Auto merge of #91469 - matthiaskrgr:rollup-xom3j55, r=matthiaskrgr

Rollup of 12 pull requests

Successful merges:

 - #89954 (Fix legacy_const_generic doc arguments display)
 - #91321 (Handle placeholder regions in NLL type outlive constraints)
 - #91329 (Fix incorrect usage of `EvaluatedToOk` when evaluating `TypeOutlives`)
 - #91364 (Improve error message for incorrect field accesses through raw pointers)
 - #91387 (Clarify and tidy up explanation of E0038)
 - #91410 (Move `#![feature(const_precise_live_drops)]` checks earlier in the pipeline)
 - #91435 (Improve diagnostic for missing half of binary operator in `if` condition)
 - #91444 (disable tests in Miri that take too long)
 - #91457 (Add additional test from rust issue number 91068)
 - #91460 (Document how `last_os_error` should be used)
 - #91464 (Document file path case sensitivity)
 - #91466 (Improve the comments in `Symbol::interner`.)

Failed merges:

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

compiler/rustc_borrowck/src/type_check/constraint_conversion.rs

@@ -6,6 +6,7 @@ use rustc_infer::infer::region_constraints::{GenericKind, VerifyBound};
 use rustc_infer::infer::{self, InferCtxt, SubregionOrigin};
 use rustc_middle::mir::ConstraintCategory;
 use rustc_middle::ty::subst::GenericArgKind;
+use rustc_middle::ty::TypeFoldable;
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_span::DUMMY_SP;
 
@@ -95,11 +96,23 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
                 self.add_outlives(r1_vid, r2_vid);
             }
 
-            GenericArgKind::Type(t1) => {
+            GenericArgKind::Type(mut t1) => {
                 // we don't actually use this for anything, but
                 // the `TypeOutlives` code needs an origin.
                 let origin = infer::RelateParamBound(DUMMY_SP, t1, None);
 
+                // Placeholder regions need to be converted now because it may
+                // create new region variables, which can't be done later when
+                // verifying these bounds.
+                if t1.has_placeholders() {
+                    t1 = tcx.fold_regions(&t1, &mut false, |r, _| match *r {
+                        ty::RegionKind::RePlaceholder(placeholder) => {
+                            self.constraints.placeholder_region(self.infcx, placeholder)
+                        }
+                        _ => r,
+                    });
+                }
+
                 TypeOutlives::new(
                     &mut *self,
                     tcx,

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
+For any given trait `Trait` there may be a related _type_ called the _trait
-requirements are satisfied by the trait in question.
+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`.
 
-Trait objects are a form of dynamic dispatch and use a dynamically sized type
+Some traits are not allowed to be used as trait object types. The traits that
-for the inner type. So, for a given trait `Trait`, when `Trait` is treated as a
+are allowed to be used as trait object types are called "object-safe" traits.
-type, as in `Box<Trait>`, the inner type is 'unsized'. In such cases the boxed
+Attempting to use a trait object type for a trait that is not object-safe will
-pointer is a 'fat pointer' that contains an extra pointer to a table of methods
+trigger error E0038.
-(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
+Two general aspects of trait object types give rise to the restrictions:
-this error.
 
-There are various rules:
+  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.
 
-### The trait cannot require `Self: Sized`
+  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.
 
-When `Trait` is treated as a type, the type does not implement the special
+The specific conditions that violate object-safety follow, most of which relate
-`Sized` trait, because the type does not have a known size at compile time and
+to missing size information and vtable polymorphism arising from these aspects.
-can only be accessed behind a pointer. Thus, if we have a trait like the
+
-following:
+### 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
+This code works because `dyn Trait`, if it exists, always implements `Trait`.
-`Self` would not be `Sized`.
+
+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_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();
 
-    run_post_borrowck_cleanup_passes(tcx, &mut body);
+    // 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);
     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,
+            }
+        }
+    }
+}

compiler/rustc_mir_transform/src/remove_uninit_drops.rs

@@ -0,0 +1,171 @@
+use rustc_index::bit_set::BitSet;
+use rustc_middle::mir::{Body, Field, Rvalue, Statement, StatementKind, TerminatorKind};
+use rustc_middle::ty::subst::SubstsRef;
+use rustc_middle::ty::{self, ParamEnv, Ty, TyCtxt, VariantDef};
+use rustc_mir_dataflow::impls::MaybeInitializedPlaces;
+use rustc_mir_dataflow::move_paths::{LookupResult, MoveData, MovePathIndex};
+use rustc_mir_dataflow::{self, move_path_children_matching, Analysis, MoveDataParamEnv};
+
+use crate::MirPass;
+
+/// Removes `Drop` and `DropAndReplace` terminators whose target is known to be uninitialized at
+/// that point.
+///
+/// This is redundant with drop elaboration, but we need to do it prior to const-checking, and
+/// running const-checking after drop elaboration makes it opimization dependent, causing issues
+/// like [#90770].
+///
+/// [#90770]: https://github.com/rust-lang/rust/issues/90770
+pub struct RemoveUninitDrops;
+
+impl<'tcx> MirPass<'tcx> for RemoveUninitDrops {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        let param_env = tcx.param_env(body.source.def_id());
+        let Ok(move_data) = MoveData::gather_moves(body, tcx, param_env) else {
+            // We could continue if there are move errors, but there's not much point since our
+            // init data isn't complete.
+            return;
+        };
+
+        let mdpe = MoveDataParamEnv { move_data, param_env };
+        let mut maybe_inits = MaybeInitializedPlaces::new(tcx, body, &mdpe)
+            .into_engine(tcx, body)
+            .pass_name("remove_uninit_drops")
+            .iterate_to_fixpoint()
+            .into_results_cursor(body);
+
+        let mut to_remove = vec![];
+        for (bb, block) in body.basic_blocks().iter_enumerated() {
+            let terminator = block.terminator();
+            let (TerminatorKind::Drop { place, .. } | TerminatorKind::DropAndReplace { place, .. })
+                = &terminator.kind
+            else { continue };
+
+            maybe_inits.seek_before_primary_effect(body.terminator_loc(bb));
+
+            // If there's no move path for the dropped place, it's probably a `Deref`. Let it alone.
+            let LookupResult::Exact(mpi) = mdpe.move_data.rev_lookup.find(place.as_ref()) else {
+                continue;
+            };
+
+            let should_keep = is_needs_drop_and_init(
+                tcx,
+                param_env,
+                maybe_inits.get(),
+                &mdpe.move_data,
+                place.ty(body, tcx).ty,
+                mpi,
+            );
+            if !should_keep {
+                to_remove.push(bb)
+            }
+        }
+
+        for bb in to_remove {
+            let block = &mut body.basic_blocks_mut()[bb];
+
+            let (TerminatorKind::Drop { target, .. } | TerminatorKind::DropAndReplace { target, .. })
+                = &block.terminator().kind
+            else { unreachable!() };
+
+            // Replace block terminator with `Goto`.
+            let target = *target;
+            let old_terminator_kind = std::mem::replace(
+                &mut block.terminator_mut().kind,
+                TerminatorKind::Goto { target },
+            );
+
+            // If this is a `DropAndReplace`, we need to emulate the assignment to the return place.
+            if let TerminatorKind::DropAndReplace { place, value, .. } = old_terminator_kind {
+                block.statements.push(Statement {
+                    source_info: block.terminator().source_info,
+                    kind: StatementKind::Assign(Box::new((place, Rvalue::Use(value)))),
+                });
+            }
+        }
+    }
+}
+
+fn is_needs_drop_and_init(
+    tcx: TyCtxt<'tcx>,
+    param_env: ParamEnv<'tcx>,
+    maybe_inits: &BitSet<MovePathIndex>,
+    move_data: &MoveData<'tcx>,
+    ty: Ty<'tcx>,
+    mpi: MovePathIndex,
+) -> bool {
+    // No need to look deeper if the root is definitely uninit or if it has no `Drop` impl.
+    if !maybe_inits.contains(mpi) || !ty.needs_drop(tcx, param_env) {
+        return false;
+    }
+
+    let field_needs_drop_and_init = |(f, f_ty, mpi)| {
+        let child = move_path_children_matching(move_data, mpi, |x| x.is_field_to(f));
+        let Some(mpi) = child else {
+            return f_ty.needs_drop(tcx, param_env);
+        };
+
+        is_needs_drop_and_init(tcx, param_env, maybe_inits, move_data, f_ty, mpi)
+    };
+
+    // This pass is only needed for const-checking, so it doesn't handle as many cases as
+    // `DropCtxt::open_drop`, since they aren't relevant in a const-context.
+    match ty.kind() {
+        ty::Adt(adt, substs) => {
+            let dont_elaborate = adt.is_union() || adt.is_manually_drop() || adt.has_dtor(tcx);
+            if dont_elaborate {
+                return true;
+            }
+
+            // Look at all our fields, or if we are an enum all our variants and their fields.
+            //
+            // If a field's projection *is not* present in `MoveData`, it has the same
+            // initializedness as its parent (maybe init).
+            //
+            // If its projection *is* present in `MoveData`, then the field may have been moved
+            // from separate from its parent. Recurse.
+            adt.variants.iter_enumerated().any(|(vid, variant)| {
+                // Enums have multiple variants, which are discriminated with a `Downcast` projection.
+                // Structs have a single variant, and don't use a `Downcast` projection.
+                let mpi = if adt.is_enum() {
+                    let downcast =
+                        move_path_children_matching(move_data, mpi, |x| x.is_downcast_to(vid));
+                    let Some(dc_mpi) = downcast else {
+                        return variant_needs_drop(tcx, param_env, substs, variant);
+                    };
+
+                    dc_mpi
+                } else {
+                    mpi
+                };
+
+                variant
+                    .fields
+                    .iter()
+                    .enumerate()
+                    .map(|(f, field)| (Field::from_usize(f), field.ty(tcx, substs), mpi))
+                    .any(field_needs_drop_and_init)
+            })
+        }
+
+        ty::Tuple(_) => ty
+            .tuple_fields()
+            .enumerate()
+            .map(|(f, f_ty)| (Field::from_usize(f), f_ty, mpi))
+            .any(field_needs_drop_and_init),
+
+        _ => true,
+    }
+}
+
+fn variant_needs_drop(
+    tcx: TyCtxt<'tcx>,
+    param_env: ParamEnv<'tcx>,
+    substs: SubstsRef<'tcx>,
+    variant: &VariantDef,
+) -> bool {
+    variant.fields.iter().any(|field| {
+        let f_ty = field.ty(tcx, substs);
+        f_ty.needs_drop(tcx, param_env)
+    })
+}

compiler/rustc_mir_transform/src/remove_unneeded_drops.rs

@@ -1,4 +1,8 @@
-//! This pass replaces a drop of a type that does not need dropping, with a goto
+//! This pass replaces a drop of a type that does not need dropping, with a goto.
+//!
+//! When the MIR is built, we check `needs_drop` before emitting a `Drop` for a place. This pass is
+//! useful because (unlike MIR building) it runs after type checking, so it can make use of
+//! `Reveal::All` to provide more precies type information.
 
 use crate::MirPass;
 use rustc_middle::mir::*;

compiler/rustc_mir_transform/src/simplify_branches.rs

@@ -1,22 +1,21 @@
-//! A pass that simplifies branches when their condition is known.
-
 use crate::MirPass;
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
 
 use std::borrow::Cow;
 
-pub struct SimplifyBranches {
+/// A pass that replaces a branch with a goto when its condition is known.
+pub struct SimplifyConstCondition {
     label: String,
 }
 
-impl SimplifyBranches {
+impl SimplifyConstCondition {
     pub fn new(label: &str) -> Self {
-        SimplifyBranches { label: format!("SimplifyBranches-{}", label) }
+        SimplifyConstCondition { label: format!("SimplifyConstCondition-{}", label) }
     }
 }
 
-impl<'tcx> MirPass<'tcx> for SimplifyBranches {
+impl<'tcx> MirPass<'tcx> for SimplifyConstCondition {
     fn name(&self) -> Cow<'_, str> {
         Cow::Borrowed(&self.label)
     }
@@ -53,12 +52,6 @@ impl<'tcx> MirPass<'tcx> for SimplifyBranches {
                     Some(v) if v == expected => TerminatorKind::Goto { target },
                     _ => continue,
                 },
-                TerminatorKind::FalseEdge { real_target, .. } => {
-                    TerminatorKind::Goto { target: real_target }
-                }
-                TerminatorKind::FalseUnwind { real_target, .. } => {
-                    TerminatorKind::Goto { target: real_target }
-                }
                 _ => continue,
             };
         }

compiler/rustc_parse/src/parser/expr.rs

@@ -1988,25 +1988,34 @@ impl<'a> Parser<'a> {
         let lo = self.prev_token.span;
         let cond = self.parse_cond_expr()?;
 
+        let missing_then_block_binop_span = || {
+            match cond.kind {
+                ExprKind::Binary(Spanned { span: binop_span, .. }, _, ref right)
+                    if let ExprKind::Block(..) = right.kind => Some(binop_span),
+                _ => None
+            }
+        };
+
         // Verify that the parsed `if` condition makes sense as a condition. If it is a block, then
         // verify that the last statement is either an implicit return (no `;`) or an explicit
         // return. This won't catch blocks with an explicit `return`, but that would be caught by
         // the dead code lint.
-        let thn = if self.eat_keyword(kw::Else) || !cond.returns() {
+        let thn = if self.token.is_keyword(kw::Else) || !cond.returns() {
+            if let Some(binop_span) = missing_then_block_binop_span() {
+                self.error_missing_if_then_block(lo, None, Some(binop_span)).emit();
+                self.mk_block_err(cond.span)
+            } else {
                 self.error_missing_if_cond(lo, cond.span)
+            }
         } else {
             let attrs = self.parse_outer_attributes()?.take_for_recovery(); // For recovery.
             let not_block = self.token != token::OpenDelim(token::Brace);
-            let block = self.parse_block().map_err(|mut err| {
+            let block = self.parse_block().map_err(|err| {
                 if not_block {
-                    err.span_label(lo, "this `if` expression has a condition, but no block");
+                    self.error_missing_if_then_block(lo, Some(err), missing_then_block_binop_span())
-                    if let ExprKind::Binary(_, _, ref right) = cond.kind {
+                } else {
-                        if let ExprKind::Block(_, _) = right.kind {
-                            err.help("maybe you forgot the right operand of the condition?");
-                        }
-                    }
-                }
                     err
+                }
             })?;
             self.error_on_if_block_attrs(lo, false, block.span, &attrs);
             block
@@ -2015,6 +2024,28 @@ impl<'a> Parser<'a> {
         Ok(self.mk_expr(lo.to(self.prev_token.span), ExprKind::If(cond, thn, els), attrs))
     }
 
+    fn error_missing_if_then_block(
+        &self,
+        if_span: Span,
+        err: Option<DiagnosticBuilder<'a>>,
+        binop_span: Option<Span>,
+    ) -> DiagnosticBuilder<'a> {
+        let msg = "this `if` expression has a condition, but no block";
+
+        let mut err = if let Some(mut err) = err {
+            err.span_label(if_span, msg);
+            err
+        } else {
+            self.struct_span_err(if_span, msg)
+        };
+
+        if let Some(binop_span) = binop_span {
+            err.span_help(binop_span, "maybe you forgot the right operand of the condition?");
+        }
+
+        err
+    }
+
     fn error_missing_if_cond(&self, lo: Span, span: Span) -> P<ast::Block> {
         let sp = self.sess.source_map().next_point(lo);
         self.struct_span_err(sp, "missing condition for `if` expression")

compiler/rustc_span/src/symbol.rs

@@ -1716,8 +1716,9 @@ pub(crate) struct Interner(Lock<InternerInner>);
 // found that to regress performance up to 2% in some cases. This might be
 // revisited after further improvements to `indexmap`.
 //
-// This type is private to prevent accidentally constructing more than one `Interner` on the same
+// This type is private to prevent accidentally constructing more than one
-// thread, which makes it easy to mixup `Symbol`s between `Interner`s.
+// `Interner` on the same thread, which makes it easy to mixup `Symbol`s
+// between `Interner`s.
 #[derive(Default)]
 struct InternerInner {
     arena: DroplessArena,
@@ -1743,14 +1744,20 @@ impl Interner {
 
         let name = Symbol::new(inner.strings.len() as u32);
 
-        // `from_utf8_unchecked` is safe since we just allocated a `&str` which is known to be
+        // SAFETY: we convert from `&str` to `&[u8]`, clone it into the arena,
-        // UTF-8.
+        // and immediately convert the clone back to `&[u8], all because there
+        // is no `inner.arena.alloc_str()` method. This is clearly safe.
         let string: &str =
             unsafe { str::from_utf8_unchecked(inner.arena.alloc_slice(string.as_bytes())) };
-        // It is safe to extend the arena allocation to `'static` because we only access
+
-        // these while the arena is still alive.
+        // SAFETY: we can extend the arena allocation to `'static` because we
+        // only access these while the arena is still alive.
         let string: &'static str = unsafe { &*(string as *const str) };
         inner.strings.push(string);
+
+        // This second hash table lookup can be avoided by using `RawEntryMut`,
+        // but this code path isn't hot enough for it to be worth it. See
+        // #91445 for details.
         inner.names.insert(string, name);
         name
     }

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -521,7 +521,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 },
 
                 ty::PredicateKind::TypeOutlives(pred) => {
-                    if pred.0.is_known_global() {
+                    // A global type with no late-bound regions can only
+                    // contain the "'static" lifetime (any other lifetime
+                    // would either be late-bound or local), so it is guaranteed
+                    // to outlive any other lifetime
+                    if pred.0.is_global(self.infcx.tcx) && !pred.0.has_late_bound_regions() {
                         Ok(EvaluatedToOk)
                     } else {
                         Ok(EvaluatedToOkModuloRegions)

compiler/rustc_typeck/src/check/expr.rs

@@ -46,6 +46,7 @@ use rustc_span::hygiene::DesugaringKind;
 use rustc_span::lev_distance::find_best_match_for_name;
 use rustc_span::source_map::Span;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
+use rustc_span::{BytePos, Pos};
 use rustc_trait_selection::traits::{self, ObligationCauseCode};
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
@@ -2063,8 +2064,37 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 Some(span),
             );
         } else {
+            let mut found = false;
+
+            if let ty::RawPtr(ty_and_mut) = expr_t.kind() {
+                if let ty::Adt(adt_def, _) = ty_and_mut.ty.kind() {
+                    if adt_def.variants.len() == 1
+                        && adt_def
+                            .variants
+                            .iter()
+                            .next()
+                            .unwrap()
+                            .fields
+                            .iter()
+                            .any(|f| f.ident == field)
+                    {
+                        if let Some(dot_loc) = expr_snippet.rfind('.') {
+                            found = true;
+                            err.span_suggestion(
+                                expr.span.with_hi(expr.span.lo() + BytePos::from_usize(dot_loc)),
+                                "to access the field, dereference first",
+                                format!("(*{})", &expr_snippet[0..dot_loc]),
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
+                    }
+                }
+            }
+
+            if !found {
                 err.help("methods are immutable and cannot be assigned to");
             }
+        }
 
         err.emit();
     }

library/core/tests/slice.rs

@@ -2330,6 +2330,7 @@ macro_rules! empty_max_mut {
     };
 }
 
+#[cfg(not(miri))] // Comparing usize::MAX many elements takes forever in Miri (and in rustc without optimizations)
 take_tests! {
     slice: &[(); usize::MAX], method: take,
     (take_in_bounds_max_range_to, (..usize::MAX), Some(EMPTY_MAX), &[(); 0]),
@@ -2337,6 +2338,7 @@ take_tests! {
     (take_in_bounds_max_range_from, (usize::MAX..), Some(&[] as _), EMPTY_MAX),
 }
 
+#[cfg(not(miri))] // Comparing usize::MAX many elements takes forever in Miri (and in rustc without optimizations)
 take_tests! {
     slice: &mut [(); usize::MAX], method: take_mut,
     (take_mut_in_bounds_max_range_to, (..usize::MAX), Some(empty_max_mut!()), &mut [(); 0]),

library/std/src/io/error.rs

@@ -440,12 +440,18 @@ impl Error {
     /// `GetLastError` on Windows) and will return a corresponding instance of
     /// [`Error`] for the error code.
     ///
+    /// This should be called immediately after a call to a platform function,
+    /// otherwise the state of the error value is indeterminate. In particular,
+    /// other standard library functions may call platform functions that may
+    /// (or may not) reset the error value even if they succeed.
+    ///
     /// # Examples
     ///
     /// ```
     /// use std::io::Error;
     ///
-    /// println!("last OS error: {:?}", Error::last_os_error());
+    /// let os_error = Error::last_os_error();
+    /// println!("last OS error: {:?}", os_error);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use]

library/std/src/path.rs

@@ -12,6 +12,13 @@
 //! [`PathBuf`]; note that the paths may differ syntactically by the
 //! normalization described in the documentation for the [`components`] method.
 //!
+//! ## Case sensitivity
+//!
+//! Unless otherwise indicated path methods that do not access the filesystem,
+//! such as [`Path::starts_with`] and [`Path::ends_with`], are case sensitive no
+//! matter the platform or filesystem. An exception to this is made for Windows
+//! drive letters.
+//!
 //! ## Simple usage
 //!
 //! Path manipulation includes both parsing components from slices and building

src/librustdoc/clean/mod.rs

@@ -749,11 +749,42 @@ fn clean_fn_or_proc_macro(
                 } else {
                     hir::Constness::NotConst
                 };
+            clean_fn_decl_legacy_const_generics(&mut func, attrs);
             FunctionItem(func)
         }
     }
 }
 
+/// This is needed to make it more "readable" when documenting functions using
+/// `rustc_legacy_const_generics`. More information in
+/// <https://github.com/rust-lang/rust/issues/83167>.
+fn clean_fn_decl_legacy_const_generics(func: &mut Function, attrs: &[ast::Attribute]) {
+    for meta_item_list in attrs
+        .iter()
+        .filter(|a| a.has_name(sym::rustc_legacy_const_generics))
+        .filter_map(|a| a.meta_item_list())
+    {
+        for (pos, literal) in meta_item_list.iter().filter_map(|meta| meta.literal()).enumerate() {
+            match literal.kind {
+                ast::LitKind::Int(a, _) => {
+                    let gen = func.generics.params.remove(0);
+                    if let GenericParamDef { name, kind: GenericParamDefKind::Const { ty, .. } } =
+                        gen
+                    {
+                        func.decl
+                            .inputs
+                            .values
+                            .insert(a as _, Argument { name, type_: *ty, is_const: true });
+                    } else {
+                        panic!("unexpected non const in position {}", pos);
+                    }
+                }
+                _ => panic!("invalid arg index"),
+            }
+        }
+    }
+}
+
 impl<'a> Clean<Function> for (&'a hir::FnSig<'a>, &'a hir::Generics<'a>, hir::BodyId) {
     fn clean(&self, cx: &mut DocContext<'_>) -> Function {
         let (generics, decl) = enter_impl_trait(cx, |cx| {
@@ -779,7 +810,7 @@ impl<'a> Clean<Arguments> for (&'a [hir::Ty<'a>], &'a [Ident]) {
                     if name.is_empty() {
                         name = kw::Underscore;
                     }
-                    Argument { name, type_: ty.clean(cx) }
+                    Argument { name, type_: ty.clean(cx), is_const: false }
                 })
                 .collect(),
         }
@@ -798,6 +829,7 @@ impl<'a> Clean<Arguments> for (&'a [hir::Ty<'a>], hir::BodyId) {
                 .map(|(i, ty)| Argument {
                     name: name_from_pat(body.params[i].pat),
                     type_: ty.clean(cx),
+                    is_const: false,
                 })
                 .collect(),
         }
@@ -828,6 +860,7 @@ impl<'tcx> Clean<FnDecl> for (DefId, ty::PolyFnSig<'tcx>) {
                     .map(|t| Argument {
                         type_: t.clean(cx),
                         name: names.next().map_or(kw::Empty, |i| i.name),
+                        is_const: false,
                     })
                     .collect(),
             },

src/librustdoc/clean/types.rs

@@ -1353,6 +1353,9 @@ crate struct Arguments {
 crate struct Argument {
     crate type_: Type,
     crate name: Symbol,
+    /// This field is used to represent "const" arguments from the `rustc_legacy_const_generics`
+    /// feature. More information in <https://github.com/rust-lang/rust/issues/83167>.
+    crate is_const: bool,
 }
 
 #[derive(Clone, PartialEq, Debug)]

src/librustdoc/html/format.rs

@@ -1177,6 +1177,10 @@ impl clean::FnDecl {
                     args.push_str(" <br>");
                     args_plain.push(' ');
                 }
+                if input.is_const {
+                    args.push_str("const ");
+                    args_plain.push_str("const ");
+                }
                 if !input.name.is_empty() {
                     args.push_str(&format!("{}: ", input.name));
                     args_plain.push_str(&format!("{}: ", input.name));

src/test/mir-opt/const_prop/switch_int.main.SimplifyConstCondition-after-const-prop.diff

@@ -1,5 +1,5 @@
-- // MIR for `main` before SimplifyBranches-after-const-prop
+- // MIR for `main` before SimplifyConstCondition-after-const-prop
-+ // MIR for `main` after SimplifyBranches-after-const-prop
++ // MIR for `main` after SimplifyConstCondition-after-const-prop
   
   fn main() -> () {
       let mut _0: ();                      // return place in scope 0 at $DIR/switch_int.rs:6:11: 6:11

src/test/mir-opt/const_prop/switch_int.rs

@@ -2,7 +2,7 @@
 fn foo(_: i32) { }
 
 // EMIT_MIR switch_int.main.ConstProp.diff
-// EMIT_MIR switch_int.main.SimplifyBranches-after-const-prop.diff
+// EMIT_MIR switch_int.main.SimplifyConstCondition-after-const-prop.diff
 fn main() {
     match 1 {
         1 => foo(0),

src/test/mir-opt/early_otherwise_branch_68867.rs

@@ -11,7 +11,7 @@ pub enum ViewportPercentageLength {
 }
 
 // EMIT_MIR early_otherwise_branch_68867.try_sum.EarlyOtherwiseBranch.diff
-// EMIT_MIR early_otherwise_branch_68867.try_sum EarlyOtherwiseBranch.before SimplifyBranches-final.after
+// EMIT_MIR early_otherwise_branch_68867.try_sum EarlyOtherwiseBranch.before SimplifyConstCondition-final.after
 #[no_mangle]
 pub extern "C" fn try_sum(
     x: &ViewportPercentageLength,

src/test/mir-opt/early_otherwise_branch_68867.try_sum.EarlyOtherwiseBranch.before-SimplifyConstCondition-final.after.diff

@@ -1,5 +1,5 @@
 - // MIR for `try_sum` before EarlyOtherwiseBranch
-+ // MIR for `try_sum` after SimplifyBranches-final
++ // MIR for `try_sum` after SimplifyConstCondition-final
   
   fn try_sum(_1: &ViewportPercentageLength, _2: &ViewportPercentageLength) -> Result<ViewportPercentageLength, ()> {
       debug x => _1;                       // in scope 0 at $DIR/early_otherwise_branch_68867.rs:17:5: 17:6

src/test/mir-opt/simplify_if.main.SimplifyConstCondition-after-const-prop.diff

@@ -1,5 +1,5 @@
-- // MIR for `main` before SimplifyBranches-after-const-prop
+- // MIR for `main` before SimplifyConstCondition-after-const-prop
-+ // MIR for `main` after SimplifyBranches-after-const-prop
++ // MIR for `main` after SimplifyConstCondition-after-const-prop
   
   fn main() -> () {
       let mut _0: ();                      // return place in scope 0 at $DIR/simplify_if.rs:5:11: 5:11

src/test/mir-opt/simplify_if.rs

@@ -1,7 +1,7 @@
 #[inline(never)]
 fn noop() {}
 
-// EMIT_MIR simplify_if.main.SimplifyBranches-after-const-prop.diff
+// EMIT_MIR simplify_if.main.SimplifyConstCondition-after-const-prop.diff
 fn main() {
     if false {
         noop();

src/test/rustdoc/legacy-const-generic.rs

@@ -0,0 +1,16 @@
+#![crate_name = "foo"]
+#![feature(rustc_attrs)]
+
+// @has 'foo/fn.foo.html'
+// @has - '//*[@class="rust fn"]' 'fn foo(x: usize, const Y: usize, z: usize) -> [usize; 3]'
+#[rustc_legacy_const_generics(1)]
+pub fn foo<const Y: usize>(x: usize, z: usize) -> [usize; 3] {
+    [x, Y, z]
+}
+
+// @has 'foo/fn.bar.html'
+// @has - '//*[@class="rust fn"]' 'fn bar(x: usize, const Y: usize, const Z: usize) -> [usize; 3]'
+#[rustc_legacy_const_generics(1, 2)]
+pub fn bar<const Y: usize, const Z: usize>(x: usize) -> [usize; 3] {
+    [x, Y, z]
+}

src/test/ui/consts/drop_zst.rs

@@ -0,0 +1,17 @@
+// check-fail
+
+#![feature(const_precise_live_drops)]
+
+struct S;
+
+impl Drop for S {
+    fn drop(&mut self) {
+        println!("Hello!");
+    }
+}
+
+const fn foo() {
+    let s = S; //~ destructor
+}
+
+fn main() {}

src/test/ui/consts/drop_zst.stderr

@@ -0,0 +1,9 @@
+error[E0493]: destructors cannot be evaluated at compile-time
+  --> $DIR/drop_zst.rs:14:9
+   |
+LL |     let s = S;
+   |         ^ constant functions cannot evaluate destructors
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0493`.

src/test/ui/expr/if/if-without-block.stderr

@@ -7,7 +7,11 @@ LL |     if 5 == {
 LL | }
    | ^ expected `{`
    |
-   = help: maybe you forgot the right operand of the condition?
+help: maybe you forgot the right operand of the condition?
+  --> $DIR/if-without-block.rs:3:10
+   |
+LL |     if 5 == {
+   |          ^^
 
 error: aborting due to previous error
 

src/test/ui/fn/implied-bounds-unnorm-associated-type-2.rs

@@ -0,0 +1,22 @@
+// check-pass
+
+trait Trait {
+    type Type;
+}
+
+impl<T> Trait for T {
+    type Type = ();
+}
+
+fn f<'a, 'b>(_: <&'a &'b () as Trait>::Type)
+where
+    'a: 'a,
+    'b: 'b,
+{
+}
+
+fn g<'a, 'b>() {
+    f::<'a, 'b>(());
+}
+
+fn main() {}

src/test/ui/lifetimes/issue-76168-hr-outlives.rs

@@ -0,0 +1,19 @@
+// edition:2018
+// check-pass
+
+#![feature(unboxed_closures)]
+use std::future::Future;
+
+async fn wrapper<F>(f: F)
+where for<'a> F: FnOnce<(&'a mut i32,)>,
+    for<'a> <F as FnOnce<(&'a mut i32,)>>::Output: Future<Output=()> + 'a
+{
+    let mut i = 41;
+    f(&mut i).await;
+}
+
+async fn add_one(i: &mut i32) {
+    *i = *i + 1;
+}
+
+fn main() {}

src/test/ui/parser/issue-91421.rs

@@ -0,0 +1,10 @@
+// Regression test for issue #91421.
+
+fn main() {
+    let value = if true && {
+    //~^ ERROR: this `if` expression has a condition, but no block
+    //~| HELP: maybe you forgot the right operand of the condition?
+        3
+        //~^ ERROR: mismatched types [E0308]
+    } else { 4 };
+}

src/test/ui/parser/issue-91421.stderr

@@ -0,0 +1,21 @@
+error: this `if` expression has a condition, but no block
+  --> $DIR/issue-91421.rs:4:17
+   |
+LL |     let value = if true && {
+   |                 ^^
+   |
+help: maybe you forgot the right operand of the condition?
+  --> $DIR/issue-91421.rs:4:25
+   |
+LL |     let value = if true && {
+   |                         ^^
+
+error[E0308]: mismatched types
+  --> $DIR/issue-91421.rs:7:9
+   |
+LL |         3
+   |         ^ expected `bool`, found integer
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/traits/project-modulo-regions.rs

@@ -0,0 +1,55 @@
+// revisions: with_clause without_clause
+// Tests that `EvaluatedToOkModuloRegions` from a projection sub-obligation
+// is correctly propagated
+
+#![feature(rustc_attrs)]
+
+trait MyTrait {
+    type Assoc;
+}
+
+struct MyStruct;
+
+impl MyTrait for MyStruct {
+    // Evaluating this projection will result in `EvaluatedToOkModuloRegions`
+    // (when `with_clause` is enabled)
+    type Assoc = <Bar as MyTrait>::Assoc;
+}
+
+struct Bar;
+
+// The `where` clause on this impl will cause us to produce `EvaluatedToOkModuloRegions`
+// when evaluating a projection involving this impl
+#[cfg(with_clause)]
+impl MyTrait for Bar where for<'b> &'b (): 'b {
+    type Assoc = bool;
+}
+
+// This impl tests that the `EvaluatedToOkModuoRegions` result that we get
+// is really due to the `where` clause on the `with_clause` impl
+#[cfg(without_clause)]
+impl MyTrait for Bar {
+    type Assoc = bool;
+}
+
+// The implementation of `#[rustc_evaluate_where_clauses]` doesn't perform
+// normalization, so we need to place the projection predicate behind a normal
+// trait predicate
+struct Helper {}
+trait HelperTrait {}
+impl HelperTrait for Helper where <MyStruct as MyTrait>::Assoc: Sized {}
+
+// Evaluating this 'where' clause will (recursively) end up evaluating
+// `for<'b> &'b (): 'b`, which will produce `EvaluatedToOkModuloRegions`
+#[rustc_evaluate_where_clauses]
+fn test(val: MyStruct) where Helper: HelperTrait  {
+    panic!()
+}
+
+fn foo(val: MyStruct) {
+    test(val);
+    //[with_clause]~^     ERROR evaluate(Binder(TraitPredicate(<Helper as HelperTrait>, polarity:Positive), [])) = Ok(EvaluatedToOkModuloRegions)
+    //[without_clause]~^^ ERROR evaluate(Binder(TraitPredicate(<Helper as HelperTrait>, polarity:Positive), [])) = Ok(EvaluatedToOk)
+}
+
+fn main() {}

src/test/ui/traits/project-modulo-regions.with_clause.stderr

@@ -0,0 +1,11 @@
+error: evaluate(Binder(TraitPredicate(<Helper as HelperTrait>, polarity:Positive), [])) = Ok(EvaluatedToOkModuloRegions)
+  --> $DIR/project-modulo-regions.rs:50:5
+   |
+LL | fn test(val: MyStruct) where Helper: HelperTrait  {
+   |                                      ----------- predicate
+...
+LL |     test(val);
+   |     ^^^^
+
+error: aborting due to previous error
+

src/test/ui/traits/project-modulo-regions.without_clause.stderr

@@ -0,0 +1,11 @@
+error: evaluate(Binder(TraitPredicate(<Helper as HelperTrait>, polarity:Positive), [])) = Ok(EvaluatedToOk)
+  --> $DIR/project-modulo-regions.rs:50:5
+   |
+LL | fn test(val: MyStruct) where Helper: HelperTrait  {
+   |                                      ----------- predicate
+...
+LL |     test(val);
+   |     ^^^^
+
+error: aborting due to previous error
+

src/test/ui/typeck/issue-91210-ptr-method.fixed

@@ -0,0 +1,15 @@
+// Regression test for issue #91210.
+
+// run-rustfix
+
+#![allow(unused)]
+
+struct Foo { read: i32 }
+
+unsafe fn blah(x: *mut Foo) {
+    (*x).read = 4;
+    //~^ ERROR: attempted to take value of method
+    //~| HELP: to access the field, dereference first
+}
+
+fn main() {}

src/test/ui/typeck/issue-91210-ptr-method.rs

@@ -0,0 +1,15 @@
+// Regression test for issue #91210.
+
+// run-rustfix
+
+#![allow(unused)]
+
+struct Foo { read: i32 }
+
+unsafe fn blah(x: *mut Foo) {
+    x.read = 4;
+    //~^ ERROR: attempted to take value of method
+    //~| HELP: to access the field, dereference first
+}
+
+fn main() {}

src/test/ui/typeck/issue-91210-ptr-method.stderr

@@ -0,0 +1,11 @@
+error[E0615]: attempted to take value of method `read` on type `*mut Foo`
+  --> $DIR/issue-91210-ptr-method.rs:10:7
+   |
+LL |     x.read = 4;
+   |     - ^^^^ method, not a field
+   |     |
+   |     help: to access the field, dereference first: `(*x)`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0615`.