Trim trailing whitespace in Rust code blocks

Before, a code block would always end with a final newline. The
newline was added unconditionally by `hide_lines`.

When the code block is syntax highlighted by highlight.js, this is not
a problem, no empty line is added for a final trailing `\n` character.
However, when the code block is editable and thus handled by the ACE
editor, a trailing newline _is_ significant. I believe this issue is
most closely described by ajaxorg/ace#2083
in the upstream repository.

The effect of the way ACE handles newlines is that a code block like

    <pre>
      Some code
    </pre>

will create an editor with _two_ lines, not just one.

By trimming trailing whitespace, we ensure that we don’t accidentally
create more lines in the ACE editor than necessary.

Author: mgeisler

src/renderer/html_handlebars/hbs_renderer.rs

@@ -906,28 +906,42 @@ fn hide_lines(content: &str) -> String {
     }
 
     let mut result = String::with_capacity(content.len());
-    for line in content.lines() {
+    let mut lines = content.lines();
+    let last_line = lines.next_back();
+    let mut handle_line = |line: &str, newline: &str| {
         if let Some(caps) = BORING_LINES_REGEX.captures(line) {
             if &caps[2] == "#" {
                 result += &caps[1];
                 result += &caps[2];
                 result += &caps[3];
                 result += "\n";
-                continue;
-            } else if &caps[2] != "!" && &caps[2] != "[" {
+                return;
+            }
+
+            if &caps[2] != "!" && &caps[2] != "[" {
                 result += "<span class=\"boring\">";
                 result += &caps[1];
                 if &caps[2] != " " {
                     result += &caps[2];
                 }
                 result += &caps[3];
-                result += "\n";
+                result += newline;
                 result += "</span>";
-                continue;
+                return;
             }
         }
+
         result += line;
-        result += "\n";
+        result += newline;
+    };
+
+    // Iterate over all lines except the last:
+    for line in lines {
+        handle_line(line, "\n");
+    }
+    // Handle the last line, if any:
+    if let Some(line) = last_line {
+        handle_line(line, "");
     }
     result
 }
@@ -1007,19 +1021,19 @@ mod tests {
     fn add_playground() {
         let inputs = [
           ("<code class=\"language-rust\">x()</code>",
-           "<pre class=\"playground\"><code class=\"language-rust\">\n<span class=\"boring\">#![allow(unused)]\n</span><span class=\"boring\">fn main() {\n</span>x()\n<span class=\"boring\">}\n</span></code></pre>"),
+           "<pre class=\"playground\"><code class=\"language-rust\">\n<span class=\"boring\">#![allow(unused)]\n</span><span class=\"boring\">fn main() {\n</span>x()\n<span class=\"boring\">}</span></code></pre>"),
           ("<code class=\"language-rust\">fn main() {}</code>",
-           "<pre class=\"playground\"><code class=\"language-rust\">fn main() {}\n</code></pre>"),
+           "<pre class=\"playground\"><code class=\"language-rust\">fn main() {}</code></pre>"),
           ("<code class=\"language-rust editable\">let s = \"foo\n # bar\n\";</code>",
-           "<pre class=\"playground\"><code class=\"language-rust editable\">let s = \"foo\n<span class=\"boring\"> bar\n</span>\";\n</code></pre>"),
+           "<pre class=\"playground\"><code class=\"language-rust editable\">let s = \"foo\n<span class=\"boring\"> bar\n</span>\";</code></pre>"),
           ("<code class=\"language-rust editable\">let s = \"foo\n ## bar\n\";</code>",
-           "<pre class=\"playground\"><code class=\"language-rust editable\">let s = \"foo\n # bar\n\";\n</code></pre>"),
+           "<pre class=\"playground\"><code class=\"language-rust editable\">let s = \"foo\n # bar\n\";</code></pre>"),
           ("<code class=\"language-rust editable\">let s = \"foo\n # bar\n#\n\";</code>",
-           "<pre class=\"playground\"><code class=\"language-rust editable\">let s = \"foo\n<span class=\"boring\"> bar\n</span><span class=\"boring\">\n</span>\";\n</code></pre>"),
+           "<pre class=\"playground\"><code class=\"language-rust editable\">let s = \"foo\n<span class=\"boring\"> bar\n</span><span class=\"boring\">\n</span>\";</code></pre>"),
           ("<code class=\"language-rust ignore\">let s = \"foo\n # bar\n\";</code>",
-           "<code class=\"language-rust ignore\">let s = \"foo\n<span class=\"boring\"> bar\n</span>\";\n</code>"),
+           "<code class=\"language-rust ignore\">let s = \"foo\n<span class=\"boring\"> bar\n</span>\";</code>"),
           ("<code class=\"language-rust editable\">#![no_std]\nlet s = \"foo\";\n #[some_attr]</code>",
-           "<pre class=\"playground\"><code class=\"language-rust editable\">#![no_std]\nlet s = \"foo\";\n #[some_attr]\n</code></pre>"),
+           "<pre class=\"playground\"><code class=\"language-rust editable\">#![no_std]\nlet s = \"foo\";\n #[some_attr]</code></pre>"),
         ];
         for (src, should_be) in &inputs {
             let got = add_playground_pre(
@@ -1037,13 +1051,13 @@ mod tests {
     fn add_playground_edition2015() {
         let inputs = [
           ("<code class=\"language-rust\">x()</code>",
-           "<pre class=\"playground\"><code class=\"language-rust edition2015\">\n<span class=\"boring\">#![allow(unused)]\n</span><span class=\"boring\">fn main() {\n</span>x()\n<span class=\"boring\">}\n</span></code></pre>"),
+           "<pre class=\"playground\"><code class=\"language-rust edition2015\">\n<span class=\"boring\">#![allow(unused)]\n</span><span class=\"boring\">fn main() {\n</span>x()\n<span class=\"boring\">}</span></code></pre>"),
           ("<code class=\"language-rust\">fn main() {}</code>",
-           "<pre class=\"playground\"><code class=\"language-rust edition2015\">fn main() {}\n</code></pre>"),
+           "<pre class=\"playground\"><code class=\"language-rust edition2015\">fn main() {}</code></pre>"),
           ("<code class=\"language-rust edition2015\">fn main() {}</code>",
-           "<pre class=\"playground\"><code class=\"language-rust edition2015\">fn main() {}\n</code></pre>"),
+           "<pre class=\"playground\"><code class=\"language-rust edition2015\">fn main() {}</code></pre>"),
           ("<code class=\"language-rust edition2018\">fn main() {}</code>",
-           "<pre class=\"playground\"><code class=\"language-rust edition2018\">fn main() {}\n</code></pre>"),
+           "<pre class=\"playground\"><code class=\"language-rust edition2018\">fn main() {}</code></pre>"),
         ];
         for (src, should_be) in &inputs {
             let got = add_playground_pre(
@@ -1061,13 +1075,13 @@ mod tests {
     fn add_playground_edition2018() {
         let inputs = [
           ("<code class=\"language-rust\">x()</code>",
-           "<pre class=\"playground\"><code class=\"language-rust edition2018\">\n<span class=\"boring\">#![allow(unused)]\n</span><span class=\"boring\">fn main() {\n</span>x()\n<span class=\"boring\">}\n</span></code></pre>"),
+           "<pre class=\"playground\"><code class=\"language-rust edition2018\">\n<span class=\"boring\">#![allow(unused)]\n</span><span class=\"boring\">fn main() {\n</span>x()\n<span class=\"boring\">}</span></code></pre>"),
           ("<code class=\"language-rust\">fn main() {}</code>",
-           "<pre class=\"playground\"><code class=\"language-rust edition2018\">fn main() {}\n</code></pre>"),
+           "<pre class=\"playground\"><code class=\"language-rust edition2018\">fn main() {}</code></pre>"),
           ("<code class=\"language-rust edition2015\">fn main() {}</code>",
-           "<pre class=\"playground\"><code class=\"language-rust edition2015\">fn main() {}\n</code></pre>"),
+           "<pre class=\"playground\"><code class=\"language-rust edition2015\">fn main() {}</code></pre>"),
           ("<code class=\"language-rust edition2018\">fn main() {}</code>",
-           "<pre class=\"playground\"><code class=\"language-rust edition2018\">fn main() {}\n</code></pre>"),
+           "<pre class=\"playground\"><code class=\"language-rust edition2018\">fn main() {}</code></pre>"),
         ];
         for (src, should_be) in &inputs {
             let got = add_playground_pre(
@@ -1085,13 +1099,13 @@ mod tests {
     fn add_playground_edition2021() {
         let inputs = [
             ("<code class=\"language-rust\">x()</code>",
-             "<pre class=\"playground\"><code class=\"language-rust edition2021\">\n<span class=\"boring\">#![allow(unused)]\n</span><span class=\"boring\">fn main() {\n</span>x()\n<span class=\"boring\">}\n</span></code></pre>"),
+             "<pre class=\"playground\"><code class=\"language-rust edition2021\">\n<span class=\"boring\">#![allow(unused)]\n</span><span class=\"boring\">fn main() {\n</span>x()\n<span class=\"boring\">}</span></code></pre>"),
             ("<code class=\"language-rust\">fn main() {}</code>",
-             "<pre class=\"playground\"><code class=\"language-rust edition2021\">fn main() {}\n</code></pre>"),
+             "<pre class=\"playground\"><code class=\"language-rust edition2021\">fn main() {}</code></pre>"),
             ("<code class=\"language-rust edition2015\">fn main() {}</code>",
-             "<pre class=\"playground\"><code class=\"language-rust edition2015\">fn main() {}\n</code></pre>"),
+             "<pre class=\"playground\"><code class=\"language-rust edition2015\">fn main() {}</code></pre>"),
             ("<code class=\"language-rust edition2018\">fn main() {}</code>",
-             "<pre class=\"playground\"><code class=\"language-rust edition2018\">fn main() {}\n</code></pre>"),
+             "<pre class=\"playground\"><code class=\"language-rust edition2018\">fn main() {}</code></pre>"),
         ];
         for (src, should_be) in &inputs {
             let got = add_playground_pre(