Rollup merge of rust-lang#68195 - estebank:impl-trait-2000, r=Centril

Account for common `impl Trait`/`dyn Trait` return type errors

- When all return paths have the same type, suggest `impl Trait`.
- When all return paths implement the expected `trait`, suggest `Box<dyn Trait>` and mention using an `enum`.
- When multiple different types are returned and `impl Trait` is expected, extend the explanation.
- When return type is `impl Trait` and the return paths do not implement `Trait`, point at the returned values.
- Split `src/librustc/traits/error_reporting.rs` into multiple files to keep size under control.

Fix rust-lang#68110, cc rust-lang#66523.

src/librustc/traits/error_reporting/on_unimplemented.rs

@@ -0,0 +1,199 @@
+use super::{
+    ObligationCauseCode, OnUnimplementedDirective, OnUnimplementedNote, PredicateObligation,
+};
+use crate::infer::InferCtxt;
+use crate::ty::subst::Subst;
+use crate::ty::{self, GenericParamDefKind};
+use rustc_hir as hir;
+use rustc_hir::def_id::DefId;
+use rustc_span::symbol::sym;
+
+impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
+    fn impl_similar_to(
+        &self,
+        trait_ref: ty::PolyTraitRef<'tcx>,
+        obligation: &PredicateObligation<'tcx>,
+    ) -> Option<DefId> {
+        let tcx = self.tcx;
+        let param_env = obligation.param_env;
+        let trait_ref = tcx.erase_late_bound_regions(&trait_ref);
+        let trait_self_ty = trait_ref.self_ty();
+
+        let mut self_match_impls = vec![];
+        let mut fuzzy_match_impls = vec![];
+
+        self.tcx.for_each_relevant_impl(trait_ref.def_id, trait_self_ty, |def_id| {
+            let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
+            let impl_trait_ref = tcx.impl_trait_ref(def_id).unwrap().subst(tcx, impl_substs);
+
+            let impl_self_ty = impl_trait_ref.self_ty();
+
+            if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
+                self_match_impls.push(def_id);
+
+                if trait_ref
+                    .substs
+                    .types()
+                    .skip(1)
+                    .zip(impl_trait_ref.substs.types().skip(1))
+                    .all(|(u, v)| self.fuzzy_match_tys(u, v))
+                {
+                    fuzzy_match_impls.push(def_id);
+                }
+            }
+        });
+
+        let impl_def_id = if self_match_impls.len() == 1 {
+            self_match_impls[0]
+        } else if fuzzy_match_impls.len() == 1 {
+            fuzzy_match_impls[0]
+        } else {
+            return None;
+        };
+
+        tcx.has_attr(impl_def_id, sym::rustc_on_unimplemented).then_some(impl_def_id)
+    }
+
+    /// Used to set on_unimplemented's `ItemContext`
+    /// to be the enclosing (async) block/function/closure
+    fn describe_enclosure(&self, hir_id: hir::HirId) -> Option<&'static str> {
+        let hir = &self.tcx.hir();
+        let node = hir.find(hir_id)?;
+        if let hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(sig, _, body_id), .. }) = &node {
+            self.describe_generator(*body_id).or_else(|| {
+                Some(if let hir::FnHeader { asyncness: hir::IsAsync::Async, .. } = sig.header {
+                    "an async function"
+                } else {
+                    "a function"
+                })
+            })
+        } else if let hir::Node::Expr(hir::Expr {
+            kind: hir::ExprKind::Closure(_is_move, _, body_id, _, gen_movability),
+            ..
+        }) = &node
+        {
+            self.describe_generator(*body_id).or_else(|| {
+                Some(if gen_movability.is_some() { "an async closure" } else { "a closure" })
+            })
+        } else if let hir::Node::Expr(hir::Expr { .. }) = &node {
+            let parent_hid = hir.get_parent_node(hir_id);
+            if parent_hid != hir_id {
+                return self.describe_enclosure(parent_hid);
+            } else {
+                None
+            }
+        } else {
+            None
+        }
+    }
+
+    crate fn on_unimplemented_note(
+        &self,
+        trait_ref: ty::PolyTraitRef<'tcx>,
+        obligation: &PredicateObligation<'tcx>,
+    ) -> OnUnimplementedNote {
+        let def_id =
+            self.impl_similar_to(trait_ref, obligation).unwrap_or_else(|| trait_ref.def_id());
+        let trait_ref = *trait_ref.skip_binder();
+
+        let mut flags = vec![];
+        flags.push((
+            sym::item_context,
+            self.describe_enclosure(obligation.cause.body_id).map(|s| s.to_owned()),
+        ));
+
+        match obligation.cause.code {
+            ObligationCauseCode::BuiltinDerivedObligation(..)
+            | ObligationCauseCode::ImplDerivedObligation(..) => {}
+            _ => {
+                // this is a "direct", user-specified, rather than derived,
+                // obligation.
+                flags.push((sym::direct, None));
+            }
+        }
+
+        if let ObligationCauseCode::ItemObligation(item) = obligation.cause.code {
+            // FIXME: maybe also have some way of handling methods
+            // from other traits? That would require name resolution,
+            // which we might want to be some sort of hygienic.
+            //
+            // Currently I'm leaving it for what I need for `try`.
+            if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
+                let method = self.tcx.item_name(item);
+                flags.push((sym::from_method, None));
+                flags.push((sym::from_method, Some(method.to_string())));
+            }
+        }
+        if let Some((t, _)) = self.get_parent_trait_ref(&obligation.cause.code) {
+            flags.push((sym::parent_trait, Some(t)));
+        }
+
+        if let Some(k) = obligation.cause.span.desugaring_kind() {
+            flags.push((sym::from_desugaring, None));
+            flags.push((sym::from_desugaring, Some(format!("{:?}", k))));
+        }
+        let generics = self.tcx.generics_of(def_id);
+        let self_ty = trait_ref.self_ty();
+        // This is also included through the generics list as `Self`,
+        // but the parser won't allow you to use it
+        flags.push((sym::_Self, Some(self_ty.to_string())));
+        if let Some(def) = self_ty.ty_adt_def() {
+            // We also want to be able to select self's original
+            // signature with no type arguments resolved
+            flags.push((sym::_Self, Some(self.tcx.type_of(def.did).to_string())));
+        }
+
+        for param in generics.params.iter() {
+            let value = match param.kind {
+                GenericParamDefKind::Type { .. } | GenericParamDefKind::Const => {
+                    trait_ref.substs[param.index as usize].to_string()
+                }
+                GenericParamDefKind::Lifetime => continue,
+            };
+            let name = param.name;
+            flags.push((name, Some(value)));
+        }
+
+        if let Some(true) = self_ty.ty_adt_def().map(|def| def.did.is_local()) {
+            flags.push((sym::crate_local, None));
+        }
+
+        // Allow targeting all integers using `{integral}`, even if the exact type was resolved
+        if self_ty.is_integral() {
+            flags.push((sym::_Self, Some("{integral}".to_owned())));
+        }
+
+        if let ty::Array(aty, len) = self_ty.kind {
+            flags.push((sym::_Self, Some("[]".to_owned())));
+            flags.push((sym::_Self, Some(format!("[{}]", aty))));
+            if let Some(def) = aty.ty_adt_def() {
+                // We also want to be able to select the array's type's original
+                // signature with no type arguments resolved
+                flags.push((
+                    sym::_Self,
+                    Some(format!("[{}]", self.tcx.type_of(def.did).to_string())),
+                ));
+                let tcx = self.tcx;
+                if let Some(len) = len.try_eval_usize(tcx, ty::ParamEnv::empty()) {
+                    flags.push((
+                        sym::_Self,
+                        Some(format!("[{}; {}]", self.tcx.type_of(def.did).to_string(), len)),
+                    ));
+                } else {
+                    flags.push((
+                        sym::_Self,
+                        Some(format!("[{}; _]", self.tcx.type_of(def.did).to_string())),
+                    ));
+                }
+            }
+        }
+
+        if let Ok(Some(command)) =
+            OnUnimplementedDirective::of_item(self.tcx, trait_ref.def_id, def_id)
+        {
+            command.evaluate(self.tcx, trait_ref, &flags[..])
+        } else {
+            OnUnimplementedNote::default()
+        }
+    }
+}