Implement for-else

This PR implements for-else statements like in Jinja. They make it easy
to print an alternative message if the loop iterator was empty. E.g.

```rs
{% for result in result %}
  <li>{{ result }}</li>
{% else %}
  <li><em>no results</em></li>
{% endfor %}
```

3 files changed, 72 insertions, 29 deletions

diff --git a/askama_shared/src/generator.rs b/askama_shared/src/generator.rs
index 6c1b151..0c2e854 100644
--- a/askama_shared/src/generator.rs
+++ b/askama_shared/src/generator.rs
@@ -2,7 +2,7 @@ use super::{get_template_source, CompileError, Integrations};
 use crate::filters;
 use crate::heritage::{Context, Heritage};
 use crate::input::{Source, TemplateInput};
-use crate::parser::{parse, Cond, CondTest, Expr, Node, Target, When, Ws};
+use crate::parser::{parse, Cond, CondTest, Expr, Loop, Node, Target, When, Ws};
 
 use proc_macro2::Span;
 
@@ -420,8 +420,8 @@ impl<'a, S: std::hash::BuildHasher> Generator<'a, S> {
                 Node::Match(ws1, ref expr, ref arms, ws2) => {
                     self.write_match(ctx, buf, ws1, expr, arms, ws2)?;
                 }
-                Node::Loop(ws1, ref var, ref iter, ref body, ws2) => {
-                    self.write_loop(ctx, buf, ws1, var, iter, body, ws2)?;
+                Node::Loop(ref loop_block) => {
+                    self.write_loop(ctx, buf, loop_block)?;
                 }
                 Node::BlockDef(ws1, name, _, ws2) => {
                     self.write_block(buf, Some(name), Ws(ws1.0, ws2.1))?;
@@ -596,21 +596,19 @@ impl<'a, S: std::hash::BuildHasher> Generator<'a, S> {
         &mut self,
         ctx: &'a Context<'_>,
         buf: &mut Buffer,
-        ws1: Ws,
-        var: &'a Target<'_>,
-        iter: &Expr<'_>,
-        body: &'a [Node<'_>],
-        ws2: Ws,
+        loop_block: &'a Loop<'_>,
     ) -> Result<usize, CompileError> {
-        self.handle_ws(ws1);
+        self.handle_ws(loop_block.ws1);
         self.locals.push();
 
-        let expr_code = self.visit_expr_root(iter)?;
+        let expr_code = self.visit_expr_root(&loop_block.iter)?;
 
         let flushed = self.write_buf_writable(buf)?;
+        buf.writeln("{")?;
+        buf.writeln("let mut _did_not_loop = true;")?;
         buf.write("for (");
-        self.visit_target(buf, true, true, var);
-        match iter {
+        self.visit_target(buf, true, true, &loop_block.var);
+        match loop_block.iter {
             Expr::Range(_, _, _) => buf.writeln(&format!(
                 ", _loop_item) in ::askama::helpers::TemplateLoop::new({}) {{",
                 expr_code
@@ -645,13 +643,24 @@ impl<'a, S: std::hash::BuildHasher> Generator<'a, S> {
             )),
         }?;
 
-        let mut size_hint = self.handle(ctx, body, buf, AstLevel::Nested)?;
-        self.handle_ws(ws2);
-
-        size_hint += self.write_buf_writable(buf)?;
+        buf.writeln("_did_not_loop = false;")?;
+        let mut size_hint1 = self.handle(ctx, &loop_block.body, buf, AstLevel::Nested)?;
+        self.handle_ws(loop_block.ws2);
+        size_hint1 += self.write_buf_writable(buf)?;
+        self.locals.pop();
         buf.writeln("}")?;
+
+        buf.writeln("if _did_not_loop {")?;
+        self.locals.push();
+        let mut size_hint2 = self.handle(ctx, &loop_block.else_block, buf, AstLevel::Nested)?;
+        self.handle_ws(loop_block.ws3);
+        size_hint2 += self.write_buf_writable(buf)?;
         self.locals.pop();
-        Ok(flushed + (size_hint * 3))
+        buf.writeln("}")?;
+
+        buf.writeln("}")?;
+
+        Ok(flushed + ((size_hint1 * 3) + size_hint2) / 2)
     }
 
     fn write_call(
diff --git a/askama_shared/src/heritage.rs b/askama_shared/src/heritage.rs
index 9f90273..a0b5460 100644
--- a/askama_shared/src/heritage.rs
+++ b/askama_shared/src/heritage.rs
@@ -1,7 +1,7 @@
 use std::collections::HashMap;
 use std::path::{Path, PathBuf};
 
-use crate::parser::{Expr, Macro, Node};
+use crate::parser::{Expr, Loop, Macro, Node};
 use crate::{CompileError, Config};
 
 pub struct Heritage<'a> {
@@ -86,8 +86,11 @@ impl<'a> Context<'a> {
                             nested.push(nodes);
                         }
                     }
-                    Node::Loop(_, _, _, nodes, _) => {
-                        nested.push(nodes);
+                    Node::Loop(Loop {
+                        body, else_block, ..
+                    }) => {
+                        nested.push(body);
+                        nested.push(else_block);
                     }
                     Node::Match(_, _, arms, _) => {
                         for (_, _, arm) in arms {
diff --git a/askama_shared/src/parser.rs b/askama_shared/src/parser.rs
index d1bc425..90c2fe3 100644
--- a/askama_shared/src/parser.rs
+++ b/askama_shared/src/parser.rs
@@ -22,7 +22,7 @@ pub enum Node<'a> {
     Let(Ws, Target<'a>, Expr<'a>),
     Cond(Vec<Cond<'a>>, Ws),
     Match(Ws, Expr<'a>, Vec<When<'a>>, Ws),
-    Loop(Ws, Target<'a>, Expr<'a>, Vec<Node<'a>>, Ws),
+    Loop(Loop<'a>),
     Extends(Expr<'a>),
     BlockDef(Ws, &'a str, Vec<Node<'a>>, Ws),
     Include(Ws, &'a str),
@@ -34,6 +34,17 @@ pub enum Node<'a> {
 }
 
 #[derive(Debug, PartialEq)]
+pub struct Loop<'a> {
+    pub ws1: Ws,
+    pub var: Target<'a>,
+    pub iter: Expr<'a>,
+    pub body: Vec<Node<'a>>,
+    pub ws2: Ws,
+    pub else_block: Vec<Node<'a>>,
+    pub ws3: Ws,
+}
+
+#[derive(Debug, PartialEq)]
 pub enum Expr<'a> {
     BoolLit(&'a str),
     NumLit(&'a str),
@@ -897,12 +908,28 @@ fn block_let(i: &[u8]) -> IResult<&[u8], Node<'_>> {
 
 fn parse_loop_content<'a>(i: &'a [u8], s: &State<'_>) -> IResult<&'a [u8], Vec<Node<'a>>> {
     s.loop_depth.set(s.loop_depth.get() + 1);
-    let (i, node) = parse_template(i, s)?;
+    let result = parse_template(i, s);
     s.loop_depth.set(s.loop_depth.get() - 1);
-    Ok((i, node))
+    result
 }
 
 fn block_for<'a>(i: &'a [u8], s: &State<'_>) -> IResult<&'a [u8], Node<'a>> {
+    let else_block = |i| {
+        let mut p = preceded(
+            ws(tag("else")),
+            cut(tuple((
+                opt(tag("-")),
+                delimited(
+                    |i| tag_block_end(i, s),
+                    |i| parse_template(i, s),
+                    |i| tag_block_start(i, s),
+                ),
+                opt(tag("-")),
+            ))),
+        );
+        let (i, (pws, nodes, nws)) = p(i)?;
+        Ok((i, (pws.is_some(), nodes, nws.is_some())))
+    };
     let mut p = tuple((
         opt(char('-')),
         ws(tag("for")),
@@ -918,6 +945,7 @@ fn block_for<'a>(i: &'a [u8], s: &State<'_>) -> IResult<&'a [u8], Node<'a>> {
                     cut(tuple((
                         |i| tag_block_start(i, s),
                         opt(char('-')),
+                        opt(else_block),
                         ws(tag("endfor")),
                         opt(char('-')),
                     ))),
@@ -925,16 +953,19 @@ fn block_for<'a>(i: &'a [u8], s: &State<'_>) -> IResult<&'a [u8], Node<'a>> {
             ))),
         ))),
     ));
-    let (i, (pws1, _, (var, _, (iter, nws1, _, (block, (_, pws2, _, nws2)))))) = p(i)?;
+    let (i, (pws1, _, (var, _, (iter, nws1, _, (body, (_, pws2, else_block, _, nws2)))))) = p(i)?;
+    let (nws3, else_block, pws3) = else_block.unwrap_or_default();
     Ok((
         i,
-        Node::Loop(
-            Ws(pws1.is_some(), nws1.is_some()),
+        Node::Loop(Loop {
+            ws1: Ws(pws1.is_some(), nws1.is_some()),
             var,
             iter,
-            block,
-            Ws(pws2.is_some(), nws2.is_some()),
-        ),
+            body,
+            ws2: Ws(pws2.is_some(), nws3),
+            else_block,
+            ws3: Ws(pws3, nws2.is_some()),
+        }),
     ))
 }