rustdoc: simplify clean by removing FnRetTy

The default fn ret ty is always unit. Just use that.

Looking back at the time when `FnRetTy` (then called
`FunctionRetTy`) was first added to rustdoc, it seems to originally
be there because `-> !` was a special form: the never type didn't
exist back then.

eb01b17#diff-384affc1b4190940f114f3fcebbf969e7e18657a71ef9001da6b223a036687d9L921-L924

src/librustdoc/clean/mod.rs

@@ -1111,8 +1111,8 @@ fn clean_fn_decl_with_args<'tcx>(
     args: Arguments,
 ) -> FnDecl {
     let output = match decl.output {
-        hir::FnRetTy::Return(typ) => Return(clean_ty(typ, cx)),
-        hir::FnRetTy::DefaultReturn(..) => DefaultReturn,
+        hir::FnRetTy::Return(typ) => clean_ty(typ, cx),
+        hir::FnRetTy::DefaultReturn(..) => Type::Tuple(Vec::new()),
     };
     FnDecl { inputs: args, output, c_variadic: decl.c_variadic }
 }
@@ -1126,10 +1126,7 @@ fn clean_fn_decl_from_did_and_sig<'tcx>(
 
     // We assume all empty tuples are default return type. This theoretically can discard `-> ()`,
     // but shouldn't change any code meaning.
-    let output = match clean_middle_ty(sig.output(), cx, None) {
-        Type::Tuple(inner) if inner.is_empty() => DefaultReturn,
-        ty => Return(ty),
-    };
+    let output = clean_middle_ty(sig.output(), cx, None);
 
     FnDecl {
         output,

src/librustdoc/clean/types.rs

@@ -42,7 +42,6 @@ use crate::formats::item_type::ItemType;
 use crate::html::render::Context;
 use crate::passes::collect_intra_doc_links::UrlFragment;
 
-pub(crate) use self::FnRetTy::*;
 pub(crate) use self::ItemKind::*;
 pub(crate) use self::SelfTy::*;
 pub(crate) use self::Type::{
@@ -1353,7 +1352,7 @@ pub(crate) struct Function {
 #[derive(Clone, PartialEq, Eq, Debug, Hash)]
 pub(crate) struct FnDecl {
     pub(crate) inputs: Arguments,
-    pub(crate) output: FnRetTy,
+    pub(crate) output: Type,
     pub(crate) c_variadic: bool,
 }
 
@@ -1371,18 +1370,16 @@ impl FnDecl {
     ///
     /// This function will panic if the return type does not match the expected sugaring for async
     /// functions.
-    pub(crate) fn sugared_async_return_type(&self) -> FnRetTy {
-        match &self.output {
-            FnRetTy::Return(Type::ImplTrait(bounds)) => match &bounds[0] {
-                GenericBound::TraitBound(PolyTrait { trait_, .. }, ..) => {
-                    let bindings = trait_.bindings().unwrap();
-                    let ret_ty = bindings[0].term();
-                    let ty = ret_ty.ty().expect("Unexpected constant return term");
-                    FnRetTy::Return(ty.clone())
-                }
-                _ => panic!("unexpected desugaring of async function"),
-            },
-            _ => panic!("unexpected desugaring of async function"),
+    pub(crate) fn sugared_async_return_type(&self) -> Type {
+        if let Type::ImplTrait(v) = &self.output &&
+            let [GenericBound::TraitBound(PolyTrait { trait_, .. }, _ )] = &v[..]
+        {
+            let bindings = trait_.bindings().unwrap();
+            let ret_ty = bindings[0].term();
+            let ty = ret_ty.ty().expect("Unexpected constant return term");
+            ty.clone()
+        } else {
+            panic!("unexpected desugaring of async function")
         }
     }
 }
@@ -1425,21 +1422,6 @@ impl Argument {
     }
 }
 
-#[derive(Clone, PartialEq, Eq, Debug, Hash)]
-pub(crate) enum FnRetTy {
-    Return(Type),
-    DefaultReturn,
-}
-
-impl FnRetTy {
-    pub(crate) fn as_return(&self) -> Option<&Type> {
-        match self {
-            Return(ret) => Some(ret),
-            DefaultReturn => None,
-        }
-    }
-}
-
 #[derive(Clone, Debug)]
 pub(crate) struct Trait {
     pub(crate) def_id: DefId,
@@ -1641,6 +1623,10 @@ impl Type {
         matches!(self, Type::ImplTrait(_))
     }
 
+    pub(crate) fn is_unit(&self) -> bool {
+        matches!(self, Type::Tuple(v) if v.is_empty())
+    }
+
     pub(crate) fn projection(&self) -> Option<(&Type, DefId, PathSegment)> {
         if let QPath(box QPathData { self_type, trait_, assoc, .. }) = self {
             Some((self_type, trait_.as_ref()?.def_id(), assoc.clone()))

src/librustdoc/html/format.rs

@@ -1257,9 +1257,9 @@ impl clean::Impl {
                 };
                 primitive_link_fragment(f, PrimitiveType::Tuple, &format!("fn ({name}₁, {name}₂, …, {name}ₙ{ellipsis})"), "#trait-implementations-1", cx)?;
                 // Write output.
-                if let clean::FnRetTy::Return(ty) = &bare_fn.decl.output {
+                if !bare_fn.decl.output.is_unit() {
                     write!(f, " -> ")?;
-                    fmt_type(ty, f, use_absolute, cx)?;
+                    fmt_type(&bare_fn.decl.output, f, use_absolute, cx)?;
                 }
             } else if let Some(ty) = self.kind.as_blanket_ty() {
                 fmt_type(ty, f, use_absolute, cx)?;
@@ -1296,22 +1296,6 @@ impl clean::Arguments {
     }
 }
 
-impl clean::FnRetTy {
-    pub(crate) fn print<'a, 'tcx: 'a>(
-        &'a self,
-        cx: &'a Context<'tcx>,
-    ) -> impl fmt::Display + 'a + Captures<'tcx> {
-        display_fn(move |f| match self {
-            clean::Return(clean::Tuple(tys)) if tys.is_empty() => Ok(()),
-            clean::Return(ty) if f.alternate() => {
-                write!(f, " -> {:#}", ty.print(cx))
-            }
-            clean::Return(ty) => write!(f, " -&gt; {}", ty.print(cx)),
-            clean::DefaultReturn => Ok(()),
-        })
-    }
-}
-
 impl clean::BareFunctionDecl {
     fn print_hrtb_with_space<'a, 'tcx: 'a>(
         &'a self,
@@ -1366,15 +1350,15 @@ impl clean::FnDecl {
                     "({args:#}{ellipsis}){arrow:#}",
                     args = self.inputs.print(cx),
                     ellipsis = ellipsis,
-                    arrow = self.output.print(cx)
+                    arrow = self.print_output(cx)
                 )
             } else {
                 write!(
                     f,
                     "({args}{ellipsis}){arrow}",
                     args = self.inputs.print(cx),
                     ellipsis = ellipsis,
-                    arrow = self.output.print(cx)
+                    arrow = self.print_output(cx)
                 )
             }
         })
@@ -1464,9 +1448,22 @@ impl clean::FnDecl {
             Some(n) => write!(f, "\n{})", Indent(n))?,
         };
 
-        fmt::Display::fmt(&self.output.print(cx), f)?;
+        fmt::Display::fmt(&self.print_output(cx), f)?;
         Ok(())
     }
+
+    pub(crate) fn print_output<'a, 'tcx: 'a>(
+        &'a self,
+        cx: &'a Context<'tcx>,
+    ) -> impl fmt::Display + 'a + Captures<'tcx> {
+        display_fn(move |f| match &self.output {
+            clean::Tuple(tys) if tys.is_empty() => Ok(()),
+            ty if f.alternate() => {
+                write!(f, " -> {:#}", ty.print(cx))
+            }
+            ty => write!(f, " -&gt; {}", ty.print(cx)),
+        })
+    }
 }
 
 pub(crate) fn visibility_print_with_space<'a, 'tcx: 'a>(

src/librustdoc/html/render/mod.rs

@@ -844,7 +844,7 @@ fn assoc_method(
         + name.as_str().len()
         + generics_len;
 
-    let notable_traits = d.output.as_return().and_then(|output| notable_traits_button(output, cx));
+    let notable_traits = notable_traits_button(&d.output, cx);
 
     let (indent, indent_str, end_newline) = if parent == ItemType::Trait {
         header_len += 4;
@@ -1282,6 +1282,11 @@ fn should_render_item(item: &clean::Item, deref_mut_: bool, tcx: TyCtxt<'_>) ->
 pub(crate) fn notable_traits_button(ty: &clean::Type, cx: &mut Context<'_>) -> Option<String> {
     let mut has_notable_trait = false;
 
+    if ty.is_unit() {
+        // Very common fast path.
+        return None;
+    }
+
     let did = ty.def_id(cx.cache())?;
 
     // Box has pass-through impls for Read, Write, Iterator, and Future when the

src/librustdoc/html/render/print_item.rs

@@ -587,8 +587,7 @@ fn item_function(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, f: &cle
         + name.as_str().len()
         + generics_len;
 
-    let notable_traits =
-        f.decl.output.as_return().and_then(|output| notable_traits_button(output, cx));
+    let notable_traits = notable_traits_button(&f.decl.output, cx);
 
     wrap_item(w, |w| {
         w.reserve(header_len);

src/librustdoc/html/render/search_index.rs

@@ -7,7 +7,7 @@ use rustc_span::symbol::Symbol;
 use serde::ser::{Serialize, SerializeStruct, Serializer};
 
 use crate::clean;
-use crate::clean::types::{FnRetTy, Function, Generics, ItemId, Type, WherePredicate};
+use crate::clean::types::{Function, Generics, ItemId, Type, WherePredicate};
 use crate::formats::cache::{Cache, OrphanImplItem};
 use crate::formats::item_type::ItemType;
 use crate::html::format::join_with_double_colon;
@@ -656,22 +656,9 @@ fn get_fn_inputs_and_outputs<'tcx>(
     }
 
     let mut ret_types = Vec::new();
-    match decl.output {
-        FnRetTy::Return(ref return_type) => {
-            add_generics_and_bounds_as_types(
-                self_,
-                generics,
-                return_type,
-                tcx,
-                0,
-                &mut ret_types,
-                cache,
-            );
-            if ret_types.is_empty() {
-                ret_types.push(get_index_type(return_type, vec![]));
-            }
-        }
-        _ => {}
-    };
+    add_generics_and_bounds_as_types(self_, generics, &decl.output, tcx, 0, &mut ret_types, cache);
+    if ret_types.is_empty() {
+        ret_types.push(get_index_type(&decl.output, vec![]));
+    }
     (all_types, ret_types)
 }

src/librustdoc/json/conversions.rs

@@ -624,10 +624,7 @@ impl FromWithTcx<clean::FnDecl> for FnDecl {
                 .into_iter()
                 .map(|arg| (arg.name.to_string(), arg.type_.into_tcx(tcx)))
                 .collect(),
-            output: match output {
-                clean::FnRetTy::Return(t) => Some(t.into_tcx(tcx)),
-                clean::FnRetTy::DefaultReturn => None,
-            },
+            output: if output.is_unit() { None } else { Some(output.into_tcx(tcx)) },
             c_variadic,
         }
     }