Auto merge of rust-lang#117590 - matthiaskrgr:rollup-9cqh1q8, r=matth…

…iaskrgr

Rollup of 6 pull requests

Successful merges:

 - rust-lang#110340 (Deref docs: expand and remove "smart pointer" qualifier)
 - rust-lang#116894 (Guarantee that `char` has the same size and alignment as `u32`)
 - rust-lang#117534 (clarify that the str invariant is a safety, not validity, invariant)
 - rust-lang#117562 (triagebot no-merges: exclude different case)
 - rust-lang#117570 (fallback for `construct_generic_bound_failure`)
 - rust-lang#117583 (Remove `'tcx` lifetime on `PlaceholderConst`)

r? `@ghost`
`@rustbot` modify labels: rollup

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2444,18 +2444,22 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 let suggestion =
                     if has_lifetimes { format!(" + {lt_name}") } else { format!(": {lt_name}") };
                 suggs.push((sp, suggestion))
-            } else {
-                let generics = self.tcx.hir().get_generics(suggestion_scope).unwrap();
+            } else if let Some(generics) = self.tcx.hir().get_generics(suggestion_scope) {
                 let pred = format!("{bound_kind}: {lt_name}");
-                let suggestion = format!("{} {}", generics.add_where_or_trailing_comma(), pred,);
+                let suggestion = format!("{} {}", generics.add_where_or_trailing_comma(), pred);
                 suggs.push((generics.tail_span_for_predicate_suggestion(), suggestion))
+            } else {
+                let consider = format!("{msg} `{bound_kind}: {sub}`...");
+                err.help(consider);
             }
 
-            err.multipart_suggestion_verbose(
-                format!("{msg}"),
-                suggs,
-                Applicability::MaybeIncorrect, // Issue #41966
-            );
+            if !suggs.is_empty() {
+                err.multipart_suggestion_verbose(
+                    format!("{msg}"),
+                    suggs,
+                    Applicability::MaybeIncorrect, // Issue #41966
+                );
+            }
         }
 
         err

compiler/rustc_middle/src/infer/canonical.rs

@@ -222,7 +222,7 @@ pub enum CanonicalVarKind<'tcx> {
     Effect,
 
     /// A "placeholder" that represents "any const".
-    PlaceholderConst(ty::PlaceholderConst<'tcx>, Ty<'tcx>),
+    PlaceholderConst(ty::PlaceholderConst, Ty<'tcx>),
 }
 
 impl<'tcx> CanonicalVarKind<'tcx> {

compiler/rustc_middle/src/ty/consts.rs

@@ -84,7 +84,7 @@ impl<'tcx> Const<'tcx> {
     #[inline]
     pub fn new_placeholder(
         tcx: TyCtxt<'tcx>,
-        placeholder: ty::PlaceholderConst<'tcx>,
+        placeholder: ty::PlaceholderConst,
         ty: Ty<'tcx>,
     ) -> Const<'tcx> {
         Const::new(tcx, ty::ConstKind::Placeholder(placeholder), ty)

compiler/rustc_middle/src/ty/context.rs

@@ -106,7 +106,7 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
     type Const = ty::Const<'tcx>;
     type InferConst = ty::InferConst;
     type AliasConst = ty::UnevaluatedConst<'tcx>;
-    type PlaceholderConst = ty::PlaceholderConst<'tcx>;
+    type PlaceholderConst = ty::PlaceholderConst;
     type ParamConst = ty::ParamConst;
     type BoundConst = ty::BoundVar;
     type ValueConst = ty::ValTree<'tcx>;

compiler/rustc_middle/src/ty/mod.rs

@@ -1527,7 +1527,7 @@ pub struct BoundConst<'tcx> {
     pub ty: Ty<'tcx>,
 }
 
-pub type PlaceholderConst<'tcx> = Placeholder<BoundVar>;
+pub type PlaceholderConst = Placeholder<BoundVar>;
 
 /// When type checking, we use the `ParamEnv` to track
 /// details about the set of where-clauses that are in scope at this

compiler/rustc_trait_selection/src/traits/project.rs

@@ -783,7 +783,7 @@ pub struct BoundVarReplacer<'me, 'tcx> {
     // the `var` (but we *could* bring that into scope if we were to track them as we pass them).
     mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
     mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy>,
-    mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
+    mapped_consts: BTreeMap<ty::PlaceholderConst, ty::BoundVar>,
     // The current depth relative to *this* folding, *not* the entire normalization. In other words,
     // the depth of binders we've passed here.
     current_index: ty::DebruijnIndex,
@@ -843,11 +843,11 @@ impl<'me, 'tcx> BoundVarReplacer<'me, 'tcx> {
         T,
         BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
         BTreeMap<ty::PlaceholderType, ty::BoundTy>,
-        BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
+        BTreeMap<ty::PlaceholderConst, ty::BoundVar>,
     ) {
         let mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion> = BTreeMap::new();
         let mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy> = BTreeMap::new();
-        let mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar> = BTreeMap::new();
+        let mapped_consts: BTreeMap<ty::PlaceholderConst, ty::BoundVar> = BTreeMap::new();
 
         let mut replacer = BoundVarReplacer {
             infcx,
@@ -966,7 +966,7 @@ pub struct PlaceholderReplacer<'me, 'tcx> {
     infcx: &'me InferCtxt<'tcx>,
     mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
     mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy>,
-    mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
+    mapped_consts: BTreeMap<ty::PlaceholderConst, ty::BoundVar>,
     universe_indices: &'me [Option<ty::UniverseIndex>],
     current_index: ty::DebruijnIndex,
 }
@@ -976,7 +976,7 @@ impl<'me, 'tcx> PlaceholderReplacer<'me, 'tcx> {
         infcx: &'me InferCtxt<'tcx>,
         mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
         mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy>,
-        mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
+        mapped_consts: BTreeMap<ty::PlaceholderConst, ty::BoundVar>,
         universe_indices: &'me [Option<ty::UniverseIndex>],
         value: T,
     ) -> T {

library/core/src/convert/mod.rs

@@ -138,7 +138,7 @@ pub const fn identity<T>(x: T) -> T {
 ///
 /// [dereferenceable types]: core::ops::Deref
 /// [pointed-to value]: core::ops::Deref::Target
-/// ['`Deref` coercion']: core::ops::Deref#more-on-deref-coercion
+/// ['`Deref` coercion']: core::ops::Deref#deref-coercion
 ///
 /// ```
 /// let x = Box::new(5i32);
@@ -244,7 +244,7 @@ pub trait AsRef<T: ?Sized> {
 ///
 /// [mutably dereferenceable types]: core::ops::DerefMut
 /// [pointed-to value]: core::ops::Deref::Target
-/// ['`Deref` coercion']: core::ops::DerefMut#more-on-deref-coercion
+/// ['`Deref` coercion']: core::ops::DerefMut#mutable-deref-coercion
 ///
 /// ```
 /// let mut x = Box::new(5i32);

library/core/src/ops/deref.rs

@@ -3,40 +3,107 @@
 /// In addition to being used for explicit dereferencing operations with the
 /// (unary) `*` operator in immutable contexts, `Deref` is also used implicitly
 /// by the compiler in many circumstances. This mechanism is called
-/// ['`Deref` coercion'][more]. In mutable contexts, [`DerefMut`] is used.
+/// ["`Deref` coercion"][coercion]. In mutable contexts, [`DerefMut`] is used and
+/// mutable deref coercion similarly occurs.
 ///
-/// Implementing `Deref` for smart pointers makes accessing the data behind them
-/// convenient, which is why they implement `Deref`. On the other hand, the
-/// rules regarding `Deref` and [`DerefMut`] were designed specifically to
-/// accommodate smart pointers. Because of this, **`Deref` should only be
-/// implemented for smart pointers** to avoid confusion.
+/// **Warning:** Deref coercion is a powerful language feature which has
+/// far-reaching implications for every type that implements `Deref`. The
+/// compiler will silently insert calls to `Deref::deref`. For this reason, one
+/// should be careful about implementing `Deref` and only do so when deref
+/// coercion is desirable. See [below][implementing] for advice on when this is
+/// typically desirable or undesirable.
 ///
-/// For similar reasons, **this trait should never fail**. Failure during
-/// dereferencing can be extremely confusing when `Deref` is invoked implicitly.
+/// Types that implement `Deref` or `DerefMut` are often called "smart
+/// pointers" and the mechanism of deref coercion has been specifically designed
+/// to facilitate the pointer-like behaviour that name suggests. Often, the
+/// purpose of a "smart pointer" type is to change the ownership semantics
+/// of a contained value (for example, [`Rc`][rc] or [`Cow`][cow]) or the
+/// storage semantics of a contained value (for example, [`Box`][box]).
 ///
-/// Violating these requirements is a logic error. The behavior resulting from a logic error is not
-/// specified, but users of the trait must ensure that such logic errors do *not* result in
-/// undefined behavior. This means that `unsafe` code **must not** rely on the correctness of this
-/// method.
+/// # Deref coercion
 ///
-/// # More on `Deref` coercion
+/// If `T` implements `Deref<Target = U>`, and `v` is a value of type `T`, then:
 ///
-/// If `T` implements `Deref<Target = U>`, and `x` is a value of type `T`, then:
-///
-/// * In immutable contexts, `*x` (where `T` is neither a reference nor a raw pointer)
-///   is equivalent to `*Deref::deref(&x)`.
+/// * In immutable contexts, `*v` (where `T` is neither a reference nor a raw
+///   pointer) is equivalent to `*Deref::deref(&v)`.
 /// * Values of type `&T` are coerced to values of type `&U`
-/// * `T` implicitly implements all the (immutable) methods of the type `U`.
+/// * `T` implicitly implements all the methods of the type `U` which take the
+///   `&self` receiver.
 ///
 /// For more details, visit [the chapter in *The Rust Programming Language*][book]
 /// as well as the reference sections on [the dereference operator][ref-deref-op],
-/// [method resolution] and [type coercions].
+/// [method resolution], and [type coercions].
+///
+/// # When to implement `Deref` or `DerefMut`
+///
+/// The same advice applies to both deref traits. In general, deref traits
+/// **should** be implemented if:
+///
+/// 1. a value of the type transparently behaves like a value of the target
+///    type;
+/// 1. the implementation of the deref function is cheap; and
+/// 1. users of the type will not be surprised by any deref coercion behaviour.
+///
+/// In general, deref traits **should not** be implemented if:
+///
+/// 1. the deref implementations could fail unexpectedly; or
+/// 1. the type has methods that are likely to collide with methods on the
+///    target type; or
+/// 1. committing to deref coercion as part of the public API is not desirable.
+///
+/// Note that there's a large difference between implementing deref traits
+/// generically over many target types, and doing so only for specific target
+/// types.
+///
+/// Generic implementations, such as for [`Box<T>`][box] (which is generic over
+/// every type and dereferences to `T`) should be careful to provide few or no
+/// methods, since the target type is unknown and therefore every method could
+/// collide with one on the target type, causing confusion for users.
+/// `impl<T> Box<T>` has no methods (though several associated functions),
+/// partly for this reason.
+///
+/// Specific implementations, such as for [`String`][string] (whose `Deref`
+/// implementation has `Target = str`) can have many methods, since avoiding
+/// collision is much easier. `String` and `str` both have many methods, and
+/// `String` additionally behaves as if it has every method of `str` because of
+/// deref coercion. The implementing type may also be generic while the
+/// implementation is still specific in this sense; for example, [`Vec<T>`][vec]
+/// dereferences to `[T]`, so methods of `T` are not applicable.
+///
+/// Consider also that deref coericion means that deref traits are a much larger
+/// part of a type's public API than any other trait as it is implicitly called
+/// by the compiler. Therefore, it is advisable to consider whether this is
+/// something you are comfortable supporting as a public API.
+///
+/// The [`AsRef`] and [`Borrow`][core::borrow::Borrow] traits have very similar
+/// signatures to `Deref`. It may be desirable to implement either or both of
+/// these, whether in addition to or rather than deref traits. See their
+/// documentation for details.
+///
+/// # Fallibility
+///
+/// **This trait's method should never unexpectedly fail**. Deref coercion means
+/// the compiler will often insert calls to `Deref::deref` implicitly. Failure
+/// during dereferencing can be extremely confusing when `Deref` is invoked
+/// implicitly. In the majority of uses it should be infallible, though it may
+/// be acceptable to panic if the type is misused through programmer error, for
+/// example.
+///
+/// However, infallibility is not enforced and therefore not guaranteed.
+/// As such, `unsafe` code should not rely on infallibility in general for
+/// soundness.
 ///
 /// [book]: ../../book/ch15-02-deref.html
-/// [more]: #more-on-deref-coercion
+/// [coercion]: #deref-coercion
+/// [implementing]: #when-to-implement-deref-or-derefmut
 /// [ref-deref-op]: ../../reference/expressions/operator-expr.html#the-dereference-operator
 /// [method resolution]: ../../reference/expressions/method-call-expr.html
 /// [type coercions]: ../../reference/type-coercions.html
+/// [box]: ../../alloc/boxed/struct.Box.html
+/// [string]: ../../alloc/string/struct.String.html
+/// [vec]: ../../alloc/vec/struct.Vec.html
+/// [rc]: ../../alloc/rc/struct.Rc.html
+/// [cow]: ../../alloc/borrow/enum.Cow.html
 ///
 /// # Examples
 ///
@@ -107,42 +174,59 @@ impl<T: ?Sized> Deref for &mut T {
 /// In addition to being used for explicit dereferencing operations with the
 /// (unary) `*` operator in mutable contexts, `DerefMut` is also used implicitly
 /// by the compiler in many circumstances. This mechanism is called
-/// ['`Deref` coercion'][more]. In immutable contexts, [`Deref`] is used.
-///
-/// Implementing `DerefMut` for smart pointers makes mutating the data behind
-/// them convenient, which is why they implement `DerefMut`. On the other hand,
-/// the rules regarding [`Deref`] and `DerefMut` were designed specifically to
-/// accommodate smart pointers. Because of this, **`DerefMut` should only be
-/// implemented for smart pointers** to avoid confusion.
+/// ["mutable deref coercion"][coercion]. In immutable contexts, [`Deref`] is used.
 ///
-/// For similar reasons, **this trait should never fail**. Failure during
-/// dereferencing can be extremely confusing when `DerefMut` is invoked
-/// implicitly.
+/// **Warning:** Deref coercion is a powerful language feature which has
+/// far-reaching implications for every type that implements `DerefMut`. The
+/// compiler will silently insert calls to `DerefMut::deref_mut`. For this
+/// reason, one should be careful about implementing `DerefMut` and only do so
+/// when mutable deref coercion is desirable. See [the `Deref` docs][implementing]
+/// for advice on when this is typically desirable or undesirable.
 ///
-/// Violating these requirements is a logic error. The behavior resulting from a logic error is not
-/// specified, but users of the trait must ensure that such logic errors do *not* result in
-/// undefined behavior. This means that `unsafe` code **must not** rely on the correctness of this
-/// method.
+/// Types that implement `DerefMut` or `Deref` are often called "smart
+/// pointers" and the mechanism of deref coercion has been specifically designed
+/// to facilitate the pointer-like behaviour that name suggests. Often, the
+/// purpose of a "smart pointer" type is to change the ownership semantics
+/// of a contained value (for example, [`Rc`][rc] or [`Cow`][cow]) or the
+/// storage semantics of a contained value (for example, [`Box`][box]).
 ///
-/// # More on `Deref` coercion
+/// # Mutable deref coercion
 ///
-/// If `T` implements `DerefMut<Target = U>`, and `x` is a value of type `T`,
+/// If `T` implements `DerefMut<Target = U>`, and `v` is a value of type `T`,
 /// then:
 ///
-/// * In mutable contexts, `*x` (where `T` is neither a reference nor a raw pointer)
-///   is equivalent to `*DerefMut::deref_mut(&mut x)`.
+/// * In mutable contexts, `*v` (where `T` is neither a reference nor a raw pointer)
+///   is equivalent to `*DerefMut::deref_mut(&mut v)`.
 /// * Values of type `&mut T` are coerced to values of type `&mut U`
 /// * `T` implicitly implements all the (mutable) methods of the type `U`.
 ///
 /// For more details, visit [the chapter in *The Rust Programming Language*][book]
 /// as well as the reference sections on [the dereference operator][ref-deref-op],
 /// [method resolution] and [type coercions].
 ///
+/// # Fallibility
+///
+/// **This trait's method should never unexpectedly fail**. Deref coercion means
+/// the compiler will often insert calls to `DerefMut::deref_mut` implicitly.
+/// Failure during dereferencing can be extremely confusing when `DerefMut` is
+/// invoked implicitly. In the majority of uses it should be infallible, though
+/// it may be acceptable to panic if the type is misused through programmer
+/// error, for example.
+///
+/// However, infallibility is not enforced and therefore not guaranteed.
+/// As such, `unsafe` code should not rely on infallibility in general for
+/// soundness.
+///
 /// [book]: ../../book/ch15-02-deref.html
-/// [more]: #more-on-deref-coercion
+/// [coercion]: #mutable-deref-coercion
+/// [implementing]: Deref#when-to-implement-deref-or-derefmut
 /// [ref-deref-op]: ../../reference/expressions/operator-expr.html#the-dereference-operator
 /// [method resolution]: ../../reference/expressions/method-call-expr.html
 /// [type coercions]: ../../reference/type-coercions.html
+/// [box]: ../../alloc/boxed/struct.Box.html
+/// [string]: ../../alloc/string/struct.String.html
+/// [rc]: ../../alloc/rc/struct.Rc.html
+/// [cow]: ../../alloc/borrow/enum.Cow.html
 ///
 /// # Examples
 ///