Suggest Box::pin when Pin::new is used instead
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3 changed files with 68 additions and 44 deletions
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@ -8,11 +8,11 @@ use rustc_errors::{Applicability, DiagnosticBuilder};
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use rustc_hir as hir;
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use rustc_hir::def::{CtorOf, DefKind};
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use rustc_hir::lang_items::LangItem;
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use rustc_hir::{Expr, ExprKind, ItemKind, Node, Stmt, StmtKind};
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use rustc_hir::{Expr, ExprKind, ItemKind, Node, Path, QPath, Stmt, StmtKind, TyKind};
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use rustc_infer::infer;
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use rustc_middle::lint::in_external_macro;
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use rustc_middle::ty::{self, Binder, Ty};
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use rustc_span::symbol::kw;
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use rustc_span::symbol::{kw, sym};
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use std::iter;
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@ -350,6 +350,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
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}
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/// When encountering an `impl Future` where `BoxFuture` is expected, suggest `Box::pin`.
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#[instrument(skip(self, err))]
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pub(in super::super) fn suggest_calling_boxed_future_when_appropriate(
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&self,
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err: &mut DiagnosticBuilder<'_>,
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@ -368,41 +369,70 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
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if pin_did.is_none() || self.tcx.lang_items().owned_box().is_none() {
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return false;
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}
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match expected.kind() {
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ty::Adt(def, _) if Some(def.did) == pin_did => (),
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_ => return false,
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}
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let box_found = self.tcx.mk_box(found);
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let pin_box_found = self.tcx.mk_lang_item(box_found, LangItem::Pin).unwrap();
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let pin_found = self.tcx.mk_lang_item(found, LangItem::Pin).unwrap();
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if self.can_coerce(pin_box_found, expected) {
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debug!("can coerce {:?} to {:?}, suggesting Box::pin", pin_box_found, expected);
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match found.kind() {
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ty::Adt(def, _) if def.is_box() => {
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err.help("use `Box::pin`");
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}
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_ => {
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err.multipart_suggestion(
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"you need to pin and box this expression",
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vec![
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(expr.span.shrink_to_lo(), "Box::pin(".to_string()),
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(expr.span.shrink_to_hi(), ")".to_string()),
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],
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Applicability::MaybeIncorrect,
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);
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}
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}
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true
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} else if self.can_coerce(pin_found, expected) {
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match found.kind() {
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ty::Adt(def, _) if def.is_box() => {
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err.help("use `Box::pin`");
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match expected.kind() {
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ty::Adt(def, _) if Some(def.did) == pin_did => {
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if self.can_coerce(pin_box_found, expected) {
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debug!("can coerce {:?} to {:?}, suggesting Box::pin", pin_box_found, expected);
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match found.kind() {
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ty::Adt(def, _) if def.is_box() => {
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err.help("use `Box::pin`");
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}
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_ => {
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err.multipart_suggestion(
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"you need to pin and box this expression",
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vec![
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(expr.span.shrink_to_lo(), "Box::pin(".to_string()),
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(expr.span.shrink_to_hi(), ")".to_string()),
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],
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Applicability::MaybeIncorrect,
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);
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}
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}
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true
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} else if self.can_coerce(pin_found, expected) {
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match found.kind() {
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ty::Adt(def, _) if def.is_box() => {
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err.help("use `Box::pin`");
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true
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}
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_ => false,
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}
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} else {
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false
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}
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_ => false,
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}
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} else {
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false
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ty::Adt(def, _) if def.is_box() && self.can_coerce(box_found, expected) => {
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// Check if the parent expression is a call to Pin::new. If it
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// is and we were expecting a Box, ergo Pin<Box<expected>>, we
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// can suggest Box::pin.
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let parent = self.tcx.hir().get_parent_node(expr.hir_id);
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let fn_name = match self.tcx.hir().find(parent) {
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Some(Node::Expr(Expr { kind: ExprKind::Call(fn_name, _), .. })) => fn_name,
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_ => return false,
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};
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match fn_name.kind {
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ExprKind::Path(QPath::TypeRelative(
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hir::Ty {
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kind: TyKind::Path(QPath::Resolved(_, Path { res: recv_ty, .. })),
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..
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},
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method,
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)) if Some(recv_ty.def_id()) == pin_did && method.ident.name == sym::new => {
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err.span_suggestion(
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fn_name.span,
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"use `Box::pin` to pin and box this expression",
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"Box::pin".to_string(),
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Applicability::MachineApplicable,
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);
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true
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}
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_ => false,
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}
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}
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_ => false,
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}
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}
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@ -15,9 +15,6 @@ fn bar<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32>
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Box::new(x) //~ ERROR mismatched types
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}
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// This case is still subpar:
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// `Pin::new(x)`: store this in the heap by calling `Box::new`: `Box::new(x)`
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// Should suggest changing the code from `Pin::new` to `Box::pin`.
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fn baz<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
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Pin::new(x) //~ ERROR mismatched types
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//~^ ERROR E0277
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@ -27,23 +27,20 @@ LL | Box::new(x)
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= help: use `Box::pin`
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error[E0308]: mismatched types
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--> $DIR/expected-boxed-future-isnt-pinned.rs:22:14
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--> $DIR/expected-boxed-future-isnt-pinned.rs:19:14
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LL | fn baz<F: Future<Output=i32> + Send + 'static>(x: F) -> BoxFuture<'static, i32> {
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| - this type parameter
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LL | Pin::new(x)
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| ^ expected struct `Box`, found type parameter `F`
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| -------- ^ expected struct `Box`, found type parameter `F`
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| |
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| help: use `Box::pin` to pin and box this expression: `Box::pin`
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= note: expected struct `Box<dyn Future<Output = i32> + Send>`
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found type parameter `F`
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= 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
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help: store this in the heap by calling `Box::new`
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LL | Pin::new(Box::new(x))
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| +++++++++ +
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error[E0277]: `dyn Future<Output = i32> + Send` cannot be unpinned
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--> $DIR/expected-boxed-future-isnt-pinned.rs:22:5
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--> $DIR/expected-boxed-future-isnt-pinned.rs:19:5
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LL | Pin::new(x)
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| ^^^^^^^^ the trait `Unpin` is not implemented for `dyn Future<Output = i32> + Send`
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@ -56,7 +53,7 @@ LL | pub const fn new(pointer: P) -> Pin<P> {
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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error[E0277]: `dyn Future<Output = i32> + Send` cannot be unpinned
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--> $DIR/expected-boxed-future-isnt-pinned.rs:27:5
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--> $DIR/expected-boxed-future-isnt-pinned.rs:24:5
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LL | Pin::new(Box::new(x))
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| ^^^^^^^^ the trait `Unpin` is not implemented for `dyn Future<Output = i32> + Send`
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@ -69,7 +66,7 @@ LL | pub const fn new(pointer: P) -> Pin<P> {
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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error[E0308]: mismatched types
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--> $DIR/expected-boxed-future-isnt-pinned.rs:31:5
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--> $DIR/expected-boxed-future-isnt-pinned.rs:28:5
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LL | fn zap() -> BoxFuture<'static, i32> {
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| ----------------------- expected `Pin<Box<(dyn Future<Output = i32> + Send + 'static)>>` because of return type
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