rustdoc: cross-crate re-exports: correctly render late-bound params in source order even if early-bound params are present

1 files changed, 36 insertions, 14 deletions

diff --git a/src/librustdoc/clean/inline.rs b/src/librustdoc/clean/inline.rs
index 23a059c6e03..e2f2c9a5e56 100644
--- a/src/librustdoc/clean/inline.rs
+++ b/src/librustdoc/clean/inline.rs
@@ -18,8 +18,8 @@ use rustc_span::hygiene::MacroKind;
 use rustc_span::symbol::{kw, sym, Symbol};
 
 use crate::clean::{
-    self, clean_bound_vars, clean_fn_decl_from_did_and_sig, clean_generics, clean_impl_item,
-    clean_middle_assoc_item, clean_middle_field, clean_middle_ty, clean_trait_ref_with_bindings,
+    self, clean_bound_vars, clean_generics, clean_impl_item, clean_middle_assoc_item,
+    clean_middle_field, clean_middle_ty, clean_poly_fn_sig, clean_trait_ref_with_bindings,
     clean_ty, clean_ty_alias_inner_type, clean_ty_generics, clean_variant_def, utils, Attributes,
     AttributesExt, ImplKind, ItemId, Type,
 };
@@ -72,7 +72,9 @@ pub(crate) fn try_inline(
         }
         Res::Def(DefKind::Fn, did) => {
             record_extern_fqn(cx, did, ItemType::Function);
-            cx.with_param_env(did, |cx| clean::FunctionItem(build_external_function(cx, did)))
+            cx.with_param_env(did, |cx| {
+                clean::enter_impl_trait(cx, |cx| clean::FunctionItem(build_function(cx, did)))
+            })
         }
         Res::Def(DefKind::Struct, did) => {
             record_extern_fqn(cx, did, ItemType::Struct);
@@ -274,18 +276,38 @@ pub(crate) fn build_external_trait(cx: &mut DocContext<'_>, did: DefId) -> clean
     clean::Trait { def_id: did, generics, items: trait_items, bounds: supertrait_bounds }
 }
 
-fn build_external_function<'tcx>(cx: &mut DocContext<'tcx>, did: DefId) -> Box<clean::Function> {
-    let sig = cx.tcx.fn_sig(did).instantiate_identity();
-    let predicates = cx.tcx.explicit_predicates_of(did);
+pub(crate) fn build_function<'tcx>(
+    cx: &mut DocContext<'tcx>,
+    def_id: DefId,
+) -> Box<clean::Function> {
+    let sig = cx.tcx.fn_sig(def_id).instantiate_identity();
+    // The generics need to be cleaned before the signature.
+    let mut generics =
+        clean_ty_generics(cx, cx.tcx.generics_of(def_id), cx.tcx.explicit_predicates_of(def_id));
+    let bound_vars = clean_bound_vars(sig.bound_vars());
+
+    // At the time of writing early & late-bound params are stored separately in rustc,
+    // namely in `generics.params` and `bound_vars` respectively.
+    //
+    // To reestablish the original source code order of the generic parameters, we
+    // need to manually sort them by their definition span after concatenation.
+    //
+    // See also:
+    // * https://rustc-dev-guide.rust-lang.org/bound-vars-and-params.html
+    // * https://rustc-dev-guide.rust-lang.org/what-does-early-late-bound-mean.html
+    let has_early_bound_params = !generics.params.is_empty();
+    let has_late_bound_params = !bound_vars.is_empty();
+    generics.params.extend(bound_vars);
+    if has_early_bound_params && has_late_bound_params {
+        // If this ever becomes a performances bottleneck either due to the sorting
+        // or due to the query calls, consider inserting the late-bound lifetime params
+        // right after the last early-bound lifetime param followed by only sorting
+        // the slice of lifetime params.
+        generics.params.sort_by_key(|param| cx.tcx.def_ident_span(param.def_id).unwrap());
+    }
+
+    let decl = clean_poly_fn_sig(cx, Some(def_id), sig);
 
-    let (generics, decl) = clean::enter_impl_trait(cx, |cx| {
-        // NOTE: generics need to be cleaned before the decl!
-        let mut generics = clean_ty_generics(cx, cx.tcx.generics_of(did), predicates);
-        // FIXME: This does not place parameters in source order (late-bound ones come last)
-        generics.params.extend(clean_bound_vars(sig.bound_vars()));
-        let decl = clean_fn_decl_from_did_and_sig(cx, Some(did), sig);
-        (generics, decl)
-    });
     Box::new(clean::Function { decl, generics })
 }