rustdoc: avoid allocating strings primitive link printing

This is aimed at hitting the allocator less in a function that
gets called a lot.

src/librustdoc/html/format.rs

@@ -847,7 +847,7 @@ fn resolved_path<'cx>(
 fn primitive_link(
     f: &mut fmt::Formatter<'_>,
     prim: clean::PrimitiveType,
-    name: &str,
+    name: &dyn std::fmt::Display,
     cx: &Context<'_>,
 ) -> fmt::Result {
     primitive_link_fragment(f, prim, name, "", cx)
@@ -856,7 +856,7 @@ fn primitive_link(
 fn primitive_link_fragment(
     f: &mut fmt::Formatter<'_>,
     prim: clean::PrimitiveType,
-    name: &str,
+    name: &dyn std::fmt::Display,
     fragment: &str,
     cx: &Context<'_>,
 ) -> fmt::Result {
@@ -907,7 +907,7 @@ fn primitive_link_fragment(
             None => {}
         }
     }
-    f.write_str(name)?;
+    name.fmt(f)?;
     if needs_termination {
         write!(f, "</a>")?;
     }
@@ -977,9 +977,9 @@ fn fmt_type<'cx>(
         }
         clean::Infer => write!(f, "_"),
         clean::Primitive(clean::PrimitiveType::Never) => {
-            primitive_link(f, PrimitiveType::Never, "!", cx)
+            primitive_link(f, PrimitiveType::Never, &"!", cx)
         }
-        clean::Primitive(prim) => primitive_link(f, prim, prim.as_sym().as_str(), cx),
+        clean::Primitive(prim) => primitive_link(f, prim, &prim.as_sym().as_str(), cx),
         clean::BareFunction(ref decl) => {
             if f.alternate() {
                 write!(
@@ -998,16 +998,21 @@ fn fmt_type<'cx>(
                     decl.unsafety.print_with_space(),
                     print_abi_with_space(decl.abi)
                 )?;
-                primitive_link(f, PrimitiveType::Fn, "fn", cx)?;
+                primitive_link(f, PrimitiveType::Fn, &"fn", cx)?;
                 write!(f, "{}", decl.decl.print(cx))
             }
         }
         clean::Tuple(ref typs) => {
             match &typs[..] {
-                &[] => primitive_link(f, PrimitiveType::Unit, "()", cx),
+                &[] => primitive_link(f, PrimitiveType::Unit, &"()", cx),
                 [one] => {
                     if let clean::Generic(name) = one {
-                        primitive_link(f, PrimitiveType::Tuple, &format!("({name},)"), cx)
+                        primitive_link(
+                            f,
+                            PrimitiveType::Tuple,
+                            &display_fn(|f| write!(f, "({name},)")),
+                            cx,
+                        )
                     } else {
                         write!(f, "(")?;
                         // Carry `f.alternate()` into this display w/o branching manually.
@@ -1028,7 +1033,13 @@ fn fmt_type<'cx>(
                         primitive_link(
                             f,
                             PrimitiveType::Tuple,
-                            &format!("({})", generic_names.iter().map(|s| s.as_str()).join(", ")),
+                            &display_fn(|f| {
+                                write!(
+                                    f,
+                                    "({})",
+                                    generic_names.iter().map(|s| s.as_str()).join(", ")
+                                )
+                            }),
                             cx,
                         )
                     } else {
@@ -1047,7 +1058,7 @@ fn fmt_type<'cx>(
         }
         clean::Slice(ref t) => match **t {
             clean::Generic(name) => {
-                primitive_link(f, PrimitiveType::Slice, &format!("[{name}]"), cx)
+                primitive_link(f, PrimitiveType::Slice, &display_fn(|f| write!(f, "[{name}]")), cx)
             }
             _ => {
                 write!(f, "[")?;
@@ -1059,7 +1070,7 @@ fn fmt_type<'cx>(
             clean::Generic(name) if !f.alternate() => primitive_link(
                 f,
                 PrimitiveType::Array,
-                &format!("[{name}; {n}]", n = Escape(n)),
+                &display_fn(|f| write!(f, "[{name}; {n}]", n = Escape(n))),
                 cx,
             ),
             _ => {
@@ -1069,7 +1080,12 @@ fn fmt_type<'cx>(
                     write!(f, "; {n}")?;
                 } else {
                     write!(f, "; ")?;
-                    primitive_link(f, PrimitiveType::Array, &format!("{n}", n = Escape(n)), cx)?;
+                    primitive_link(
+                        f,
+                        PrimitiveType::Array,
+                        &display_fn(|f| write!(f, "{n}", n = Escape(n))),
+                        cx,
+                    )?;
                 }
                 write!(f, "]")
             }
@@ -1081,30 +1097,34 @@ fn fmt_type<'cx>(
             };
 
             if matches!(**t, clean::Generic(_)) || t.is_assoc_ty() {
-                let text = if f.alternate() {
-                    format!("*{m} {ty:#}", ty = t.print(cx))
+                let (f1, f2);
+                let display: &dyn std::fmt::Display = if f.alternate() {
+                    f1 = display_fn(|f| write!(f, "*{m} {ty:#}", ty = t.print(cx)));
+                    &f1
                 } else {
-                    format!("*{m} {ty}", ty = t.print(cx))
+                    f2 = display_fn(|f| write!(f, "*{m} {ty}", ty = t.print(cx)));
+                    &f2
                 };
-                primitive_link(f, clean::PrimitiveType::RawPointer, &text, cx)
+                primitive_link(f, clean::PrimitiveType::RawPointer, display, cx)
             } else {
-                primitive_link(f, clean::PrimitiveType::RawPointer, &format!("*{m} "), cx)?;
+                let display = display_fn(|f| write!(f, "*{m} "));
+                primitive_link(f, clean::PrimitiveType::RawPointer, &display, cx)?;
                 fmt::Display::fmt(&t.print(cx), f)
             }
         }
         clean::BorrowedRef { lifetime: ref l, mutability, type_: ref ty } => {
-            let lt = match l {
-                Some(l) => format!("{} ", l.print()),
-                _ => String::new(),
-            };
+            let lt = display_fn(|f| match l {
+                Some(l) => write!(f, "{} ", l.print()),
+                _ => Ok(()),
+            });
             let m = mutability.print_with_space();
             let amp = if f.alternate() { "&" } else { "&amp;" };
 
             if let clean::Generic(name) = **ty {
                 return primitive_link(
                     f,
                     PrimitiveType::Reference,
-                    &format!("{amp}{lt}{m}{name}"),
+                    &display_fn(|f| write!(f, "{amp}{lt}{m}{name}")),
                     cx,
                 );
             }
@@ -1665,7 +1685,7 @@ impl clean::ImportSource {
                 }
                 let name = self.path.last();
                 if let hir::def::Res::PrimTy(p) = self.path.res {
-                    primitive_link(f, PrimitiveType::from(p), name.as_str(), cx)?;
+                    primitive_link(f, PrimitiveType::from(p), &name.as_str(), cx)?;
                 } else {
                     f.write_str(name.as_str())?;
                 }