Rollup of 6 pull requests

compiler/rustc_const_eval/src/transform/check_consts/post_drop_elaboration.rs

@@ -80,7 +80,8 @@ impl Visitor<'tcx> for CheckLiveDrops<'mir, 'tcx> {
         trace!("visit_terminator: terminator={:?} location={:?}", terminator, location);
 
         match &terminator.kind {
-            mir::TerminatorKind::Drop { place: dropped_place, .. } => {
+            mir::TerminatorKind::Drop { place: dropped_place, .. }
+            | mir::TerminatorKind::DropAndReplace { place: dropped_place, .. } => {
                 let dropped_ty = dropped_place.ty(self.body, self.tcx).ty;
                 if !NeedsNonConstDrop::in_any_value_of_ty(self.ccx, dropped_ty) {
                     // Instead of throwing a bug, we just return here. This is because we have to
@@ -104,11 +105,6 @@ impl Visitor<'tcx> for CheckLiveDrops<'mir, 'tcx> {
                 }
             }
 
-            mir::TerminatorKind::DropAndReplace { .. } => span_bug!(
-                terminator.source_info.span,
-                "`DropAndReplace` should be removed by drop elaboration",
-            ),
-
             mir::TerminatorKind::Abort
             | mir::TerminatorKind::Call { .. }
             | mir::TerminatorKind::Assert { .. }

compiler/rustc_error_codes/src/error_codes/E0038.md

@@ -1,34 +1,64 @@
-Trait objects like `Box<Trait>` can only be constructed when certain
-requirements are satisfied by the trait in question.
-
-Trait objects are a form of dynamic dispatch and use a dynamically sized type
-for the inner type. So, for a given trait `Trait`, when `Trait` is treated as a
-type, as in `Box<Trait>`, the inner type is 'unsized'. In such cases the boxed
-pointer is a 'fat pointer' that contains an extra pointer to a table of methods
-(among other things) for dynamic dispatch. This design mandates some
-restrictions on the types of traits that are allowed to be used in trait
-objects, which are collectively termed as 'object safety' rules.
-
-Attempting to create a trait object for a non object-safe trait will trigger
-this error.
-
-There are various rules:
-
-### The trait cannot require `Self: Sized`
-
-When `Trait` is treated as a type, the type does not implement the special
-`Sized` trait, because the type does not have a known size at compile time and
-can only be accessed behind a pointer. Thus, if we have a trait like the
-following:
+For any given trait `Trait` there may be a related _type_ called the _trait
+object type_ which is typically written as `dyn Trait`. In earlier editions of
+Rust, trait object types were written as plain `Trait` (just the name of the
+trait, written in type positions) but this was a bit too confusing, so we now
+write `dyn Trait`.
+
+Some traits are not allowed to be used as trait object types. The traits that
+are allowed to be used as trait object types are called "object-safe" traits.
+Attempting to use a trait object type for a trait that is not object-safe will
+trigger error E0038.
+
+Two general aspects of trait object types give rise to the restrictions:
+
+  1. Trait object types are dynamically sized types (DSTs), and trait objects of
+     these types can only be accessed through pointers, such as `&dyn Trait` or
+     `Box<dyn Trait>`. The size of such a pointer is known, but the size of the
+     `dyn Trait` object pointed-to by the pointer is _opaque_ to code working
+     with it, and different tait objects with the same trait object type may
+     have different sizes.
+
+  2. The pointer used to access a trait object is paired with an extra pointer
+     to a "virtual method table" or "vtable", which is used to implement dynamic
+     dispatch to the object's implementations of the trait's methods. There is a
+     single such vtable for each trait implementation, but different trait
+     objects with the same trait object type may point to vtables from different
+     implementations.
+
+The specific conditions that violate object-safety follow, most of which relate
+to missing size information and vtable polymorphism arising from these aspects.
+
+### The trait requires `Self: Sized`
+
+Traits that are declared as `Trait: Sized` or which otherwise inherit a
+constraint of `Self:Sized` are not object-safe.
+
+The reasoning behind this is somewhat subtle. It derives from the fact that Rust
+requires (and defines) that every trait object type `dyn Trait` automatically
+implements `Trait`. Rust does this to simplify error reporting and ease
+interoperation between static and dynamic polymorphism. For example, this code
+works:
 
 ```
-trait Foo where Self: Sized {
+trait Trait {
+}
+
+fn static_foo<T:Trait + ?Sized>(b: &T) {
+}
 
+fn dynamic_bar(a: &dyn Trait) {
+    static_foo(a)
 }
 ```
 
-We cannot create an object of type `Box<Foo>` or `&Foo` since in this case
-`Self` would not be `Sized`.
+This code works because `dyn Trait`, if it exists, always implements `Trait`.
+
+However as we know, any `dyn Trait` is also unsized, and so it can never
+implement a sized trait like `Trait:Sized`. So, rather than allow an exception
+to the rule that `dyn Trait` always implements `Trait`, Rust chooses to prohibit
+such a `dyn Trait` from existing at all.
+
+Only unsized traits are considered object-safe.
 
 Generally, `Self: Sized` is used to indicate that the trait should not be used
 as a trait object. If the trait comes from your own crate, consider removing
@@ -67,7 +97,7 @@ trait Trait {
     fn foo(&self) -> Self;
 }
 
-fn call_foo(x: Box<Trait>) {
+fn call_foo(x: Box<dyn Trait>) {
     let y = x.foo(); // What type is y?
     // ...
 }
@@ -76,7 +106,8 @@ fn call_foo(x: Box<Trait>) {
 If only some methods aren't object-safe, you can add a `where Self: Sized` bound
 on them to mark them as explicitly unavailable to trait objects. The
 functionality will still be available to all other implementers, including
-`Box<Trait>` which is itself sized (assuming you `impl Trait for Box<Trait>`).
+`Box<dyn Trait>` which is itself sized (assuming you `impl Trait for Box<dyn
+Trait>`).
 
 ```
 trait Trait {
@@ -115,7 +146,9 @@ impl Trait for u8 {
 ```
 
 At compile time each implementation of `Trait` will produce a table containing
-the various methods (and other items) related to the implementation.
+the various methods (and other items) related to the implementation, which will
+be used as the virtual method table for a `dyn Trait` object derived from that
+implementation.
 
 This works fine, but when the method gains generic parameters, we can have a
 problem.
@@ -174,7 +207,7 @@ Now, if we have the following code:
 # impl Trait for u8 { fn foo<T>(&self, on: T) {} }
 # impl Trait for bool { fn foo<T>(&self, on: T) {} }
 # // etc.
-fn call_foo(thing: Box<Trait>) {
+fn call_foo(thing: Box<dyn Trait>) {
     thing.foo(true); // this could be any one of the 8 types above
     thing.foo(1);
     thing.foo("hello");
@@ -200,7 +233,7 @@ trait Trait {
 ```
 
 If this is not an option, consider replacing the type parameter with another
-trait object (e.g., if `T: OtherTrait`, use `on: Box<OtherTrait>`). If the
+trait object (e.g., if `T: OtherTrait`, use `on: Box<dyn OtherTrait>`). If the
 number of types you intend to feed to this method is limited, consider manually
 listing out the methods of different types.
 
@@ -226,7 +259,7 @@ trait Foo {
 }
 ```
 
-### The trait cannot contain associated constants
+### Trait contains associated constants
 
 Just like static functions, associated constants aren't stored on the method
 table. If the trait or any subtrait contain an associated constant, they cannot
@@ -248,7 +281,7 @@ trait Foo {
 }
 ```
 
-### The trait cannot use `Self` as a type parameter in the supertrait listing
+### Trait uses `Self` as a type parameter in the supertrait listing
 
 This is similar to the second sub-error, but subtler. It happens in situations
 like the following:

compiler/rustc_middle/src/mir/mod.rs

@@ -1803,6 +1803,16 @@ impl<V, T> ProjectionElem<V, T> {
             | Self::Downcast(_, _) => false,
         }
     }
+
+    /// Returns `true` if this is a `Downcast` projection with the given `VariantIdx`.
+    pub fn is_downcast_to(&self, v: VariantIdx) -> bool {
+        matches!(*self, Self::Downcast(_, x) if x == v)
+    }
+
+    /// Returns `true` if this is a `Field` projection with the given index.
+    pub fn is_field_to(&self, f: Field) -> bool {
+        matches!(*self, Self::Field(x, _) if x == f)
+    }
 }
 
 /// Alias for projections as they appear in places, where the base is a place

compiler/rustc_middle/src/ty/adt.rs

@@ -64,6 +64,28 @@ bitflags! {
 /// Moreover, Rust only allows recursive data types through indirection.
 ///
 /// [adt]: https://en.wikipedia.org/wiki/Algebraic_data_type
+///
+/// # Recursive types
+///
+/// It may seem impossible to represent recursive types using [`Ty`],
+/// since [`TyKind::Adt`] includes [`AdtDef`], which includes its fields,
+/// creating a cycle. However, `AdtDef` does not actually include the *types*
+/// of its fields; it includes just their [`DefId`]s.
+///
+/// For example, the following type:
+///
+/// ```
+/// struct S { x: Box<S> }
+/// ```
+///
+/// is essentially represented with [`Ty`] as the following pseudocode:
+///
+/// ```
+/// struct S { x }
+/// ```
+///
+/// where `x` here represents the `DefId` of `S.x`. Then, the `DefId`
+/// can be used with [`TyCtxt::type_of()`] to get the type of the field.
 pub struct AdtDef {
     /// The `DefId` of the struct, enum or union item.
     pub did: DefId,

compiler/rustc_middle/src/ty/mod.rs

@@ -1735,7 +1735,7 @@ impl ReprOptions {
 
 impl<'tcx> FieldDef {
     /// Returns the type of this field. The resulting type is not normalized. The `subst` is
-    /// typically obtained via the second field of `TyKind::AdtDef`.
+    /// typically obtained via the second field of [`TyKind::Adt`].
     pub fn ty(&self, tcx: TyCtxt<'tcx>, subst: SubstsRef<'tcx>) -> Ty<'tcx> {
         tcx.type_of(self.did).subst(tcx, subst)
     }

compiler/rustc_mir_transform/src/lib.rs

@@ -60,8 +60,10 @@ mod match_branches;
 mod multiple_return_terminators;
 mod normalize_array_len;
 mod nrvo;
+mod remove_false_edges;
 mod remove_noop_landing_pads;
 mod remove_storage_markers;
+mod remove_uninit_drops;
 mod remove_unneeded_drops;
 mod remove_zsts;
 mod required_consts;
@@ -75,7 +77,7 @@ mod simplify_try;
 mod uninhabited_enum_branching;
 mod unreachable_prop;
 
-use rustc_const_eval::transform::check_consts;
+use rustc_const_eval::transform::check_consts::{self, ConstCx};
 use rustc_const_eval::transform::promote_consts;
 use rustc_const_eval::transform::validate;
 use rustc_mir_dataflow::rustc_peek;
@@ -444,8 +446,20 @@ fn mir_drops_elaborated_and_const_checked<'tcx>(
     let (body, _) = tcx.mir_promoted(def);
     let mut body = body.steal();
 
+    // IMPORTANT
+    remove_false_edges::RemoveFalseEdges.run_pass(tcx, &mut body);
+
+    // Do a little drop elaboration before const-checking if `const_precise_live_drops` is enabled.
+    //
+    // FIXME: Can't use `run_passes` for these, since `run_passes` SILENTLY DOES NOTHING IF THE MIR
+    // PHASE DOESN'T CHANGE.
+    if check_consts::post_drop_elaboration::checking_enabled(&ConstCx::new(tcx, &body)) {
+        simplify::SimplifyCfg::new("remove-false-edges").run_pass(tcx, &mut body);
+        remove_uninit_drops::RemoveUninitDrops.run_pass(tcx, &mut body);
+        check_consts::post_drop_elaboration::check_live_drops(tcx, &body);
+    }
+
     run_post_borrowck_cleanup_passes(tcx, &mut body);
-    check_consts::post_drop_elaboration::check_live_drops(tcx, &body);
     tcx.alloc_steal_mir(body)
 }
 
@@ -455,7 +469,7 @@ fn run_post_borrowck_cleanup_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tc
 
     let post_borrowck_cleanup: &[&dyn MirPass<'tcx>] = &[
         // Remove all things only needed by analysis
-        &simplify_branches::SimplifyBranches::new("initial"),
+        &simplify_branches::SimplifyConstCondition::new("initial"),
         &remove_noop_landing_pads::RemoveNoopLandingPads,
         &cleanup_post_borrowck::CleanupNonCodegenStatements,
         &simplify::SimplifyCfg::new("early-opt"),
@@ -514,13 +528,13 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         &instcombine::InstCombine,
         &separate_const_switch::SeparateConstSwitch,
         &const_prop::ConstProp,
-        &simplify_branches::SimplifyBranches::new("after-const-prop"),
+        &simplify_branches::SimplifyConstCondition::new("after-const-prop"),
         &early_otherwise_branch::EarlyOtherwiseBranch,
         &simplify_comparison_integral::SimplifyComparisonIntegral,
         &simplify_try::SimplifyArmIdentity,
         &simplify_try::SimplifyBranchSame,
         &dest_prop::DestinationPropagation,
-        &simplify_branches::SimplifyBranches::new("final"),
+        &simplify_branches::SimplifyConstCondition::new("final"),
         &remove_noop_landing_pads::RemoveNoopLandingPads,
         &simplify::SimplifyCfg::new("final"),
         &nrvo::RenameReturnPlace,

compiler/rustc_mir_transform/src/remove_false_edges.rs

@@ -0,0 +1,29 @@
+use rustc_middle::mir::{Body, TerminatorKind};
+use rustc_middle::ty::TyCtxt;
+
+use crate::MirPass;
+
+/// Removes `FalseEdge` and `FalseUnwind` terminators from the MIR.
+///
+/// These are only needed for borrow checking, and can be removed afterwards.
+///
+/// FIXME: This should probably have its own MIR phase.
+pub struct RemoveFalseEdges;
+
+impl<'tcx> MirPass<'tcx> for RemoveFalseEdges {
+    fn run_pass(&self, _: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        for block in body.basic_blocks_mut() {
+            let terminator = block.terminator_mut();
+            terminator.kind = match terminator.kind {
+                TerminatorKind::FalseEdge { real_target, .. } => {
+                    TerminatorKind::Goto { target: real_target }
+                }
+                TerminatorKind::FalseUnwind { real_target, .. } => {
+                    TerminatorKind::Goto { target: real_target }
+                }
+
+                _ => continue,
+            }
+        }
+    }
+}