rustdoc: elide cross-crate default generic arguments

Author: fmease

src/librustdoc/clean/utils.rs

@@ -14,10 +14,16 @@ use rustc_ast::tokenstream::TokenTree;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId, LOCAL_CRATE};
+use rustc_infer::infer::at::ToTrace;
+use rustc_infer::infer::outlives::env::OutlivesEnvironment;
 use rustc_metadata::rendered_const;
 use rustc_middle::mir;
+use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::{self, GenericArgKind, GenericArgsRef, TyCtxt};
+use rustc_middle::ty::{TypeVisitable, TypeVisitableExt};
 use rustc_span::symbol::{kw, sym, Symbol};
+use rustc_trait_selection::infer::TyCtxtInferExt;
+use rustc_trait_selection::traits::ObligationCtxt;
 use std::fmt::Write as _;
 use std::mem;
 use std::sync::LazyLock as Lazy;
@@ -76,44 +82,142 @@ 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>,
 ) -> 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
+    let param_env = ty::ParamEnv::empty();
+    let cause = ObligationCause::dummy();
+    let params = container.map(|container| &cx.tcx.generics_of(container).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(params) = params
+                && 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(
+                    cx.tcx,
+                    &cause,
+                    param_env,
+                    ty_args.rebind(ty),
+                    default,
+                    params[index].def_id,
+                ) {
+                    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,
+                    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(params) = params
+                && 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(
+                    cx.tcx,
+                    &cause,
+                    param_env,
+                    ty_args.rebind(ct),
+                    default,
+                    params[index].def_id,
+                ) {
+                    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.
+fn can_elide_generic_arg<'tcx, T: ToTrace<'tcx> + TypeVisitable<TyCtxt<'tcx>>>(
+    tcx: TyCtxt<'tcx>,
+    cause: &ObligationCause<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    actual: ty::Binder<'tcx, T>,
+    default: ty::Binder<'tcx, T>,
+    did: DefId,
+) -> bool {
+    // The operations below are only correct if we don't have any inference variables.
+    debug_assert!(!actual.has_infer());
+    debug_assert!(!default.has_infer());
+
+    // Since we don't properly keep track of bound variables, don't attempt to make
+    // any sense out of escaping bound variables (we just don't have enough context).
+    if actual.has_escaping_bound_vars() || default.has_escaping_bound_vars() {
+        return false;
+    }
+
+    // If the arguments contain projections or (non-escaping) late-bound regions, we have to examine
+    // them more closely and can't take the fast path.
+    // Having projections means that there's potential to be further normalized thereby revealing if
+    // they are equal after all. Regarding late-bound regions, they can be liberated allowing us to
+    // consider more types to be equal by ignoring the names of binders.
+    if !actual.has_late_bound_regions()
+        && !actual.has_projections()
+        && !default.has_late_bound_regions()
+        && !default.has_projections()
+    {
+        // Check the memory addresses of the interned arguments for equality.
+        return actual.skip_binder() == default.skip_binder();
+    }
+
+    let actual = tcx.liberate_late_bound_regions(did, actual);
+    let default = tcx.liberate_late_bound_regions(did, default);
+
+    let infcx = tcx.infer_ctxt().build();
+    let ocx = ObligationCtxt::new(&infcx);
+
+    let actual = ocx.normalize(cause, param_env, actual);
+    let default = ocx.normalize(cause, param_env, default);
+
+    ocx.eq(cause, param_env, actual, default).is_ok()
+        && ocx.select_all_or_error().is_empty()
+        && infcx.resolve_regions(&OutlivesEnvironment::new(param_env)).is_empty()
 }
 
 fn external_generic_args<'tcx>(

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 +1,108 @@
+#![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
+// Test that we normalize constants in this case:
+// FIXME: Ideally, we would render `3` here instead of the def-path str of the normalized constant.
+// @has - '//*[@class="rust item-decl"]//code' "CtPair<default_generic_args::::C2::{constant#0}>"
+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:
+// FIXME: Ideally we would also print the *binders* here.
+// @has - '//*[@class="rust item-decl"]//code' "fn(_: 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):
+// FIXME: Ideally we would also print the *binders* here.
+// @has - '//*[@class="rust item-decl"]//code' "fn(_: 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' "fn(_: 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<()>"
+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::AmbiguousBoundWrappedEarly1;
 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::func4;
 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;