prefer `if let` to match with `None => {}` arm in some places

This is a spiritual succesor to #34268/8531d581, in which we replaced a
number of matches of None to the unit value with `if let` conditionals
where it was judged that this made for clearer/simpler code (as would be
recommended by Manishearth/rust-clippy's `single_match` lint). The same
rationale applies to matches of None to the empty block.

3 files changed, 25 insertions, 36 deletions

diff --git a/src/librustdoc/html/format.rs b/src/librustdoc/html/format.rs
index 760e84622cf..77c4a0f8174 100644
--- a/src/librustdoc/html/format.rs
+++ b/src/librustdoc/html/format.rs
@@ -131,9 +131,8 @@ impl fmt::Display for clean::Generics {
                     write!(f, ": {}", TyParamBounds(&tp.bounds))?;
                 }
 
-                match tp.default {
-                    Some(ref ty) => { write!(f, " = {}", ty)?; },
-                    None => {}
+                if let Some(ref ty) = tp.default {
+                    write!(f, " = {}", ty)?;
                 };
             }
         }
@@ -401,15 +400,12 @@ fn primitive_link(f: &mut fmt::Formatter,
                 }
                 (_, render::Unknown) => None,
             };
-            match loc {
-                Some(root) => {
-                    write!(f, "<a class='primitive' href='{}{}/primitive.{}.html'>",
-                           root,
-                           path.0.first().unwrap(),
-                           prim.to_url_str())?;
-                    needs_termination = true;
-                }
-                None => {}
+            if let Some(root) = loc {
+                write!(f, "<a class='primitive' href='{}{}/primitive.{}.html'>",
+                       root,
+                       path.0.first().unwrap(),
+                       prim.to_url_str())?;
+                needs_termination = true;
             }
         }
         None => {}
diff --git a/src/librustdoc/html/highlight.rs b/src/librustdoc/html/highlight.rs
index 2e2f9989773..84e98a67391 100644
--- a/src/librustdoc/html/highlight.rs
+++ b/src/librustdoc/html/highlight.rs
@@ -352,9 +352,8 @@ fn write_header(class: Option<&str>,
                 out: &mut Write)
                 -> io::Result<()> {
     write!(out, "<pre ")?;
-    match id {
-        Some(id) => write!(out, "id='{}' ", id)?,
-        None => {}
+    if let Some(id) = id {
+        write!(out, "id='{}' ", id)?;
     }
     write!(out, "class='rust {}'>\n", class.unwrap_or(""))
 }
diff --git a/src/librustdoc/html/render.rs b/src/librustdoc/html/render.rs
index 9f2b33c0282..acf867561a6 100644
--- a/src/librustdoc/html/render.rs
+++ b/src/librustdoc/html/render.rs
@@ -589,19 +589,16 @@ fn build_index(krate: &clean::Crate, cache: &mut Cache) -> String {
     // Attach all orphan methods to the type's definition if the type
     // has since been learned.
     for &(did, ref item) in orphan_methods {
-        match paths.get(&did) {
-            Some(&(ref fqp, _)) => {
-                search_index.push(IndexItem {
-                    ty: shortty(item),
-                    name: item.name.clone().unwrap(),
-                    path: fqp[..fqp.len() - 1].join("::"),
-                    desc: Escape(&shorter(item.doc_value())).to_string(),
-                    parent: Some(did),
-                    parent_idx: None,
-                    search_type: get_index_search_type(&item),
-                });
-            },
-            None => {}
+        if let Some(&(ref fqp, _)) = paths.get(&did) {
+            search_index.push(IndexItem {
+                ty: shortty(item),
+                name: item.name.clone().unwrap(),
+                path: fqp[..fqp.len() - 1].join("::"),
+                desc: Escape(&shorter(item.doc_value())).to_string(),
+                parent: Some(did),
+                parent_idx: None,
+                search_type: get_index_search_type(&item),
+            });
         }
     }
 
@@ -2093,15 +2090,12 @@ fn item_trait(w: &mut fmt::Formatter, cx: &Context, it: &clean::Item,
         <h2 id='implementors'>Implementors</h2>
         <ul class='item-list' id='implementors-list'>
     ")?;
-    match cache.implementors.get(&it.def_id) {
-        Some(implementors) => {
-            for i in implementors {
-                write!(w, "<li><code>")?;
-                fmt_impl_for_trait_page(&i.impl_, w)?;
-                writeln!(w, "</code></li>")?;
-            }
+    if let Some(implementors) = cache.implementors.get(&it.def_id) {
+        for i in implementors {
+            write!(w, "<li><code>")?;
+            fmt_impl_for_trait_page(&i.impl_, w)?;
+            writeln!(w, "</code></li>")?;
         }
-        None => {}
     }
     write!(w, "</ul>")?;
     write!(w, r#"<script type="text/javascript" async