Add warnings when rustdoc html rendering differs

src/librustdoc/Cargo.toml

@@ -13,6 +13,9 @@ env_logger = { version = "0.4", default-features = false }
 log = "0.3"
 pulldown-cmark = { version = "0.0.14", default-features = false }
 
+[target.'cfg(not(stage0))'.dependencies]
+html-diff = "0.0.2"
+
 [build-dependencies]
 build_helper = { path = "../build_helper" }
 gcc = "0.3.50"

src/librustdoc/html/render.rs

@@ -75,6 +75,9 @@ use html::item_type::ItemType;
 use html::markdown::{self, Markdown, MarkdownHtml, MarkdownSummaryLine, RenderType};
 use html::{highlight, layout};
 
+#[cfg(not(stage0))]
+use html_diff;
+
 /// A pair of name and its optional document.
 pub type NameDoc = (String, Option<String>);
 
@@ -1645,6 +1648,34 @@ fn document(w: &mut fmt::Formatter, cx: &Context, item: &clean::Item) -> fmt::Re
     Ok(())
 }
 
+#[cfg(not(stage0))]
+fn get_html_diff(w: &mut fmt::Formatter, md_text: &str, render_type: RenderType,
+                 prefix: &str) -> fmt::Result {
+    if render_type == RenderType::Pulldown {
+        let output = format!("{}", Markdown(md_text, render_type));
+        let old = format!("{}", Markdown(md_text, RenderType::Hoedown));
+        let differences = html_diff::get_differences(&output, &old);
+        if !differences.is_empty() {
+            println!("Differences spotted in {:?}:\n{}",
+                     md_text,
+                     differences.iter()
+                                .map(|s| format!("=> {}", s.to_string()))
+                                .collect::<Vec<String>>()
+                                .join("\n"));
+        }
+        write!(w, "<div class='docblock'>{}{}</div>", prefix, output)
+    } else {
+        write!(w, "<div class='docblock'>{}{}</div>",
+               prefix,
+               Markdown(md_text, render_type))
+    }
+}
+
+#[cfg(stage0)]
+fn get_html_diff(w: &mut fmt::Formatter, md_text: &str, render_type: RenderType) -> fmt::Result {
+    write!(w, "<div class='docblock'>{}</div>", Markdown(md_text, render_type))
+}
+
 fn document_short(w: &mut fmt::Formatter, item: &clean::Item, link: AssocItemLink,
                   render_type: RenderType, prefix: &str) -> fmt::Result {
     if let Some(s) = item.doc_value() {
@@ -1654,7 +1685,7 @@ fn document_short(w: &mut fmt::Formatter, item: &clean::Item, link: AssocItemLin
         } else {
             format!("{}", &plain_summary_line(Some(s)))
         };
-        write!(w, "<div class='docblock'>{}{}</div>", prefix, Markdown(&markdown, render_type))?;
+        get_html_diff(&markdown, render_type, prefix)?;
     } else if !prefix.is_empty() {
         write!(w, "<div class='docblock'>{}</div>", prefix)?;
     }
@@ -1678,7 +1709,7 @@ fn render_assoc_const_value(item: &clean::Item) -> String {
 fn document_full(w: &mut fmt::Formatter, item: &clean::Item,
                  render_type: RenderType, prefix: &str) -> fmt::Result {
     if let Some(s) = item.doc_value() {
-        write!(w, "<div class='docblock'>{}{}</div>", prefix, Markdown(s, render_type))?;
+        get_html_diff(format!("{}{}", md_render_assoc_item(item), s), render_type, prefix)?;
     } else if !prefix.is_empty() {
         write!(w, "<div class='docblock'>{}</div>", prefix)?;
     }

src/librustdoc/lib.rs

@@ -28,6 +28,8 @@
 extern crate arena;
 extern crate getopts;
 extern crate env_logger;
+#[cfg(not(stage0))]
+extern crate html_diff;
 extern crate libc;
 extern crate rustc;
 extern crate rustc_data_structures;

src/test/rustdoc/issue-12834.rs

@@ -15,7 +15,7 @@
 // @has issue_12834/fn.foo.html
 // @has - //pre 'a + b '
 
-/// ```
+/// ```text
 /// a + b ∈ Self ∀ a, b ∈ Self
 /// ```
 pub fn foo() {}