan::Collapsed(next_node) => next_node,
@@ -2158,6 +2155,7 @@ impl<'a> Parser<'a> {
                                 RefScan::Collapsed(..) => LinkType::Collapsed,
                                 RefScan::Failed => LinkType::Shortcut,
                             };
+
                             let label: Option<ReferenceLabel<'a>> = match scan_result {
                                 RefScan::LinkLabel(l, ..) => Some(ReferenceLabel::Link(l)),
                                 RefScan::Collapsed(..) | RefScan::Failed => {
@@ -2217,17 +2215,20 @@ impl<'a> Parser<'a> {
                                     } else {
                                         ItemBody::Link(link_ix)
                                     };
+                                    let label_node = self.tree[tos.node].next;
 
                                     // lets do some tree surgery to add the link to the tree
                                     // 1st: skip the label node and close node
                                     self.tree[tos.node].next = node_after_link;
 
-                                    // then, add the label node as a child to the link node
-                                    self.tree[tos.node].child = label_node;
+                                    // then, if it exists, add the label node as a child to the link node
+                                    if label_node != cur {
+                                        self.tree[tos.node].child = label_node;
 
-                                    // finally: disconnect list of children
-                                    if let TreePointer::Valid(prev_ix) = prev {
-                                        self.tree[prev_ix].next = TreePointer::Nil;
+                                        // finally: disconnect list of children
+                                        if let TreePointer::Valid(prev_ix) = prev {
+                                            self.tree[prev_ix].next = TreePointer::Nil;
+                                        }
                                     }
 
                                     // set up cur so next node will be node_after_link
@@ -2349,10 +2350,9 @@ impl<'a> Parser<'a> {
     fn scan_inline_link(
         &self,
         underlying: &'a str,
-        start_ix: usize,
+        mut ix: usize,
         node: TreePointer,
     ) -> Option<(usize, CowStr<'a>, CowStr<'a>)> {
-        let mut ix = start_ix;
         if scan_ch(&underlying.as_bytes()[ix..], b'(') == 0 {
             return None;
         }

tests/suite/regression.rs

@@ -828,3 +828,22 @@ An unordered list before the footnotes:
 
     test_markdown_html(original, expected);
 }
+
+#[test]
+fn regression_test_61() {
+    let original = r##"[][a]
+
+[a]: b
+
+# assimp-rs [![][crates-badge]][crates]
+
+[crates]: https://crates.io/crates/assimp
+[crates-badge]: http://meritbadge.herokuapp.com/assimp
+"##;
+    let expected = r##"<p><a href="b"></a></p>
+
+<h1>assimp-rs <a href="https://crates.io/crates/assimp"><img alt="" src="http://meritbadge.herokuapp.com/assimp"></a></h1>
+"##;
+
+    test_markdown_html(original, expected);
+}