Support HIR wf checking for function signatures

compiler/rustc_middle/src/hir/map/mod.rs

@@ -29,7 +29,10 @@ fn fn_decl<'hir>(node: Node<'hir>) -> Option<&'hir FnDecl<'hir>> {
         Node::Item(Item { kind: ItemKind::Fn(sig, _, _), .. })
         | Node::TraitItem(TraitItem { kind: TraitItemKind::Fn(sig, _), .. })
         | Node::ImplItem(ImplItem { kind: ImplItemKind::Fn(sig, _), .. }) => Some(&sig.decl),
-        Node::Expr(Expr { kind: ExprKind::Closure(_, fn_decl, ..), .. }) => Some(fn_decl),
+        Node::Expr(Expr { kind: ExprKind::Closure(_, fn_decl, ..), .. })
+        | Node::ForeignItem(ForeignItem { kind: ForeignItemKind::Fn(fn_decl, ..), .. }) => {
+            Some(fn_decl)
+        }
         _ => None,
     }
 }

compiler/rustc_middle/src/query/mod.rs

@@ -1722,7 +1722,7 @@ rustc_queries! {
     /// span) for an *existing* error. Therefore, it is best-effort, and may never handle
     /// all of the cases that the normal `ty::Ty`-based wfcheck does. This is fine,
     /// because the `ty::Ty`-based wfcheck is always run.
-    query diagnostic_hir_wf_check(key: (ty::Predicate<'tcx>, hir::HirId)) -> Option<traits::ObligationCause<'tcx>> {
+    query diagnostic_hir_wf_check(key: (ty::Predicate<'tcx>, traits::WellFormedLoc)) -> Option<traits::ObligationCause<'tcx>> {
         eval_always
         no_hash
         desc { "performing HIR wf-checking for predicate {:?} at item {:?}", key.0, key.1 }

compiler/rustc_middle/src/traits/mod.rs

@@ -16,7 +16,7 @@ use crate::ty::{self, AdtKind, Ty, TyCtxt};
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
-use rustc_hir::def_id::DefId;
+use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::Constness;
 use rustc_span::symbol::Symbol;
 use rustc_span::{Span, DUMMY_SP};
@@ -327,17 +327,39 @@ pub enum ObligationCauseCode<'tcx> {
     /// If `X` is the concrete type of an opaque type `impl Y`, then `X` must implement `Y`
     OpaqueType,
 
-    /// Well-formed checking. If a `HirId` is provided,
-    /// it is used to perform HIR-based wf checking if an error
-    /// occurs, in order to generate a more precise error message.
+    /// Well-formed checking. If a `WellFormedLoc` is provided,
+    /// then it will be used to eprform HIR-based wf checking
+    /// after an error occurs, in order to generate a more precise error span.
     /// This is purely for diagnostic purposes - it is always
-    /// correct to use `MiscObligation` instead
-    WellFormed(Option<hir::HirId>),
+    /// correct to use `MiscObligation` instead, or to specify
+    /// `WellFormed(None)`
+    WellFormed(Option<WellFormedLoc>),
 
     /// From `match_impl`. The cause for us having to match an impl, and the DefId we are matching against.
     MatchImpl(Lrc<ObligationCauseCode<'tcx>>, DefId),
 }
 
+/// The 'location' at which we try to perform HIR-based wf checking.
+/// This information is used to obtain an `hir::Ty`, which
+/// we can walk in order to obtain precise spans for any
+/// 'nested' types (e.g. `Foo` in `Option<Foo>`).
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, HashStable)]
+pub enum WellFormedLoc {
+    /// Use the type of the provided definition.
+    Ty(LocalDefId),
+    /// Use the type of the parameter of the provided function.
+    /// We cannot use `hir::Param`, since the function may
+    /// not have a body (e.g. a trait method definition)
+    Param {
+        /// The function to lookup the parameter in
+        function: LocalDefId,
+        /// The index of the parameter to use.
+        /// Parameters are indexed from 0, with the return type
+        /// being the last 'parameter'
+        param_idx: u16,
+    },
+}
+
 impl ObligationCauseCode<'_> {
     // Return the base obligation, ignoring derived obligations.
     pub fn peel_derives(&self) -> &Self {

compiler/rustc_middle/src/ty/context.rs

@@ -1682,7 +1682,7 @@ nop_list_lift! {bound_variable_kinds; ty::BoundVariableKind => ty::BoundVariable
 // This is the impl for `&'a InternalSubsts<'a>`.
 nop_list_lift! {substs; GenericArg<'a> => GenericArg<'tcx>}
 
-CloneLiftImpls! { for<'tcx> { Constness, } }
+CloneLiftImpls! { for<'tcx> { Constness, traits::WellFormedLoc, } }
 
 pub mod tls {
     use super::{ptr_eq, GlobalCtxt, TyCtxt};

compiler/rustc_query_impl/src/keys.rs

@@ -1,9 +1,9 @@
 //! Defines the set of legal keys that can be used in queries.
 
 use rustc_hir::def_id::{CrateNum, DefId, LocalDefId, LOCAL_CRATE};
-use rustc_hir::HirId;
 use rustc_middle::infer::canonical::Canonical;
 use rustc_middle::mir;
+use rustc_middle::traits;
 use rustc_middle::ty::fast_reject::SimplifiedType;
 use rustc_middle::ty::subst::{GenericArg, SubstsRef};
 use rustc_middle::ty::{self, Ty, TyCtxt};
@@ -397,7 +397,7 @@ impl<'tcx> Key for (DefId, Ty<'tcx>, SubstsRef<'tcx>, ty::ParamEnv<'tcx>) {
     }
 }
 
-impl<'tcx> Key for (ty::Predicate<'tcx>, HirId) {
+impl<'tcx> Key for (ty::Predicate<'tcx>, traits::WellFormedLoc) {
     #[inline(always)]
     fn query_crate_is_local(&self) -> bool {
         true

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -242,11 +242,11 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             SelectionError::Unimplemented => {
                 // If this obligation was generated as a result of well-formed checking, see if we
                 // can get a better error message by performing HIR-based well formed checking.
-                if let ObligationCauseCode::WellFormed(Some(wf_hir_id)) =
+                if let ObligationCauseCode::WellFormed(Some(wf_loc)) =
                     root_obligation.cause.code.peel_derives()
                 {
                     if let Some(cause) =
-                        self.tcx.diagnostic_hir_wf_check((obligation.predicate, *wf_hir_id))
+                        self.tcx.diagnostic_hir_wf_check((obligation.predicate, wf_loc.clone()))
                     {
                         obligation.cause = cause;
                         span = obligation.cause.span;

compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs

@@ -40,6 +40,7 @@ use rustc_trait_selection::opaque_types::InferCtxtExt as _;
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
 use rustc_trait_selection::traits::{
     self, ObligationCause, ObligationCauseCode, StatementAsExpression, TraitEngine, TraitEngineExt,
+    WellFormedLoc,
 };
 
 use std::collections::hash_map::Entry;
@@ -419,13 +420,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         &self,
         span: Span,
         value: T,
-        hir_id: hir::HirId,
+        loc: WellFormedLoc,
     ) -> T
     where
         T: TypeFoldable<'tcx>,
     {
         self.inh.normalize_associated_types_in_with_cause(
-            ObligationCause::new(span, self.body_id, ObligationCauseCode::WellFormed(Some(hir_id))),
+            ObligationCause::new(span, self.body_id, ObligationCauseCode::WellFormed(Some(loc))),
             self.param_env,
             value,
         )

compiler/rustc_typeck/src/check/wfcheck.rs

@@ -22,8 +22,9 @@ use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::Span;
 use rustc_trait_selection::opaque_types::may_define_opaque_type;
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
-use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode};
+use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode, WellFormedLoc};
 
+use std::convert::TryInto;
 use std::iter;
 use std::ops::ControlFlow;
 
@@ -386,7 +387,7 @@ fn check_associated_item(
     span: Span,
     sig_if_method: Option<&hir::FnSig<'_>>,
 ) {
-    let code = ObligationCauseCode::WellFormed(Some(item_id));
+    let code = ObligationCauseCode::WellFormed(Some(WellFormedLoc::Ty(item_id.expect_owner())));
     for_id(tcx, item_id, span).with_fcx(|fcx| {
         let item = fcx.tcx.associated_item(fcx.tcx.hir().local_def_id(item_id));
 
@@ -400,7 +401,11 @@ fn check_associated_item(
         match item.kind {
             ty::AssocKind::Const => {
                 let ty = fcx.tcx.type_of(item.def_id);
-                let ty = fcx.normalize_associated_types_in_wf(span, ty, item_id);
+                let ty = fcx.normalize_associated_types_in_wf(
+                    span,
+                    ty,
+                    WellFormedLoc::Ty(item_id.expect_owner()),
+                );
                 fcx.register_wf_obligation(ty.into(), span, code.clone());
             }
             ty::AssocKind::Fn => {
@@ -422,7 +427,11 @@ fn check_associated_item(
                 }
                 if item.defaultness.has_value() {
                     let ty = fcx.tcx.type_of(item.def_id);
-                    let ty = fcx.normalize_associated_types_in_wf(span, ty, item_id);
+                    let ty = fcx.normalize_associated_types_in_wf(
+                        span,
+                        ty,
+                        WellFormedLoc::Ty(item_id.expect_owner()),
+                    );
                     fcx.register_wf_obligation(ty.into(), span, code.clone());
                 }
             }
@@ -621,7 +630,11 @@ fn check_item_type(tcx: TyCtxt<'_>, item_id: hir::HirId, ty_span: Span, allow_fo
 
     for_id(tcx, item_id, ty_span).with_fcx(|fcx| {
         let ty = tcx.type_of(tcx.hir().local_def_id(item_id));
-        let item_ty = fcx.normalize_associated_types_in_wf(ty_span, ty, item_id);
+        let item_ty = fcx.normalize_associated_types_in_wf(
+            ty_span,
+            ty,
+            WellFormedLoc::Ty(item_id.expect_owner()),
+        );
 
         let mut forbid_unsized = true;
         if allow_foreign_ty {
@@ -634,7 +647,7 @@ fn check_item_type(tcx: TyCtxt<'_>, item_id: hir::HirId, ty_span: Span, allow_fo
         fcx.register_wf_obligation(
             item_ty.into(),
             ty_span,
-            ObligationCauseCode::WellFormed(Some(item_id)),
+            ObligationCauseCode::WellFormed(Some(WellFormedLoc::Ty(item_id.expect_owner()))),
         );
         if forbid_unsized {
             fcx.register_bound(
@@ -684,7 +697,9 @@ fn check_impl<'tcx>(
                 fcx.register_wf_obligation(
                     self_ty.into(),
                     ast_self_ty.span,
-                    ObligationCauseCode::WellFormed(Some(item.hir_id())),
+                    ObligationCauseCode::WellFormed(Some(WellFormedLoc::Ty(
+                        item.hir_id().expect_owner(),
+                    ))),
                 );
             }
         }
@@ -901,11 +916,48 @@ fn check_fn_or_method<'fcx, 'tcx>(
     implied_bounds: &mut Vec<Ty<'tcx>>,
 ) {
     let sig = fcx.tcx.liberate_late_bound_regions(def_id, sig);
-    let sig = fcx.normalize_associated_types_in(span, sig);
 
-    for (&input_ty, ty) in iter::zip(sig.inputs(), hir_decl.inputs) {
-        fcx.register_wf_obligation(input_ty.into(), ty.span, ObligationCauseCode::WellFormed(None));
+    // Normalize the input and output types one at a time, using a different
+    // `WellFormedLoc` for each. We cannot call `normalize_associated_types`
+    // on the entire `FnSig`, since this would use the same `WellFormedLoc`
+    // for each type, preventing the HIR wf check from generating
+    // a nice error message.
+    let ty::FnSig { mut inputs_and_output, c_variadic, unsafety, abi } = sig;
+    inputs_and_output =
+        fcx.tcx.mk_type_list(inputs_and_output.iter().enumerate().map(|(i, ty)| {
+            fcx.normalize_associated_types_in_wf(
+                span,
+                ty,
+                WellFormedLoc::Param {
+                    function: def_id.expect_local(),
+                    // Note that the `param_idx` of the output type is
+                    // one greater than the index of the last input type.
+                    param_idx: i.try_into().unwrap(),
+                },
+            )
+        }));
+    // Manually call `normalize_assocaited_types_in` on the other types
+    // in `FnSig`. This ensures that if the types of these fields
+    // ever change to include projections, we will start normalizing
+    // them automatically.
+    let sig = ty::FnSig {
+        inputs_and_output,
+        c_variadic: fcx.normalize_associated_types_in(span, c_variadic),
+        unsafety: fcx.normalize_associated_types_in(span, unsafety),
+        abi: fcx.normalize_associated_types_in(span, abi),
+    };
+
+    for (i, (&input_ty, ty)) in iter::zip(sig.inputs(), hir_decl.inputs).enumerate() {
+        fcx.register_wf_obligation(
+            input_ty.into(),
+            ty.span,
+            ObligationCauseCode::WellFormed(Some(WellFormedLoc::Param {
+                function: def_id.expect_local(),
+                param_idx: i.try_into().unwrap(),
+            })),
+        );
     }
+
     implied_bounds.extend(sig.inputs());
 
     fcx.register_wf_obligation(