fix(grid): stop iterating a grid without columns

src/grid.rs

@@ -600,7 +600,9 @@ impl Iterator for GridIntoIterator {
     fn next(&mut self) -> Option<Self::Item> {
         if self.grid.dense {
             loop {
-                if self.y == self.grid.height {
+                // With no columns, `x` would never come back to `width` and the walk would
+                // never move on to the next row.
+                if self.y == self.grid.height || self.grid.width == 0 {
                     return None;
                 }
                 let r = (self.grid.has_vertex((self.x, self.y))).then_some((self.x, self.y));
@@ -646,7 +648,9 @@ impl Iterator for GridIterator<'_> {
     fn next(&mut self) -> Option<Self::Item> {
         if self.grid.dense {
             loop {
-                if self.y == self.grid.height {
+                // With no columns, `x` would never come back to `width` and the walk would
+                // never move on to the next row.
+                if self.y == self.grid.height || self.grid.width == 0 {
                     return None;
                 }
                 let r = (self.grid.has_vertex((self.x, self.y))).then_some((self.x, self.y));
@@ -698,7 +702,9 @@ impl Iterator for EdgesIterator<'_> {
 
     fn next(&mut self) -> Option<Self::Item> {
         loop {
-            if self.y == self.grid.height {
+            // With no columns, `x` would never come back to `width` and the walk would never
+            // move on to the next row.
+            if self.y == self.grid.height || self.grid.width == 0 {
                 return None;
             }
             let x = self.x;

tests/grid.rs

@@ -666,3 +666,24 @@ fn iter_full_grid_yields_all_vertices() {
     via_into_iter.sort_unstable();
     assert_eq!(via_into_iter, expected);
 }
+
+/// A grid without columns has no vertices. Walking it used to step `x` until it came back to
+/// `width`, which with a width of zero never happens, so these loops never ended.
+#[test]
+fn a_grid_without_columns_iterates_to_nothing() {
+    let mut g = Grid::new(0, 5);
+    assert_eq!(g.edges().count(), 0);
+    g.fill();
+    assert_eq!(g.iter().count(), 0);
+    assert_eq!(g.edges().count(), 0);
+    assert_eq!(g.into_iter().count(), 0);
+
+    // The same state reached by shrinking a full grid.
+    let mut g = Grid::new(3, 3);
+    g.fill();
+    assert!(g.resize(0, 3));
+    assert_eq!(g.iter().count(), 0);
+    assert_eq!(g.edges().count(), 0);
+    assert_eq!(g.vertices_len(), 0);
+    assert_eq!(g.into_iter().count(), 0);
+}