rustdoc: render unnamed arguments as underscores in cross-crate functions & function pointers

for consistency with the way we display local definitions (cleaned from HIR, not from rustc_middle).

src/librustdoc/clean/mod.rs

@@ -957,12 +957,14 @@ fn clean_args_from_types_and_names<'tcx>(
         values: types
             .iter()
             .enumerate()
-            .map(|(i, ty)| {
+            .map(|(i, ty)| Argument {
-                let mut name = names.get(i).map_or(kw::Empty, |ident| ident.name);
+                type_: clean_ty(ty, cx),
-                if name.is_empty() {
+                name: names
-                    name = kw::Underscore;
+                    .get(i)
-                }
+                    .map(|ident| ident.name)
-                Argument { name, type_: clean_ty(ty, cx), is_const: false }
+                    .filter(|ident| !ident.is_empty())
+                    .unwrap_or(kw::Underscore),
+                is_const: false,
             })
             .collect(),
     }
@@ -1024,7 +1026,11 @@ fn clean_fn_decl_from_did_and_sig<'tcx>(
                 .iter()
                 .map(|t| Argument {
                     type_: clean_middle_ty(*t, cx, None),
-                    name: names.next().map_or(kw::Empty, |i| i.name),
+                    name: names
+                        .next()
+                        .map(|i| i.name)
+                        .filter(|i| !i.is_empty())
+                        .unwrap_or(kw::Underscore),
                     is_const: false,
                 })
                 .collect(),

src/librustdoc/html/format.rs

@@ -1232,9 +1232,8 @@ pub(crate) fn print<'a, 'tcx: 'a>(
     ) -> impl fmt::Display + 'a + Captures<'tcx> {
         display_fn(move |f| {
             for (i, input) in self.values.iter().enumerate() {
-                if !input.name.is_empty() {
+                write!(f, "{}: ", input.name)?;
-                    write!(f, "{}: ", input.name)?;
+
-                }
                 if f.alternate() {
                     write!(f, "{:#}", input.type_.print(cx))?;
                 } else {
@@ -1367,10 +1366,8 @@ fn inner_full_print(
                     args.push_str("const ");
                     args_plain.push_str("const ");
                 }
-                if !input.name.is_empty() {
+                write!(args, "{}: ", input.name);
-                    write!(args, "{}: ", input.name);
+                write!(args_plain, "{}: ", input.name);
-                    write!(args_plain, "{}: ", input.name);
-                }
 
                 if f.alternate() {
                     write!(args, "{:#}", input.type_.print(cx));

src/test/rustdoc/inline_cross/assoc_item_trait_bounds.rs

@@ -33,7 +33,7 @@
 // @snapshot out9 - '//*[@id="associatedtype.Out9"]/*[@class="code-header"]'
 //
 // @has - '//*[@id="tymethod.make"]' \
-// "fn make<F>(F, impl FnMut(&str) -> bool)\
+// "fn make<F>(_: F, _: impl FnMut(&str) -> bool)\
 // where \
 //     F: FnOnce(u32) -> String, \
 //     Self::Out2<()>: Protocol<u8, Q0 = Self::Item, Q1 = ()>"

src/test/rustdoc/inline_cross/impl_trait.rs

@@ -29,7 +29,7 @@
 // @has impl_trait/fn.func5.html
 // @has - '//pre[@class="rust fn"]' "func5("
 // @has - '//pre[@class="rust fn"]' "_f: impl for<'any> Fn(&'any str, &'any str) -> bool + for<'r> Other<T<'r> = ()>,"
-// @has - '//pre[@class="rust fn"]' "_a: impl for<'alpha, 'beta> Auxiliary<'alpha, Item<'beta> = fn(&'beta ())>"
+// @has - '//pre[@class="rust fn"]' "_a: impl for<'alpha, 'beta> Auxiliary<'alpha, Item<'beta> = fn(_: &'beta ())>"
 // @!has - '//pre[@class="rust fn"]' 'where'
 pub use impl_trait_aux::func5;