Rollup of 5 pull requests

compiler/rustc_middle/src/ty/sty.rs

@@ -977,7 +977,6 @@ impl<'tcx> TraitRef<'tcx> {
         substs: SubstsRef<'tcx>,
     ) -> ty::TraitRef<'tcx> {
         let defs = tcx.generics_of(trait_id);
-
         ty::TraitRef { def_id: trait_id, substs: tcx.intern_substs(&substs[..defs.params.len()]) }
     }
 }

compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs

@@ -4,7 +4,7 @@ use super::{
 use crate::infer::InferCtxt;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
-use rustc_middle::ty::subst::Subst;
+use rustc_middle::ty::subst::{Subst, SubstsRef};
 use rustc_middle::ty::{self, GenericParamDefKind};
 use rustc_span::symbol::sym;
 use std::iter;
@@ -17,7 +17,7 @@ crate trait InferCtxtExt<'tcx> {
         &self,
         trait_ref: ty::PolyTraitRef<'tcx>,
         obligation: &PredicateObligation<'tcx>,
-    ) -> Option<DefId>;
+    ) -> Option<(DefId, SubstsRef<'tcx>)>;
 
     /*private*/
     fn describe_enclosure(&self, hir_id: hir::HirId) -> Option<&'static str>;
@@ -34,7 +34,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         &self,
         trait_ref: ty::PolyTraitRef<'tcx>,
         obligation: &PredicateObligation<'tcx>,
-    ) -> Option<DefId> {
+    ) -> Option<(DefId, SubstsRef<'tcx>)> {
         let tcx = self.tcx;
         let param_env = obligation.param_env;
         let trait_ref = tcx.erase_late_bound_regions(trait_ref);
@@ -50,28 +50,29 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             let impl_self_ty = impl_trait_ref.self_ty();
 
             if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
-                self_match_impls.push(def_id);
+                self_match_impls.push((def_id, impl_substs));
 
                 if iter::zip(
                     trait_ref.substs.types().skip(1),
                     impl_trait_ref.substs.types().skip(1),
                 )
                 .all(|(u, v)| self.fuzzy_match_tys(u, v, false).is_some())
                 {
-                    fuzzy_match_impls.push(def_id);
+                    fuzzy_match_impls.push((def_id, impl_substs));
                 }
             }
         });
 
-        let impl_def_id = if self_match_impls.len() == 1 {
+        let impl_def_id_and_substs = if self_match_impls.len() == 1 {
             self_match_impls[0]
         } else if fuzzy_match_impls.len() == 1 {
             fuzzy_match_impls[0]
         } else {
             return None;
         };
 
-        tcx.has_attr(impl_def_id, sym::rustc_on_unimplemented).then_some(impl_def_id)
+        tcx.has_attr(impl_def_id_and_substs.0, sym::rustc_on_unimplemented)
+            .then_some(impl_def_id_and_substs)
     }
 
     /// Used to set on_unimplemented's `ItemContext`
@@ -120,8 +121,9 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         trait_ref: ty::PolyTraitRef<'tcx>,
         obligation: &PredicateObligation<'tcx>,
     ) -> OnUnimplementedNote {
-        let def_id =
-            self.impl_similar_to(trait_ref, obligation).unwrap_or_else(|| trait_ref.def_id());
+        let (def_id, substs) = self
+            .impl_similar_to(trait_ref, obligation)
+            .unwrap_or_else(|| (trait_ref.def_id(), trait_ref.skip_binder().substs));
         let trait_ref = trait_ref.skip_binder();
 
         let mut flags = vec![(
@@ -176,15 +178,15 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             for param in generics.params.iter() {
                 let value = match param.kind {
                     GenericParamDefKind::Type { .. } | GenericParamDefKind::Const { .. } => {
-                        trait_ref.substs[param.index as usize].to_string()
+                        substs[param.index as usize].to_string()
                     }
                     GenericParamDefKind::Lifetime => continue,
                 };
                 let name = param.name;
                 flags.push((name, Some(value)));
 
                 if let GenericParamDefKind::Type { .. } = param.kind {
-                    let param_ty = trait_ref.substs[param.index as usize].expect_ty();
+                    let param_ty = substs[param.index as usize].expect_ty();
                     if let Some(def) = param_ty.ty_adt_def() {
                         // We also want to be able to select the parameter's
                         // original signature with no type arguments resolved
@@ -229,9 +231,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             }
         });
 
-        if let Ok(Some(command)) =
-            OnUnimplementedDirective::of_item(self.tcx, trait_ref.def_id, def_id)
-        {
+        if let Ok(Some(command)) = OnUnimplementedDirective::of_item(self.tcx, def_id) {
             command.evaluate(self.tcx, trait_ref, &flags)
         } else {
             OnUnimplementedNote::default()

compiler/rustc_trait_selection/src/traits/on_unimplemented.rs

@@ -54,7 +54,7 @@ fn parse_error(
 impl<'tcx> OnUnimplementedDirective {
     fn parse(
         tcx: TyCtxt<'tcx>,
-        trait_def_id: DefId,
+        item_def_id: DefId,
         items: &[NestedMetaItem],
         span: Span,
         is_root: bool,
@@ -63,7 +63,7 @@ impl<'tcx> OnUnimplementedDirective {
         let mut item_iter = items.iter();
 
         let parse_value = |value_str| {
-            OnUnimplementedFormatString::try_parse(tcx, trait_def_id, value_str, span).map(Some)
+            OnUnimplementedFormatString::try_parse(tcx, item_def_id, value_str, span).map(Some)
         };
 
         let condition = if is_root {
@@ -135,7 +135,7 @@ impl<'tcx> OnUnimplementedDirective {
             {
                 if let Some(items) = item.meta_item_list() {
                     if let Ok(subcommand) =
-                        Self::parse(tcx, trait_def_id, &items, item.span(), false)
+                        Self::parse(tcx, item_def_id, &items, item.span(), false)
                     {
                         subcommands.push(subcommand);
                     } else {
@@ -178,27 +178,23 @@ impl<'tcx> OnUnimplementedDirective {
         }
     }
 
-    pub fn of_item(
-        tcx: TyCtxt<'tcx>,
-        trait_def_id: DefId,
-        impl_def_id: DefId,
-    ) -> Result<Option<Self>, ErrorGuaranteed> {
-        let attrs = tcx.get_attrs(impl_def_id);
+    pub fn of_item(tcx: TyCtxt<'tcx>, item_def_id: DefId) -> Result<Option<Self>, ErrorGuaranteed> {
+        let attrs = tcx.get_attrs(item_def_id);
 
         let Some(attr) = tcx.sess.find_by_name(&attrs, sym::rustc_on_unimplemented) else {
             return Ok(None);
         };
 
         let result = if let Some(items) = attr.meta_item_list() {
-            Self::parse(tcx, trait_def_id, &items, attr.span, true).map(Some)
+            Self::parse(tcx, item_def_id, &items, attr.span, true).map(Some)
         } else if let Some(value) = attr.value_str() {
             Ok(Some(OnUnimplementedDirective {
                 condition: None,
                 message: None,
                 subcommands: vec![],
                 label: Some(OnUnimplementedFormatString::try_parse(
                     tcx,
-                    trait_def_id,
+                    item_def_id,
                     value,
                     attr.span,
                 )?),
@@ -209,7 +205,7 @@ impl<'tcx> OnUnimplementedDirective {
         } else {
             return Err(ErrorGuaranteed);
         };
-        debug!("of_item({:?}/{:?}) = {:?}", trait_def_id, impl_def_id, result);
+        debug!("of_item({:?}) = {:?}", item_def_id, result);
         result
     }
 
@@ -280,23 +276,29 @@ impl<'tcx> OnUnimplementedDirective {
 impl<'tcx> OnUnimplementedFormatString {
     fn try_parse(
         tcx: TyCtxt<'tcx>,
-        trait_def_id: DefId,
+        item_def_id: DefId,
         from: Symbol,
         err_sp: Span,
     ) -> Result<Self, ErrorGuaranteed> {
         let result = OnUnimplementedFormatString(from);
-        result.verify(tcx, trait_def_id, err_sp)?;
+        result.verify(tcx, item_def_id, err_sp)?;
         Ok(result)
     }
 
     fn verify(
         &self,
         tcx: TyCtxt<'tcx>,
-        trait_def_id: DefId,
+        item_def_id: DefId,
         span: Span,
     ) -> Result<(), ErrorGuaranteed> {
-        let name = tcx.item_name(trait_def_id);
-        let generics = tcx.generics_of(trait_def_id);
+        let trait_def_id = if tcx.is_trait(item_def_id) {
+            item_def_id
+        } else {
+            tcx.trait_id_of_impl(item_def_id)
+                .expect("expected `on_unimplemented` to correspond to a trait")
+        };
+        let trait_name = tcx.item_name(trait_def_id);
+        let generics = tcx.generics_of(item_def_id);
         let s = self.0.as_str();
         let parser = Parser::new(s, None, None, false, ParseMode::Format);
         let mut result = Ok(());
@@ -307,7 +309,7 @@ impl<'tcx> OnUnimplementedFormatString {
                     // `{Self}` is allowed
                     Position::ArgumentNamed(s, _) if s == kw::SelfUpper => (),
                     // `{ThisTraitsName}` is allowed
-                    Position::ArgumentNamed(s, _) if s == name => (),
+                    Position::ArgumentNamed(s, _) if s == trait_name => (),
                     // `{from_method}` is allowed
                     Position::ArgumentNamed(s, _) if s == sym::from_method => (),
                     // `{from_desugaring}` is allowed
@@ -329,9 +331,13 @@ impl<'tcx> OnUnimplementedFormatString {
                                     tcx.sess,
                                     span,
                                     E0230,
-                                    "there is no parameter `{}` on trait `{}`",
+                                    "there is no parameter `{}` on {}",
                                     s,
-                                    name
+                                    if trait_def_id == item_def_id {
+                                        format!("trait `{}`", trait_name)
+                                    } else {
+                                        "impl".to_string()
+                                    }
                                 )
                                 .emit();
                                 result = Err(ErrorGuaranteed);

compiler/rustc_typeck/src/check/check.rs

@@ -742,12 +742,11 @@ pub fn check_item_type<'tcx>(tcx: TyCtxt<'tcx>, it: &'tcx hir::Item<'tcx>) {
                     impl_trait_ref,
                     &impl_.items,
                 );
-                let trait_def_id = impl_trait_ref.def_id;
-                check_on_unimplemented(tcx, trait_def_id, it);
+                check_on_unimplemented(tcx, it);
             }
         }
         hir::ItemKind::Trait(_, _, _, _, ref items) => {
-            check_on_unimplemented(tcx, it.def_id.to_def_id(), it);
+            check_on_unimplemented(tcx, it);
 
             for item in items.iter() {
                 let item = tcx.hir().trait_item(item.id);
@@ -857,9 +856,9 @@ pub fn check_item_type<'tcx>(tcx: TyCtxt<'tcx>, it: &'tcx hir::Item<'tcx>) {
     }
 }
 
-pub(super) fn check_on_unimplemented(tcx: TyCtxt<'_>, trait_def_id: DefId, item: &hir::Item<'_>) {
+pub(super) fn check_on_unimplemented(tcx: TyCtxt<'_>, item: &hir::Item<'_>) {
     // an error would be reported if this fails.
-    let _ = traits::OnUnimplementedDirective::of_item(tcx, trait_def_id, item.def_id.to_def_id());
+    let _ = traits::OnUnimplementedDirective::of_item(tcx, item.def_id.to_def_id());
 }
 
 pub(super) fn check_specialization_validity<'tcx>(

library/core/src/hint.rs

@@ -173,3 +173,126 @@ pub fn spin_loop() {
 pub const fn black_box<T>(dummy: T) -> T {
     crate::intrinsics::black_box(dummy)
 }
+
+/// An identity function that causes an `unused_must_use` warning to be
+/// triggered if the given value is not used (returned, stored in a variable,
+/// etc) by the caller.
+///
+/// This is primarily intended for use in macro-generated code, in which a
+/// [`#[must_use]` attribute][must_use] either on a type or a function would not
+/// be convenient.
+///
+/// [must_use]: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute
+///
+/// # Example
+///
+/// ```
+/// #![feature(hint_must_use)]
+///
+/// use core::fmt;
+///
+/// pub struct Error(/* ... */);
+///
+/// #[macro_export]
+/// macro_rules! make_error {
+///     ($($args:expr),*) => {
+///         core::hint::must_use({
+///             let error = $crate::make_error(core::format_args!($($args),*));
+///             error
+///         })
+///     };
+/// }
+///
+/// // Implementation detail of make_error! macro.
+/// #[doc(hidden)]
+/// pub fn make_error(args: fmt::Arguments<'_>) -> Error {
+///     Error(/* ... */)
+/// }
+///
+/// fn demo() -> Option<Error> {
+///     if true {
+///         // Oops, meant to write `return Some(make_error!("..."));`
+///         Some(make_error!("..."));
+///     }
+///     None
+/// }
+/// #
+/// # // Make rustdoc not wrap the whole snippet in fn main, so that $crate::make_error works
+/// # fn main() {}
+/// ```
+///
+/// In the above example, we'd like an `unused_must_use` lint to apply to the
+/// value created by `make_error!`. However, neither `#[must_use]` on a struct
+/// nor `#[must_use]` on a function is appropriate here, so the macro expands
+/// using `core::hint::must_use` instead.
+///
+/// - We wouldn't want `#[must_use]` on the `struct Error` because that would
+///   make the following unproblematic code trigger a warning:
+///
+///   ```
+///   # struct Error;
+///   #
+///   fn f(arg: &str) -> Result<(), Error>
+///   # { Ok(()) }
+///
+///   #[test]
+///   fn t() {
+///       // Assert that `f` returns error if passed an empty string.
+///       // A value of type `Error` is unused here but that's not a problem.
+///       f("").unwrap_err();
+///   }
+///   ```
+///
+/// - Using `#[must_use]` on `fn make_error` can't help because the return value
+///   *is* used, as the right-hand side of a `let` statement. The `let`
+///   statement looks useless but is in fact necessary for ensuring that
+///   temporaries within the `format_args` expansion are not kept alive past the
+///   creation of the `Error`, as keeping them alive past that point can cause
+///   autotrait issues in async code:
+///
+///   ```
+///   # #![feature(hint_must_use)]
+///   #
+///   # struct Error;
+///   #
+///   # macro_rules! make_error {
+///   #     ($($args:expr),*) => {
+///   #         core::hint::must_use({
+///   #             // If `let` isn't used, then `f()` produces a non-Send future.
+///   #             let error = make_error(core::format_args!($($args),*));
+///   #             error
+///   #         })
+///   #     };
+///   # }
+///   #
+///   # fn make_error(args: core::fmt::Arguments<'_>) -> Error {
+///   #     Error
+///   # }
+///   #
+///   async fn f() {
+///       // Using `let` inside the make_error expansion causes temporaries like
+///       // `unsync()` to drop at the semicolon of that `let` statement, which
+///       // is prior to the await point. They would otherwise stay around until
+///       // the semicolon on *this* statement, which is after the await point,
+///       // and the enclosing Future would not implement Send.
+///       log(make_error!("look: {:p}", unsync())).await;
+///   }
+///
+///   async fn log(error: Error) {/* ... */}
+///
+///   // Returns something without a Sync impl.
+///   fn unsync() -> *const () {
+///       0 as *const ()
+///   }
+///   #
+///   # fn test() {
+///   #     fn assert_send(_: impl Send) {}
+///   #     assert_send(f());
+///   # }
+///   ```
+#[unstable(feature = "hint_must_use", issue = "94745")]
+#[rustc_const_unstable(feature = "hint_must_use", issue = "94745")]
+#[must_use] // <-- :)
+pub const fn must_use<T>(value: T) -> T {
+    value
+}

library/std/src/lib.rs

@@ -231,6 +231,7 @@
 #![feature(assert_matches)]
 #![feature(associated_type_bounds)]
 #![feature(async_iterator)]
+#![feature(atomic_mut_ptr)]
 #![feature(bench_black_box)]
 #![feature(box_syntax)]
 #![feature(c_unwind)]