Unimplement unsized_locals

Author: mejrs

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -373,8 +373,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
     }
 
     fn unsized_feature_enabled(&self) -> bool {
-        let features = self.tcx().features();
-        features.unsized_locals() || features.unsized_fn_params()
+        self.tcx().features().unsized_fn_params()
     }
 
     /// Equate the inferred type and the annotated type for user type annotations
@@ -957,7 +956,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
             }
         }
 
-        // When `unsized_fn_params` or `unsized_locals` is enabled, only function calls
+        // When `unsized_fn_params` is enabled, only function calls
         // and nullary ops are checked in `check_call_dest`.
         if !self.unsized_feature_enabled() {
             match self.body.local_kind(local) {
@@ -1941,7 +1940,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                     );
                 }
 
-                // When `unsized_fn_params` and `unsized_locals` are both not enabled,
+                // When `unsized_fn_params` is not enabled,
                 // this check is done at `check_local`.
                 if self.unsized_feature_enabled() {
                     let span = term.source_info.span;

compiler/rustc_codegen_cranelift/example/arbitrary_self_types_pointers_and_wrappers.rs

@@ -32,10 +32,6 @@ impl<T: CoerceUnsized<U>, U> CoerceUnsized<Wrapper<U>> for Wrapper<T> {}
 impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<Wrapper<U>> for Wrapper<T> {}
 
 trait Trait {
-    // This method isn't object-safe yet. Unsized by-value `self` is object-safe (but not callable
-    // without unsized_locals), but wrappers around `Self` currently are not.
-    // FIXME (mikeyhew) uncomment this when unsized rvalues object-safety is implemented
-    // fn wrapper(self: Wrapper<Self>) -> i32;
     fn ptr_wrapper(self: Ptr<Wrapper<Self>>) -> i32;
     fn wrapper_ptr(self: Wrapper<Ptr<Self>>) -> i32;
     fn wrapper_ptr_wrapper(self: Wrapper<Ptr<Wrapper<Self>>>) -> i32;

compiler/rustc_codegen_gcc/example/arbitrary_self_types_pointers_and_wrappers.rs

@@ -37,10 +37,6 @@ impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<Wrapper<U>> for Wrapper<T> {}
 
 
 trait Trait {
-    // This method isn't object-safe yet. Unsized by-value `self` is object-safe (but not callable
-    // without unsized_locals), but wrappers around `Self` currently are not.
-    // FIXME (mikeyhew) uncomment this when unsized rvalues object-safety is implemented
-    // fn wrapper(self: Wrapper<Self>) -> i32;
     fn ptr_wrapper(self: Ptr<Wrapper<Self>>) -> i32;
     fn wrapper_ptr(self: Wrapper<Ptr<Self>>) -> i32;
     fn wrapper_ptr_wrapper(self: Wrapper<Ptr<Wrapper<Self>>>) -> i32;

compiler/rustc_feature/src/removed.rs

@@ -263,6 +263,8 @@ declare_features! (
     /// Allows unnamed fields of struct and union type
     (removed, unnamed_fields, "1.83.0", Some(49804), Some("feature needs redesign"), 131045),
     (removed, unsafe_no_drop_flag, "1.0.0", None, None),
+    /// Allows unsized rvalues at arguments and parameters.
+    (removed, unsized_locals, "CURRENT_RUSTC_VERSION", Some(48055), Some("removed due to implementation concerns; see https://github.com/rust-lang/rust/issues/111942")),
     (removed, unsized_tuple_coercion, "1.87.0", Some(42877),
      Some("The feature restricts possible layouts for tuples, and this restriction is not worth it."), 137728),
     /// Allows `union` fields that don't implement `Copy` as long as they don't have any drop glue.

compiler/rustc_feature/src/unstable.rs

@@ -665,8 +665,6 @@ declare_features! (
     (incomplete, unsized_const_params, "1.82.0", Some(95174)),
     /// Allows unsized fn parameters.
     (internal, unsized_fn_params, "1.49.0", Some(48055)),
-    /// Allows unsized rvalues at arguments and parameters.
-    (incomplete, unsized_locals, "1.30.0", Some(48055)),
     /// Allows using the `#[used(linker)]` (or `#[used(compiler)]`) attribute.
     (unstable, used_with_arg, "1.60.0", Some(93798)),
     /// Allows use of attributes in `where` clauses.

compiler/rustc_hir_typeck/src/coercion.rs

@@ -1662,9 +1662,7 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                             blk_id,
                             expression,
                         );
-                        if !fcx.tcx.features().unsized_locals() {
-                            unsized_return = self.is_return_ty_definitely_unsized(fcx);
-                        }
+                        unsized_return = self.is_return_ty_definitely_unsized(fcx);
                     }
                     ObligationCauseCode::ReturnValue(return_expr_id) => {
                         err = self.report_return_mismatched_types(
@@ -1676,9 +1674,7 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                             return_expr_id,
                             expression,
                         );
-                        if !fcx.tcx.features().unsized_locals() {
-                            unsized_return = self.is_return_ty_definitely_unsized(fcx);
-                        }
+                        unsized_return = self.is_return_ty_definitely_unsized(fcx);
                     }
                     ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
                         arm_span,

compiler/rustc_hir_typeck/src/expr.rs

@@ -809,9 +809,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     );
                 }
             }
-            // Here we want to prevent struct constructors from returning unsized types.
-            // There were two cases this happened: fn pointer coercion in stable
-            // and usual function call in presence of unsized_locals.
+            // Here we want to prevent struct constructors from returning unsized types,
+            // which can happen with fn pointer coercion on stable.
             // Also, as we just want to check sizedness, instead of introducing
             // placeholder lifetimes with probing, we just replace higher lifetimes
             // with fresh vars.

compiler/rustc_hir_typeck/src/gather_locals.rs

@@ -202,7 +202,7 @@ impl<'a, 'tcx> Visitor<'tcx> for GatherLocalsVisitor<'a, 'tcx> {
                         ),
                     );
                 }
-            } else if !self.fcx.tcx.features().unsized_locals() {
+            } else {
                 self.fcx.require_type_is_sized(
                     var_ty,
                     p.span,

compiler/rustc_hir_typeck/src/upvar.rs

@@ -492,7 +492,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let final_upvar_tys = self.final_upvar_tys(closure_def_id);
         debug!(?closure_hir_id, ?args, ?final_upvar_tys);
 
-        if self.tcx.features().unsized_locals() || self.tcx.features().unsized_fn_params() {
+        if self.tcx.features().unsized_fn_params() {
             for capture in
                 self.typeck_results.borrow().closure_min_captures_flattened(closure_def_id)
             {

compiler/rustc_middle/src/mir/syntax.rs

@@ -1133,13 +1133,6 @@ pub type AssertMessage<'tcx> = AssertKind<Operand<'tcx>>;
 /// Each local naturally corresponds to the place `Place { local, projection: [] }`. This place has
 /// the address of the local's allocation and the type of the local.
 ///
-/// **Needs clarification:** Unsized locals seem to present a bit of an issue. Their allocation
-/// can't actually be created on `StorageLive`, because it's unclear how big to make the allocation.
-/// Furthermore, MIR produces assignments to unsized locals, although that is not permitted under
-/// `#![feature(unsized_locals)]` in Rust. Besides just putting "unsized locals are special and
-/// different" in a bunch of places, I (JakobDegen) don't know how to incorporate this behavior into
-/// the current MIR semantics in a clean way - possibly this needs some design work first.
-///
 /// For places that are not locals, ie they have a non-empty list of projections, we define the
 /// values as a function of the parent place, that is the place with its last [`ProjectionElem`]
 /// stripped. The way this is computed of course depends on the kind of that last projection

compiler/rustc_middle/src/ty/instance.rs

@@ -79,7 +79,7 @@ pub enum InstanceKind<'tcx> {
     Intrinsic(DefId),
 
     /// `<T as Trait>::method` where `method` receives unsizeable `self: Self` (part of the
-    /// `unsized_locals` feature).
+    /// `unsized_fn_params` feature).
     ///
     /// The generated shim will take `Self` via `*mut Self` - conceptually this is `&owned Self` -
     /// and dereference the argument to call the original function.

compiler/rustc_mir_build/src/builder/expr/as_operand.rs

@@ -55,9 +55,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// local variable of unsized type. For example, consider this program:
     ///
     /// ```
-    /// #![feature(unsized_locals, unsized_fn_params)]
+    /// #![feature(unsized_fn_params)]
     /// # use core::fmt::Debug;
-    /// fn foo(p: dyn Debug) { dbg!(p); }
+    /// fn foo(_p: dyn Debug) { /* ... */ }
     ///
     /// fn bar(box_p: Box<dyn Debug>) { foo(*box_p); }
     /// ```
@@ -84,7 +84,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// will actually provide a pointer to the interior of the box, and not move the `dyn Debug`
     /// value to the stack.
     ///
-    /// See #68304 for more details.
+    /// See <https://github.com/rust-lang/rust/issues/68304> for more details.
     pub(crate) fn as_local_call_operand(
         &mut self,
         block: BasicBlock,

compiler/rustc_mir_transform/src/coroutine.rs

@@ -1417,9 +1417,9 @@ fn check_field_tys_sized<'tcx>(
     coroutine_layout: &CoroutineLayout<'tcx>,
     def_id: LocalDefId,
 ) {
-    // No need to check if unsized_locals/unsized_fn_params is disabled,
+    // No need to check if unsized_fn_params is disabled,
     // since we will error during typeck.
-    if !tcx.features().unsized_locals() && !tcx.features().unsized_fn_params() {
+    if !tcx.features().unsized_fn_params() {
         return;
     }
 

compiler/rustc_mir_transform/src/gvn.rs

@@ -240,8 +240,6 @@ struct VnState<'body, 'tcx> {
     next_opaque: usize,
     /// Cache the deref values.
     derefs: Vec<VnIndex>,
-    /// Cache the value of the `unsized_locals` features, to avoid fetching it repeatedly in a loop.
-    feature_unsized_locals: bool,
     ssa: &'body SsaLocals,
     dominators: Dominators<BasicBlock>,
     reused_locals: DenseBitSet<Local>,
@@ -273,7 +271,6 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             evaluated: IndexVec::with_capacity(num_values),
             next_opaque: 1,
             derefs: Vec::new(),
-            feature_unsized_locals: tcx.features().unsized_locals(),
             ssa,
             dominators,
             reused_locals: DenseBitSet::new_empty(local_decls.len()),
@@ -329,13 +326,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
     fn assign(&mut self, local: Local, value: VnIndex) {
         debug_assert!(self.ssa.is_ssa(local));
         self.locals[local] = Some(value);
-
-        // Only register the value if its type is `Sized`, as we will emit copies of it.
-        let is_sized = !self.feature_unsized_locals
-            || self.local_decls[local].ty.is_sized(self.tcx, self.typing_env());
-        if is_sized {
-            self.rev_locals[value].push(local);
-        }
+        self.rev_locals[value].push(local);
     }
 
     fn insert_constant(&mut self, value: Const<'tcx>) -> VnIndex {

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

@@ -2995,9 +2995,6 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 if local {
                     err.note("all local variables must have a statically known size");
                 }
-                if !tcx.features().unsized_locals() {
-                    err.help("unsized locals are gated as an unstable feature");
-                }
             }
             ObligationCauseCode::SizedArgumentType(hir_id) => {
                 let mut ty = None;

compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs

@@ -414,8 +414,8 @@ fn virtual_call_violations_for_method<'tcx>(
 
     let receiver_ty = tcx.liberate_late_bound_regions(method.def_id, sig.input(0));
 
-    // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
-    // However, this is already considered dyn compatible. We allow it as a special case here.
+    // `self: Self` can't be dispatched on.
+    // However, this is considered dyn compatible. We allow it as a special case here.
     // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
     // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
     if receiver_ty != tcx.types.self_param {

library/core/src/mem/mod.rs

@@ -149,8 +149,32 @@ pub const fn forget<T>(t: T) {
 
 /// Like [`forget`], but also accepts unsized values.
 ///
-/// This function is just a shim intended to be removed when the `unsized_locals` feature gets
-/// stabilized.
+/// While Rust does not permit unsized locals since its removal in [#111942] it is
+/// still possible to call functions with unsized values from a function argument
+/// or in-place construction.
+///
+/// ```rust
+/// #![feature(unsized_fn_params, forget_unsized)]
+/// #![allow(internal_features)]
+///
+/// use std::mem::forget_unsized;
+///
+/// pub fn in_place() {
+///     forget_unsized(*Box::new("str"));
+/// }
+///
+/// pub fn param(x: str) {
+///     forget_unsized(x);
+/// }
+/// ```
+///
+/// This works because the compiler will alter these functions to pass the parameter
+/// by reference instead. This trick is necessary to support `Box<dyn FnOnce()>: FnOnce()`.
+/// See [#68304] and [#71170] for more information.
+///
+/// [#111942]: https://github.com/rust-lang/rust/issues/111942
+/// [#68304]: https://github.com/rust-lang/rust/issues/68304
+/// [#71170]: https://github.com/rust-lang/rust/pull/71170
 #[inline]
 #[unstable(feature = "forget_unsized", issue = "none")]
 pub fn forget_unsized<T: ?Sized>(t: T) {

library/coretests/tests/ptr.rs

@@ -653,7 +653,6 @@ fn thin_box() {
     //   if `{size,align}_of_for_meta<T: ?Sized>(T::Metadata)` are added.
     // * Constructing a `ThinBox` without consuming and deallocating a `Box`
     //   requires either the unstable `Unsize` marker trait,
-    //   or the unstable `unsized_locals` language feature,
     //   or taking `&dyn T` and restricting to `T: Copy`.
 
     use std::alloc::*;

src/doc/rustc-dev-guide/src/implementing_new_features.md

@@ -156,8 +156,8 @@ a new unstable feature:
    [`incomplete_features` lint]: https://doc.rust-lang.org/rustc/lints/listing/warn-by-default.html#incomplete-features
 
    ```rust ignore
-   /// Allows unsized rvalues at arguments and parameters.
-   (incomplete, unsized_locals, "CURRENT_RUSTC_VERSION", Some(48055), None),
+   /// Allows deref patterns.
+   (incomplete, deref_patterns, "CURRENT_RUSTC_VERSION", Some(87121), None),
    ```
 
    To avoid [semantic merge conflicts], please use `CURRENT_RUSTC_VERSION` instead of `1.70` or