//! sort of a minor point so I've opted to leave it for later -- after all,
//! we may want to adjust precisely when coercions occur.
+use crate::astconv::AstConv;
use crate::check::{FnCtxt, Needs};
-use errors::{struct_span_err, DiagnosticBuilder};
use rustc::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
use rustc::infer::{Coercion, InferOk, InferResult};
+use rustc::session::parse::feature_err;
+use rustc::traits::object_safety_violations;
use rustc::traits::{self, ObligationCause, ObligationCauseCode};
use rustc::ty::adjustment::{
Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, PointerCast,
use rustc::ty::relate::RelateResult;
use rustc::ty::subst::SubstsRef;
use rustc::ty::{self, Ty, TypeAndMut};
+use rustc_errors::{struct_span_err, DiagnosticBuilder};
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
-use rustc_span;
use rustc_span::symbol::sym;
+use rustc_span::{self, Span};
use rustc_target::spec::abi::Abi;
use smallvec::{smallvec, SmallVec};
use std::ops::Deref;
-use syntax::feature_gate;
-
-use rustc_error_codes::*;
struct Coerce<'a, 'tcx> {
fcx: &'a FnCtxt<'a, 'tcx>,
}
if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion {
- feature_gate::feature_err(
+ feature_err(
&self.tcx.sess.parse_sess,
sym::unsized_tuple_coercion,
self.cause.span,
};
let mut err;
+ let mut unsized_return = false;
match cause.code {
ObligationCauseCode::ReturnNoExpression => {
err = struct_span_err!(
parent_id,
expression.map(|expr| (expr, blk_id)),
);
+ if !fcx.tcx.features().unsized_locals {
+ unsized_return = self.is_return_ty_unsized(fcx, blk_id);
+ }
}
ObligationCauseCode::ReturnValue(id) => {
err = self.report_return_mismatched_types(
id,
None,
);
+ if !fcx.tcx.features().unsized_locals {
+ let id = fcx.tcx.hir().get_parent_node(id);
+ unsized_return = self.is_return_ty_unsized(fcx, id);
+ }
}
_ => {
err = fcx.report_mismatched_types(cause, expected, found, coercion_error);
.filter(|e| fcx.is_assign_to_bool(e, self.expected_ty()))
.is_some();
- err.emit_unless(assign_to_bool);
+ err.emit_unless(assign_to_bool || unsized_return);
self.final_ty = Some(fcx.tcx.types.err);
}
let mut err = fcx.report_mismatched_types(cause, expected, found, ty_err);
let mut pointing_at_return_type = false;
- let mut return_sp = None;
+ let mut fn_output = None;
// Verify that this is a tail expression of a function, otherwise the
// label pointing out the cause for the type coercion will be wrong
);
}
if !pointing_at_return_type {
- return_sp = Some(fn_decl.output.span()); // `impl Trait` return type
+ fn_output = Some(&fn_decl.output); // `impl Trait` return type
}
}
- if let (Some(sp), Some(return_sp)) = (fcx.ret_coercion_span.borrow().as_ref(), return_sp) {
- err.span_label(return_sp, "expected because this return type...");
- err.span_label( *sp, format!(
- "...is found to be `{}` here",
- fcx.resolve_vars_with_obligations(expected),
- ));
+ if let (Some(sp), Some(fn_output)) = (fcx.ret_coercion_span.borrow().as_ref(), fn_output) {
+ self.add_impl_trait_explanation(&mut err, fcx, expected, *sp, fn_output);
}
err
}
+ fn add_impl_trait_explanation<'a>(
+ &self,
+ err: &mut DiagnosticBuilder<'a>,
+ fcx: &FnCtxt<'a, 'tcx>,
+ expected: Ty<'tcx>,
+ sp: Span,
+ fn_output: &hir::FunctionRetTy<'_>,
+ ) {
+ let return_sp = fn_output.span();
+ err.span_label(return_sp, "expected because this return type...");
+ err.span_label(
+ sp,
+ format!("...is found to be `{}` here", fcx.resolve_vars_with_obligations(expected)),
+ );
+ let impl_trait_msg = "for information on `impl Trait`, see \
+ <https://doc.rust-lang.org/book/ch10-02-traits.html\
+ #returning-types-that-implement-traits>";
+ let trait_obj_msg = "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>";
+ err.note("to return `impl Trait`, all returned values must be of the same type");
+ err.note(impl_trait_msg);
+ let snippet = fcx
+ .tcx
+ .sess
+ .source_map()
+ .span_to_snippet(return_sp)
+ .unwrap_or_else(|_| "dyn Trait".to_string());
+ let mut snippet_iter = snippet.split_whitespace();
+ let has_impl = snippet_iter.next().map_or(false, |s| s == "impl");
+ // Only suggest `Box<dyn Trait>` if `Trait` in `impl Trait` is object safe.
+ let mut is_object_safe = false;
+ if let hir::FunctionRetTy::Return(ty) = fn_output {
+ // Get the return type.
+ if let hir::TyKind::Def(..) = ty.kind {
+ let ty = AstConv::ast_ty_to_ty(fcx, ty);
+ // Get the `impl Trait`'s `DefId`.
+ if let ty::Opaque(def_id, _) = ty.kind {
+ let hir_id = fcx.tcx.hir().as_local_hir_id(def_id).unwrap();
+ // Get the `impl Trait`'s `Item` so that we can get its trait bounds and
+ // get the `Trait`'s `DefId`.
+ if let hir::ItemKind::OpaqueTy(hir::OpaqueTy { bounds, .. }) =
+ fcx.tcx.hir().expect_item(hir_id).kind
+ {
+ // Are of this `impl Trait`'s traits object safe?
+ is_object_safe = bounds.iter().all(|bound| {
+ bound.trait_def_id().map_or(false, |def_id| {
+ object_safety_violations(fcx.tcx, def_id).is_empty()
+ })
+ })
+ }
+ }
+ }
+ };
+ if has_impl {
+ if is_object_safe {
+ err.help(&format!(
+ "you can instead return a boxed trait object using `Box<dyn {}>`",
+ &snippet[5..]
+ ));
+ } else {
+ err.help(&format!(
+ "if the trait `{}` were object safe, you could return a boxed trait object",
+ &snippet[5..]
+ ));
+ }
+ err.note(trait_obj_msg);
+ }
+ err.help("alternatively, create a new `enum` with a variant for each returned type");
+ }
+
+ fn is_return_ty_unsized(&self, fcx: &FnCtxt<'a, 'tcx>, blk_id: hir::HirId) -> bool {
+ if let Some((fn_decl, _)) = fcx.get_fn_decl(blk_id) {
+ if let hir::FunctionRetTy::Return(ty) = fn_decl.output {
+ let ty = AstConv::ast_ty_to_ty(fcx, ty);
+ if let ty::Dynamic(..) = ty.kind {
+ return true;
+ }
+ }
+ }
+ false
+ }
+
pub fn complete<'a>(self, fcx: &FnCtxt<'a, 'tcx>) -> Ty<'tcx> {
if let Some(final_ty) = self.final_ty {
final_ty
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