Handle and test wildcard arguments

Author: jyn514

compiler/rustc_ast_lowering/src/item.rs

@@ -1102,10 +1102,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         ident,
                         _,
                     ) => (ident, true),
-                    // For `ref mut` arguments, we can't reuse the binding, but
+                    // For `ref mut` or wildcard arguments, we can't reuse the binding, but
                     // we can keep the same name for the parameter.
                     // This lets rustdoc render it correctly in documentation.
                     hir::PatKind::Binding(_, _, ident, _) => (ident, false),
+                    hir::PatKind::Wild => {
+                        (Ident::with_dummy_span(rustc_span::symbol::kw::Underscore), false)
+                    }
                     _ => {
                         // Replace the ident for bindings that aren't simple.
                         let name = format!("__arg{}", index);

src/test/rustdoc/async-fn.rs

@@ -1,4 +1,5 @@
 // edition:2018
+#![feature(min_const_generics)]
 
 // @has async_fn/fn.foo.html '//pre[@class="rust fn"]' 'pub async fn foo() -> Option<Foo>'
 pub async fn foo() -> Option<Foo> {
@@ -46,3 +47,8 @@ impl Foo {
     pub async unsafe fn g() {}
     pub async fn mut_self(mut self, mut first: usize) {}
 }
+
+pub trait Trait<const N: usize> {}
+// @has async_fn/fn.const_generics.html
+// @has - '//pre[@class="rust fn"]' 'pub async fn const_generics<const N: usize>(_: impl Trait<N>)'
+pub async fn const_generics<const N: usize>(_: impl Trait<N>) {}

src/test/rustdoc/const-generics/const-generics-docs.rs

@@ -70,8 +70,7 @@ pub async fn a_sink<const N: usize>(v: [u8; N]) -> impl Trait<N> {
 }
 
 // @has foo/fn.b_sink.html '//pre[@class="rust fn"]' \
-//      'pub async fn b_sink<const N: usize>(__arg0: impl Trait<N>)'
-// FIXME(const_generics): This should be `_` not `__arg0`.
+//      'pub async fn b_sink<const N: usize>(_: impl Trait<N>)'
 pub async fn b_sink<const N: usize>(_: impl Trait<N>) {}
 
 // @has foo/fn.concrete.html '//pre[@class="rust fn"]' \