Specific error for unsized dyn Trait return type
Suggest `impl Trait` when possible, and `Box<dyn Trait>` otherwise.
This commit is contained in:
Esteban Küber
parent
9fe05e9456
commit
6fd564112f
@ -1,3 +1,4 @@
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// ignore-tidy-filelength
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use super::{
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ConstEvalFailure, EvaluationResult, FulfillmentError, FulfillmentErrorCode,
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MismatchedProjectionTypes, ObjectSafetyViolation, Obligation, ObligationCause,
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@ -22,9 +23,12 @@ use crate::ty::TypeckTables;
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use crate::ty::{self, AdtKind, DefIdTree, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable};
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use rustc_data_structures::fx::{FxHashMap, FxHashSet};
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use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticBuilder, Style};
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use rustc_errors::{
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error_code, pluralize, struct_span_err, Applicability, DiagnosticBuilder, Style,
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};
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use rustc_hir as hir;
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use rustc_hir::def_id::{DefId, LOCAL_CRATE};
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use rustc_hir::intravisit::Visitor;
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use rustc_hir::Node;
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use rustc_span::source_map::SourceMap;
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use rustc_span::symbol::{kw, sym};
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@ -758,7 +762,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
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)),
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Some(
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"the question mark operation (`?`) implicitly performs a \
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conversion on the error value using the `From` trait"
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conversion on the error value using the `From` trait"
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.to_owned(),
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),
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)
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@ -835,6 +839,10 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
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self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
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self.suggest_semicolon_removal(&obligation, &mut err, span, &trait_ref);
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self.note_version_mismatch(&mut err, &trait_ref);
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if self.suggest_impl_trait(&mut err, span, &obligation, &trait_ref) {
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err.emit();
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return;
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}
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// Try to report a help message
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if !trait_ref.has_infer_types()
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@ -1696,6 +1704,195 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
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}
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}
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fn suggest_impl_trait(
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&self,
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err: &mut DiagnosticBuilder<'tcx>,
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span: Span,
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obligation: &PredicateObligation<'tcx>,
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trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
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) -> bool {
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if let ObligationCauseCode::SizedReturnType = obligation.cause.code.peel_derives() {
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} else {
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return false;
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}
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let hir = self.tcx.hir();
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let parent_node = hir.get_parent_node(obligation.cause.body_id);
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let node = hir.find(parent_node);
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if let Some(hir::Node::Item(hir::Item {
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kind: hir::ItemKind::Fn(sig, _, body_id), ..
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})) = node
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{
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let body = hir.body(*body_id);
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let trait_ref = self.resolve_vars_if_possible(trait_ref);
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let ty = trait_ref.skip_binder().self_ty();
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if let ty::Dynamic(..) = ty.kind {
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} else {
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// We only want to suggest `impl Trait` to `dyn Trait`s.
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// For example, `fn foo() -> str` needs to be filtered out.
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return false;
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}
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// Use `TypeVisitor` instead of the output type directly to find the span of `ty` for
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// cases like `fn foo() -> (dyn Trait, i32) {}`.
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// Recursively look for `TraitObject` types and if there's only one, use that span to
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// suggest `impl Trait`.
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struct ReturnsVisitor<'v>(Vec<&'v hir::Expr<'v>>);
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impl<'v> Visitor<'v> for ReturnsVisitor<'v> {
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type Map = rustc::hir::map::Map<'v>;
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fn nested_visit_map(&mut self) -> hir::intravisit::NestedVisitorMap<'_, Self::Map> {
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hir::intravisit::NestedVisitorMap::None
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}
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fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
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match ex.kind {
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hir::ExprKind::Ret(Some(ex)) => self.0.push(ex),
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_ => {}
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}
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hir::intravisit::walk_expr(self, ex);
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}
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fn visit_body(&mut self, body: &'v hir::Body<'v>) {
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if body.generator_kind().is_none() {
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if let hir::ExprKind::Block(block, None) = body.value.kind {
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if let Some(expr) = block.expr {
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self.0.push(expr);
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}
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}
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}
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hir::intravisit::walk_body(self, body);
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}
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}
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// Visit to make sure there's a single `return` type to suggest `impl Trait`,
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// otherwise suggest using `Box<dyn Trait>` or an enum.
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let mut visitor = ReturnsVisitor(vec![]);
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visitor.visit_body(&body);
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let tables = self.in_progress_tables.map(|t| t.borrow()).unwrap();
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if let hir::FunctionRetTy::Return(ret_ty) = sig.decl.output {
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let mut all_returns_conform_to_trait = true;
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let mut all_returns_have_same_type = true;
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let mut last_ty = None;
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if let Some(ty_ret_ty) = tables.node_type_opt(ret_ty.hir_id) {
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let cause = ObligationCause::misc(ret_ty.span, ret_ty.hir_id);
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if let ty::Dynamic(predicates, _) = &ty_ret_ty.kind {
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for predicate in predicates.iter() {
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for expr in &visitor.0 {
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if let Some(returned_ty) = tables.node_type_opt(expr.hir_id) {
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if let Some(ty) = last_ty {
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all_returns_have_same_type &= ty == returned_ty;
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}
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last_ty = Some(returned_ty);
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let param_env = ty::ParamEnv::empty();
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let pred = predicate.with_self_ty(self.tcx, returned_ty);
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let obligation =
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Obligation::new(cause.clone(), param_env, pred);
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all_returns_conform_to_trait &=
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self.predicate_may_hold(&obligation);
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}
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}
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}
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}
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} else {
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// We still want to verify whether all the return types conform to each other.
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for expr in &visitor.0 {
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if let Some(returned_ty) = tables.node_type_opt(expr.hir_id) {
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if let Some(ty) = last_ty {
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all_returns_have_same_type &= ty == returned_ty;
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}
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last_ty = Some(returned_ty);
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}
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}
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}
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if let (true, hir::TyKind::TraitObject(..), Ok(snippet), true, Some(last_ty)) = (
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ret_ty.span.overlaps(span),
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&ret_ty.kind,
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self.tcx.sess.source_map().span_to_snippet(ret_ty.span),
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all_returns_conform_to_trait,
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last_ty,
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) {
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err.code = Some(error_code!(E0746));
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err.set_primary_message(
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"return type cannot have a bare trait because it must be `Sized`",
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);
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err.children.clear();
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let impl_trait_msg = "for information on `impl Trait`, see \
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<https://doc.rust-lang.org/book/ch10-02-traits.html\
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#returning-types-that-implement-traits>";
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let trait_obj_msg = "for information on trait objects, see \
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<https://doc.rust-lang.org/book/ch17-02-trait-objects.html\
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#using-trait-objects-that-allow-for-values-of-different-types>";
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let has_dyn = snippet.split_whitespace().next().map_or(false, |s| s == "dyn");
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let trait_obj = if has_dyn { &snippet[4..] } else { &snippet[..] };
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if all_returns_have_same_type {
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err.span_suggestion(
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ret_ty.span,
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&format!(
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"you can use the `impl Trait` feature \
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in the return type because all the return paths are of type \
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`{}`, which implements `dyn {}`",
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last_ty, trait_obj,
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),
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format!("impl {}", trait_obj),
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Applicability::MaybeIncorrect,
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);
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err.note(impl_trait_msg);
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} else {
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let mut suggestions = visitor
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.0
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.iter()
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.map(|expr| {
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(
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expr.span,
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format!(
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"Box::new({})",
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self.tcx
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.sess
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.source_map()
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.span_to_snippet(expr.span)
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.unwrap()
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),
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)
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})
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.collect::<Vec<_>>();
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suggestions.push((
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ret_ty.span,
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format!("Box<{}{}>", if has_dyn { "" } else { "dyn " }, snippet),
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));
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err.multipart_suggestion(
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"if the performance implications are acceptable, you can return a \
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trait object",
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suggestions,
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Applicability::MaybeIncorrect,
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);
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err.span_help(
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visitor.0.iter().map(|expr| expr.span).collect::<Vec<_>>(),
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&format!(
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"if all the returned values were of the same type you could use \
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`impl {}` as the return type",
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trait_obj,
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),
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);
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err.help(
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"alternatively, you can always create a new `enum` with a variant \
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for each returned type",
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);
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err.note(impl_trait_msg);
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err.note(trait_obj_msg);
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}
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return true;
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}
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}
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}
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false
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}
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/// Given some node representing a fn-like thing in the HIR map,
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/// returns a span and `ArgKind` information that describes the
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/// arguments it expects. This can be supplied to
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@ -1171,6 +1171,17 @@ impl<'tcx> ObligationCause<'tcx> {
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}
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}
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impl<'tcx> ObligationCauseCode<'tcx> {
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pub fn peel_derives(&self) -> &Self {
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match self {
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BuiltinDerivedObligation(cause) | ImplDerivedObligation(cause) => {
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cause.parent_code.peel_derives()
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}
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_ => self,
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}
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}
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}
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impl<'tcx, N> Vtable<'tcx, N> {
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pub fn nested_obligations(self) -> Vec<N> {
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match self {
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@ -608,4 +608,5 @@ E0745: include_str!("./error_codes/E0745.md"),
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E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
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E0727, // `async` generators are not yet supported
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E0739, // invalid track_caller application/syntax
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E0746, // `dyn Trait` return type
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}
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17
src/test/ui/error-codes/E0746.rs
Normal file
17
src/test/ui/error-codes/E0746.rs
Normal file
@ -0,0 +1,17 @@
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struct Struct;
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trait Trait {}
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impl Trait for Struct {}
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impl Trait for u32 {}
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fn foo() -> dyn Trait { Struct }
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//~^ ERROR E0746
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//~| ERROR E0308
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fn bar() -> dyn Trait { //~ ERROR E0746
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if true {
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return 0; //~ ERROR E0308
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}
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42 //~ ERROR E0308
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}
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fn main() {}
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62
src/test/ui/error-codes/E0746.stderr
Normal file
62
src/test/ui/error-codes/E0746.stderr
Normal file
@ -0,0 +1,62 @@
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error[E0308]: mismatched types
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--> $DIR/E0746.rs:6:25
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LL | fn foo() -> dyn Trait { Struct }
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| --------- ^^^^^^ expected trait `Trait`, found struct `Struct`
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| |
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| expected `(dyn Trait + 'static)` because of return type
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= note: expected trait object `(dyn Trait + 'static)`
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found struct `Struct`
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error[E0746]: return type cannot have a bare trait because it must be `Sized`
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--> $DIR/E0746.rs:6:13
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LL | fn foo() -> dyn Trait { Struct }
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| ^^^^^^^^^ doesn't have a size known at compile-time
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|
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= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
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help: you can use the `impl Trait` feature in the return type because all the return paths are of type `Struct`, which implements `dyn Trait`
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|
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LL | fn foo() -> impl Trait { Struct }
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| ^^^^^^^^^^
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error[E0746]: return type cannot have a bare trait because it must be `Sized`
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--> $DIR/E0746.rs:10:13
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LL | fn bar() -> dyn Trait {
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| ^^^^^^^^^ doesn't have a size known at compile-time
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|
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= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
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help: you can use the `impl Trait` feature in the return type because all the return paths are of type `{integer}`, which implements `dyn Trait`
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|
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LL | fn bar() -> impl Trait {
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| ^^^^^^^^^^
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error[E0308]: mismatched types
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--> $DIR/E0746.rs:12:16
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|
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LL | fn bar() -> dyn Trait {
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| --------- expected `(dyn Trait + 'static)` because of return type
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LL | if true {
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LL | return 0;
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| ^ expected trait `Trait`, found integer
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|
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= note: expected trait object `(dyn Trait + 'static)`
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found type `{integer}`
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error[E0308]: mismatched types
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--> $DIR/E0746.rs:14:5
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|
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LL | fn bar() -> dyn Trait {
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| --------- expected `(dyn Trait + 'static)` because of return type
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...
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LL | 42
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| ^^ expected trait `Trait`, found integer
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|
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= note: expected trait object `(dyn Trait + 'static)`
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found type `{integer}`
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error: aborting due to 5 previous errors
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For more information about this error, try `rustc --explain E0308`.
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@ -0,0 +1,36 @@
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#![allow(bare_trait_objects)]
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struct Struct;
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trait Trait {}
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impl Trait for Struct {}
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impl Trait for u32 {}
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fn fuz() -> (usize, Trait) { (42, Struct) }
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//~^ ERROR E0277
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//~| ERROR E0308
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fn bar() -> (usize, dyn Trait) { (42, Struct) }
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//~^ ERROR E0277
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//~| ERROR E0308
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fn bap() -> Trait { Struct }
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//~^ ERROR E0746
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//~| ERROR E0308
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fn ban() -> dyn Trait { Struct }
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//~^ ERROR E0746
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//~| ERROR E0308
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fn bak() -> dyn Trait { unimplemented!() } //~ ERROR E0277
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// Suggest using `Box<dyn Trait>`
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fn bal() -> dyn Trait { //~ ERROR E0746
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if true {
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return Struct; //~ ERROR E0308
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}
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42 //~ ERROR E0308
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}
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// Suggest using `impl Trait`
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fn bat() -> dyn Trait { //~ ERROR E0746
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if true {
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return 0; //~ ERROR E0308
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}
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42 //~ ERROR E0308
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}
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fn main() {}
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@ -0,0 +1,186 @@
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error[E0308]: mismatched types
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:7:35
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|
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LL | fn fuz() -> (usize, Trait) { (42, Struct) }
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| ^^^^^^ expected trait `Trait`, found struct `Struct`
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|
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= note: expected trait object `(dyn Trait + 'static)`
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found struct `Struct`
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error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:7:13
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|
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LL | fn fuz() -> (usize, Trait) { (42, Struct) }
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| ^^^^^^^^^^^^^^ doesn't have a size known at compile-time
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|
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= help: within `(usize, (dyn Trait + 'static))`, the trait `std::marker::Sized` is not implemented for `(dyn Trait + 'static)`
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= note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
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= note: required because it appears within the type `(usize, (dyn Trait + 'static))`
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= note: the return type of a function must have a statically known size
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error[E0308]: mismatched types
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:10:39
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|
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LL | fn bar() -> (usize, dyn Trait) { (42, Struct) }
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| ^^^^^^ expected trait `Trait`, found struct `Struct`
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|
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= note: expected trait object `(dyn Trait + 'static)`
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found struct `Struct`
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error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:10:13
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|
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LL | fn bar() -> (usize, dyn Trait) { (42, Struct) }
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| ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
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|
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= help: within `(usize, (dyn Trait + 'static))`, the trait `std::marker::Sized` is not implemented for `(dyn Trait + 'static)`
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= note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
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= note: required because it appears within the type `(usize, (dyn Trait + 'static))`
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= note: the return type of a function must have a statically known size
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error[E0308]: mismatched types
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--> $DIR/dyn-trait-return-should-be-impl-trait.rs:13:21
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|
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LL | fn bap() -> Trait { Struct }
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||||
| ----- ^^^^^^ expected trait `Trait`, found struct `Struct`
|
||||
| |
|
||||
| expected `(dyn Trait + 'static)` because of return type
|
||||
|
|
||||
= note: expected trait object `(dyn Trait + 'static)`
|
||||
found struct `Struct`
|
||||
|
||||
error[E0746]: return type cannot have a bare trait because it must be `Sized`
|
||||
--> $DIR/dyn-trait-return-should-be-impl-trait.rs:13:13
|
||||
|
|
||||
LL | fn bap() -> Trait { Struct }
|
||||
| ^^^^^ doesn't have a size known at compile-time
|
||||
|
|
||||
= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
|
||||
help: you can use the `impl Trait` feature in the return type because all the return paths are of type `Struct`, which implements `dyn Trait`
|
||||
|
|
||||
LL | fn bap() -> impl Trait { Struct }
|
||||
| ^^^^^^^^^^
|
||||
|
||||
error[E0308]: mismatched types
|
||||
--> $DIR/dyn-trait-return-should-be-impl-trait.rs:16:25
|
||||
|
|
||||
LL | fn ban() -> dyn Trait { Struct }
|
||||
| --------- ^^^^^^ expected trait `Trait`, found struct `Struct`
|
||||
| |
|
||||
| expected `(dyn Trait + 'static)` because of return type
|
||||
|
|
||||
= note: expected trait object `(dyn Trait + 'static)`
|
||||
found struct `Struct`
|
||||
|
||||
error[E0746]: return type cannot have a bare trait because it must be `Sized`
|
||||
--> $DIR/dyn-trait-return-should-be-impl-trait.rs:16:13
|
||||
|
|
||||
LL | fn ban() -> dyn Trait { Struct }
|
||||
| ^^^^^^^^^ doesn't have a size known at compile-time
|
||||
|
|
||||
= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
|
||||
help: you can use the `impl Trait` feature in the return type because all the return paths are of type `Struct`, which implements `dyn Trait`
|
||||
|
|
||||
LL | fn ban() -> impl Trait { Struct }
|
||||
| ^^^^^^^^^^
|
||||
|
||||
error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
|
||||
--> $DIR/dyn-trait-return-should-be-impl-trait.rs:19:13
|
||||
|
|
||||
LL | fn bak() -> dyn Trait { unimplemented!() }
|
||||
| ^^^^^^^^^ doesn't have a size known at compile-time
|
||||
|
|
||||
= help: the trait `std::marker::Sized` is not implemented for `(dyn Trait + 'static)`
|
||||
= note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
|
||||
= note: the return type of a function must have a statically known size
|
||||
|
||||
error[E0308]: mismatched types
|
||||
--> $DIR/dyn-trait-return-should-be-impl-trait.rs:23:16
|
||||
|
|
||||
LL | fn bal() -> dyn Trait {
|
||||
| --------- expected `(dyn Trait + 'static)` because of return type
|
||||
LL | if true {
|
||||
LL | return Struct;
|
||||
| ^^^^^^ expected trait `Trait`, found struct `Struct`
|
||||
|
|
||||
= note: expected trait object `(dyn Trait + 'static)`
|
||||
found struct `Struct`
|
||||
|
||||
error[E0746]: return type cannot have a bare trait because it must be `Sized`
|
||||
--> $DIR/dyn-trait-return-should-be-impl-trait.rs:21:13
|
||||
|
|
||||
LL | fn bal() -> dyn Trait {
|
||||
| ^^^^^^^^^ doesn't have a size known at compile-time
|
||||
|
|
||||
help: if all the returned values were of the same type you could use `impl Trait` as the return type
|
||||
--> $DIR/dyn-trait-return-should-be-impl-trait.rs:25:5
|
||||
|
|
||||
LL | return Struct;
|
||||
| ^^^^^^
|
||||
LL | }
|
||||
LL | 42
|
||||
| ^^
|
||||
= help: alternatively, you can always create a new `enum` with a variant for each returned type
|
||||
= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
|
||||
= note: for information on trait objects, see <https://doc.rust-lang.org/book/ch17-02-trait-objects.html#using-trait-objects-that-allow-for-values-of-different-types>
|
||||
help: if the performance implications are acceptable, you can return a trait object
|
||||
|
|
||||
LL | fn bal() -> Box<dyn Trait> {
|
||||
LL | if true {
|
||||
LL | return Box::new(Struct);
|
||||
LL | }
|
||||
LL | Box::new(42)
|
||||
|
|
||||
|
||||
error[E0308]: mismatched types
|
||||
--> $DIR/dyn-trait-return-should-be-impl-trait.rs:25:5
|
||||
|
|
||||
LL | fn bal() -> dyn Trait {
|
||||
| --------- expected `(dyn Trait + 'static)` because of return type
|
||||
...
|
||||
LL | 42
|
||||
| ^^ expected trait `Trait`, found integer
|
||||
|
|
||||
= note: expected trait object `(dyn Trait + 'static)`
|
||||
found type `{integer}`
|
||||
|
||||
error[E0746]: return type cannot have a bare trait because it must be `Sized`
|
||||
--> $DIR/dyn-trait-return-should-be-impl-trait.rs:29:13
|
||||
|
|
||||
LL | fn bat() -> dyn Trait {
|
||||
| ^^^^^^^^^ doesn't have a size known at compile-time
|
||||
|
|
||||
= note: for information on `impl Trait`, see <https://doc.rust-lang.org/book/ch10-02-traits.html#returning-types-that-implement-traits>
|
||||
help: you can use the `impl Trait` feature in the return type because all the return paths are of type `{integer}`, which implements `dyn Trait`
|
||||
|
|
||||
LL | fn bat() -> impl Trait {
|
||||
| ^^^^^^^^^^
|
||||
|
||||
error[E0308]: mismatched types
|
||||
--> $DIR/dyn-trait-return-should-be-impl-trait.rs:31:16
|
||||
|
|
||||
LL | fn bat() -> dyn Trait {
|
||||
| --------- expected `(dyn Trait + 'static)` because of return type
|
||||
LL | if true {
|
||||
LL | return 0;
|
||||
| ^ expected trait `Trait`, found integer
|
||||
|
|
||||
= note: expected trait object `(dyn Trait + 'static)`
|
||||
found type `{integer}`
|
||||
|
||||
error[E0308]: mismatched types
|
||||
--> $DIR/dyn-trait-return-should-be-impl-trait.rs:33:5
|
||||
|
|
||||
LL | fn bat() -> dyn Trait {
|
||||
| --------- expected `(dyn Trait + 'static)` because of return type
|
||||
...
|
||||
LL | 42
|
||||
| ^^ expected trait `Trait`, found integer
|
||||
|
|
||||
= note: expected trait object `(dyn Trait + 'static)`
|
||||
found type `{integer}`
|
||||
|
||||
error: aborting due to 15 previous errors
|
||||
|
||||
Some errors have detailed explanations: E0277, E0308.
|
||||
For more information about an error, try `rustc --explain E0277`.
|
||||
Loading…
Reference in New Issue
Block a user