Auto merge of #100456 - Dylan-DPC:rollup-fn17z9f, r=Dylan-DPC

Rollup of 9 pull requests

Successful merges:

 - #100022 (Optimize thread ID generation)
 - #100030 (cleanup code w/ pointers in std a little)
 - #100229 (add -Zextra-const-ub-checks to enable more UB checking in const-eval)
 - #100247 (Generalize trait object generic param check to aliases.)
 - #100255 (Adding more verbose documentation for `std::fmt::Write`)
 - #100366 (errors: don't fail on broken primary translations)
 - #100396 (Suggest const and static for global variable)
 - #100409 (rustdoc: don't generate DOM element for operator)
 - #100443 (Add two let else regression tests)

Failed merges:

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

50 files changed, 584 insertions, 258 deletions

diff --git a/compiler/rustc_const_eval/src/const_eval/machine.rs b/compiler/rustc_const_eval/src/const_eval/machine.rs
index fc2e6652a3d..684877cae76 100644
--- a/compiler/rustc_const_eval/src/const_eval/machine.rs
+++ b/compiler/rustc_const_eval/src/const_eval/machine.rs
@@ -236,6 +236,16 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
 
     const PANIC_ON_ALLOC_FAIL: bool = false; // will be raised as a proper error
 
+    #[inline(always)]
+    fn enforce_alignment(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        ecx.tcx.sess.opts.unstable_opts.extra_const_ub_checks
+    }
+
+    #[inline(always)]
+    fn enforce_validity(ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        ecx.tcx.sess.opts.unstable_opts.extra_const_ub_checks
+    }
+
     fn load_mir(
         ecx: &InterpCx<'mir, 'tcx, Self>,
         instance: ty::InstanceDef<'tcx>,
diff --git a/compiler/rustc_const_eval/src/interpret/machine.rs b/compiler/rustc_const_eval/src/interpret/machine.rs
index 71ccd1799fa..9b9919fcc2a 100644
--- a/compiler/rustc_const_eval/src/interpret/machine.rs
+++ b/compiler/rustc_const_eval/src/interpret/machine.rs
@@ -437,24 +437,12 @@ pub macro compile_time_machine(<$mir: lifetime, $tcx: lifetime>) {
     type FrameExtra = ();
 
     #[inline(always)]
-    fn enforce_alignment(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
-        // We do not check for alignment to avoid having to carry an `Align`
-        // in `ConstValue::ByRef`.
-        false
-    }
-
-    #[inline(always)]
     fn force_int_for_alignment_check(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
         // We do not support `force_int`.
         false
     }
 
     #[inline(always)]
-    fn enforce_validity(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
-        false // for now, we don't enforce validity
-    }
-
-    #[inline(always)]
     fn enforce_number_init(_ecx: &InterpCx<$mir, $tcx, Self>) -> bool {
         true
     }
diff --git a/compiler/rustc_const_eval/src/interpret/validity.rs b/compiler/rustc_const_eval/src/interpret/validity.rs
index 6f6717721fb..f1b1855c3ec 100644
--- a/compiler/rustc_const_eval/src/interpret/validity.rs
+++ b/compiler/rustc_const_eval/src/interpret/validity.rs
@@ -1005,6 +1005,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     /// It will error if the bits at the destination do not match the ones described by the layout.
     #[inline(always)]
     pub fn validate_operand(&self, op: &OpTy<'tcx, M::Provenance>) -> InterpResult<'tcx> {
+        // Note that we *could* actually be in CTFE here with `-Zextra-const-ub-checks`, but it's
+        // still correct to not use `ctfe_mode`: that mode is for validation of the final constant
+        // value, it rules out things like `UnsafeCell` in awkward places. It also can make checking
+        // recurse through references which, for now, we don't want here, either.
         self.validate_operand_internal(op, vec![], None, None)
     }
 }
diff --git a/compiler/rustc_errors/src/emitter.rs b/compiler/rustc_errors/src/emitter.rs
index 61d953cd6f1..753e2f07c04 100644
--- a/compiler/rustc_errors/src/emitter.rs
+++ b/compiler/rustc_errors/src/emitter.rs
@@ -273,40 +273,58 @@ pub trait Emitter {
             DiagnosticMessage::FluentIdentifier(identifier, attr) => (identifier, attr),
         };
 
-        let bundle = match self.fluent_bundle() {
-            Some(bundle) if bundle.has_message(&identifier) => bundle,
-            _ => self.fallback_fluent_bundle(),
-        };
+        let translate_with_bundle = |bundle: &'a FluentBundle| -> Option<(Cow<'_, str>, Vec<_>)> {
+            let message = bundle.get_message(&identifier)?;
+            let value = match attr {
+                Some(attr) => message.get_attribute(attr)?.value(),
+                None => message.value()?,
+            };
+            debug!(?message, ?value);
 
-        let message = bundle.get_message(&identifier).expect("missing diagnostic in fluent bundle");
-        let value = match attr {
-            Some(attr) => {
-                if let Some(attr) = message.get_attribute(attr) {
-                    attr.value()
-                } else {
-                    panic!("missing attribute `{attr}` in fluent message `{identifier}`")
-                }
-            }
-            None => {
-                if let Some(value) = message.value() {
-                    value
-                } else {
-                    panic!("missing value in fluent message `{identifier}`")
-                }
-            }
+            let mut errs = vec![];
+            let translated = bundle.format_pattern(value, Some(&args), &mut errs);
+            debug!(?translated, ?errs);
+            Some((translated, errs))
         };
 
-        let mut err = vec![];
-        let translated = bundle.format_pattern(value, Some(&args), &mut err);
-        trace!(?translated, ?err);
-        debug_assert!(
-            err.is_empty(),
-            "identifier: {:?}, args: {:?}, errors: {:?}",
-            identifier,
-            args,
-            err
-        );
-        translated
+        self.fluent_bundle()
+            .and_then(|bundle| translate_with_bundle(bundle))
+            // If `translate_with_bundle` returns `None` with the primary bundle, this is likely
+            // just that the primary bundle doesn't contain the message being translated, so
+            // proceed to the fallback bundle.
+            //
+            // However, when errors are produced from translation, then that means the translation
+            // is broken (e.g. `{$foo}` exists in a translation but `foo` isn't provided).
+            //
+            // In debug builds, assert so that compiler devs can spot the broken translation and
+            // fix it..
+            .inspect(|(_, errs)| {
+                debug_assert!(
+                    errs.is_empty(),
+                    "identifier: {:?}, attr: {:?}, args: {:?}, errors: {:?}",
+                    identifier,
+                    attr,
+                    args,
+                    errs
+                );
+            })
+            // ..otherwise, for end users, an error about this wouldn't be useful or actionable, so
+            // just hide it and try with the fallback bundle.
+            .filter(|(_, errs)| errs.is_empty())
+            .or_else(|| translate_with_bundle(self.fallback_fluent_bundle()))
+            .map(|(translated, errs)| {
+                // Always bail out for errors with the fallback bundle.
+                assert!(
+                    errs.is_empty(),
+                    "identifier: {:?}, attr: {:?}, args: {:?}, errors: {:?}",
+                    identifier,
+                    attr,
+                    args,
+                    errs
+                );
+                translated
+            })
+            .expect("failed to find message in primary or fallback fluent bundles")
     }
 
     /// Formats the substitutions of the primary_span
diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index 15c1858023d..f83e972efd5 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -6,9 +6,10 @@
 #![feature(drain_filter)]
 #![feature(if_let_guard)]
 #![cfg_attr(bootstrap, feature(let_chains))]
+#![feature(adt_const_params)]
 #![feature(let_else)]
 #![feature(never_type)]
-#![feature(adt_const_params)]
+#![feature(result_option_inspect)]
 #![feature(rustc_attrs)]
 #![allow(incomplete_features)]
 #![allow(rustc::potential_query_instability)]
diff --git a/compiler/rustc_mir_transform/src/const_prop.rs b/compiler/rustc_mir_transform/src/const_prop.rs
index 98016659a05..1c087b93b49 100644
--- a/compiler/rustc_mir_transform/src/const_prop.rs
+++ b/compiler/rustc_mir_transform/src/const_prop.rs
@@ -183,6 +183,18 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx>
 
     type MemoryKind = !;
 
+    #[inline(always)]
+    fn enforce_alignment(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        // We do not check for alignment to avoid having to carry an `Align`
+        // in `ConstValue::ByRef`.
+        false
+    }
+
+    #[inline(always)]
+    fn enforce_validity(_ecx: &InterpCx<'mir, 'tcx, Self>) -> bool {
+        false // for now, we don't enforce validity
+    }
+
     fn load_mir(
         _ecx: &InterpCx<'mir, 'tcx, Self>,
         _instance: ty::InstanceDef<'tcx>,
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index 72c23776d33..197c0384898 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -68,7 +68,12 @@ impl<'a> Parser<'a> {
             if !self.maybe_consume_incorrect_semicolon(&items) {
                 let msg = &format!("expected item, found {token_str}");
                 let mut err = self.struct_span_err(self.token.span, msg);
-                err.span_label(self.token.span, "expected item");
+                let label = if self.is_kw_followed_by_ident(kw::Let) {
+                    "consider using `const` or `static` instead of `let` for global variables"
+                } else {
+                    "expected item"
+                };
+                err.span_label(self.token.span, label);
                 return Err(err);
             }
         }
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index 1827f1c208d..0032dd7d113 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -1310,6 +1310,8 @@ options! {
         "emit the bc module with thin LTO info (default: yes)"),
     export_executable_symbols: bool = (false, parse_bool, [TRACKED],
         "export symbols from executables, as if they were dynamic libraries"),
+    extra_const_ub_checks: bool = (false, parse_bool, [TRACKED],
+        "turns on more checks to detect const UB, which can be slow (default: no)"),
     #[cfg_attr(not(bootstrap), rustc_lint_opt_deny_field_access("use `Session::fewer_names` instead of this field"))]
     fewer_names: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "reduce memory use by retaining fewer names within compilation artifacts (LLVM-IR) \
diff --git a/compiler/rustc_typeck/src/astconv/mod.rs b/compiler/rustc_typeck/src/astconv/mod.rs
index 758f1ef9766..1e6cb53f3ee 100644
--- a/compiler/rustc_typeck/src/astconv/mod.rs
+++ b/compiler/rustc_typeck/src/astconv/mod.rs
@@ -44,7 +44,7 @@ use rustc_trait_selection::traits::error_reporting::{
 };
 use rustc_trait_selection::traits::wf::object_region_bounds;
 
-use smallvec::SmallVec;
+use smallvec::{smallvec, SmallVec};
 use std::collections::BTreeSet;
 use std::slice;
 
@@ -368,36 +368,13 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             return (tcx.intern_substs(&[]), arg_count);
         }
 
-        let is_object = self_ty.map_or(false, |ty| ty == self.tcx().types.trait_object_dummy_self);
-
         struct SubstsForAstPathCtxt<'a, 'tcx> {
             astconv: &'a (dyn AstConv<'tcx> + 'a),
             def_id: DefId,
             generic_args: &'a GenericArgs<'a>,
             span: Span,
-            missing_type_params: Vec<Symbol>,
             inferred_params: Vec<Span>,
             infer_args: bool,
-            is_object: bool,
-        }
-
-        impl<'tcx, 'a> SubstsForAstPathCtxt<'tcx, 'a> {
-            fn default_needs_object_self(&mut self, param: &ty::GenericParamDef) -> bool {
-                let tcx = self.astconv.tcx();
-                if let GenericParamDefKind::Type { has_default, .. } = param.kind {
-                    if self.is_object && has_default {
-                        let default_ty = tcx.at(self.span).type_of(param.def_id);
-                        let self_param = tcx.types.self_param;
-                        if default_ty.walk().any(|arg| arg == self_param.into()) {
-                            // There is no suitable inference default for a type parameter
-                            // that references self, in an object type.
-                            return true;
-                        }
-                    }
-                }
-
-                false
-            }
         }
 
         impl<'a, 'tcx> CreateSubstsForGenericArgsCtxt<'a, 'tcx> for SubstsForAstPathCtxt<'a, 'tcx> {
@@ -500,41 +477,23 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     GenericParamDefKind::Type { has_default, .. } => {
                         if !infer_args && has_default {
                             // No type parameter provided, but a default exists.
-
-                            // If we are converting an object type, then the
-                            // `Self` parameter is unknown. However, some of the
-                            // other type parameters may reference `Self` in their
-                            // defaults. This will lead to an ICE if we are not
-                            // careful!
-                            if self.default_needs_object_self(param) {
-                                self.missing_type_params.push(param.name);
-                                tcx.ty_error().into()
-                            } else {
-                                // This is a default type parameter.
-                                let substs = substs.unwrap();
-                                if substs.iter().any(|arg| match arg.unpack() {
-                                    GenericArgKind::Type(ty) => ty.references_error(),
-                                    _ => false,
-                                }) {
-                                    // Avoid ICE #86756 when type error recovery goes awry.
-                                    return tcx.ty_error().into();
-                                }
-                                self.astconv
-                                    .normalize_ty(
-                                        self.span,
-                                        EarlyBinder(tcx.at(self.span).type_of(param.def_id))
-                                            .subst(tcx, substs),
-                                    )
-                                    .into()
+                            let substs = substs.unwrap();
+                            if substs.iter().any(|arg| match arg.unpack() {
+                                GenericArgKind::Type(ty) => ty.references_error(),
+                                _ => false,
+                            }) {
+                                // Avoid ICE #86756 when type error recovery goes awry.
+                                return tcx.ty_error().into();
                             }
+                            self.astconv
+                                .normalize_ty(
+                                    self.span,
+                                    EarlyBinder(tcx.at(self.span).type_of(param.def_id))
+                                        .subst(tcx, substs),
+                                )
+                                .into()
                         } else if infer_args {
-                            // No type parameters were provided, we can infer all.
-                            let param = if !self.default_needs_object_self(param) {
-                                Some(param)
-                            } else {
-                                None
-                            };
-                            self.astconv.ty_infer(param, self.span).into()
+                            self.astconv.ty_infer(Some(param), self.span).into()
                         } else {
                             // We've already errored above about the mismatch.
                             tcx.ty_error().into()
@@ -564,10 +523,8 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             def_id,
             span,
             generic_args,
-            missing_type_params: vec![],
             inferred_params: vec![],
             infer_args,
-            is_object,
         };
         let substs = Self::create_substs_for_generic_args(
             tcx,
@@ -579,13 +536,6 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             &mut substs_ctx,
         );
 
-        self.complain_about_missing_type_params(
-            substs_ctx.missing_type_params,
-            def_id,
-            span,
-            generic_args.args.is_empty(),
-        );
-
         debug!(
             "create_substs_for_ast_path(generic_params={:?}, self_ty={:?}) -> {:?}",
             generics, self_ty, substs
@@ -1490,23 +1440,71 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         // Erase the `dummy_self` (`trait_object_dummy_self`) used above.
         let existential_trait_refs = regular_traits.iter().map(|i| {
             i.trait_ref().map_bound(|trait_ref: ty::TraitRef<'tcx>| {
-                if trait_ref.self_ty() != dummy_self {
-                    // FIXME: There appears to be a missing filter on top of `expand_trait_aliases`,
-                    // which picks up non-supertraits where clauses - but also, the object safety
-                    // completely ignores trait aliases, which could be object safety hazards. We
-                    // `delay_span_bug` here to avoid an ICE in stable even when the feature is
-                    // disabled. (#66420)
-                    tcx.sess.delay_span_bug(
-                        DUMMY_SP,
-                        &format!(
-                            "trait_ref_to_existential called on {:?} with non-dummy Self",
-                            trait_ref,
-                        ),
+                assert_eq!(trait_ref.self_ty(), dummy_self);
+
+                // Verify that `dummy_self` did not leak inside default type parameters.  This
+                // could not be done at path creation, since we need to see through trait aliases.
+                let mut missing_type_params = vec![];
+                let mut references_self = false;
+                let generics = tcx.generics_of(trait_ref.def_id);
+                let substs: Vec<_> = trait_ref
+                    .substs
+                    .iter()
+                    .enumerate()
+                    .skip(1) // Remove `Self` for `ExistentialPredicate`.
+                    .map(|(index, arg)| {
+                        if let ty::GenericArgKind::Type(ty) = arg.unpack() {
+                            debug!(?ty);
+                            if ty == dummy_self {
+                                let param = &generics.params[index];
+                                missing_type_params.push(param.name);
+                                tcx.ty_error().into()
+                            } else if ty.walk().any(|arg| arg == dummy_self.into()) {
+                                references_self = true;
+                                tcx.ty_error().into()
+                            } else {
+                                arg
+                            }
+                        } else {
+                            arg
+                        }
+                    })
+                    .collect();
+                let substs = tcx.intern_substs(&substs[..]);
+
+                let span = i.bottom().1;
+                let empty_generic_args = trait_bounds.iter().any(|hir_bound| {
+                    hir_bound.trait_ref.path.res == Res::Def(DefKind::Trait, trait_ref.def_id)
+                        && hir_bound.span.contains(span)
+                });
+                self.complain_about_missing_type_params(
+                    missing_type_params,
+                    trait_ref.def_id,
+                    span,
+                    empty_generic_args,
+                );
+
+                if references_self {
+                    let def_id = i.bottom().0.def_id();
+                    let mut err = struct_span_err!(
+                        tcx.sess,
+                        i.bottom().1,
+                        E0038,
+                        "the {} `{}` cannot be made into an object",
+                        tcx.def_kind(def_id).descr(def_id),
+                        tcx.item_name(def_id),
+                    );
+                    err.note(
+                        rustc_middle::traits::ObjectSafetyViolation::SupertraitSelf(smallvec![])
+                            .error_msg(),
                     );
+                    err.emit();
                 }
-                ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
+
+                ty::ExistentialTraitRef { def_id: trait_ref.def_id, substs }
             })
         });
+
         let existential_projections = bounds.projection_bounds.iter().map(|(bound, _)| {
             bound.map_bound(|b| {
                 if b.projection_ty.self_ty() != dummy_self {
diff --git a/library/core/src/alloc/global.rs b/library/core/src/alloc/global.rs
index 887246c6001..6756eecd0e0 100644
--- a/library/core/src/alloc/global.rs
+++ b/library/core/src/alloc/global.rs
@@ -74,7 +74,7 @@ use crate::ptr;
 ///         {
 ///             return null_mut();
 ///         };
-///         (self.arena.get() as *mut u8).add(allocated)
+///         self.arena.get().cast::<u8>().add(allocated)
 ///     }
 ///     unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
 /// }
diff --git a/library/core/src/fmt/mod.rs b/library/core/src/fmt/mod.rs
index 372141e0933..7ec565edb34 100644
--- a/library/core/src/fmt/mod.rs
+++ b/library/core/src/fmt/mod.rs
@@ -119,6 +119,10 @@ pub trait Write {
     ///
     /// This function will return an instance of [`Error`] on error.
     ///
+    /// The purpose of std::fmt::Error is to abort the formatting operation when the underlying
+    /// destination encounters some error preventing it from accepting more text; it should
+    /// generally be propagated rather than handled, at least when implementing formatting traits.
+    ///
     /// # Examples
     ///
     /// ```
diff --git a/library/core/src/ptr/const_ptr.rs b/library/core/src/ptr/const_ptr.rs
index c0f609a01b5..c25b159c533 100644
--- a/library/core/src/ptr/const_ptr.rs
+++ b/library/core/src/ptr/const_ptr.rs
@@ -1267,20 +1267,21 @@ impl<T: ?Sized> *const T {
     /// Accessing adjacent `u8` as `u16`
     ///
     /// ```
-    /// # fn foo(n: usize) {
-    /// # use std::mem::align_of;
+    /// use std::mem::align_of;
+    ///
     /// # unsafe {
-    /// let x = [5u8, 6u8, 7u8, 8u8, 9u8];
-    /// let ptr = x.as_ptr().add(n) as *const u8;
+    /// let x = [5_u8, 6, 7, 8, 9];
+    /// let ptr = x.as_ptr();
     /// let offset = ptr.align_offset(align_of::<u16>());
-    /// if offset < x.len() - n - 1 {
-    ///     let u16_ptr = ptr.add(offset) as *const u16;
-    ///     assert_ne!(*u16_ptr, 500);
+    ///
+    /// if offset < x.len() - 1 {
+    ///     let u16_ptr = ptr.add(offset).cast::<u16>();
+    ///     assert!(*u16_ptr == u16::from_ne_bytes([5, 6]) || *u16_ptr == u16::from_ne_bytes([6, 7]));
     /// } else {
     ///     // while the pointer can be aligned via `offset`, it would point
     ///     // outside the allocation
     /// }
-    /// # } }
+    /// # }
     /// ```
     #[stable(feature = "align_offset", since = "1.36.0")]
     #[rustc_const_unstable(feature = "const_align_offset", issue = "90962")]
diff --git a/library/core/src/ptr/mut_ptr.rs b/library/core/src/ptr/mut_ptr.rs
index 7059d0008c4..fff06b458c7 100644
--- a/library/core/src/ptr/mut_ptr.rs
+++ b/library/core/src/ptr/mut_ptr.rs
@@ -1545,20 +1545,23 @@ impl<T: ?Sized> *mut T {
     /// Accessing adjacent `u8` as `u16`
     ///
     /// ```
-    /// # fn foo(n: usize) {
-    /// # use std::mem::align_of;
+    /// use std::mem::align_of;
+    ///
     /// # unsafe {
-    /// let x = [5u8, 6u8, 7u8, 8u8, 9u8];
-    /// let ptr = x.as_ptr().add(n) as *const u8;
+    /// let mut x = [5_u8, 6, 7, 8, 9];
+    /// let ptr = x.as_mut_ptr();
     /// let offset = ptr.align_offset(align_of::<u16>());
-    /// if offset < x.len() - n - 1 {
-    ///     let u16_ptr = ptr.add(offset) as *const u16;
-    ///     assert_ne!(*u16_ptr, 500);
+    ///
+    /// if offset < x.len() - 1 {
+    ///     let u16_ptr = ptr.add(offset).cast::<u16>();
+    ///     *u16_ptr = 0;
+    ///
+    ///     assert!(x == [0, 0, 7, 8, 9] || x == [5, 0, 0, 8, 9]);
     /// } else {
     ///     // while the pointer can be aligned via `offset`, it would point
     ///     // outside the allocation
     /// }
-    /// # } }
+    /// # }
     /// ```
     #[stable(feature = "align_offset", since = "1.36.0")]
     #[rustc_const_unstable(feature = "const_align_offset", issue = "90962")]
diff --git a/library/core/src/slice/iter.rs b/library/core/src/slice/iter.rs
index f1e65930967..f43b780ec9a 100644
--- a/library/core/src/slice/iter.rs
+++ b/library/core/src/slice/iter.rs
@@ -92,7 +92,7 @@ impl<'a, T> Iter<'a, T> {
             assume(!ptr.is_null());
 
             let end = if mem::size_of::<T>() == 0 {
-                (ptr as *const u8).wrapping_add(slice.len()) as *const T
+                ptr.wrapping_byte_add(slice.len())
             } else {
                 ptr.add(slice.len())
             };
@@ -228,7 +228,7 @@ impl<'a, T> IterMut<'a, T> {
             assume(!ptr.is_null());
 
             let end = if mem::size_of::<T>() == 0 {
-                (ptr as *mut u8).wrapping_add(slice.len()) as *mut T
+                ptr.wrapping_byte_add(slice.len())
             } else {
                 ptr.add(slice.len())
             };
diff --git a/library/core/tests/const_ptr.rs b/library/core/tests/const_ptr.rs
index 152fed803ec..d874f08317f 100644
--- a/library/core/tests/const_ptr.rs
+++ b/library/core/tests/const_ptr.rs
@@ -3,7 +3,7 @@ const DATA: [u16; 2] = [u16::from_ne_bytes([0x01, 0x23]), u16::from_ne_bytes([0x
 
 const fn unaligned_ptr() -> *const u16 {
     // Since DATA.as_ptr() is aligned to two bytes, adding 1 byte to that produces an unaligned *const u16
-    unsafe { (DATA.as_ptr() as *const u8).add(1) as *const u16 }
+    unsafe { DATA.as_ptr().byte_add(1) }
 }
 
 #[test]
@@ -67,7 +67,7 @@ fn write() {
     const fn write_unaligned() -> [u16; 2] {
         let mut two_aligned = [0u16; 2];
         unsafe {
-            let unaligned_ptr = (two_aligned.as_mut_ptr() as *mut u8).add(1) as *mut u16;
+            let unaligned_ptr = two_aligned.as_mut_ptr().byte_add(1);
             ptr::write_unaligned(unaligned_ptr, u16::from_ne_bytes([0x23, 0x45]));
         }
         two_aligned
@@ -91,7 +91,7 @@ fn mut_ptr_write() {
     const fn write_unaligned() -> [u16; 2] {
         let mut two_aligned = [0u16; 2];
         unsafe {
-            let unaligned_ptr = (two_aligned.as_mut_ptr() as *mut u8).add(1) as *mut u16;
+            let unaligned_ptr = two_aligned.as_mut_ptr().byte_add(1);
             unaligned_ptr.write_unaligned(u16::from_ne_bytes([0x23, 0x45]));
         }
         two_aligned
diff --git a/library/core/tests/lib.rs b/library/core/tests/lib.rs
index db94368f6e0..df9b1073a09 100644
--- a/library/core/tests/lib.rs
+++ b/library/core/tests/lib.rs
@@ -14,6 +14,7 @@
 #![feature(const_maybe_uninit_assume_init_read)]
 #![feature(const_nonnull_new)]
 #![feature(const_num_from_num)]
+#![feature(const_pointer_byte_offsets)]
 #![feature(const_ptr_as_ref)]
 #![feature(const_ptr_read)]
 #![feature(const_ptr_write)]
@@ -74,6 +75,7 @@
 #![feature(never_type)]
 #![feature(unwrap_infallible)]
 #![feature(result_into_ok_or_err)]
+#![feature(pointer_byte_offsets)]
 #![feature(portable_simd)]
 #![feature(ptr_metadata)]
 #![feature(once_cell)]
diff --git a/library/std/src/sys/sgx/abi/usercalls/alloc.rs b/library/std/src/sys/sgx/abi/usercalls/alloc.rs
index ea24fedd0eb..66fa1efbf10 100644
--- a/library/std/src/sys/sgx/abi/usercalls/alloc.rs
+++ b/library/std/src/sys/sgx/abi/usercalls/alloc.rs
@@ -115,7 +115,7 @@ pub unsafe trait UserSafe {
     /// * the pointer is null.
     /// * the pointed-to range is not in user memory.
     unsafe fn check_ptr(ptr: *const Self) {
-        let is_aligned = |p| -> bool { 0 == (p as usize) & (Self::align_of() - 1) };
+        let is_aligned = |p: *const u8| -> bool { 0 == p.addr() & (Self::align_of() - 1) };
 
         assert!(is_aligned(ptr as *const u8));
         assert!(is_user_range(ptr as _, mem::size_of_val(unsafe { &*ptr })));
diff --git a/library/std/src/sys/unsupported/alloc.rs b/library/std/src/sys/unsupported/alloc.rs
index 8d5d0a2f5cc..d715ae45401 100644
--- a/library/std/src/sys/unsupported/alloc.rs
+++ b/library/std/src/sys/unsupported/alloc.rs
@@ -1,15 +1,16 @@
 use crate::alloc::{GlobalAlloc, Layout, System};
+use crate::ptr::null_mut;
 
 #[stable(feature = "alloc_system_type", since = "1.28.0")]
 unsafe impl GlobalAlloc for System {
     #[inline]
     unsafe fn alloc(&self, _layout: Layout) -> *mut u8 {
-        0 as *mut u8
+        null_mut()
     }
 
     #[inline]
     unsafe fn alloc_zeroed(&self, _layout: Layout) -> *mut u8 {
-        0 as *mut u8
+        null_mut()
     }
 
     #[inline]
@@ -17,6 +18,6 @@ unsafe impl GlobalAlloc for System {
 
     #[inline]
     unsafe fn realloc(&self, _ptr: *mut u8, _layout: Layout, _new_size: usize) -> *mut u8 {
-        0 as *mut u8
+        null_mut()
     }
 }
diff --git a/library/std/src/thread/mod.rs b/library/std/src/thread/mod.rs
index 44c8a50fd86..479669647c1 100644
--- a/library/std/src/thread/mod.rs
+++ b/library/std/src/thread/mod.rs
@@ -170,7 +170,6 @@ use crate::ptr::addr_of_mut;
 use crate::str;
 use crate::sync::Arc;
 use crate::sys::thread as imp;
-use crate::sys_common::mutex;
 use crate::sys_common::thread;
 use crate::sys_common::thread_info;
 use crate::sys_common::thread_parker::Parker;
@@ -1033,24 +1032,48 @@ pub struct ThreadId(NonZeroU64);
 impl ThreadId {
     // Generate a new unique thread ID.
     fn new() -> ThreadId {
-        // It is UB to attempt to acquire this mutex reentrantly!
-        static GUARD: mutex::StaticMutex = mutex::StaticMutex::new();
-        static mut COUNTER: u64 = 1;
-
-        unsafe {
-            let guard = GUARD.lock();
-
-            // If we somehow use up all our bits, panic so that we're not
-            // covering up subtle bugs of IDs being reused.
-            if COUNTER == u64::MAX {
-                drop(guard); // in case the panic handler ends up calling `ThreadId::new()`, avoid reentrant lock acquire.
-                panic!("failed to generate unique thread ID: bitspace exhausted");
-            }
-
-            let id = COUNTER;
-            COUNTER += 1;
+        #[cold]
+        fn exhausted() -> ! {
+            panic!("failed to generate unique thread ID: bitspace exhausted")
+        }
 
-            ThreadId(NonZeroU64::new(id).unwrap())
+        cfg_if::cfg_if! {
+            if #[cfg(target_has_atomic = "64")] {
+                use crate::sync::atomic::{AtomicU64, Ordering::Relaxed};
+
+                static COUNTER: AtomicU64 = AtomicU64::new(0);
+
+                let mut last = COUNTER.load(Relaxed);
+                loop {
+                    let Some(id) = last.checked_add(1) else {
+                        exhausted();
+                    };
+
+                    match COUNTER.compare_exchange_weak(last, id, Relaxed, Relaxed) {
+                        Ok(_) => return ThreadId(NonZeroU64::new(id).unwrap()),
+                        Err(id) => last = id,
+                    }
+                }
+            } else {
+                use crate::sys_common::mutex::StaticMutex;
+
+                // It is UB to attempt to acquire this mutex reentrantly!
+                static GUARD: StaticMutex = StaticMutex::new();
+                static mut COUNTER: u64 = 0;
+
+                unsafe {
+                    let guard = GUARD.lock();
+
+                    let Some(id) = COUNTER.checked_add(1) else {
+                        drop(guard); // in case the panic handler ends up calling `ThreadId::new()`, avoid reentrant lock acquire.
+                        exhausted();
+                    };
+
+                    COUNTER = id;
+                    drop(guard);
+                    ThreadId(NonZeroU64::new(id).unwrap())
+                }
+            }
         }
     }
 
diff --git a/src/librustdoc/html/highlight.rs b/src/librustdoc/html/highlight.rs
index 5a6720a8dd9..27ccff9a276 100644
--- a/src/librustdoc/html/highlight.rs
+++ b/src/librustdoc/html/highlight.rs
@@ -163,7 +163,6 @@ enum Class {
     // Keywords that do pointer/reference stuff.
     RefKeyWord,
     Self_(Span),
-    Op,
     Macro(Span),
     MacroNonTerminal,
     String,
@@ -187,7 +186,6 @@ impl Class {
             Class::KeyWord => "kw",
             Class::RefKeyWord => "kw-2",
             Class::Self_(_) => "self",
-            Class::Op => "op",
             Class::Macro(_) => "macro",
             Class::MacroNonTerminal => "macro-nonterminal",
             Class::String => "string",
@@ -212,7 +210,6 @@ impl Class {
             | Self::Attribute
             | Self::KeyWord
             | Self::RefKeyWord
-            | Self::Op
             | Self::MacroNonTerminal
             | Self::String
             | Self::Number
@@ -516,7 +513,7 @@ impl<'a> Classifier<'a> {
             // or a reference or pointer type. Unless, of course, it looks like
             // a logical and or a multiplication operator: `&&` or `* `.
             TokenKind::Star => match self.tokens.peek() {
-                Some((TokenKind::Whitespace, _)) => Class::Op,
+                Some((TokenKind::Whitespace, _)) => return no_highlight(sink),
                 Some((TokenKind::Ident, "mut")) => {
                     self.next();
                     sink(Highlight::Token { text: "*mut", class: Some(Class::RefKeyWord) });
@@ -532,15 +529,15 @@ impl<'a> Classifier<'a> {
             TokenKind::And => match self.tokens.peek() {
                 Some((TokenKind::And, _)) => {
                     self.next();
-                    sink(Highlight::Token { text: "&&", class: Some(Class::Op) });
+                    sink(Highlight::Token { text: "&&", class: None });
                     return;
                 }
                 Some((TokenKind::Eq, _)) => {
                     self.next();
-                    sink(Highlight::Token { text: "&=", class: Some(Class::Op) });
+                    sink(Highlight::Token { text: "&=", class: None });
                     return;
                 }
-                Some((TokenKind::Whitespace, _)) => Class::Op,
+                Some((TokenKind::Whitespace, _)) => return no_highlight(sink),
                 Some((TokenKind::Ident, "mut")) => {
                     self.next();
                     sink(Highlight::Token { text: "&mut", class: Some(Class::RefKeyWord) });
@@ -553,7 +550,7 @@ impl<'a> Classifier<'a> {
             TokenKind::Eq => match lookahead {
                 Some(TokenKind::Eq) => {
                     self.next();
-                    sink(Highlight::Token { text: "==", class: Some(Class::Op) });
+                    sink(Highlight::Token { text: "==", class: None });
                     return;
                 }
                 Some(TokenKind::Gt) => {
@@ -561,7 +558,7 @@ impl<'a> Classifier<'a> {
                     sink(Highlight::Token { text: "=>", class: None });
                     return;
                 }
-                _ => Class::Op,
+                _ => return no_highlight(sink),
             },
             TokenKind::Minus if lookahead == Some(TokenKind::Gt) => {
                 self.next();
@@ -578,7 +575,7 @@ impl<'a> Classifier<'a> {
             | TokenKind::Percent
             | TokenKind::Bang
             | TokenKind::Lt
-            | TokenKind::Gt => Class::Op,
+            | TokenKind::Gt => return no_highlight(sink),
 
             // Miscellaneous, no highlighting.
             TokenKind::Dot
diff --git a/src/librustdoc/html/highlight/fixtures/decorations.html b/src/librustdoc/html/highlight/fixtures/decorations.html
index 45f567880c9..ae4dba116d6 100644
--- a/src/librustdoc/html/highlight/fixtures/decorations.html
+++ b/src/librustdoc/html/highlight/fixtures/decorations.html
@@ -1,2 +1,2 @@
-<span class="example"><span class="kw">let</span> <span class="ident">x</span> <span class="op">=</span> <span class="number">1</span>;</span>
-<span class="kw">let</span> <span class="ident">y</span> <span class="op">=</span> <span class="number">2</span>;
\ No newline at end of file
+<span class="example"><span class="kw">let</span> <span class="ident">x</span> = <span class="number">1</span>;</span>
+<span class="kw">let</span> <span class="ident">y</span> = <span class="number">2</span>;
\ No newline at end of file
diff --git a/src/librustdoc/html/highlight/fixtures/highlight.html b/src/librustdoc/html/highlight/fixtures/highlight.html
index abc2db1790c..17f23278ec1 100644
--- a/src/librustdoc/html/highlight/fixtures/highlight.html
+++ b/src/librustdoc/html/highlight/fixtures/highlight.html
@@ -1,4 +1,4 @@
 <span class="kw">use</span> <span class="ident"><span class="kw">crate</span>::a::foo</span>;
 <span class="kw">use</span> <span class="ident"><span class="self">self</span>::whatever</span>;
-<span class="kw">let</span> <span class="ident">x</span> <span class="op">=</span> <span class="ident"><span class="kw">super</span>::b::foo</span>;
-<span class="kw">let</span> <span class="ident">y</span> <span class="op">=</span> <span class="ident"><span class="self">Self</span>::whatever</span>;
\ No newline at end of file
+<span class="kw">let</span> <span class="ident">x</span> = <span class="ident"><span class="kw">super</span>::b::foo</span>;
+<span class="kw">let</span> <span class="ident">y</span> = <span class="ident"><span class="self">Self</span>::whatever</span>;
\ No newline at end of file
diff --git a/src/librustdoc/html/highlight/fixtures/sample.html b/src/librustdoc/html/highlight/fixtures/sample.html
index b117a12e39f..ea797fd99d3 100644
--- a/src/librustdoc/html/highlight/fixtures/sample.html
+++ b/src/librustdoc/html/highlight/fixtures/sample.html
@@ -8,23 +8,23 @@
 .lifetime { color: #B76514; }
 .question-mark { color: #ff9011; }
 </style>
-<pre><code><span class="attribute">#![<span class="ident">crate_type</span> <span class="op">=</span> <span class="string">&quot;lib&quot;</span>]</span>
+<pre><code><span class="attribute">#![<span class="ident">crate_type</span> = <span class="string">&quot;lib&quot;</span>]</span>
 
 <span class="kw">use</span> <span class="ident">std::path</span>::{<span class="ident">Path</span>, <span class="ident">PathBuf</span>};
 
-<span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">target_os</span> <span class="op">=</span> <span class="string">&quot;linux&quot;</span>)]</span>
+<span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">target_os</span> = <span class="string">&quot;linux&quot;</span>)]</span>
 <span class="kw">fn</span> <span class="ident">main</span>() -&gt; () {
-    <span class="kw">let</span> <span class="ident">foo</span> <span class="op">=</span> <span class="bool-val">true</span> <span class="op">&amp;&amp;</span> <span class="bool-val">false</span> <span class="op">|</span><span class="op">|</span> <span class="bool-val">true</span>;
-    <span class="kw">let</span> <span class="kw">_</span>: <span class="kw-2">*const</span> () <span class="op">=</span> <span class="number">0</span>;
-    <span class="kw">let</span> <span class="kw">_</span> <span class="op">=</span> <span class="kw-2">&amp;</span><span class="ident">foo</span>;
-    <span class="kw">let</span> <span class="kw">_</span> <span class="op">=</span> <span class="op">&amp;&amp;</span><span class="ident">foo</span>;
-    <span class="kw">let</span> <span class="kw">_</span> <span class="op">=</span> <span class="kw-2">*</span><span class="ident">foo</span>;
+    <span class="kw">let</span> <span class="ident">foo</span> = <span class="bool-val">true</span> &amp;&amp; <span class="bool-val">false</span> || <span class="bool-val">true</span>;
+    <span class="kw">let</span> <span class="kw">_</span>: <span class="kw-2">*const</span> () = <span class="number">0</span>;
+    <span class="kw">let</span> <span class="kw">_</span> = <span class="kw-2">&amp;</span><span class="ident">foo</span>;
+    <span class="kw">let</span> <span class="kw">_</span> = &amp;&amp;<span class="ident">foo</span>;
+    <span class="kw">let</span> <span class="kw">_</span> = <span class="kw-2">*</span><span class="ident">foo</span>;
     <span class="macro">mac!</span>(<span class="ident">foo</span>, <span class="kw-2">&amp;mut</span> <span class="ident">bar</span>);
-    <span class="macro">assert!</span>(<span class="self">self</span>.<span class="ident">length</span> <span class="op">&lt;</span> <span class="ident">N</span> <span class="op">&amp;&amp;</span> <span class="ident">index</span> <span class="op">&lt;</span><span class="op">=</span> <span class="self">self</span>.<span class="ident">length</span>);
+    <span class="macro">assert!</span>(<span class="self">self</span>.<span class="ident">length</span> &lt; <span class="ident">N</span> &amp;&amp; <span class="ident">index</span> &lt;= <span class="self">self</span>.<span class="ident">length</span>);
     <span class="ident">::std::env::var</span>(<span class="string">&quot;gateau&quot;</span>).<span class="ident">is_ok</span>();
     <span class="attribute">#[<span class="ident">rustfmt::skip</span>]</span>
-    <span class="kw">let</span> <span class="ident">s</span>:<span class="ident">std::path::PathBuf</span> <span class="op">=</span> <span class="ident">std::path::PathBuf::new</span>();
-    <span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">s</span> <span class="op">=</span> <span class="ident">String::new</span>();
+    <span class="kw">let</span> <span class="ident">s</span>:<span class="ident">std::path::PathBuf</span> = <span class="ident">std::path::PathBuf::new</span>();
+    <span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">s</span> = <span class="ident">String::new</span>();
 
     <span class="kw">match</span> <span class="kw-2">&amp;</span><span class="ident">s</span> {
         <span class="kw-2">ref</span> <span class="kw-2">mut</span> <span class="ident">x</span> =&gt; {}
diff --git a/src/librustdoc/html/highlight/fixtures/union.html b/src/librustdoc/html/highlight/fixtures/union.html
index c0acf31a05d..ac8bd28f6c3 100644
--- a/src/librustdoc/html/highlight/fixtures/union.html
+++ b/src/librustdoc/html/highlight/fixtures/union.html
@@ -4,5 +4,5 @@
 }
 
 <span class="kw">fn</span> <span class="ident">main</span>() {
-    <span class="kw">let</span> <span class="ident">union</span> <span class="op">=</span> <span class="number">0</span>;
+    <span class="kw">let</span> <span class="ident">union</span> = <span class="number">0</span>;
 }
diff --git a/src/librustdoc/html/static/css/themes/ayu.css b/src/librustdoc/html/static/css/themes/ayu.css
index 39a4dae3348..b8218867a8b 100644
--- a/src/librustdoc/html/static/css/themes/ayu.css
+++ b/src/librustdoc/html/static/css/themes/ayu.css
@@ -238,7 +238,7 @@ details.rustdoc-toggle > summary::before {
 pre.rust .number, pre.rust .string { color: #b8cc52; }
 pre.rust .kw, pre.rust .kw-2, pre.rust .prelude-ty,
 pre.rust .bool-val, pre.rust .prelude-val,
-pre.rust .op, pre.rust .lifetime { color: #ff7733; }
+pre.rust .lifetime { color: #ff7733; }
 pre.rust .macro, pre.rust .macro-nonterminal { color: #a37acc; }
 pre.rust .question-mark {
 	color: #ff9011;
@@ -250,7 +250,7 @@ pre.rust .self {
 pre.rust .attribute {
 	color: #e6e1cf;
 }
-pre.rust .attribute .ident, pre.rust .attribute .op {
+pre.rust .attribute .ident {
 	color: #e6e1cf;
 }
 
diff --git a/src/test/run-make/translation/Makefile b/src/test/run-make/translation/Makefile
index bfff75e7acb..20e86c7f9a0 100644
--- a/src/test/run-make/translation/Makefile
+++ b/src/test/run-make/translation/Makefile
@@ -9,16 +9,29 @@ FAKEROOT=$(TMPDIR)/fakeroot
 
 all: normal custom sysroot
 
-normal: basic-translation.rs
+# Check that the test works normally, using the built-in fallback bundle.
+normal: test.rs
 	$(RUSTC) $< 2>&1 | grep "struct literal body without path"
 
-custom: basic-translation.rs basic-translation.ftl
-	$(RUSTC) $< -Ztranslate-additional-ftl=$(CURDIR)/basic-translation.ftl 2>&1 | grep "this is a test message"
+# Check that a primary bundle can be loaded and will be preferentially used
+# where possible.
+custom: test.rs working.ftl
+	$(RUSTC) $< -Ztranslate-additional-ftl=$(CURDIR)/working.ftl 2>&1 | grep "this is a test message"
+
+# Check that a primary bundle with a broken message (e.g. a interpolated
+# variable is missing) will use the fallback bundle.
+missing: test.rs missing.ftl
+	$(RUSTC) $< -Ztranslate-additional-ftl=$(CURDIR)/missing.ftl 2>&1 | grep "struct literal body without path"
+
+# Check that a primary bundle without the desired message will use the fallback
+# bundle.
+broken: test.rs broken.ftl
+	$(RUSTC) $< -Ztranslate-additional-ftl=$(CURDIR)/broken.ftl 2>&1 | grep "struct literal body without path"
 
 # Check that a locale can be loaded from the sysroot given a language
 # identifier by making a local copy of the sysroot and adding the custom locale
 # to it.
-sysroot: basic-translation.rs basic-translation.ftl
+sysroot: test.rs working.ftl
 	mkdir $(FAKEROOT)
 	ln -s $(SYSROOT)/* $(FAKEROOT)
 	rm -f $(FAKEROOT)/lib
@@ -31,7 +44,7 @@ sysroot: basic-translation.rs basic-translation.ftl
 	mkdir $(FAKEROOT)/lib/rustlib/src
 	ln -s $(SYSROOT)/lib/rustlib/src/* $(FAKEROOT)/lib/rustlib/src
 	mkdir -p $(FAKEROOT)/share/locale/zh-CN/
-	ln -s $(CURDIR)/basic-translation.ftl $(FAKEROOT)/share/locale/zh-CN/basic-translation.ftl
+	ln -s $(CURDIR)/working.ftl $(FAKEROOT)/share/locale/zh-CN/basic-translation.ftl
 	$(RUSTC) $< --sysroot $(FAKEROOT) -Ztranslate-lang=zh-CN 2>&1 | grep "this is a test message"
 
 # Check that the compiler errors out when the sysroot requested cannot be
@@ -43,7 +56,7 @@ sysroot-missing:
 # Check that the compiler errors out when the sysroot requested cannot be
 # found. This test might start failing if there actually exists a Klingon
 # translation of rustc's error messages.
-sysroot-invalid: basic-translation.rs basic-translation.ftl
+sysroot-invalid: test.rs working.ftl
 	mkdir $(FAKEROOT)
 	ln -s $(SYSROOT)/* $(FAKEROOT)
 	rm -f $(FAKEROOT)/lib
diff --git a/src/test/run-make/translation/broken.ftl b/src/test/run-make/translation/broken.ftl
new file mode 100644
index 00000000000..1482dd2824a
--- /dev/null
+++ b/src/test/run-make/translation/broken.ftl
@@ -0,0 +1,3 @@
+# `foo` isn't provided by this diagnostic so it is expected that the fallback message is used.
+parser-struct-literal-body-without-path = this is a {$foo} message
+    .suggestion = this is a test suggestion
diff --git a/src/test/run-make/translation/missing.ftl b/src/test/run-make/translation/missing.ftl
new file mode 100644
index 00000000000..43076b1d6ae
--- /dev/null
+++ b/src/test/run-make/translation/missing.ftl
@@ -0,0 +1,3 @@
+# `parser-struct-literal-body-without-path` isn't provided by this resource at all, so the
+# fallback should be used.
+foo = bar
diff --git a/src/test/run-make/translation/basic-translation.rs b/src/test/run-make/translation/test.rs
index b8f5bff3153..b8f5bff3153 100644
--- a/src/test/run-make/translation/basic-translation.rs
+++ b/src/test/run-make/translation/test.rs
diff --git a/src/test/run-make/translation/basic-translation.ftl b/src/test/run-make/translation/working.ftl
index 4681b879cda..4681b879cda 100644
--- a/src/test/run-make/translation/basic-translation.ftl
+++ b/src/test/run-make/translation/working.ftl
diff --git a/src/test/rustdoc-ui/z-help.stdout b/src/test/rustdoc-ui/z-help.stdout
index 6dc41231559..236469ce979 100644
--- a/src/test/rustdoc-ui/z-help.stdout
+++ b/src/test/rustdoc-ui/z-help.stdout
@@ -38,6 +38,7 @@
     -Z                        emit-stack-sizes=val -- emit a section containing stack size metadata (default: no)
     -Z                           emit-thin-lto=val -- emit the bc module with thin LTO info (default: yes)
     -Z               export-executable-symbols=val -- export symbols from executables, as if they were dynamic libraries
+    -Z                   extra-const-ub-checks=val -- turns on more checks to detect const UB, which can be slow (default: no)
     -Z                             fewer-names=val -- reduce memory use by retaining fewer names within compilation artifacts (LLVM-IR) (default: no)
     -Z              force-unstable-if-unmarked=val -- force all crates to be `rustc_private` unstable (default: no)
     -Z                                    fuel=val -- set the optimization fuel quota for a crate
diff --git a/src/test/rustdoc/macro_rules-matchers.rs b/src/test/rustdoc/macro_rules-matchers.rs
index efc3b21e6da..131c53ec24b 100644
--- a/src/test/rustdoc/macro_rules-matchers.rs
+++ b/src/test/rustdoc/macro_rules-matchers.rs
@@ -6,16 +6,13 @@
 // @has 'foo/macro.todo.html'
 // @has - '//span[@class="macro"]' 'macro_rules!'
 // @has - '//span[@class="ident"]' 'todo'
-// Note: the only op is the `+`
-// @count - '//pre[@class="rust macro"]//span[@class="op"]' 1
 
 // @has - '{ () =&gt; { ... }; ($('
 // @has - '//span[@class="macro-nonterminal"]' '$'
 // @has - '//span[@class="macro-nonterminal"]' 'arg'
 // @has - ':'
 // @has - '//span[@class="ident"]' 'tt'
-// @has - '),'
-// @has - '//span[@class="op"]' '+'
+// @has - ')+'
 // @has - ') =&gt; { ... }; }'
 pub use std::todo;
 
diff --git a/src/test/ui/associated-types/issue-22560.stderr b/src/test/ui/associated-types/issue-22560.stderr
index 700923c1b3f..2b88cf0b441 100644
--- a/src/test/ui/associated-types/issue-22560.stderr
+++ b/src/test/ui/associated-types/issue-22560.stderr
@@ -1,25 +1,3 @@
-error[E0393]: the type parameter `Rhs` must be explicitly specified
-  --> $DIR/issue-22560.rs:9:23
-   |
-LL | trait Sub<Rhs=Self> {
-   | ------------------- type parameter `Rhs` must be specified for this
-...
-LL | type Test = dyn Add + Sub;
-   |                       ^^^ help: set the type parameter to the desired type: `Sub<Rhs>`
-   |
-   = note: because of the default `Self` reference, type parameters must be specified on object types
-
-error[E0393]: the type parameter `Rhs` must be explicitly specified
-  --> $DIR/issue-22560.rs:9:17
-   |
-LL | trait Add<Rhs=Self> {
-   | ------------------- type parameter `Rhs` must be specified for this
-...
-LL | type Test = dyn Add + Sub;
-   |                 ^^^ help: set the type parameter to the desired type: `Add<Rhs>`
-   |
-   = note: because of the default `Self` reference, type parameters must be specified on object types
-
 error[E0225]: only auto traits can be used as additional traits in a trait object
   --> $DIR/issue-22560.rs:9:23
    |
@@ -28,7 +6,7 @@ LL | type Test = dyn Add + Sub;
    |                 |
    |                 first non-auto trait
    |
-   = help: consider creating a new trait with all of these as supertraits and using that trait here instead: `trait NewTrait: Add<[type error]> + Sub<[type error]> {}`
+   = help: consider creating a new trait with all of these as supertraits and using that trait here instead: `trait NewTrait: Add + Sub {}`
    = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
 
 error[E0191]: the value of the associated types `Output` (from trait `Add`), `Output` (from trait `Sub`) must be specified
@@ -50,6 +28,28 @@ help: specify the associated types
 LL | type Test = dyn Add<Output = Type> + Sub<Output = Type>;
    |                 ~~~~~~~~~~~~~~~~~~   ~~~~~~~~~~~~~~~~~~
 
+error[E0393]: the type parameter `Rhs` must be explicitly specified
+  --> $DIR/issue-22560.rs:9:17
+   |
+LL | trait Add<Rhs=Self> {
+   | ------------------- type parameter `Rhs` must be specified for this
+...
+LL | type Test = dyn Add + Sub;
+   |                 ^^^ help: set the type parameter to the desired type: `Add<Rhs>`
+   |
+   = note: because of the default `Self` reference, type parameters must be specified on object types
+
+error[E0393]: the type parameter `Rhs` must be explicitly specified
+  --> $DIR/issue-22560.rs:9:23
+   |
+LL | trait Sub<Rhs=Self> {
+   | ------------------- type parameter `Rhs` must be specified for this
+...
+LL | type Test = dyn Add + Sub;
+   |                       ^^^ help: set the type parameter to the desired type: `Sub<Rhs>`
+   |
+   = note: because of the default `Self` reference, type parameters must be specified on object types
+
 error: aborting due to 4 previous errors
 
 Some errors have detailed explanations: E0191, E0225, E0393.
diff --git a/src/test/ui/consts/extra-const-ub/detect-extra-ub.rs b/src/test/ui/consts/extra-const-ub/detect-extra-ub.rs
new file mode 100644
index 00000000000..97c9e150519
--- /dev/null
+++ b/src/test/ui/consts/extra-const-ub/detect-extra-ub.rs
@@ -0,0 +1,45 @@
+// revisions: no_flag with_flag
+// [no_flag] check-pass
+// [with_flag] compile-flags: -Zextra-const-ub-checks
+#![feature(const_ptr_read)]
+
+use std::mem::transmute;
+
+const INVALID_BOOL: () = unsafe {
+    let _x: bool = transmute(3u8);
+    //[with_flag]~^ ERROR: evaluation of constant value failed
+    //[with_flag]~| invalid value
+};
+
+const INVALID_PTR_IN_INT: () = unsafe {
+    let _x: usize = transmute(&3u8);
+    //[with_flag]~^ ERROR: evaluation of constant value failed
+    //[with_flag]~| invalid value
+};
+
+const INVALID_SLICE_TO_USIZE_TRANSMUTE: () = unsafe {
+    let x: &[u8] = &[0; 32];
+    let _x: (usize, usize) = transmute(x);
+    //[with_flag]~^ ERROR: evaluation of constant value failed
+    //[with_flag]~| invalid value
+};
+
+const UNALIGNED_PTR: () = unsafe {
+    let _x: &u32 = transmute(&[0u8; 4]);
+    //[with_flag]~^ ERROR: evaluation of constant value failed
+    //[with_flag]~| invalid value
+};
+
+const UNALIGNED_READ: () = {
+    INNER; //[with_flag]~ERROR any use of this value will cause an error
+    //[with_flag]~| previously accepted
+    // There is an error here but its span is in the standard library so we cannot match it...
+    // so we have this in a *nested* const, such that the *outer* const fails to use it.
+    const INNER: () = unsafe {
+        let x = &[0u8; 4];
+        let ptr = x.as_ptr().cast::<u32>();
+        ptr.read();
+    };
+};
+
+fn main() {}
diff --git a/src/test/ui/consts/extra-const-ub/detect-extra-ub.with_flag.stderr b/src/test/ui/consts/extra-const-ub/detect-extra-ub.with_flag.stderr
new file mode 100644
index 00000000000..1706db7ac43
--- /dev/null
+++ b/src/test/ui/consts/extra-const-ub/detect-extra-ub.with_flag.stderr
@@ -0,0 +1,71 @@
+error[E0080]: evaluation of constant value failed
+  --> $DIR/detect-extra-ub.rs:9:20
+   |
+LL |     let _x: bool = transmute(3u8);
+   |                    ^^^^^^^^^^^^^^ constructing invalid value: encountered 0x03, but expected a boolean
+
+error[E0080]: evaluation of constant value failed
+  --> $DIR/detect-extra-ub.rs:15:21
+   |
+LL |     let _x: usize = transmute(&3u8);
+   |                     ^^^^^^^^^^^^^^^ constructing invalid value: encountered (potentially part of) a pointer, but expected plain (non-pointer) bytes
+
+error[E0080]: evaluation of constant value failed
+  --> $DIR/detect-extra-ub.rs:22:30
+   |
+LL |     let _x: (usize, usize) = transmute(x);
+   |                              ^^^^^^^^^^^^ constructing invalid value at .0: encountered (potentially part of) a pointer, but expected plain (non-pointer) bytes
+
+error[E0080]: evaluation of constant value failed
+  --> $DIR/detect-extra-ub.rs:28:20
+   |
+LL |     let _x: &u32 = transmute(&[0u8; 4]);
+   |                    ^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered an unaligned reference (required 4 byte alignment but found 1)
+
+error[E0080]: evaluation of constant value failed
+  --> $SRC_DIR/core/src/ptr/mod.rs:LL:COL
+   |
+LL |         copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |         |
+   |         accessing memory with alignment 1, but alignment 4 is required
+   |         inside `std::ptr::read::<u32>` at $SRC_DIR/core/src/ptr/mod.rs:LL:COL
+   |
+  ::: $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
+   |
+LL |         unsafe { read(self) }
+   |                  ---------- inside `ptr::const_ptr::<impl *const u32>::read` at $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
+   |
+  ::: $DIR/detect-extra-ub.rs:41:9
+   |
+LL |         ptr.read();
+   |         ---------- inside `INNER` at $DIR/detect-extra-ub.rs:41:9
+
+error: any use of this value will cause an error
+  --> $DIR/detect-extra-ub.rs:34:5
+   |
+LL | const UNALIGNED_READ: () = {
+   | ------------------------
+LL |     INNER;
+   |     ^^^^^ referenced constant has errors
+   |
+   = note: `#[deny(const_err)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0080`.
+Future incompatibility report: Future breakage diagnostic:
+error: any use of this value will cause an error
+  --> $DIR/detect-extra-ub.rs:34:5
+   |
+LL | const UNALIGNED_READ: () = {
+   | ------------------------
+LL |     INNER;
+   |     ^^^^^ referenced constant has errors
+   |
+   = note: `#[deny(const_err)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+
diff --git a/src/test/ui/cycle-trait/cycle-trait-default-type-trait.rs b/src/test/ui/cycle-trait/cycle-trait-default-type-trait.rs
index b2edc1a1f66..6175b7df110 100644
--- a/src/test/ui/cycle-trait/cycle-trait-default-type-trait.rs
+++ b/src/test/ui/cycle-trait/cycle-trait-default-type-trait.rs
@@ -3,7 +3,6 @@
 
 trait Foo<X = Box<dyn Foo>> {
     //~^ ERROR cycle detected
-    //~| ERROR cycle detected
 }
 
 fn main() { }
diff --git a/src/test/ui/cycle-trait/cycle-trait-default-type-trait.stderr b/src/test/ui/cycle-trait/cycle-trait-default-type-trait.stderr
index d4976a0f9c9..9d715f49471 100644
--- a/src/test/ui/cycle-trait/cycle-trait-default-type-trait.stderr
+++ b/src/test/ui/cycle-trait/cycle-trait-default-type-trait.stderr
@@ -10,30 +10,11 @@ note: cycle used when collecting item types in top-level module
    |
 LL | / trait Foo<X = Box<dyn Foo>> {
 LL | |
-LL | |
-LL | | }
-LL | |
-LL | | fn main() { }
-   | |_____________^
-
-error[E0391]: cycle detected when computing type of `Foo::X`
-  --> $DIR/cycle-trait-default-type-trait.rs:4:23
-   |
-LL | trait Foo<X = Box<dyn Foo>> {
-   |                       ^^^
-   |
-   = note: ...which immediately requires computing type of `Foo::X` again
-note: cycle used when collecting item types in top-level module
-  --> $DIR/cycle-trait-default-type-trait.rs:4:1
-   |
-LL | / trait Foo<X = Box<dyn Foo>> {
-LL | |
-LL | |
 LL | | }
 LL | |
 LL | | fn main() { }
    | |_____________^
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0391`.
diff --git a/src/test/ui/issues/issue-21950.stderr b/src/test/ui/issues/issue-21950.stderr
index 4909398bb84..731615a6bd8 100644
--- a/src/test/ui/issues/issue-21950.stderr
+++ b/src/test/ui/issues/issue-21950.stderr
@@ -1,3 +1,12 @@
+error[E0191]: the value of the associated type `Output` (from trait `Add`) must be specified
+  --> $DIR/issue-21950.rs:10:25
+   |
+LL |     type Output;
+   |     ----------- `Output` defined here
+...
+LL |     let x = &10 as &dyn Add;
+   |                         ^^^ help: specify the associated type: `Add<Output = Type>`
+
 error[E0393]: the type parameter `Rhs` must be explicitly specified
   --> $DIR/issue-21950.rs:10:25
    |
@@ -9,15 +18,6 @@ LL |     let x = &10 as &dyn Add;
    |
    = note: because of the default `Self` reference, type parameters must be specified on object types
 
-error[E0191]: the value of the associated type `Output` (from trait `Add`) must be specified
-  --> $DIR/issue-21950.rs:10:25
-   |
-LL |     type Output;
-   |     ----------- `Output` defined here
-...
-LL |     let x = &10 as &dyn Add;
-   |                         ^^^ help: specify the associated type: `Add<Output = Type>`
-
 error: aborting due to 2 previous errors
 
 Some errors have detailed explanations: E0191, E0393.
diff --git a/src/test/ui/let-else/issue-94176.rs b/src/test/ui/let-else/issue-94176.rs
new file mode 100644
index 00000000000..e35bbd88830
--- /dev/null
+++ b/src/test/ui/let-else/issue-94176.rs
@@ -0,0 +1,10 @@
+// Issue #94176: wrong span for the error message of a mismatched type error,
+// if the function uses a `let else` construct.
+#![feature(let_else)]
+
+pub fn test(a: Option<u32>) -> Option<u32> { //~ ERROR mismatched types
+    let Some(_) = a else { return None; };
+    println!("Foo");
+}
+
+fn main() {}
diff --git a/src/test/ui/let-else/issue-94176.stderr b/src/test/ui/let-else/issue-94176.stderr
new file mode 100644
index 00000000000..0cb97aceebf
--- /dev/null
+++ b/src/test/ui/let-else/issue-94176.stderr
@@ -0,0 +1,19 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-94176.rs:5:32
+   |
+LL | pub fn test(a: Option<u32>) -> Option<u32> {
+   |        ----                    ^^^^^^^^^^^ expected enum `Option`, found `()`
+   |        |
+   |        implicitly returns `()` as its body has no tail or `return` expression
+   |
+   = note:   expected enum `Option<u32>`
+           found unit type `()`
+help: consider returning the local binding `a`
+   |
+LL ~     println!("Foo");
+LL +     a
+   |
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/let-else/let-else-then-diverge.rs b/src/test/ui/let-else/let-else-then-diverge.rs
new file mode 100644
index 00000000000..49633d943be
--- /dev/null
+++ b/src/test/ui/let-else/let-else-then-diverge.rs
@@ -0,0 +1,19 @@
+//
+// popped up in in #94012, where an alternative desugaring was
+// causing unreachable code errors
+
+#![feature(let_else)]
+#![deny(unused_variables)]
+#![deny(unreachable_code)]
+
+fn let_else_diverge() -> bool {
+    let Some(_) = Some("test") else {
+        let x = 5; //~ ERROR unused variable: `x`
+        return false;
+    };
+    return true;
+}
+
+fn main() {
+    let_else_diverge();
+}
diff --git a/src/test/ui/let-else/let-else-then-diverge.stderr b/src/test/ui/let-else/let-else-then-diverge.stderr
new file mode 100644
index 00000000000..ceb61029d38
--- /dev/null
+++ b/src/test/ui/let-else/let-else-then-diverge.stderr
@@ -0,0 +1,14 @@
+error: unused variable: `x`
+  --> $DIR/let-else-then-diverge.rs:11:13
+   |
+LL |         let x = 5;
+   |             ^ help: if this is intentional, prefix it with an underscore: `_x`
+   |
+note: the lint level is defined here
+  --> $DIR/let-else-then-diverge.rs:6:9
+   |
+LL | #![deny(unused_variables)]
+   |         ^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/parser/suggest-const-for-global-var.rs b/src/test/ui/parser/suggest-const-for-global-var.rs
new file mode 100644
index 00000000000..d6216cb7ac2
--- /dev/null
+++ b/src/test/ui/parser/suggest-const-for-global-var.rs
@@ -0,0 +1,6 @@
+let X: i32 = 12;
+//~^ ERROR expected item, found keyword `let`
+
+fn main() {
+    println!("{}", X);
+}
diff --git a/src/test/ui/parser/suggest-const-for-global-var.stderr b/src/test/ui/parser/suggest-const-for-global-var.stderr
new file mode 100644
index 00000000000..94e44ec7f6c
--- /dev/null
+++ b/src/test/ui/parser/suggest-const-for-global-var.stderr
@@ -0,0 +1,8 @@
+error: expected item, found keyword `let`
+  --> $DIR/suggest-const-for-global-var.rs:1:1
+   |
+LL | let X: i32 = 12;
+   | ^^^ consider using `const` or `static` instead of `let` for global variables
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/traits/alias/generic-default-in-dyn.rs b/src/test/ui/traits/alias/generic-default-in-dyn.rs
new file mode 100644
index 00000000000..d44e1c2a975
--- /dev/null
+++ b/src/test/ui/traits/alias/generic-default-in-dyn.rs
@@ -0,0 +1,10 @@
+trait SendEqAlias<T> = PartialEq;
+//~^ ERROR trait aliases are experimental
+
+struct Foo<T>(dyn SendEqAlias<T>);
+//~^ ERROR the type parameter `Rhs` must be explicitly specified [E0393]
+
+struct Bar<T>(dyn SendEqAlias<T>, T);
+//~^ ERROR the type parameter `Rhs` must be explicitly specified [E0393]
+
+fn main() {}
diff --git a/src/test/ui/traits/alias/generic-default-in-dyn.stderr b/src/test/ui/traits/alias/generic-default-in-dyn.stderr
new file mode 100644
index 00000000000..76a068e864a
--- /dev/null
+++ b/src/test/ui/traits/alias/generic-default-in-dyn.stderr
@@ -0,0 +1,39 @@
+error[E0658]: trait aliases are experimental
+  --> $DIR/generic-default-in-dyn.rs:1:1
+   |
+LL | trait SendEqAlias<T> = PartialEq;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #41517 <https://github.com/rust-lang/rust/issues/41517> for more information
+   = help: add `#![feature(trait_alias)]` to the crate attributes to enable
+
+error[E0393]: the type parameter `Rhs` must be explicitly specified
+  --> $DIR/generic-default-in-dyn.rs:4:19
+   |
+LL | struct Foo<T>(dyn SendEqAlias<T>);
+   |                   ^^^^^^^^^^^^^^ missing reference to `Rhs`
+   |
+  ::: $SRC_DIR/core/src/cmp.rs:LL:COL
+   |
+LL | pub trait PartialEq<Rhs: ?Sized = Self> {
+   | --------------------------------------- type parameter `Rhs` must be specified for this
+   |
+   = note: because of the default `Self` reference, type parameters must be specified on object types
+
+error[E0393]: the type parameter `Rhs` must be explicitly specified
+  --> $DIR/generic-default-in-dyn.rs:7:19
+   |
+LL | struct Bar<T>(dyn SendEqAlias<T>, T);
+   |                   ^^^^^^^^^^^^^^ missing reference to `Rhs`
+   |
+  ::: $SRC_DIR/core/src/cmp.rs:LL:COL
+   |
+LL | pub trait PartialEq<Rhs: ?Sized = Self> {
+   | --------------------------------------- type parameter `Rhs` must be specified for this
+   |
+   = note: because of the default `Self` reference, type parameters must be specified on object types
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0393, E0658.
+For more information about an error, try `rustc --explain E0393`.
diff --git a/src/test/ui/traits/alias/self-in-generics.rs b/src/test/ui/traits/alias/self-in-generics.rs
new file mode 100644
index 00000000000..6b99431f5bb
--- /dev/null
+++ b/src/test/ui/traits/alias/self-in-generics.rs
@@ -0,0 +1,8 @@
+#![feature(trait_alias)]
+
+pub trait SelfInput = Fn(&mut Self);
+
+pub fn f(_f: &dyn SelfInput) {}
+//~^ ERROR the trait alias `SelfInput` cannot be made into an object [E0038]
+
+fn main() {}
diff --git a/src/test/ui/traits/alias/self-in-generics.stderr b/src/test/ui/traits/alias/self-in-generics.stderr
new file mode 100644
index 00000000000..a1056872ea6
--- /dev/null
+++ b/src/test/ui/traits/alias/self-in-generics.stderr
@@ -0,0 +1,11 @@
+error[E0038]: the trait alias `SelfInput` cannot be made into an object
+  --> $DIR/self-in-generics.rs:5:19
+   |
+LL | pub fn f(_f: &dyn SelfInput) {}
+   |                   ^^^^^^^^^
+   |
+   = note: it cannot use `Self` as a type parameter in a supertrait or `where`-clause
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0038`.
diff --git a/src/test/ui/unspecified-self-in-trait-ref.rs b/src/test/ui/traits/unspecified-self-in-trait-ref.rs
index 158b5a98557..158b5a98557 100644
--- a/src/test/ui/unspecified-self-in-trait-ref.rs
+++ b/src/test/ui/traits/unspecified-self-in-trait-ref.rs
diff --git a/src/test/ui/unspecified-self-in-trait-ref.stderr b/src/test/ui/traits/unspecified-self-in-trait-ref.stderr
index 7869176bb3a..7869176bb3a 100644
--- a/src/test/ui/unspecified-self-in-trait-ref.stderr
+++ b/src/test/ui/traits/unspecified-self-in-trait-ref.stderr