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Rollup merge of rust-lang#57793 - estebank:impl-trait-resolve, r=oli-obk
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Explain type mismatch cause pointing to return type when it is `impl Trait`

Fix rust-lang#57743.
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@Centril
Centril authored Jan 23, 2019
2 parents afc632b + ab2479b commit cf308c5
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Showing 3 changed files with 78 additions and 58 deletions.
117 changes: 63 additions & 54 deletions src/librustc_typeck/check/coercion.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1199,7 +1199,6 @@ impl<'gcx, 'tcx, 'exprs, E> CoerceMany<'gcx, 'tcx, 'exprs, E>
(self.final_ty.unwrap_or(self.expected_ty), expression_ty)
};

let reason_label = "expected because of this statement";
let mut db;
match cause.code {
ObligationCauseCode::ReturnNoExpression => {
Expand All @@ -1209,63 +1208,20 @@ impl<'gcx, 'tcx, 'exprs, E> CoerceMany<'gcx, 'tcx, 'exprs, E>
db.span_label(cause.span, "return type is not `()`");
}
ObligationCauseCode::BlockTailExpression(blk_id) => {
db = fcx.report_mismatched_types(cause, expected, found, err);

let expr = expression.unwrap_or_else(|| {
span_bug!(cause.span,
"supposed to be part of a block tail expression, but the \
expression is empty");
});
let pointing_at_return_type = fcx.suggest_mismatched_types_on_tail(
&mut db,
expr,
let parent_id = fcx.tcx.hir().get_parent_node(blk_id);
db = self.report_return_mismatched_types(
cause,
expected,
found,
cause.span,
blk_id,
err,
fcx,
parent_id,
expression.map(|expr| (expr, blk_id)),
);
// FIXME: replace with navigating up the chain until hitting an fn or
// bailing if no "pass-through" Node is found, in order to provide a
// suggestion when encountering something like:
// ```
// fn foo(a: bool) -> impl Debug {
// if a {
// bar()?;
// }
// {
// let x = unsafe { bar() };
// x
// }
// }
// ```
//
// Verify that this is a tail expression of a function, otherwise the
// label pointing out the cause for the type coercion will be wrong
// as prior return coercions would not be relevant (#57664).
let parent_id = fcx.tcx.hir().get_parent_node(blk_id);
let parent = fcx.tcx.hir().get(fcx.tcx.hir().get_parent_node(parent_id));
if fcx.get_node_fn_decl(parent).is_some() && !pointing_at_return_type {
if let Some(sp) = fcx.ret_coercion_span.borrow().as_ref() {
db.span_label(*sp, reason_label);
}
}
}
ObligationCauseCode::ReturnType(_id) => {
db = fcx.report_mismatched_types(cause, expected, found, err);
let _id = fcx.tcx.hir().get_parent_node(_id);
let mut pointing_at_return_type = false;
if let Some((fn_decl, can_suggest)) = fcx.get_fn_decl(_id) {
pointing_at_return_type = fcx.suggest_missing_return_type(
&mut db, &fn_decl, expected, found, can_suggest);
}
if let (Some(sp), false) = (
fcx.ret_coercion_span.borrow().as_ref(),
pointing_at_return_type,
) {
if !sp.overlaps(cause.span) {
db.span_label(*sp, reason_label);
}
}
ObligationCauseCode::ReturnType(id) => {
db = self.report_return_mismatched_types(
cause, expected, found, err, fcx, id, None);
}
_ => {
db = fcx.report_mismatched_types(cause, expected, found, err);
Expand All @@ -1283,6 +1239,59 @@ impl<'gcx, 'tcx, 'exprs, E> CoerceMany<'gcx, 'tcx, 'exprs, E>
}
}

fn report_return_mismatched_types<'a>(
&self,
cause: &ObligationCause<'tcx>,
expected: Ty<'tcx>,
found: Ty<'tcx>,
err: TypeError<'tcx>,
fcx: &FnCtxt<'a, 'gcx, 'tcx>,
id: syntax::ast::NodeId,
expression: Option<(&'gcx hir::Expr, syntax::ast::NodeId)>,
) -> DiagnosticBuilder<'a> {
let mut db = fcx.report_mismatched_types(cause, expected, found, err);

let mut pointing_at_return_type = false;
let mut return_sp = 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
// as prior return coercions would not be relevant (#57664).
let parent_id = fcx.tcx.hir().get_parent_node(id);
let fn_decl = if let Some((expr, blk_id)) = expression {
pointing_at_return_type = fcx.suggest_mismatched_types_on_tail(
&mut db,
expr,
expected,
found,
cause.span,
blk_id,
);
let parent = fcx.tcx.hir().get(parent_id);
fcx.get_node_fn_decl(parent).map(|(fn_decl, _, is_main)| (fn_decl, is_main))
} else {
fcx.get_fn_decl(parent_id)
};

if let (Some((fn_decl, can_suggest)), _) = (fn_decl, pointing_at_return_type) {
if expression.is_none() {
pointing_at_return_type |= fcx.suggest_missing_return_type(
&mut db, &fn_decl, expected, found, can_suggest);
}
if !pointing_at_return_type {
return_sp = Some(fn_decl.output.span()); // `impl Trait` return type
}
}
if let (Some(sp), Some(return_sp)) = (fcx.ret_coercion_span.borrow().as_ref(), return_sp) {
db.span_label(return_sp, "expected because this return type...");
db.span_label( *sp, format!(
"...is found to be `{}` here",
fcx.resolve_type_vars_with_obligations(expected),
));
}
db
}

pub fn complete<'a>(self, fcx: &FnCtxt<'a, 'gcx, 'tcx>) -> Ty<'tcx> {
if let Some(final_ty) = self.final_ty {
final_ty
Expand Down
5 changes: 4 additions & 1 deletion src/test/ui/impl-trait/equality.stderr
View file
Original file line number Diff line number Diff line change
@@ -1,8 +1,11 @@
error[E0308]: mismatched types
--> $DIR/equality.rs:15:5
|
LL | fn two(x: bool) -> impl Foo {
| -------- expected because this return type...
LL | if x {
LL | return 1_i32;
| ----- expected because of this statement
| ----- ...is found to be `i32` here
LL | }
LL | 0_u32
| ^^^^^ expected i32, found u32
Expand Down
14 changes: 11 additions & 3 deletions src/test/ui/point-to-type-err-cause-on-impl-trait-return.stderr
View file
Original file line number Diff line number Diff line change
@@ -1,8 +1,11 @@
error[E0308]: mismatched types
--> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:5:5
|
LL | fn foo() -> impl std::fmt::Display {
| ---------------------- expected because this return type...
LL | if false {
LL | return 0i32;
| ---- expected because of this statement
| ---- ...is found to be `i32` here
LL | }
LL | 1u32
| ^^^^ expected i32, found u32
Expand All @@ -13,8 +16,11 @@ LL | 1u32
error[E0308]: mismatched types
--> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:13:16
|
LL | fn bar() -> impl std::fmt::Display {
| ---------------------- expected because this return type...
LL | if false {
LL | return 0i32;
| ---- expected because of this statement
| ---- ...is found to be `i32` here
LL | } else {
LL | return 1u32;
| ^^^^ expected i32, found u32
Expand All @@ -25,10 +31,12 @@ LL | return 1u32;
error[E0308]: mismatched types
--> $DIR/point-to-type-err-cause-on-impl-trait-return.rs:19:5
|
LL | fn baz() -> impl std::fmt::Display {
| ---------------------- expected because this return type...
LL | / if false {
LL | | //~^ ERROR mismatched types
LL | | return 0i32;
| | ---- expected because of this statement
| | ---- ...is found to be `i32` here
LL | | } else {
LL | | 1u32
LL | | }
Expand Down

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