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Suggest Box::pin when Pin::new is used instead
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@tmandry
tmandry committed Oct 14, 2021
1 parent a8558e9 commit d18502d
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Showing 3 changed files with 67 additions and 43 deletions.
90 changes: 60 additions & 30 deletions compiler/rustc_typeck/src/check/fn_ctxt/suggestions.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,11 +8,11 @@ use rustc_errors::{Applicability, DiagnosticBuilder};
use rustc_hir as hir;
use rustc_hir::def::{CtorOf, DefKind};
use rustc_hir::lang_items::LangItem;
use rustc_hir::{Expr, ExprKind, ItemKind, Node, Stmt, StmtKind};
use rustc_hir::{Expr, ExprKind, ItemKind, Node, Path, QPath, Stmt, StmtKind, TyKind};
use rustc_infer::infer;
use rustc_middle::lint::in_external_macro;
use rustc_middle::ty::{self, Binder, Ty};
use rustc_span::symbol::kw;
use rustc_span::symbol::{kw, sym};

use std::iter;

Expand Down Expand Up @@ -350,6 +350,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
}

/// When encountering an `impl Future` where `BoxFuture` is expected, suggest `Box::pin`.
#[instrument(skip(self, err))]
pub(in super::super) fn suggest_calling_boxed_future_when_appropriate(
&self,
err: &mut DiagnosticBuilder<'_>,
Expand All @@ -368,41 +369,70 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
if pin_did.is_none() || self.tcx.lang_items().owned_box().is_none() {
return false;
}
match expected.kind() {
ty::Adt(def, _) if Some(def.did) == pin_did => (),
_ => return false,
}
let box_found = self.tcx.mk_box(found);
let pin_box_found = self.tcx.mk_lang_item(box_found, LangItem::Pin).unwrap();
let pin_found = self.tcx.mk_lang_item(found, LangItem::Pin).unwrap();
if self.can_coerce(pin_box_found, expected) {
debug!("can coerce {:?} to {:?}, suggesting Box::pin", pin_box_found, expected);
match found.kind() {
ty::Adt(def, _) if def.is_box() => {
err.help("use `Box::pin`");
}
_ => {
err.multipart_suggestion(
"you need to pin and box this expression",
vec![
(expr.span.shrink_to_lo(), "Box::pin(".to_string()),
(expr.span.shrink_to_hi(), ")".to_string()),
],
Applicability::MaybeIncorrect,
);
match expected.kind() {
ty::Adt(def, _) if Some(def.did) == pin_did => {
if self.can_coerce(pin_box_found, expected) {
debug!("can coerce {:?} to {:?}, suggesting Box::pin", pin_box_found, expected);
match found.kind() {
ty::Adt(def, _) if def.is_box() => {
err.help("use `Box::pin`");
}
_ => {
err.multipart_suggestion(
"you need to pin and box this expression",
vec![
(expr.span.shrink_to_lo(), "Box::pin(".to_string()),
(expr.span.shrink_to_hi(), ")".to_string()),
],
Applicability::MaybeIncorrect,
);
}
}
true
} else if self.can_coerce(pin_found, expected) {
match found.kind() {
ty::Adt(def, _) if def.is_box() => {
err.help("use `Box::pin`");
true
}
_ => false,
}
} else {
false
}
}
true
} else if self.can_coerce(pin_found, expected) {
match found.kind() {
ty::Adt(def, _) if def.is_box() => {
err.help("use `Box::pin`");
true
ty::Adt(def, _) if def.is_box() && self.can_coerce(box_found, expected) => {
// Check if the parent expression is a call to Pin::new. If it
// is and we were expecting a Box, ergo Pin<Box<expected>>, we
// can suggest Box::pin.
let parent = self.tcx.hir().get_parent_node(expr.hir_id);
let fn_name = match self.tcx.hir().find(parent) {
Some(Node::Expr(Expr { kind: ExprKind::Call(fn_name, _), .. })) => fn_name,
_ => return false,
};
match fn_name.kind {
ExprKind::Path(QPath::TypeRelative(
hir::Ty {
kind: TyKind::Path(QPath::Resolved(_, Path { res: recv_ty, .. })),
..
},
method,
)) if Some(recv_ty.def_id()) == pin_did && method.ident.name == sym::new => {
err.span_suggestion(
fn_name.span,
"use `Box::pin` to pin and box this expression",
"Box::pin".to_string(),
Applicability::MachineApplicable,
);
true
}
_ => false,
}
_ => false,
}
} else {
false
_ => false,
}
}

Expand Down
3 changes: 0 additions & 3 deletions src/test/ui/suggestions/expected-boxed-future-isnt-pinned.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -15,9 +15,6 @@ fn bar<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32>
Box::new(x) //~ ERROR mismatched types
}

// This case is still subpar:
// `Pin::new(x)`: store this in the heap by calling `Box::new`: `Box::new(x)`
// Should suggest changing the code from `Pin::new` to `Box::pin`.
fn baz<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
Pin::new(x) //~ ERROR mismatched types
//~^ ERROR E0277
Expand Down
17 changes: 7 additions & 10 deletions src/test/ui/suggestions/expected-boxed-future-isnt-pinned.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -27,23 +27,20 @@ LL | Box::new(x)
= help: use `Box::pin`

error[E0308]: mismatched types
--> $DIR/expected-boxed-future-isnt-pinned.rs:22:14
--> $DIR/expected-boxed-future-isnt-pinned.rs:19:14
|
LL | fn baz<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
| - this type parameter
LL | Pin::new(x)
| ^ expected struct `Box`, found type parameter `F`
| -------- ^ expected struct `Box`, found type parameter `F`
| |
| help: use `Box::pin` to pin and box this expression: `Box::pin`
|
= note: expected struct `Box<dyn Future<Output = i32> + Send>`
found type parameter `F`
= note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
help: store this in the heap by calling `Box::new`
|
LL | Pin::new(Box::new(x))
| +++++++++ +

error[E0277]: `dyn Future<Output = i32> + Send` cannot be unpinned
--> $DIR/expected-boxed-future-isnt-pinned.rs:22:5
--> $DIR/expected-boxed-future-isnt-pinned.rs:19:5
|
LL | Pin::new(x)
| ^^^^^^^^ the trait `Unpin` is not implemented for `dyn Future<Output = i32> + Send`
Expand All @@ -56,7 +53,7 @@ LL | pub const fn new(pointer: P) -> Pin<P> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0277]: `dyn Future<Output = i32> + Send` cannot be unpinned
--> $DIR/expected-boxed-future-isnt-pinned.rs:27:5
--> $DIR/expected-boxed-future-isnt-pinned.rs:24:5
|
LL | Pin::new(Box::new(x))
| ^^^^^^^^ the trait `Unpin` is not implemented for `dyn Future<Output = i32> + Send`
Expand All @@ -69,7 +66,7 @@ LL | pub const fn new(pointer: P) -> Pin<P> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0308]: mismatched types
--> $DIR/expected-boxed-future-isnt-pinned.rs:31:5
--> $DIR/expected-boxed-future-isnt-pinned.rs:28:5
|
LL | fn zap() -> BoxFuture<'static, i32> {
| ----------------------- expected `Pin<Box<(dyn Future<Output = i32> + Send + 'static)>>` because of return type
Expand Down

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