Add spacing between statusline elements

Intersperses a raw space between statusline
elements. Uses a custom `IntersperseIter` iterator
because `intersperse()` has not landed in stable
Rust as of this commit.

This implementation currently resides in
`statusline.rs` as it is the only file that uses
it but it should probably be moved because it is
general but also temporary.

helix-term/src/ui/statusline.rs

@@ -49,6 +49,59 @@ pub struct RenderBuffer<'a> {
     pub right: Spans<'a>,
 }
 
+struct IntersperseIter<T, I>
+where
+    T: Clone,
+    I: Iterator<Item = T>,
+{
+    iter: I,
+    spacer: T,
+    emit_spacer: bool,
+    next_item: Option<T>,
+}
+
+impl<T, I> Iterator for IntersperseIter<T, I>
+where
+    T: Clone,
+    I: Iterator<Item = T>,
+{
+    type Item = T;
+
+    fn next(&mut self) -> Option<T> {
+        if self.next_item.is_none() {
+            Option::None
+        } else if self.emit_spacer {
+            self.emit_spacer = false;
+            Option::Some(self.spacer.clone())
+        } else {
+            self.emit_spacer = true;
+            let curr_item = self.next_item.clone();
+            self.next_item = self.iter.next();
+            curr_item
+        }
+    }
+}
+
+trait Intersperse<T>
+where
+    T: Clone,
+    Self: Iterator<Item = T> + Sized,
+{
+    fn interspersed(self, elem: T) -> IntersperseIter<T, Self>;
+}
+
+impl<T: Clone, I: Iterator<Item = T>> Intersperse<T> for I {
+    fn interspersed(mut self, elem: T) -> IntersperseIter<T, Self> {
+        let first_item = self.next();
+        IntersperseIter {
+            iter: self,
+            spacer: elem,
+            emit_spacer: false,
+            next_item: first_item,
+        }
+    }
+}
+
 pub fn render(context: &mut RenderContext, viewport: Rect, surface: &mut Surface) {
     let base_style = if context.focused {
         context.editor.theme.get("ui.statusline")
@@ -67,6 +120,7 @@ pub fn render(context: &mut RenderContext, viewport: Rect, surface: &mut Surface
         .iter()
         .map(|element_id| get_render_function(*element_id))
         .flat_map(|render| render(context).0)
+        .interspersed(Span::raw(" "))
         .collect::<Vec<Span>>()
         .into();
 
@@ -84,6 +138,7 @@ pub fn render(context: &mut RenderContext, viewport: Rect, surface: &mut Surface
         .iter()
         .map(|element_id| get_render_function(*element_id))
         .flat_map(|render| render(context).0)
+        .interspersed(Span::raw(" "))
         .collect::<Vec<Span>>()
         .into();
 
@@ -104,6 +159,7 @@ pub fn render(context: &mut RenderContext, viewport: Rect, surface: &mut Surface
         .iter()
         .map(|element_id| get_render_function(*element_id))
         .flat_map(|render| render(context).0)
+        .interspersed(Span::raw(" "))
         .collect::<Vec<Span>>()
         .into();