Auto merge of #123725 - GuillaumeGomez:rollup-gk2bbrg, r=GuillaumeGomez

Rollup of 7 pull requests

Successful merges:

 - #118391 (Add `REDUNDANT_LIFETIMES` lint to detect lifetimes which are semantically redundant)
 - #123534 (Windows: set main thread name without re-encoding)
 - #123659 (Add support to intrinsics fallback body)
 - #123689 (Add const generics support for pattern types)
 - #123701 (Only assert for child/parent projection compatibility AFTER checking that theyre coming from the same place)
 - #123702 (Further cleanup cfgs in the UI test suite)
 - #123706 (rustdoc: reduce per-page HTML overhead)

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

12 files changed, 244 insertions, 72 deletions

diff --git a/compiler/rustc_errors/src/diagnostic_impls.rs b/compiler/rustc_errors/src/diagnostic_impls.rs
index f90190797ae..6c0551848d6 100644
--- a/compiler/rustc_errors/src/diagnostic_impls.rs
+++ b/compiler/rustc_errors/src/diagnostic_impls.rs
@@ -46,6 +46,7 @@ impl<'a, T: Clone + IntoDiagArg> IntoDiagArg for &'a T {
     }
 }
 
+#[macro_export]
 macro_rules! into_diag_arg_using_display {
     ($( $ty:ty ),+ $(,)?) => {
         $(
diff --git a/compiler/rustc_hir_analysis/messages.ftl b/compiler/rustc_hir_analysis/messages.ftl
index e66a834ab9e..86b8b6d6b2b 100644
--- a/compiler/rustc_hir_analysis/messages.ftl
+++ b/compiler/rustc_hir_analysis/messages.ftl
@@ -355,6 +355,9 @@ hir_analysis_pattern_type_wild_pat = "wildcard patterns are not permitted for pa
 hir_analysis_placeholder_not_allowed_item_signatures = the placeholder `_` is not allowed within types on item signatures for {$kind}
     .label = not allowed in type signatures
 
+hir_analysis_redundant_lifetime_args = unnecessary lifetime parameter `{$victim}`
+    .note = you can use the `{$candidate}` lifetime directly, in place of `{$victim}`
+
 hir_analysis_requires_note = the `{$trait_name}` impl for `{$ty}` requires that `{$error_predicate}`
 
 hir_analysis_return_type_notation_equality_bound =
diff --git a/compiler/rustc_hir_analysis/src/check/wfcheck.rs b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
index 4fd7c870fc7..c26f982fa47 100644
--- a/compiler/rustc_hir_analysis/src/check/wfcheck.rs
+++ b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
@@ -8,11 +8,13 @@ use rustc_ast as ast;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexSet};
 use rustc_errors::{codes::*, pluralize, struct_span_code_err, Applicability, ErrorGuaranteed};
 use rustc_hir as hir;
+use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId, LocalModDefId};
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::ItemKind;
 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
 use rustc_infer::infer::{self, InferCtxt, TyCtxtInferExt};
+use rustc_macros::LintDiagnostic;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::trait_def::TraitSpecializationKind;
@@ -136,6 +138,8 @@ where
         infcx.implied_bounds_tys_compat(param_env, body_def_id, &assumed_wf_types, false);
     let outlives_env = OutlivesEnvironment::with_bounds(param_env, implied_bounds);
 
+    lint_redundant_lifetimes(tcx, body_def_id, &outlives_env);
+
     let errors = infcx.resolve_regions(&outlives_env);
     if errors.is_empty() {
         return Ok(());
@@ -2010,6 +2014,137 @@ fn check_mod_type_wf(tcx: TyCtxt<'_>, module: LocalModDefId) -> Result<(), Error
     res
 }
 
+fn lint_redundant_lifetimes<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    owner_id: LocalDefId,
+    outlives_env: &OutlivesEnvironment<'tcx>,
+) {
+    let def_kind = tcx.def_kind(owner_id);
+    match def_kind {
+        DefKind::Struct
+        | DefKind::Union
+        | DefKind::Enum
+        | DefKind::Trait
+        | DefKind::TraitAlias
+        | DefKind::Fn
+        | DefKind::Const
+        | DefKind::Impl { of_trait: _ } => {
+            // Proceed
+        }
+        DefKind::AssocFn | DefKind::AssocTy | DefKind::AssocConst => {
+            let parent_def_id = tcx.local_parent(owner_id);
+            if matches!(tcx.def_kind(parent_def_id), DefKind::Impl { of_trait: true }) {
+                // Don't check for redundant lifetimes for associated items of trait
+                // implementations, since the signature is required to be compatible
+                // with the trait, even if the implementation implies some lifetimes
+                // are redundant.
+                return;
+            }
+        }
+        DefKind::Mod
+        | DefKind::Variant
+        | DefKind::TyAlias
+        | DefKind::ForeignTy
+        | DefKind::TyParam
+        | DefKind::ConstParam
+        | DefKind::Static { .. }
+        | DefKind::Ctor(_, _)
+        | DefKind::Macro(_)
+        | DefKind::ExternCrate
+        | DefKind::Use
+        | DefKind::ForeignMod
+        | DefKind::AnonConst
+        | DefKind::InlineConst
+        | DefKind::OpaqueTy
+        | DefKind::Field
+        | DefKind::LifetimeParam
+        | DefKind::GlobalAsm
+        | DefKind::Closure => return,
+    }
+
+    // The ordering of this lifetime map is a bit subtle.
+    //
+    // Specifically, we want to find a "candidate" lifetime that precedes a "victim" lifetime,
+    // where we can prove that `'candidate = 'victim`.
+    //
+    // `'static` must come first in this list because we can never replace `'static` with
+    // something else, but if we find some lifetime `'a` where `'a = 'static`, we want to
+    // suggest replacing `'a` with `'static`.
+    let mut lifetimes = vec![tcx.lifetimes.re_static];
+    lifetimes.extend(
+        ty::GenericArgs::identity_for_item(tcx, owner_id).iter().filter_map(|arg| arg.as_region()),
+    );
+    // If we are in a function, add its late-bound lifetimes too.
+    if matches!(def_kind, DefKind::Fn | DefKind::AssocFn) {
+        for var in tcx.fn_sig(owner_id).instantiate_identity().bound_vars() {
+            let ty::BoundVariableKind::Region(kind) = var else { continue };
+            lifetimes.push(ty::Region::new_late_param(tcx, owner_id.to_def_id(), kind));
+        }
+    }
+    lifetimes.retain(|candidate| candidate.has_name());
+
+    // Keep track of lifetimes which have already been replaced with other lifetimes.
+    // This makes sure that if `'a = 'b = 'c`, we don't say `'c` should be replaced by
+    // both `'a` and `'b`.
+    let mut shadowed = FxHashSet::default();
+
+    for (idx, &candidate) in lifetimes.iter().enumerate() {
+        // Don't suggest removing a lifetime twice. We only need to check this
+        // here and not up in the `victim` loop because equality is transitive,
+        // so if A = C and B = C, then A must = B, so it'll be shadowed too in
+        // A's victim loop.
+        if shadowed.contains(&candidate) {
+            continue;
+        }
+
+        for &victim in &lifetimes[(idx + 1)..] {
+            // We should only have late-bound lifetimes of the `BrNamed` variety,
+            // since we get these signatures straight from `hir_lowering`. And any
+            // other regions (ReError/ReStatic/etc.) shouldn't matter, since we
+            // can't really suggest to remove them.
+            let (ty::ReEarlyParam(ty::EarlyParamRegion { def_id, .. })
+            | ty::ReLateParam(ty::LateParamRegion {
+                bound_region: ty::BoundRegionKind::BrNamed(def_id, _),
+                ..
+            })) = victim.kind()
+            else {
+                continue;
+            };
+
+            // Do not rename lifetimes not local to this item since they'll overlap
+            // with the lint running on the parent. We still want to consider parent
+            // lifetimes which make child lifetimes redundant, otherwise we would
+            // have truncated the `identity_for_item` args above.
+            if tcx.parent(def_id) != owner_id.to_def_id() {
+                continue;
+            }
+
+            // If `candidate <: victim` and `victim <: candidate`, then they're equal.
+            if outlives_env.free_region_map().sub_free_regions(tcx, candidate, victim)
+                && outlives_env.free_region_map().sub_free_regions(tcx, victim, candidate)
+            {
+                shadowed.insert(victim);
+                tcx.emit_node_span_lint(
+                    rustc_lint_defs::builtin::REDUNDANT_LIFETIMES,
+                    tcx.local_def_id_to_hir_id(def_id.expect_local()),
+                    tcx.def_span(def_id),
+                    RedundantLifetimeArgsLint { candidate, victim },
+                );
+            }
+        }
+    }
+}
+
+#[derive(LintDiagnostic)]
+#[diag(hir_analysis_redundant_lifetime_args)]
+#[note]
+struct RedundantLifetimeArgsLint<'tcx> {
+    /// The lifetime we have found to be redundant.
+    victim: ty::Region<'tcx>,
+    // The lifetime we can replace the victim with.
+    candidate: ty::Region<'tcx>,
+}
+
 pub fn provide(providers: &mut Providers) {
     *providers = Providers { check_mod_type_wf, check_well_formed, ..*providers };
 }
diff --git a/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs b/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
index 0b8ac9926e4..affd678fc6c 100644
--- a/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
+++ b/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
@@ -20,7 +20,6 @@ use rustc_middle::hir::nested_filter;
 use rustc_middle::middle::resolve_bound_vars::*;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::{self, TyCtxt, TypeSuperVisitable, TypeVisitor};
-use rustc_session::lint;
 use rustc_span::def_id::DefId;
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::Span;
@@ -867,31 +866,6 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
             }) => {
                 self.visit_lifetime(lifetime);
                 walk_list!(self, visit_param_bound, bounds);
-
-                if lifetime.res != hir::LifetimeName::Static {
-                    for bound in bounds {
-                        let hir::GenericBound::Outlives(lt) = bound else {
-                            continue;
-                        };
-                        if lt.res != hir::LifetimeName::Static {
-                            continue;
-                        }
-                        self.insert_lifetime(lt, ResolvedArg::StaticLifetime);
-                        self.tcx.node_span_lint(
-                            lint::builtin::UNUSED_LIFETIMES,
-                            lifetime.hir_id,
-                            lifetime.ident.span,
-                            format!("unnecessary lifetime parameter `{}`", lifetime.ident),
-                            |lint| {
-                                let help = format!(
-                                    "you can use the `'static` lifetime directly, in place of `{}`",
-                                    lifetime.ident,
-                                );
-                                lint.help(help);
-                            },
-                        );
-                    }
-                }
             }
             &hir::WherePredicate::EqPredicate(hir::WhereEqPredicate { lhs_ty, rhs_ty, .. }) => {
                 self.visit_ty(lhs_ty);
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
index 59f0fac5aa7..9fb8b4ac40e 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
@@ -2223,6 +2223,24 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                                         Err(LitToConstError::TypeError) => todo!(),
                                     }
                                 }
+
+                                hir::ExprKind::Path(hir::QPath::Resolved(
+                                    _,
+                                    &hir::Path {
+                                        res: Res::Def(DefKind::ConstParam, def_id), ..
+                                    },
+                                )) => {
+                                    let ty = tcx
+                                        .type_of(def_id)
+                                        .no_bound_vars()
+                                        .expect("const parameter types cannot be generic");
+                                    let item_def_id = tcx.parent(def_id);
+                                    let generics = tcx.generics_of(item_def_id);
+                                    let index = generics.param_def_id_to_index[&def_id];
+                                    let name = tcx.item_name(def_id);
+                                    ty::Const::new_param(tcx, ty::ParamConst::new(index, name), ty)
+                                }
+
                                 _ => {
                                     let err = tcx
                                         .dcx()
diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
index 53b5273803c..2713690f812 100644
--- a/compiler/rustc_lint_defs/src/builtin.rs
+++ b/compiler/rustc_lint_defs/src/builtin.rs
@@ -79,6 +79,7 @@ declare_lint_pass! {
         PROC_MACRO_BACK_COMPAT,
         PROC_MACRO_DERIVE_RESOLUTION_FALLBACK,
         PUB_USE_OF_PRIVATE_EXTERN_CRATE,
+        REDUNDANT_LIFETIMES,
         REFINING_IMPL_TRAIT_INTERNAL,
         REFINING_IMPL_TRAIT_REACHABLE,
         RENAMED_AND_REMOVED_LINTS,
@@ -1708,6 +1709,33 @@ declare_lint! {
 }
 
 declare_lint! {
+    /// The `redundant_lifetimes` lint detects lifetime parameters that are
+    /// redundant because they are equal to another named lifetime.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #[deny(redundant_lifetimes)]
+    ///
+    /// // `'a = 'static`, so all usages of `'a` can be replaced with `'static`
+    /// pub fn bar<'a: 'static>() {}
+    ///
+    /// // `'a = 'b`, so all usages of `'b` can be replaced with `'a`
+    /// pub fn bar<'a: 'b, 'b: 'a>() {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Unused lifetime parameters may signal a mistake or unfinished code.
+    /// Consider removing the parameter.
+    pub REDUNDANT_LIFETIMES,
+    Allow,
+    "detects lifetime parameters that are redundant because they are equal to some other named lifetime"
+}
+
+declare_lint! {
     /// The `tyvar_behind_raw_pointer` lint detects raw pointer to an
     /// inference variable.
     ///
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 0daf83162db..6275c5d2a11 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -757,7 +757,7 @@ pub struct GlobalCtxt<'tcx> {
 impl<'tcx> GlobalCtxt<'tcx> {
     /// Installs `self` in a `TyCtxt` and `ImplicitCtxt` for the duration of
     /// `f`.
-    pub fn enter<'a: 'tcx, F, R>(&'a self, f: F) -> R
+    pub fn enter<F, R>(&'tcx self, f: F) -> R
     where
         F: FnOnce(TyCtxt<'tcx>) -> R,
     {
diff --git a/compiler/rustc_middle/src/ty/diagnostics.rs b/compiler/rustc_middle/src/ty/diagnostics.rs
index ee18647cdd8..cc1d6e50f6d 100644
--- a/compiler/rustc_middle/src/ty/diagnostics.rs
+++ b/compiler/rustc_middle/src/ty/diagnostics.rs
@@ -5,13 +5,13 @@ use std::fmt::Write;
 use std::ops::ControlFlow;
 
 use crate::ty::{
-    AliasTy, Const, ConstKind, FallibleTypeFolder, InferConst, InferTy, Opaque, PolyTraitPredicate,
-    Projection, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable, TypeSuperVisitable, TypeVisitable,
-    TypeVisitor,
+    self, AliasTy, Const, ConstKind, FallibleTypeFolder, InferConst, InferTy, Opaque,
+    PolyTraitPredicate, Projection, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable,
+    TypeSuperVisitable, TypeVisitable, TypeVisitor,
 };
 
 use rustc_data_structures::fx::FxHashMap;
-use rustc_errors::{Applicability, Diag, DiagArgValue, IntoDiagArg};
+use rustc_errors::{into_diag_arg_using_display, Applicability, Diag, DiagArgValue, IntoDiagArg};
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::DefId;
@@ -19,10 +19,9 @@ use rustc_hir::{PredicateOrigin, WherePredicate};
 use rustc_span::{BytePos, Span};
 use rustc_type_ir::TyKind::*;
 
-impl<'tcx> IntoDiagArg for Ty<'tcx> {
-    fn into_diag_arg(self) -> DiagArgValue {
-        self.to_string().into_diag_arg()
-    }
+into_diag_arg_using_display! {
+    Ty<'_>,
+    ty::Region<'_>,
 }
 
 impl<'tcx> Ty<'tcx> {
diff --git a/compiler/rustc_mir_transform/src/coroutine/by_move_body.rs b/compiler/rustc_mir_transform/src/coroutine/by_move_body.rs
index ccb229616e8..b26f968bf5e 100644
--- a/compiler/rustc_mir_transform/src/coroutine/by_move_body.rs
+++ b/compiler/rustc_mir_transform/src/coroutine/by_move_body.rs
@@ -154,6 +154,14 @@ impl<'tcx> MirPass<'tcx> for ByMoveBody {
             }) {
                 let (child_field_idx, child_capture) = child_captures.next().unwrap();
 
+                // This analysis only makes sense if the parent capture is a
+                // prefix of the child capture.
+                assert!(
+                    child_capture.place.projections.len() >= parent_capture.place.projections.len(),
+                    "parent capture ({parent_capture:#?}) expected to be prefix of \
+                    child capture ({child_capture:#?})"
+                );
+
                 // Store this set of additional projections (fields and derefs).
                 // We need to re-apply them later.
                 let child_precise_captures =
@@ -244,7 +252,6 @@ fn child_prefix_matches_parent_projections(
         bug!("expected capture to be an upvar");
     };
 
-    assert!(child_capture.place.projections.len() >= parent_capture.place.projections.len());
     parent_base.var_path.hir_id == child_base.var_path.hir_id
         && std::iter::zip(&child_capture.place.projections, &parent_capture.place.projections)
             .all(|(child, parent)| child.kind == parent.kind)
diff --git a/compiler/rustc_smir/src/rustc_smir/builder.rs b/compiler/rustc_smir/src/rustc_smir/builder.rs
index 0762016ef75..221224eed01 100644
--- a/compiler/rustc_smir/src/rustc_smir/builder.rs
+++ b/compiler/rustc_smir/src/rustc_smir/builder.rs
@@ -4,9 +4,10 @@
 //! monomorphic body using internal representation.
 //! After that, we convert the internal representation into a stable one.
 use crate::rustc_smir::{Stable, Tables};
+use rustc_hir::def::DefKind;
 use rustc_middle::mir;
 use rustc_middle::mir::visit::MutVisitor;
-use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt};
+use rustc_middle::ty::{self, TyCtxt};
 
 /// Builds a monomorphic body for a given instance.
 pub struct BodyBuilder<'tcx> {
@@ -16,46 +17,43 @@ pub struct BodyBuilder<'tcx> {
 
 impl<'tcx> BodyBuilder<'tcx> {
     pub fn new(tcx: TyCtxt<'tcx>, instance: ty::Instance<'tcx>) -> Self {
+        let instance = match instance.def {
+            // To get the fallback body of an intrinsic, we need to convert it to an item.
+            ty::InstanceDef::Intrinsic(def_id) => ty::Instance::new(def_id, instance.args),
+            _ => instance,
+        };
         BodyBuilder { tcx, instance }
     }
 
     /// Build a stable monomorphic body for a given instance based on the MIR body.
     ///
-    /// Note that we skip instantiation for static and constants. Trying to do so can cause ICE.
-    ///
-    /// We do monomorphize non-generic functions to eval unevaluated constants.
+    /// All constants are also evaluated.
     pub fn build(mut self, tables: &mut Tables<'tcx>) -> stable_mir::mir::Body {
-        let mut body = self.tcx.instance_mir(self.instance.def).clone();
-        if self.tcx.def_kind(self.instance.def_id()).is_fn_like() || !self.instance.args.is_empty()
+        let body = tables.tcx.instance_mir(self.instance.def).clone();
+        let mono_body = if !self.instance.args.is_empty()
+            // Without the `generic_const_exprs` feature gate, anon consts in signatures do not
+            // get generic parameters. Which is wrong, but also not a problem without
+            // generic_const_exprs
+            || self.tcx.def_kind(self.instance.def_id()) != DefKind::AnonConst
         {
-            self.visit_body(&mut body);
-        }
-        body.stable(tables)
-    }
-
-    fn monomorphize<T>(&self, value: T) -> T
-    where
-        T: ty::TypeFoldable<TyCtxt<'tcx>>,
-    {
-        self.instance.instantiate_mir_and_normalize_erasing_regions(
-            self.tcx,
-            ty::ParamEnv::reveal_all(),
-            ty::EarlyBinder::bind(value),
-        )
+            let mut mono_body = self.instance.instantiate_mir_and_normalize_erasing_regions(
+                tables.tcx,
+                ty::ParamEnv::reveal_all(),
+                ty::EarlyBinder::bind(body),
+            );
+            self.visit_body(&mut mono_body);
+            mono_body
+        } else {
+            // Already monomorphic.
+            body
+        };
+        mono_body.stable(tables)
     }
 }
 
 impl<'tcx> MutVisitor<'tcx> for BodyBuilder<'tcx> {
-    fn visit_ty_const(&mut self, ct: &mut ty::Const<'tcx>, _location: mir::Location) {
-        *ct = self.monomorphize(*ct);
-    }
-
-    fn visit_ty(&mut self, ty: &mut Ty<'tcx>, _: mir::visit::TyContext) {
-        *ty = self.monomorphize(*ty);
-    }
-
     fn visit_constant(&mut self, constant: &mut mir::ConstOperand<'tcx>, location: mir::Location) {
-        let const_ = self.monomorphize(constant.const_);
+        let const_ = constant.const_;
         let val = match const_.eval(self.tcx, ty::ParamEnv::reveal_all(), constant.span) {
             Ok(v) => v,
             Err(mir::interpret::ErrorHandled::Reported(..)) => return,
@@ -68,10 +66,6 @@ impl<'tcx> MutVisitor<'tcx> for BodyBuilder<'tcx> {
         self.super_constant(constant, location);
     }
 
-    fn visit_args(&mut self, args: &mut GenericArgsRef<'tcx>, _: mir::Location) {
-        *args = self.monomorphize(*args);
-    }
-
     fn tcx(&self) -> TyCtxt<'tcx> {
         self.tcx
     }
diff --git a/compiler/rustc_ty_utils/src/layout.rs b/compiler/rustc_ty_utils/src/layout.rs
index 77cd4662ae5..902b76e8c1e 100644
--- a/compiler/rustc_ty_utils/src/layout.rs
+++ b/compiler/rustc_ty_utils/src/layout.rs
@@ -133,10 +133,14 @@ fn layout_of_uncached<'tcx>(
                 ty::PatternKind::Range { start, end, include_end } => {
                     if let Abi::Scalar(scalar) | Abi::ScalarPair(scalar, _) = &mut layout.abi {
                         if let Some(start) = start {
-                            scalar.valid_range_mut().start = start.eval_bits(tcx, param_env);
+                            scalar.valid_range_mut().start = start
+                                .try_eval_bits(tcx, param_env)
+                                .ok_or_else(|| error(cx, LayoutError::Unknown(ty)))?;
                         }
                         if let Some(end) = end {
-                            let mut end = end.eval_bits(tcx, param_env);
+                            let mut end = end
+                                .try_eval_bits(tcx, param_env)
+                                .ok_or_else(|| error(cx, LayoutError::Unknown(ty)))?;
                             if !include_end {
                                 end = end.wrapping_sub(1);
                             }
diff --git a/compiler/stable_mir/src/mir/mono.rs b/compiler/stable_mir/src/mir/mono.rs
index aafa89c03e0..a032a180fcf 100644
--- a/compiler/stable_mir/src/mir/mono.rs
+++ b/compiler/stable_mir/src/mir/mono.rs
@@ -41,13 +41,22 @@ impl Instance {
         with(|cx| cx.instance_args(self.def))
     }
 
-    /// Get the body of an Instance. The body will be eagerly monomorphized.
+    /// Get the body of an Instance.
+    ///
+    /// The body will be eagerly monomorphized and all constants will already be evaluated.
+    ///
+    /// This method will return the intrinsic fallback body if one was defined.
     pub fn body(&self) -> Option<Body> {
         with(|context| context.instance_body(self.def))
     }
 
     /// Check whether this instance has a body available.
     ///
+    /// For intrinsics with fallback body, this will return `true`. It is up to the user to decide
+    /// whether to specialize the intrinsic or to use its fallback body.
+    ///
+    /// For more information on fallback body, see <https://github.com/rust-lang/rust/issues/93145>.
+    ///
     /// This call is much cheaper than `instance.body().is_some()`, since it doesn't try to build
     /// the StableMIR body.
     pub fn has_body(&self) -> bool {