rustdoc: elide cross-crate default generic arguments

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(),

src/librustdoc/clean/utils.rs

@@ -17,6 +17,7 @@
 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 }));
+    if ty_args.skip_binder().is_empty() {
+        // Fast path which avoids executing the query `generics_of`.
+        return Vec::new();
+    }
 
-    ret_val.extend(args.iter().enumerate().filter_map(|(index, kind)| {
-        match kind.skip_binder().unpack() {
+    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 skip_first => {
-                skip_first = false;
-                None
+        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;
                 }
-            GenericArgKind::Type(ty) => Some(GenericArg::Type(clean_middle_ty(
-                kind.rebind(ty),
+
+                elision_has_failed_once_before = true;
+            }
+
+            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;
                 }
-    }));
-    ret_val
+
+                elision_has_failed_once_before = true;
+            }
+
+            Some(GenericArg::Const(Box::new(clean_middle_const(ty_args.rebind(ct), cx))))
+        }
+    };
+
+    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

tests/rustdoc/const-generics/add-impl.rs

@@ -7,7 +7,7 @@ pub struct Simd<T, const WIDTH: usize> {
     inner: T,
 }
 
-// @has foo/struct.Simd.html '//div[@id="trait-implementations-list"]//h3[@class="code-header"]' 'impl Add<Simd<u8, 16>> for Simd<u8, 16>'
+// @has foo/struct.Simd.html '//div[@id="trait-implementations-list"]//h3[@class="code-header"]' 'impl Add for Simd<u8, 16>'
 impl Add for Simd<u8, 16> {
     type Output = Self;
 

tests/rustdoc/inline_cross/auxiliary/default-generic-args.rs

@@ -0,0 +1,45 @@
+pub type BoxedStr = Box<str>;
+pub type IntMap = std::collections::HashMap<i64, u64>;
+
+pub struct TyPair<T, U = T>(T, U);
+
+pub type T0 = TyPair<i32>;
+pub type T1 = TyPair<i32, u32>;
+pub type T2<K> = TyPair<i32, K>;
+pub type T3<Q> = TyPair<Q, Q>;
+
+pub struct CtPair<const C: u32, const D: u32 = C>;
+
+pub type C0 = CtPair<43, 43>;
+pub type C1 = CtPair<0, 1>;
+pub type C2 = CtPair<{1 + 2}, 3>;
+
+pub struct Re<'a, U = &'a ()>(&'a (), U);
+
+pub type R0<'q> = Re<'q>;
+pub type R1<'q> = Re<'q, &'q ()>;
+pub type R2<'q> = Re<'q, &'static ()>;
+pub type H0 = fn(for<'a> fn(Re<'a>));
+pub type H1 = for<'b> fn(for<'a> fn(Re<'a, &'b ()>));
+pub type H2 = for<'a> fn(for<'b> fn(Re<'a, &'b ()>));
+
+pub struct Proj<T: Basis, U = <T as Basis>::Assoc>(T, U);
+pub trait Basis { type Assoc; }
+impl Basis for () { type Assoc = bool; }
+
+pub type P0 = Proj<()>;
+pub type P1 = Proj<(), bool>;
+pub type P2 = Proj<(), ()>;
+
+pub struct Alpha<T = for<'any> fn(&'any ())>(T);
+
+pub type A0 = Alpha;
+pub type A1 = Alpha<for<'arbitrary> fn(&'arbitrary ())>;
+
+pub struct Multi<A = u64, B = u64>(A, B);
+
+pub type M0 = Multi<u64, ()>;
+
+pub trait Trait<'a, T = &'a ()> {}
+
+pub type F = dyn for<'a> Trait<'a>;

tests/rustdoc/inline_cross/default-generic-args.rs

@@ -0,0 +1,104 @@
+#![crate_name = "user"]
+// aux-crate:default_generic_args=default-generic-args.rs
+// edition:2021
+
+// @has user/type.BoxedStr.html
+// @has - '//*[@class="rust item-decl"]//code' "Box<str>"
+pub use default_generic_args::BoxedStr;
+
+// @has user/type.IntMap.html
+// @has - '//*[@class="rust item-decl"]//code' "HashMap<i64, u64>"
+pub use default_generic_args::IntMap;
+
+// @has user/type.T0.html
+// @has - '//*[@class="rust item-decl"]//code' "TyPair<i32>"
+pub use default_generic_args::T0;
+
+// @has user/type.T1.html
+// @has - '//*[@class="rust item-decl"]//code' "TyPair<i32, u32>"
+pub use default_generic_args::T1;
+
+// @has user/type.T2.html
+// @has - '//*[@class="rust item-decl"]//code' "TyPair<i32, K>"
+pub use default_generic_args::T2;
+
+// @has user/type.T3.html
+// @has - '//*[@class="rust item-decl"]//code' "TyPair<Q>"
+pub use default_generic_args::T3;
+
+// @has user/type.C0.html
+// @has - '//*[@class="rust item-decl"]//code' "CtPair<43>"
+pub use default_generic_args::C0;
+
+// @has user/type.C1.html
+// @has - '//*[@class="rust item-decl"]//code' "CtPair<0, 1>"
+pub use default_generic_args::C1;
+
+// @has user/type.C2.html
+// @has - '//*[@class="rust item-decl"]//code' "CtPair<default_generic_args::::C2::{constant#0}, 3>"
+pub use default_generic_args::C2;
+
+// @has user/type.R0.html
+// @has - '//*[@class="rust item-decl"]//code' "Re<'q>"
+pub use default_generic_args::R0;
+
+// @has user/type.R1.html
+// @has - '//*[@class="rust item-decl"]//code' "Re<'q>"
+pub use default_generic_args::R1;
+
+// @has user/type.R2.html
+// Check that we consider regions:
+// @has - '//*[@class="rust item-decl"]//code' "Re<'q, &'static ()>"
+pub use default_generic_args::R2;
+
+// @has user/type.H0.html
+// Check that we handle higher-ranked regions correctly:
+// @has - '//*[@class="rust item-decl"]//code' "fn(_: for<'a> fn(_: Re<'a>))"
+pub use default_generic_args::H0;
+
+// @has user/type.H1.html
+// Check that we don't conflate distinct universially quantified regions (#1):
+// @has - '//*[@class="rust item-decl"]//code' "for<'b> fn(_: for<'a> fn(_: Re<'a, &'b ()>))"
+pub use default_generic_args::H1;
+
+// @has user/type.H2.html
+// Check that we don't conflate distinct universially quantified regions (#2):
+// @has - '//*[@class="rust item-decl"]//code' "for<'a> fn(_: for<'b> fn(_: Re<'a, &'b ()>))"
+pub use default_generic_args::H2;
+
+// @has user/type.P0.html
+// @has - '//*[@class="rust item-decl"]//code' "Proj<()>"
+pub use default_generic_args::P0;
+
+// @has user/type.P1.html
+// @has - '//*[@class="rust item-decl"]//code' "Proj<(), bool>"
+pub use default_generic_args::P1;
+
+// @has user/type.P2.html
+// @has - '//*[@class="rust item-decl"]//code' "Proj<(), ()>"
+pub use default_generic_args::P2;
+
+// @has user/type.A0.html
+// Ensure that we elide generic arguments that are alpha-equivalent to their respective
+// generic parameter (modulo substs) (#1):
+// @has - '//*[@class="rust item-decl"]//code' "Alpha"
+pub use default_generic_args::A0;
+
+// @has user/type.A1.html
+// Ensure that we elide generic arguments that are alpha-equivalent to their respective
+// generic parameter (modulo substs) (#1):
+// @has - '//*[@class="rust item-decl"]//code' "Alpha"
+pub use default_generic_args::A1;
+
+// @has user/type.M0.html
+// Test that we don't elide `u64` even if it coincides with `A`'s default precisely because
+// `()` is not the default of `B`. Mindlessly eliding `u64` would lead to `M<()>` which is a
+// different type (`M<(), u64>` versus `M<u64, ()>`).
+// @has - '//*[@class="rust item-decl"]//code' "Multi<u64, ()>"
+pub use default_generic_args::M0;
+
+// @has user/type.F.html
+// FIXME: Ideally, we would elide `&'a ()` but `'a` is an escaping bound var which we can't reason
+//        about at the moment since we don't keep track of bound vars.
+// @has - '//*[@class="rust item-decl"]//code' "dyn for<'a> Trait<'a, &'a ()>"
+pub use default_generic_args::F;

tests/rustdoc/inline_cross/dyn_trait.rs

@@ -75,16 +75,16 @@
 pub use dyn_trait::AmbiguousBoundWrappedStatic;
 
 // @has user/type.NoBoundsWrappedDefaulted.html
-// @has - '//*[@class="rust item-decl"]//code' "Box<dyn Trait, Global>;"
+// @has - '//*[@class="rust item-decl"]//code' "Box<dyn Trait>;"
 pub use dyn_trait::NoBoundsWrappedDefaulted;
 // @has user/type.NoBoundsWrappedEarly.html
-// @has - '//*[@class="rust item-decl"]//code' "Box<dyn Trait + 'e, Global>;"
+// @has - '//*[@class="rust item-decl"]//code' "Box<dyn Trait + 'e>;"
 pub use dyn_trait::NoBoundsWrappedEarly;
 // @has user/fn.nbwl.html
-// @has - '//pre[@class="rust item-decl"]' "nbwl<'l>(_: Box<dyn Trait + 'l, Global>)"
+// @has - '//pre[@class="rust item-decl"]' "nbwl<'l>(_: Box<dyn Trait + 'l>)"
 pub use dyn_trait::no_bounds_wrapped_late as nbwl;
 // @has user/fn.nbwel.html
-// @has - '//pre[@class="rust item-decl"]' "nbwel(_: Box<dyn Trait + '_, Global>)"
+// @has - '//pre[@class="rust item-decl"]' "nbwel(_: Box<dyn Trait + '_>)"
 // NB: It might seem counterintuitive to display the explicitly elided lifetime `'_` here instead of
 // eliding it but this behavior is correct: The default is `'static` here which != `'_`.
 pub use dyn_trait::no_bounds_wrapped_elided as nbwel;

tests/rustdoc/inline_cross/impl_trait.rs

@@ -4,7 +4,7 @@
 extern crate impl_trait_aux;
 
 // @has impl_trait/fn.func.html
-// @has - '//pre[@class="rust item-decl"]' "pub fn func<'a>(_x: impl Clone + Into<Vec<u8, Global>> + 'a)"
+// @has - '//pre[@class="rust item-decl"]' "pub fn func<'a>(_x: impl Clone + Into<Vec<u8>> + 'a)"
 // @!has - '//pre[@class="rust item-decl"]' 'where'
 pub use impl_trait_aux::func;
 
@@ -34,6 +34,6 @@
 pub use impl_trait_aux::func5;
 
 // @has impl_trait/struct.Foo.html
-// @has - '//*[@id="method.method"]//h4[@class="code-header"]' "pub fn method<'a>(_x: impl Clone + Into<Vec<u8, Global>> + 'a)"
+// @has - '//*[@id="method.method"]//h4[@class="code-header"]' "pub fn method<'a>(_x: impl Clone + Into<Vec<u8>> + 'a)"
 // @!has - '//*[@id="method.method"]//h4[@class="code-header"]' 'where'
 pub use impl_trait_aux::Foo;

tests/rustdoc/normalize-assoc-item.rs

@@ -30,7 +30,7 @@ pub fn f2() -> <isize as Trait>::X {
 }
 
 pub struct S {
-    // @has 'normalize_assoc_item/struct.S.html' '//span[@id="structfield.box_me_up"]' 'box_me_up: Box<S, Global>'
+    // @has 'normalize_assoc_item/struct.S.html' '//span[@id="structfield.box_me_up"]' 'box_me_up: Box<S>'
     pub box_me_up: <S as Trait>::X,
     // @has 'normalize_assoc_item/struct.S.html' '//span[@id="structfield.generic"]' 'generic: (usize, isize)'
     pub generic: <Generic<usize> as Trait>::X,
@@ -76,7 +76,7 @@ pub fn g() -> <usize as Lifetimes<'static>>::Y {
 // @has 'normalize_assoc_item/fn.foo.html' '//pre[@class="rust item-decl"]' "pub fn foo() -> i32"
 pub use inner::foo;
 
-// @has 'normalize_assoc_item/fn.h.html' '//pre[@class="rust item-decl"]' "pub fn h<T>() -> IntoIter<T, Global>"
+// @has 'normalize_assoc_item/fn.h.html' '//pre[@class="rust item-decl"]' "pub fn h<T>() -> IntoIter<T>"
 pub fn h<T>() -> <Vec<T> as IntoIterator>::IntoIter {
     vec![].into_iter()
 }

tests/rustdoc/where-clause-order.rs

@@ -7,7 +7,7 @@ pub trait SomeTrait<Rhs = Self>
 }
 
 // @has 'foo/trait.SomeTrait.html'
-// @has - "//*[@id='impl-SomeTrait-for-(A,+B,+C,+D,+E)']/h3" "impl<A, B, C, D, E> SomeTrait<(A, B, C, D, E)> for (A, B, C, D, E)where A: PartialOrd<A> + PartialEq<A>, B: PartialOrd<B> + PartialEq<B>, C: PartialOrd<C> + PartialEq<C>, D: PartialOrd<D> + PartialEq<D>, E: PartialOrd<E> + PartialEq<E> + ?Sized, "
+// @has - "//*[@id='impl-SomeTrait-for-(A,+B,+C,+D,+E)']/h3" "impl<A, B, C, D, E> SomeTrait for (A, B, C, D, E)where A: PartialOrd<A> + PartialEq<A>, B: PartialOrd<B> + PartialEq<B>, C: PartialOrd<C> + PartialEq<C>, D: PartialOrd<D> + PartialEq<D>, E: PartialOrd<E> + PartialEq<E> + ?Sized, "
 impl<A, B, C, D, E> SomeTrait<(A, B, C, D, E)> for (A, B, C, D, E)
 where
     A: PartialOrd<A> + PartialEq<A>,