rustdoc: elide cross-crate default generic arguments

2 files changed, 113 insertions, 31 deletions

diff --git a/src/librustdoc/clean/mod.rs b/src/librustdoc/clean/mod.rs
index f2447b877ca..445f86a0b70 100644
--- a/src/librustdoc/clean/mod.rs
+++ b/src/librustdoc/clean/mod.rs
@@ -475,8 +475,9 @@ fn projection_to_path_segment<'tcx>(
     ty: ty::Binder<'tcx, ty::AliasTy<'tcx>>,
     cx: &mut DocContext<'tcx>,
 ) -> PathSegment {
-    let item = cx.tcx.associated_item(ty.skip_binder().def_id);
-    let generics = cx.tcx.generics_of(ty.skip_binder().def_id);
+    let def_id = ty.skip_binder().def_id;
+    let item = cx.tcx.associated_item(def_id);
+    let generics = cx.tcx.generics_of(def_id);
     PathSegment {
         name: item.name,
         args: GenericArgs::AngleBracketed {
@@ -484,7 +485,7 @@ fn projection_to_path_segment<'tcx>(
                 cx,
                 ty.map_bound(|ty| &ty.args[generics.parent_count..]),
                 false,
-                None,
+                def_id,
             )
             .into(),
             bindings: Default::default(),
@@ -2200,18 +2201,19 @@ pub(crate) fn clean_middle_ty<'tcx>(
         }
 
         ty::Alias(ty::Inherent, alias_ty) => {
+            let def_id = alias_ty.def_id;
             let alias_ty = bound_ty.rebind(alias_ty);
             let self_type = clean_middle_ty(alias_ty.map_bound(|ty| ty.self_ty()), cx, None, None);
 
             Type::QPath(Box::new(QPathData {
                 assoc: PathSegment {
-                    name: cx.tcx.associated_item(alias_ty.skip_binder().def_id).name,
+                    name: cx.tcx.associated_item(def_id).name,
                     args: GenericArgs::AngleBracketed {
                         args: ty_args_to_args(
                             cx,
                             alias_ty.map_bound(|ty| ty.args.as_slice()),
                             true,
-                            None,
+                            def_id,
                         )
                         .into(),
                         bindings: Default::default(),
diff --git a/src/librustdoc/clean/utils.rs b/src/librustdoc/clean/utils.rs
index c5302570489..dea7bfaf7e2 100644
--- a/src/librustdoc/clean/utils.rs
+++ b/src/librustdoc/clean/utils.rs
@@ -17,6 +17,7 @@ use rustc_hir::def_id::{DefId, LocalDefId, LOCAL_CRATE};
 use rustc_metadata::rendered_const;
 use rustc_middle::mir;
 use rustc_middle::ty::{self, GenericArgKind, GenericArgsRef, TyCtxt};
+use rustc_middle::ty::{TypeVisitable, TypeVisitableExt};
 use rustc_span::symbol::{kw, sym, Symbol};
 use std::fmt::Write as _;
 use std::mem;
@@ -76,44 +77,123 @@ pub(crate) fn krate(cx: &mut DocContext<'_>) -> Crate {
 
 pub(crate) fn ty_args_to_args<'tcx>(
     cx: &mut DocContext<'tcx>,
-    args: ty::Binder<'tcx, &'tcx [ty::GenericArg<'tcx>]>,
+    ty_args: ty::Binder<'tcx, &'tcx [ty::GenericArg<'tcx>]>,
     has_self: bool,
-    container: Option<DefId>,
+    owner: DefId,
 ) -> Vec<GenericArg> {
-    let mut skip_first = has_self;
-    let mut ret_val =
-        Vec::with_capacity(args.skip_binder().len().saturating_sub(if skip_first { 1 } else { 0 }));
-
-    ret_val.extend(args.iter().enumerate().filter_map(|(index, kind)| {
-        match kind.skip_binder().unpack() {
-            GenericArgKind::Lifetime(lt) => {
-                Some(GenericArg::Lifetime(clean_middle_region(lt).unwrap_or(Lifetime::elided())))
-            }
-            GenericArgKind::Type(_) if skip_first => {
-                skip_first = false;
-                None
+    if ty_args.skip_binder().is_empty() {
+        // Fast path which avoids executing the query `generics_of`.
+        return Vec::new();
+    }
+
+    let params = &cx.tcx.generics_of(owner).params;
+    let mut elision_has_failed_once_before = false;
+
+    let offset = if has_self { 1 } else { 0 };
+    let mut args = Vec::with_capacity(ty_args.skip_binder().len().saturating_sub(offset));
+
+    let ty_arg_to_arg = |(index, arg): (usize, &ty::GenericArg<'tcx>)| match arg.unpack() {
+        GenericArgKind::Lifetime(lt) => {
+            Some(GenericArg::Lifetime(clean_middle_region(lt).unwrap_or(Lifetime::elided())))
+        }
+        GenericArgKind::Type(_) if has_self && index == 0 => None,
+        GenericArgKind::Type(ty) => {
+            if !elision_has_failed_once_before
+                && let Some(default) = params[index].default_value(cx.tcx)
+            {
+                let default =
+                    ty_args.map_bound(|args| default.instantiate(cx.tcx, args).expect_ty());
+
+                if can_elide_generic_arg(ty_args.rebind(ty), default) {
+                    return None;
+                }
+
+                elision_has_failed_once_before = true;
             }
-            GenericArgKind::Type(ty) => Some(GenericArg::Type(clean_middle_ty(
-                kind.rebind(ty),
+
+            Some(GenericArg::Type(clean_middle_ty(
+                ty_args.rebind(ty),
                 cx,
                 None,
-                container.map(|container| crate::clean::ContainerTy::Regular {
-                    ty: container,
-                    args,
+                Some(crate::clean::ContainerTy::Regular {
+                    ty: owner,
+                    args: ty_args,
                     has_self,
                     arg: index,
                 }),
-            ))),
+            )))
+        }
+        GenericArgKind::Const(ct) => {
             // FIXME(effects): this relies on the host effect being called `host`, which users could also name
             // their const generics.
             // FIXME(effects): this causes `host = true` and `host = false` generics to also be emitted.
-            GenericArgKind::Const(ct) if let ty::ConstKind::Param(p) = ct.kind() && p.name == sym::host => None,
-            GenericArgKind::Const(ct) => {
-                Some(GenericArg::Const(Box::new(clean_middle_const(kind.rebind(ct), cx))))
+            if let ty::ConstKind::Param(p) = ct.kind()
+                && p.name == sym::host
+            {
+                return None;
             }
+
+            if !elision_has_failed_once_before
+                && let Some(default) = params[index].default_value(cx.tcx)
+            {
+                let default =
+                    ty_args.map_bound(|args| default.instantiate(cx.tcx, args).expect_const());
+
+                if can_elide_generic_arg(ty_args.rebind(ct), default) {
+                    return None;
+                }
+
+                elision_has_failed_once_before = true;
+            }
+
+            Some(GenericArg::Const(Box::new(clean_middle_const(ty_args.rebind(ct), cx))))
         }
-    }));
-    ret_val
+    };
+
+    args.extend(ty_args.skip_binder().iter().enumerate().rev().filter_map(ty_arg_to_arg));
+    args.reverse();
+    args
+}
+
+/// Check if the generic argument `actual` coincides with the `default` and can therefore be elided.
+///
+/// This uses a very conservative approach for performance and correctness reasons, meaning for
+/// several classes of terms it claims that they cannot be elided even if they theoretically could.
+/// This is absolutely fine since it mostly concerns edge cases.
+fn can_elide_generic_arg<'tcx, Term>(
+    actual: ty::Binder<'tcx, Term>,
+    default: ty::Binder<'tcx, Term>,
+) -> bool
+where
+    Term: Eq + TypeVisitable<TyCtxt<'tcx>>,
+{
+    // In practice, we shouldn't have any inference variables at this point.
+    // However to be safe, we bail out if we do happen to stumble upon them.
+    if actual.has_infer() || default.has_infer() {
+        return false;
+    }
+
+    // Since we don't properly keep track of bound variables in rustdoc (yet), we don't attempt to
+    // make any sense out of escaping bound variables. We simply don't have enough context and it
+    // would be incorrect to try to do so anyway.
+    if actual.has_escaping_bound_vars() || default.has_escaping_bound_vars() {
+        return false;
+    }
+
+    // Theoretically we could now check if either term contains (non-escaping) late-bound regions or
+    // projections, relate the two using an `InferCtxt` and check if the resulting obligations hold.
+    // Having projections means that the terms can potentially be further normalized thereby possibly
+    // revealing that they are equal after all. Regarding late-bound regions, they could to be
+    // liberated allowing us to consider more types to be equal by ignoring the names of binders
+    // (e.g., `for<'a> TYPE<'a>` and `for<'b> TYPE<'b>`).
+    //
+    // However, we are mostly interested in “reeliding” generic args, i.e., eliding generic args that
+    // were originally elided by the user and later filled in by the compiler contrary to eliding
+    // arbitrary generic arguments if they happen to semantically coincide with the default (of course,
+    // we cannot possibly distinguish these two cases). Therefore and for performance reasons, it
+    // suffices to only perform a syntactic / structural check by comparing the memory addresses of
+    // the interned arguments.
+    actual.skip_binder() == default.skip_binder()
 }
 
 fn external_generic_args<'tcx>(
@@ -123,7 +203,7 @@ fn external_generic_args<'tcx>(
     bindings: ThinVec<TypeBinding>,
     ty_args: ty::Binder<'tcx, GenericArgsRef<'tcx>>,
 ) -> GenericArgs {
-    let args = ty_args_to_args(cx, ty_args.map_bound(|args| &args[..]), has_self, Some(did));
+    let args = ty_args_to_args(cx, ty_args.map_bound(|args| &args[..]), has_self, did);
 
     if cx.tcx.fn_trait_kind_from_def_id(did).is_some() {
         let ty = ty_args