Auto merge of #18267 - lnicola:sync-from-rust, r=lnicola

minor: Sync from downstream

15 files changed, 602 insertions, 269 deletions

diff --git a/compiler/rustc_mir_transform/src/check_undefined_transmutes.rs b/compiler/rustc_mir_transform/src/check_undefined_transmutes.rs
new file mode 100644
index 00000000000..8ba14a1158e
--- /dev/null
+++ b/compiler/rustc_mir_transform/src/check_undefined_transmutes.rs
@@ -0,0 +1,77 @@
+use rustc_middle::mir::visit::Visitor;
+use rustc_middle::mir::{Body, Location, Operand, Terminator, TerminatorKind};
+use rustc_middle::ty::{AssocItem, AssocKind, TyCtxt};
+use rustc_session::lint::builtin::PTR_TO_INTEGER_TRANSMUTE_IN_CONSTS;
+use rustc_span::sym;
+
+use crate::errors;
+
+/// Check for transmutes that exhibit undefined behavior.
+/// For example, transmuting pointers to integers in a const context.
+pub(super) struct CheckUndefinedTransmutes;
+
+impl<'tcx> crate::MirLint<'tcx> for CheckUndefinedTransmutes {
+    fn run_lint(&self, tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
+        let mut checker = UndefinedTransmutesChecker { body, tcx };
+        checker.visit_body(body);
+    }
+}
+
+struct UndefinedTransmutesChecker<'a, 'tcx> {
+    body: &'a Body<'tcx>,
+    tcx: TyCtxt<'tcx>,
+}
+
+impl<'a, 'tcx> UndefinedTransmutesChecker<'a, 'tcx> {
+    // This functions checks two things:
+    // 1. `function` takes a raw pointer as input and returns an integer as output.
+    // 2. `function` is called from a const function or an associated constant.
+    //
+    // Why do we consider const functions and associated constants only?
+    //
+    // Generally, undefined behavior in const items are handled by the evaluator.
+    // But, const functions and associated constants are evaluated only when referenced.
+    // This can result in undefined behavior in a library going unnoticed until
+    // the function or constant is actually used.
+    //
+    // Therefore, we only consider const functions and associated constants here and leave
+    // other const items to be handled by the evaluator.
+    fn is_ptr_to_int_in_const(&self, function: &Operand<'tcx>) -> bool {
+        let def_id = self.body.source.def_id();
+
+        if self.tcx.is_const_fn(def_id)
+            || matches!(
+                self.tcx.opt_associated_item(def_id),
+                Some(AssocItem { kind: AssocKind::Const, .. })
+            )
+        {
+            let fn_sig = function.ty(self.body, self.tcx).fn_sig(self.tcx).skip_binder();
+            if let [input] = fn_sig.inputs() {
+                return input.is_unsafe_ptr() && fn_sig.output().is_integral();
+            }
+        }
+        false
+    }
+}
+
+impl<'tcx> Visitor<'tcx> for UndefinedTransmutesChecker<'_, 'tcx> {
+    // Check each block's terminator for calls to pointer to integer transmutes
+    // in const functions or associated constants and emit a lint.
+    fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
+        if let TerminatorKind::Call { func, .. } = &terminator.kind
+            && let Some((func_def_id, _)) = func.const_fn_def()
+            && self.tcx.is_intrinsic(func_def_id, sym::transmute)
+            && self.is_ptr_to_int_in_const(func)
+            && let Some(call_id) = self.body.source.def_id().as_local()
+        {
+            let hir_id = self.tcx.local_def_id_to_hir_id(call_id);
+            let span = self.body.source_info(location).span;
+            self.tcx.emit_node_span_lint(
+                PTR_TO_INTEGER_TRANSMUTE_IN_CONSTS,
+                hir_id,
+                span,
+                errors::UndefinedTransmute,
+            );
+        }
+    }
+}
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 65442877d2d..cc4b7689d40 100644
--- a/compiler/rustc_mir_transform/src/coroutine/by_move_body.rs
+++ b/compiler/rustc_mir_transform/src/coroutine/by_move_body.rs
@@ -223,14 +223,14 @@ pub(crate) fn coroutine_by_move_body_def_id<'tcx>(
 
     // Inherited from the by-ref coroutine.
     body_def.codegen_fn_attrs(tcx.codegen_fn_attrs(coroutine_def_id).clone());
-    body_def.constness(tcx.constness(coroutine_def_id).clone());
-    body_def.coroutine_kind(tcx.coroutine_kind(coroutine_def_id).clone());
+    body_def.constness(tcx.constness(coroutine_def_id));
+    body_def.coroutine_kind(tcx.coroutine_kind(coroutine_def_id));
     body_def.def_ident_span(tcx.def_ident_span(coroutine_def_id));
     body_def.def_span(tcx.def_span(coroutine_def_id));
-    body_def.explicit_predicates_of(tcx.explicit_predicates_of(coroutine_def_id).clone());
+    body_def.explicit_predicates_of(tcx.explicit_predicates_of(coroutine_def_id));
     body_def.generics_of(tcx.generics_of(coroutine_def_id).clone());
-    body_def.param_env(tcx.param_env(coroutine_def_id).clone());
-    body_def.predicates_of(tcx.predicates_of(coroutine_def_id).clone());
+    body_def.param_env(tcx.param_env(coroutine_def_id));
+    body_def.predicates_of(tcx.predicates_of(coroutine_def_id));
 
     // The type of the coroutine is the `by_move_coroutine_ty`.
     body_def.type_of(ty::EarlyBinder::bind(by_move_coroutine_ty));
diff --git a/compiler/rustc_mir_transform/src/coverage/counters.rs b/compiler/rustc_mir_transform/src/coverage/counters.rs
index ef4031c5c03..94088156756 100644
--- a/compiler/rustc_mir_transform/src/coverage/counters.rs
+++ b/compiler/rustc_mir_transform/src/coverage/counters.rs
@@ -4,6 +4,7 @@ use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::graph::DirectedGraph;
 use rustc_index::IndexVec;
+use rustc_index::bit_set::BitSet;
 use rustc_middle::bug;
 use rustc_middle::mir::coverage::{CounterId, CovTerm, Expression, ExpressionId, Op};
 use tracing::{debug, debug_span, instrument};
@@ -13,13 +14,13 @@ use crate::coverage::graph::{BasicCoverageBlock, CoverageGraph, TraverseCoverage
 /// The coverage counter or counter expression associated with a particular
 /// BCB node or BCB edge.
 #[derive(Clone, Copy, PartialEq, Eq, Hash)]
-pub(super) enum BcbCounter {
+enum BcbCounter {
     Counter { id: CounterId },
     Expression { id: ExpressionId },
 }
 
 impl BcbCounter {
-    pub(super) fn as_term(&self) -> CovTerm {
+    fn as_term(&self) -> CovTerm {
         match *self {
             BcbCounter::Counter { id, .. } => CovTerm::Counter(id),
             BcbCounter::Expression { id, .. } => CovTerm::Expression(id),
@@ -78,21 +79,22 @@ impl CoverageCounters {
     /// counters or counter expressions for nodes and edges as required.
     pub(super) fn make_bcb_counters(
         basic_coverage_blocks: &CoverageGraph,
-        bcb_needs_counter: impl Fn(BasicCoverageBlock) -> bool,
+        bcb_needs_counter: &BitSet<BasicCoverageBlock>,
     ) -> Self {
-        let num_bcbs = basic_coverage_blocks.num_nodes();
+        let mut counters = MakeBcbCounters::new(basic_coverage_blocks, bcb_needs_counter);
+        counters.make_bcb_counters();
 
-        let mut this = Self {
+        counters.coverage_counters
+    }
+
+    fn with_num_bcbs(num_bcbs: usize) -> Self {
+        Self {
             counter_increment_sites: IndexVec::new(),
             bcb_counters: IndexVec::from_elem_n(None, num_bcbs),
             bcb_edge_counters: FxHashMap::default(),
             expressions: IndexVec::new(),
             expressions_memo: FxHashMap::default(),
-        };
-
-        MakeBcbCounters::new(&mut this, basic_coverage_blocks).make_bcb_counters(bcb_needs_counter);
-
-        this
+        }
     }
 
     /// Shared helper used by [`Self::make_phys_node_counter`] and
@@ -218,8 +220,8 @@ impl CoverageCounters {
         }
     }
 
-    pub(super) fn bcb_counter(&self, bcb: BasicCoverageBlock) -> Option<BcbCounter> {
-        self.bcb_counters[bcb]
+    pub(super) fn term_for_bcb(&self, bcb: BasicCoverageBlock) -> Option<CovTerm> {
+        self.bcb_counters[bcb].map(|counter| counter.as_term())
     }
 
     /// Returns an iterator over all the nodes/edges in the coverage graph that
@@ -265,19 +267,25 @@ impl CoverageCounters {
 
 /// Helper struct that allows counter creation to inspect the BCB graph.
 struct MakeBcbCounters<'a> {
-    coverage_counters: &'a mut CoverageCounters,
+    coverage_counters: CoverageCounters,
     basic_coverage_blocks: &'a CoverageGraph,
+    bcb_needs_counter: &'a BitSet<BasicCoverageBlock>,
 }
 
 impl<'a> MakeBcbCounters<'a> {
     fn new(
-        coverage_counters: &'a mut CoverageCounters,
         basic_coverage_blocks: &'a CoverageGraph,
+        bcb_needs_counter: &'a BitSet<BasicCoverageBlock>,
     ) -> Self {
-        Self { coverage_counters, basic_coverage_blocks }
+        assert_eq!(basic_coverage_blocks.num_nodes(), bcb_needs_counter.domain_size());
+        Self {
+            coverage_counters: CoverageCounters::with_num_bcbs(basic_coverage_blocks.num_nodes()),
+            basic_coverage_blocks,
+            bcb_needs_counter,
+        }
     }
 
-    fn make_bcb_counters(&mut self, bcb_needs_counter: impl Fn(BasicCoverageBlock) -> bool) {
+    fn make_bcb_counters(&mut self) {
         debug!("make_bcb_counters(): adding a counter or expression to each BasicCoverageBlock");
 
         // Traverse the coverage graph, ensuring that every node that needs a
@@ -290,7 +298,7 @@ impl<'a> MakeBcbCounters<'a> {
         let mut traversal = TraverseCoverageGraphWithLoops::new(self.basic_coverage_blocks);
         while let Some(bcb) = traversal.next() {
             let _span = debug_span!("traversal", ?bcb).entered();
-            if bcb_needs_counter(bcb) {
+            if self.bcb_needs_counter.contains(bcb) {
                 self.make_node_counter_and_out_edge_counters(&traversal, bcb);
             }
         }
diff --git a/compiler/rustc_mir_transform/src/coverage/mappings.rs b/compiler/rustc_mir_transform/src/coverage/mappings.rs
index ec5ba354805..bc86ae22a0d 100644
--- a/compiler/rustc_mir_transform/src/coverage/mappings.rs
+++ b/compiler/rustc_mir_transform/src/coverage/mappings.rs
@@ -1,10 +1,11 @@
 use std::collections::BTreeSet;
 
+use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::graph::DirectedGraph;
 use rustc_index::IndexVec;
 use rustc_index::bit_set::BitSet;
 use rustc_middle::mir::coverage::{
-    BlockMarkerId, BranchSpan, ConditionInfo, CoverageInfoHi, CoverageKind,
+    BlockMarkerId, BranchSpan, ConditionId, ConditionInfo, CoverageInfoHi, CoverageKind,
 };
 use rustc_middle::mir::{self, BasicBlock, StatementKind};
 use rustc_middle::ty::TyCtxt;
@@ -14,6 +15,7 @@ use crate::coverage::ExtractedHirInfo;
 use crate::coverage::graph::{BasicCoverageBlock, CoverageGraph, START_BCB};
 use crate::coverage::spans::extract_refined_covspans;
 use crate::coverage::unexpand::unexpand_into_body_span;
+use crate::errors::MCDCExceedsTestVectorLimit;
 
 /// Associates an ordinary executable code span with its corresponding BCB.
 #[derive(Debug)]
@@ -38,10 +40,11 @@ pub(super) struct MCDCBranch {
     pub(super) span: Span,
     pub(super) true_bcb: BasicCoverageBlock,
     pub(super) false_bcb: BasicCoverageBlock,
-    /// If `None`, this actually represents a normal branch mapping inserted
-    /// for code that was too complex for MC/DC.
-    pub(super) condition_info: Option<ConditionInfo>,
-    pub(super) decision_depth: u16,
+    pub(super) condition_info: ConditionInfo,
+    // Offset added to test vector idx if this branch is evaluated to true.
+    pub(super) true_index: usize,
+    // Offset added to test vector idx if this branch is evaluated to false.
+    pub(super) false_index: usize,
 }
 
 /// Associates an MC/DC decision with its join BCBs.
@@ -49,11 +52,15 @@ pub(super) struct MCDCBranch {
 pub(super) struct MCDCDecision {
     pub(super) span: Span,
     pub(super) end_bcbs: BTreeSet<BasicCoverageBlock>,
-    pub(super) bitmap_idx: u32,
-    pub(super) num_conditions: u16,
+    pub(super) bitmap_idx: usize,
+    pub(super) num_test_vectors: usize,
     pub(super) decision_depth: u16,
 }
 
+// LLVM uses `i32` to index the bitmap. Thus `i32::MAX` is the hard limit for number of all test vectors
+// in a function.
+const MCDC_MAX_BITMAP_SIZE: usize = i32::MAX as usize;
+
 #[derive(Default)]
 pub(super) struct ExtractedMappings {
     /// Store our own copy of [`CoverageGraph::num_nodes`], so that we don't
@@ -62,9 +69,9 @@ pub(super) struct ExtractedMappings {
     pub(super) num_bcbs: usize,
     pub(super) code_mappings: Vec<CodeMapping>,
     pub(super) branch_pairs: Vec<BranchPair>,
-    pub(super) mcdc_bitmap_bytes: u32,
-    pub(super) mcdc_branches: Vec<MCDCBranch>,
-    pub(super) mcdc_decisions: Vec<MCDCDecision>,
+    pub(super) mcdc_bitmap_bits: usize,
+    pub(super) mcdc_degraded_branches: Vec<MCDCBranch>,
+    pub(super) mcdc_mappings: Vec<(MCDCDecision, Vec<MCDCBranch>)>,
 }
 
 /// Extracts coverage-relevant spans from MIR, and associates them with
@@ -77,9 +84,9 @@ pub(super) fn extract_all_mapping_info_from_mir<'tcx>(
 ) -> ExtractedMappings {
     let mut code_mappings = vec![];
     let mut branch_pairs = vec![];
-    let mut mcdc_bitmap_bytes = 0;
-    let mut mcdc_branches = vec![];
-    let mut mcdc_decisions = vec![];
+    let mut mcdc_bitmap_bits = 0;
+    let mut mcdc_degraded_branches = vec![];
+    let mut mcdc_mappings = vec![];
 
     if hir_info.is_async_fn || tcx.sess.coverage_no_mir_spans() {
         // An async function desugars into a function that returns a future,
@@ -102,20 +109,21 @@ pub(super) fn extract_all_mapping_info_from_mir<'tcx>(
 
     extract_mcdc_mappings(
         mir_body,
+        tcx,
         hir_info.body_span,
         basic_coverage_blocks,
-        &mut mcdc_bitmap_bytes,
-        &mut mcdc_branches,
-        &mut mcdc_decisions,
+        &mut mcdc_bitmap_bits,
+        &mut mcdc_degraded_branches,
+        &mut mcdc_mappings,
     );
 
     ExtractedMappings {
         num_bcbs: basic_coverage_blocks.num_nodes(),
         code_mappings,
         branch_pairs,
-        mcdc_bitmap_bytes,
-        mcdc_branches,
-        mcdc_decisions,
+        mcdc_bitmap_bits,
+        mcdc_degraded_branches,
+        mcdc_mappings,
     }
 }
 
@@ -126,9 +134,9 @@ impl ExtractedMappings {
             num_bcbs,
             code_mappings,
             branch_pairs,
-            mcdc_bitmap_bytes: _,
-            mcdc_branches,
-            mcdc_decisions,
+            mcdc_bitmap_bits: _,
+            mcdc_degraded_branches,
+            mcdc_mappings,
         } = self;
 
         // Identify which BCBs have one or more mappings.
@@ -144,7 +152,10 @@ impl ExtractedMappings {
             insert(true_bcb);
             insert(false_bcb);
         }
-        for &MCDCBranch { true_bcb, false_bcb, .. } in mcdc_branches {
+        for &MCDCBranch { true_bcb, false_bcb, .. } in mcdc_degraded_branches
+            .iter()
+            .chain(mcdc_mappings.iter().map(|(_, branches)| branches.into_iter()).flatten())
+        {
             insert(true_bcb);
             insert(false_bcb);
         }
@@ -152,8 +163,8 @@ impl ExtractedMappings {
         // MC/DC decisions refer to BCBs, but don't require those BCBs to have counters.
         if bcbs_with_counter_mappings.is_empty() {
             debug_assert!(
-                mcdc_decisions.is_empty(),
-                "A function with no counter mappings shouldn't have any decisions: {mcdc_decisions:?}",
+                mcdc_mappings.is_empty(),
+                "A function with no counter mappings shouldn't have any decisions: {mcdc_mappings:?}",
             );
         }
 
@@ -230,11 +241,12 @@ pub(super) fn extract_branch_pairs(
 
 pub(super) fn extract_mcdc_mappings(
     mir_body: &mir::Body<'_>,
+    tcx: TyCtxt<'_>,
     body_span: Span,
     basic_coverage_blocks: &CoverageGraph,
-    mcdc_bitmap_bytes: &mut u32,
-    mcdc_branches: &mut impl Extend<MCDCBranch>,
-    mcdc_decisions: &mut impl Extend<MCDCDecision>,
+    mcdc_bitmap_bits: &mut usize,
+    mcdc_degraded_branches: &mut impl Extend<MCDCBranch>,
+    mcdc_mappings: &mut impl Extend<(MCDCDecision, Vec<MCDCBranch>)>,
 ) {
     let Some(coverage_info_hi) = mir_body.coverage_info_hi.as_deref() else { return };
 
@@ -257,43 +269,146 @@ pub(super) fn extract_mcdc_mappings(
             Some((span, true_bcb, false_bcb))
         };
 
-    mcdc_branches.extend(coverage_info_hi.mcdc_branch_spans.iter().filter_map(
-        |&mir::coverage::MCDCBranchSpan {
-             span: raw_span,
-             condition_info,
-             true_marker,
-             false_marker,
-             decision_depth,
-         }| {
-            let (span, true_bcb, false_bcb) =
-                check_branch_bcb(raw_span, true_marker, false_marker)?;
-            Some(MCDCBranch { span, true_bcb, false_bcb, condition_info, decision_depth })
-        },
-    ));
-
-    mcdc_decisions.extend(coverage_info_hi.mcdc_decision_spans.iter().filter_map(
-        |decision: &mir::coverage::MCDCDecisionSpan| {
-            let span = unexpand_into_body_span(decision.span, body_span)?;
-
-            let end_bcbs = decision
-                .end_markers
-                .iter()
-                .map(|&marker| bcb_from_marker(marker))
-                .collect::<Option<_>>()?;
-
-            // Each decision containing N conditions needs 2^N bits of space in
-            // the bitmap, rounded up to a whole number of bytes.
-            // The decision's "bitmap index" points to its first byte in the bitmap.
-            let bitmap_idx = *mcdc_bitmap_bytes;
-            *mcdc_bitmap_bytes += (1_u32 << decision.num_conditions).div_ceil(8);
-
-            Some(MCDCDecision {
+    let to_mcdc_branch = |&mir::coverage::MCDCBranchSpan {
+                              span: raw_span,
+                              condition_info,
+                              true_marker,
+                              false_marker,
+                          }| {
+        let (span, true_bcb, false_bcb) = check_branch_bcb(raw_span, true_marker, false_marker)?;
+        Some(MCDCBranch {
+            span,
+            true_bcb,
+            false_bcb,
+            condition_info,
+            true_index: usize::MAX,
+            false_index: usize::MAX,
+        })
+    };
+
+    let mut get_bitmap_idx = |num_test_vectors: usize| -> Option<usize> {
+        let bitmap_idx = *mcdc_bitmap_bits;
+        let next_bitmap_bits = bitmap_idx.saturating_add(num_test_vectors);
+        (next_bitmap_bits <= MCDC_MAX_BITMAP_SIZE).then(|| {
+            *mcdc_bitmap_bits = next_bitmap_bits;
+            bitmap_idx
+        })
+    };
+    mcdc_degraded_branches
+        .extend(coverage_info_hi.mcdc_degraded_branch_spans.iter().filter_map(to_mcdc_branch));
+
+    mcdc_mappings.extend(coverage_info_hi.mcdc_spans.iter().filter_map(|(decision, branches)| {
+        if branches.len() == 0 {
+            return None;
+        }
+        let decision_span = unexpand_into_body_span(decision.span, body_span)?;
+
+        let end_bcbs = decision
+            .end_markers
+            .iter()
+            .map(|&marker| bcb_from_marker(marker))
+            .collect::<Option<_>>()?;
+        let mut branch_mappings: Vec<_> = branches.into_iter().filter_map(to_mcdc_branch).collect();
+        if branch_mappings.len() != branches.len() {
+            mcdc_degraded_branches.extend(branch_mappings);
+            return None;
+        }
+        let num_test_vectors = calc_test_vectors_index(&mut branch_mappings);
+        let Some(bitmap_idx) = get_bitmap_idx(num_test_vectors) else {
+            tcx.dcx().emit_warn(MCDCExceedsTestVectorLimit {
+                span: decision_span,
+                max_num_test_vectors: MCDC_MAX_BITMAP_SIZE,
+            });
+            mcdc_degraded_branches.extend(branch_mappings);
+            return None;
+        };
+        // LLVM requires span of the decision contains all spans of its conditions.
+        // Usually the decision span meets the requirement well but in cases like macros it may not.
+        let span = branch_mappings
+            .iter()
+            .map(|branch| branch.span)
+            .reduce(|lhs, rhs| lhs.to(rhs))
+            .map(
+                |joint_span| {
+                    if decision_span.contains(joint_span) { decision_span } else { joint_span }
+                },
+            )
+            .expect("branch mappings are ensured to be non-empty as checked above");
+        Some((
+            MCDCDecision {
                 span,
                 end_bcbs,
                 bitmap_idx,
-                num_conditions: decision.num_conditions as u16,
+                num_test_vectors,
                 decision_depth: decision.decision_depth,
-            })
-        },
-    ));
+            },
+            branch_mappings,
+        ))
+    }));
+}
+
+// LLVM checks the executed test vector by accumulating indices of tested branches.
+// We calculate number of all possible test vectors of the decision and assign indices
+// to branches here.
+// See [the rfc](https://discourse.llvm.org/t/rfc-coverage-new-algorithm-and-file-format-for-mc-dc/76798/)
+// for more details about the algorithm.
+// This function is mostly like [`TVIdxBuilder::TvIdxBuilder`](https://github.com/llvm/llvm-project/blob/d594d9f7f4dc6eb748b3261917db689fdc348b96/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp#L226)
+fn calc_test_vectors_index(conditions: &mut Vec<MCDCBranch>) -> usize {
+    let mut indegree_stats = IndexVec::<ConditionId, usize>::from_elem_n(0, conditions.len());
+    // `num_paths` is `width` described at the llvm rfc, which indicates how many paths reaching the condition node.
+    let mut num_paths_stats = IndexVec::<ConditionId, usize>::from_elem_n(0, conditions.len());
+    let mut next_conditions = conditions
+        .iter_mut()
+        .map(|branch| {
+            let ConditionInfo { condition_id, true_next_id, false_next_id } = branch.condition_info;
+            [true_next_id, false_next_id]
+                .into_iter()
+                .filter_map(std::convert::identity)
+                .for_each(|next_id| indegree_stats[next_id] += 1);
+            (condition_id, branch)
+        })
+        .collect::<FxIndexMap<_, _>>();
+
+    let mut queue = std::collections::VecDeque::from_iter(
+        next_conditions.swap_remove(&ConditionId::START).into_iter(),
+    );
+    num_paths_stats[ConditionId::START] = 1;
+    let mut decision_end_nodes = Vec::new();
+    while let Some(branch) = queue.pop_front() {
+        let ConditionInfo { condition_id, true_next_id, false_next_id } = branch.condition_info;
+        let (false_index, true_index) = (&mut branch.false_index, &mut branch.true_index);
+        let this_paths_count = num_paths_stats[condition_id];
+        // Note. First check the false next to ensure conditions are touched in same order with llvm-cov.
+        for (next, index) in [(false_next_id, false_index), (true_next_id, true_index)] {
+            if let Some(next_id) = next {
+                let next_paths_count = &mut num_paths_stats[next_id];
+                *index = *next_paths_count;
+                *next_paths_count = next_paths_count.saturating_add(this_paths_count);
+                let next_indegree = &mut indegree_stats[next_id];
+                *next_indegree -= 1;
+                if *next_indegree == 0 {
+                    queue.push_back(next_conditions.swap_remove(&next_id).expect(
+                        "conditions with non-zero indegree before must be in next_conditions",
+                    ));
+                }
+            } else {
+                decision_end_nodes.push((this_paths_count, condition_id, index));
+            }
+        }
+    }
+    assert!(next_conditions.is_empty(), "the decision tree has untouched nodes");
+    let mut cur_idx = 0;
+    // LLVM hopes the end nodes are sorted in descending order by `num_paths` so that it can
+    // optimize bitmap size for decisions in tree form such as `a && b && c && d && ...`.
+    decision_end_nodes.sort_by_key(|(num_paths, _, _)| usize::MAX - *num_paths);
+    for (num_paths, condition_id, index) in decision_end_nodes {
+        assert_eq!(
+            num_paths, num_paths_stats[condition_id],
+            "end nodes should not be updated since they were visited"
+        );
+        assert_eq!(*index, usize::MAX, "end nodes should not be assigned index before");
+        *index = cur_idx;
+        cur_idx += num_paths;
+    }
+    cur_idx
 }
diff --git a/compiler/rustc_mir_transform/src/coverage/mod.rs b/compiler/rustc_mir_transform/src/coverage/mod.rs
index 104f340c8d6..d0f30314e79 100644
--- a/compiler/rustc_mir_transform/src/coverage/mod.rs
+++ b/compiler/rustc_mir_transform/src/coverage/mod.rs
@@ -94,9 +94,8 @@ fn instrument_function_for_coverage<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir:
         return;
     }
 
-    let bcb_has_counter_mappings = |bcb| bcbs_with_counter_mappings.contains(bcb);
     let coverage_counters =
-        CoverageCounters::make_bcb_counters(&basic_coverage_blocks, bcb_has_counter_mappings);
+        CoverageCounters::make_bcb_counters(&basic_coverage_blocks, &bcbs_with_counter_mappings);
 
     let mappings = create_mappings(tcx, &hir_info, &extracted_mappings, &coverage_counters);
     if mappings.is_empty() {
@@ -115,16 +114,16 @@ fn instrument_function_for_coverage<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir:
     inject_mcdc_statements(mir_body, &basic_coverage_blocks, &extracted_mappings);
 
     let mcdc_num_condition_bitmaps = extracted_mappings
-        .mcdc_decisions
+        .mcdc_mappings
         .iter()
-        .map(|&mappings::MCDCDecision { decision_depth, .. }| decision_depth)
+        .map(|&(mappings::MCDCDecision { decision_depth, .. }, _)| decision_depth)
         .max()
         .map_or(0, |max| usize::from(max) + 1);
 
     mir_body.function_coverage_info = Some(Box::new(FunctionCoverageInfo {
         function_source_hash: hir_info.function_source_hash,
         num_counters: coverage_counters.num_counters(),
-        mcdc_bitmap_bytes: extracted_mappings.mcdc_bitmap_bytes,
+        mcdc_bitmap_bits: extracted_mappings.mcdc_bitmap_bits,
         expressions: coverage_counters.into_expressions(),
         mappings,
         mcdc_num_condition_bitmaps,
@@ -153,12 +152,8 @@ fn create_mappings<'tcx>(
         &source_file.name.for_scope(tcx.sess, RemapPathScopeComponents::MACRO).to_string_lossy(),
     );
 
-    let term_for_bcb = |bcb| {
-        coverage_counters
-            .bcb_counter(bcb)
-            .expect("all BCBs with spans were given counters")
-            .as_term()
-    };
+    let term_for_bcb =
+        |bcb| coverage_counters.term_for_bcb(bcb).expect("all BCBs with spans were given counters");
     let region_for_span = |span: Span| make_source_region(source_map, file_name, span, body_span);
 
     // Fully destructure the mappings struct to make sure we don't miss any kinds.
@@ -166,9 +161,9 @@ fn create_mappings<'tcx>(
         num_bcbs: _,
         code_mappings,
         branch_pairs,
-        mcdc_bitmap_bytes: _,
-        mcdc_branches,
-        mcdc_decisions,
+        mcdc_bitmap_bits: _,
+        mcdc_degraded_branches,
+        mcdc_mappings,
     } = extracted_mappings;
     let mut mappings = Vec::new();
 
@@ -191,26 +186,79 @@ fn create_mappings<'tcx>(
         },
     ));
 
-    mappings.extend(mcdc_branches.iter().filter_map(
-        |&mappings::MCDCBranch { span, true_bcb, false_bcb, condition_info, decision_depth: _ }| {
+    let term_for_bcb =
+        |bcb| coverage_counters.term_for_bcb(bcb).expect("all BCBs with spans were given counters");
+
+    // MCDC branch mappings are appended with their decisions in case decisions were ignored.
+    mappings.extend(mcdc_degraded_branches.iter().filter_map(
+        |&mappings::MCDCBranch {
+             span,
+             true_bcb,
+             false_bcb,
+             condition_info: _,
+             true_index: _,
+             false_index: _,
+         }| {
             let source_region = region_for_span(span)?;
             let true_term = term_for_bcb(true_bcb);
             let false_term = term_for_bcb(false_bcb);
-            let kind = match condition_info {
-                Some(mcdc_params) => MappingKind::MCDCBranch { true_term, false_term, mcdc_params },
-                None => MappingKind::Branch { true_term, false_term },
-            };
-            Some(Mapping { kind, source_region })
+            Some(Mapping { kind: MappingKind::Branch { true_term, false_term }, source_region })
         },
     ));
 
-    mappings.extend(mcdc_decisions.iter().filter_map(
-        |&mappings::MCDCDecision { span, bitmap_idx, num_conditions, .. }| {
-            let source_region = region_for_span(span)?;
-            let kind = MappingKind::MCDCDecision(DecisionInfo { bitmap_idx, num_conditions });
-            Some(Mapping { kind, source_region })
-        },
-    ));
+    for (decision, branches) in mcdc_mappings {
+        let num_conditions = branches.len() as u16;
+        let conditions = branches
+            .into_iter()
+            .filter_map(
+                |&mappings::MCDCBranch {
+                     span,
+                     true_bcb,
+                     false_bcb,
+                     condition_info,
+                     true_index: _,
+                     false_index: _,
+                 }| {
+                    let source_region = region_for_span(span)?;
+                    let true_term = term_for_bcb(true_bcb);
+                    let false_term = term_for_bcb(false_bcb);
+                    Some(Mapping {
+                        kind: MappingKind::MCDCBranch {
+                            true_term,
+                            false_term,
+                            mcdc_params: condition_info,
+                        },
+                        source_region,
+                    })
+                },
+            )
+            .collect::<Vec<_>>();
+
+        if conditions.len() == num_conditions as usize
+            && let Some(source_region) = region_for_span(decision.span)
+        {
+            // LLVM requires end index for counter mapping regions.
+            let kind = MappingKind::MCDCDecision(DecisionInfo {
+                bitmap_idx: (decision.bitmap_idx + decision.num_test_vectors) as u32,
+                num_conditions,
+            });
+            mappings.extend(
+                std::iter::once(Mapping { kind, source_region }).chain(conditions.into_iter()),
+            );
+        } else {
+            mappings.extend(conditions.into_iter().map(|mapping| {
+                let MappingKind::MCDCBranch { true_term, false_term, mcdc_params: _ } =
+                    mapping.kind
+                else {
+                    unreachable!("all mappings here are MCDCBranch as shown above");
+                };
+                Mapping {
+                    kind: MappingKind::Branch { true_term, false_term },
+                    source_region: mapping.source_region,
+                }
+            }))
+        }
+    }
 
     mappings
 }
@@ -279,44 +327,41 @@ fn inject_mcdc_statements<'tcx>(
     basic_coverage_blocks: &CoverageGraph,
     extracted_mappings: &ExtractedMappings,
 ) {
-    // Inject test vector update first because `inject_statement` always insert new statement at
-    // head.
-    for &mappings::MCDCDecision {
-        span: _,
-        ref end_bcbs,
-        bitmap_idx,
-        num_conditions: _,
-        decision_depth,
-    } in &extracted_mappings.mcdc_decisions
-    {
-        for end in end_bcbs {
-            let end_bb = basic_coverage_blocks[*end].leader_bb();
+    for (decision, conditions) in &extracted_mappings.mcdc_mappings {
+        // Inject test vector update first because `inject_statement` always insert new statement at head.
+        for &end in &decision.end_bcbs {
+            let end_bb = basic_coverage_blocks[end].leader_bb();
             inject_statement(
                 mir_body,
-                CoverageKind::TestVectorBitmapUpdate { bitmap_idx, decision_depth },
+                CoverageKind::TestVectorBitmapUpdate {
+                    bitmap_idx: decision.bitmap_idx as u32,
+                    decision_depth: decision.decision_depth,
+                },
                 end_bb,
             );
         }
-    }
 
-    for &mappings::MCDCBranch { span: _, true_bcb, false_bcb, condition_info, decision_depth } in
-        &extracted_mappings.mcdc_branches
-    {
-        let Some(condition_info) = condition_info else { continue };
-        let id = condition_info.condition_id;
-
-        let true_bb = basic_coverage_blocks[true_bcb].leader_bb();
-        inject_statement(
-            mir_body,
-            CoverageKind::CondBitmapUpdate { id, value: true, decision_depth },
-            true_bb,
-        );
-        let false_bb = basic_coverage_blocks[false_bcb].leader_bb();
-        inject_statement(
-            mir_body,
-            CoverageKind::CondBitmapUpdate { id, value: false, decision_depth },
-            false_bb,
-        );
+        for &mappings::MCDCBranch {
+            span: _,
+            true_bcb,
+            false_bcb,
+            condition_info: _,
+            true_index,
+            false_index,
+        } in conditions
+        {
+            for (index, bcb) in [(false_index, false_bcb), (true_index, true_bcb)] {
+                let bb = basic_coverage_blocks[bcb].leader_bb();
+                inject_statement(
+                    mir_body,
+                    CoverageKind::CondBitmapUpdate {
+                        index: index as u32,
+                        decision_depth: decision.decision_depth,
+                    },
+                    bb,
+                );
+            }
+        }
     }
 }
 
diff --git a/compiler/rustc_mir_transform/src/coverage/query.rs b/compiler/rustc_mir_transform/src/coverage/query.rs
index e65a5fdd5e7..df151f8cca3 100644
--- a/compiler/rustc_mir_transform/src/coverage/query.rs
+++ b/compiler/rustc_mir_transform/src/coverage/query.rs
@@ -63,7 +63,8 @@ fn coverage_attr_on(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
             Some([item]) if item.has_name(sym::on) => return true,
             Some(_) | None => {
                 // Other possibilities should have been rejected by `rustc_parse::validate_attr`.
-                tcx.dcx().span_bug(attr.span, "unexpected value of coverage attribute");
+                // Use `span_delayed_bug` to avoid an ICE in failing builds (#127880).
+                tcx.dcx().span_delayed_bug(attr.span, "unexpected value of coverage attribute");
             }
         }
     }
diff --git a/compiler/rustc_mir_transform/src/dataflow_const_prop.rs b/compiler/rustc_mir_transform/src/dataflow_const_prop.rs
index 88dc8e74a8c..002216f50f2 100644
--- a/compiler/rustc_mir_transform/src/dataflow_const_prop.rs
+++ b/compiler/rustc_mir_transform/src/dataflow_const_prop.rs
@@ -3,7 +3,9 @@
 //! Currently, this pass only propagates scalar values.
 
 use rustc_const_eval::const_eval::{DummyMachine, throw_machine_stop_str};
-use rustc_const_eval::interpret::{ImmTy, Immediate, InterpCx, OpTy, PlaceTy, Projectable};
+use rustc_const_eval::interpret::{
+    ImmTy, Immediate, InterpCx, OpTy, PlaceTy, Projectable, interp_ok,
+};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_hir::def::DefKind;
 use rustc_middle::bug;
@@ -236,6 +238,7 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
                     FlatSet::Elem(op) => self
                         .ecx
                         .int_to_int_or_float(&op, layout)
+                        .discard_err()
                         .map_or(FlatSet::Top, |result| self.wrap_immediate(*result)),
                     FlatSet::Bottom => FlatSet::Bottom,
                     FlatSet::Top => FlatSet::Top,
@@ -249,6 +252,7 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
                     FlatSet::Elem(op) => self
                         .ecx
                         .float_to_float_or_int(&op, layout)
+                        .discard_err()
                         .map_or(FlatSet::Top, |result| self.wrap_immediate(*result)),
                     FlatSet::Bottom => FlatSet::Bottom,
                     FlatSet::Top => FlatSet::Top,
@@ -271,6 +275,7 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
                 FlatSet::Elem(value) => self
                     .ecx
                     .unary_op(*op, &value)
+                    .discard_err()
                     .map_or(FlatSet::Top, |val| self.wrap_immediate(*val)),
                 FlatSet::Bottom => FlatSet::Bottom,
                 FlatSet::Top => FlatSet::Top,
@@ -364,8 +369,8 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
                 }
             }
             Operand::Constant(box constant) => {
-                if let Ok(constant) =
-                    self.ecx.eval_mir_constant(&constant.const_, constant.span, None)
+                if let Some(constant) =
+                    self.ecx.eval_mir_constant(&constant.const_, constant.span, None).discard_err()
                 {
                     self.assign_constant(state, place, constant, &[]);
                 }
@@ -387,7 +392,7 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
         for &(mut proj_elem) in projection {
             if let PlaceElem::Index(index) = proj_elem {
                 if let FlatSet::Elem(index) = state.get(index.into(), &self.map)
-                    && let Ok(offset) = index.to_target_usize(&self.tcx)
+                    && let Some(offset) = index.to_target_usize(&self.tcx).discard_err()
                     && let Some(min_length) = offset.checked_add(1)
                 {
                     proj_elem = PlaceElem::ConstantIndex { offset, min_length, from_end: false };
@@ -395,7 +400,7 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
                     return;
                 }
             }
-            operand = if let Ok(operand) = self.ecx.project(&operand, proj_elem) {
+            operand = if let Some(operand) = self.ecx.project(&operand, proj_elem).discard_err() {
                 operand
             } else {
                 return;
@@ -406,24 +411,24 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
             place,
             operand,
             &mut |elem, op| match elem {
-                TrackElem::Field(idx) => self.ecx.project_field(op, idx.as_usize()).ok(),
-                TrackElem::Variant(idx) => self.ecx.project_downcast(op, idx).ok(),
+                TrackElem::Field(idx) => self.ecx.project_field(op, idx.as_usize()).discard_err(),
+                TrackElem::Variant(idx) => self.ecx.project_downcast(op, idx).discard_err(),
                 TrackElem::Discriminant => {
-                    let variant = self.ecx.read_discriminant(op).ok()?;
+                    let variant = self.ecx.read_discriminant(op).discard_err()?;
                     let discr_value =
-                        self.ecx.discriminant_for_variant(op.layout.ty, variant).ok()?;
+                        self.ecx.discriminant_for_variant(op.layout.ty, variant).discard_err()?;
                     Some(discr_value.into())
                 }
                 TrackElem::DerefLen => {
-                    let op: OpTy<'_> = self.ecx.deref_pointer(op).ok()?.into();
-                    let len_usize = op.len(&self.ecx).ok()?;
+                    let op: OpTy<'_> = self.ecx.deref_pointer(op).discard_err()?.into();
+                    let len_usize = op.len(&self.ecx).discard_err()?;
                     let layout =
                         self.tcx.layout_of(self.param_env.and(self.tcx.types.usize)).unwrap();
                     Some(ImmTy::from_uint(len_usize, layout).into())
                 }
             },
             &mut |place, op| {
-                if let Ok(imm) = self.ecx.read_immediate_raw(op)
+                if let Some(imm) = self.ecx.read_immediate_raw(op).discard_err()
                     && let Some(imm) = imm.right()
                 {
                     let elem = self.wrap_immediate(*imm);
@@ -447,11 +452,11 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
             (FlatSet::Bottom, _) | (_, FlatSet::Bottom) => (FlatSet::Bottom, FlatSet::Bottom),
             // Both sides are known, do the actual computation.
             (FlatSet::Elem(left), FlatSet::Elem(right)) => {
-                match self.ecx.binary_op(op, &left, &right) {
+                match self.ecx.binary_op(op, &left, &right).discard_err() {
                     // Ideally this would return an Immediate, since it's sometimes
                     // a pair and sometimes not. But as a hack we always return a pair
                     // and just make the 2nd component `Bottom` when it does not exist.
-                    Ok(val) => {
+                    Some(val) => {
                         if matches!(val.layout.abi, Abi::ScalarPair(..)) {
                             let (val, overflow) = val.to_scalar_pair();
                             (FlatSet::Elem(val), FlatSet::Elem(overflow))
@@ -470,7 +475,7 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
                 }
 
                 let arg_scalar = const_arg.to_scalar();
-                let Ok(arg_value) = arg_scalar.to_bits(layout.size) else {
+                let Some(arg_value) = arg_scalar.to_bits(layout.size).discard_err() else {
                     return (FlatSet::Top, FlatSet::Top);
                 };
 
@@ -519,7 +524,7 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
         }
         let enum_ty_layout = self.tcx.layout_of(self.param_env.and(enum_ty)).ok()?;
         let discr_value =
-            self.ecx.discriminant_for_variant(enum_ty_layout.ty, variant_index).ok()?;
+            self.ecx.discriminant_for_variant(enum_ty_layout.ty, variant_index).discard_err()?;
         Some(discr_value.to_scalar())
     }
 
@@ -595,7 +600,7 @@ impl<'a, 'tcx> Collector<'a, 'tcx> {
                 .intern_with_temp_alloc(layout, |ecx, dest| {
                     try_write_constant(ecx, dest, place, ty, state, map)
                 })
-                .ok()?;
+                .discard_err()?;
             return Some(Const::Val(ConstValue::Indirect { alloc_id, offset: Size::ZERO }, ty));
         }
 
@@ -632,7 +637,7 @@ fn try_write_constant<'tcx>(
 
     // Fast path for ZSTs.
     if layout.is_zst() {
-        return Ok(());
+        return interp_ok(());
     }
 
     // Fast path for scalars.
@@ -717,7 +722,7 @@ fn try_write_constant<'tcx>(
         ty::Error(_) | ty::Infer(..) | ty::CoroutineWitness(..) => bug!(),
     }
 
-    Ok(())
+    interp_ok(())
 }
 
 impl<'mir, 'tcx>
@@ -830,7 +835,7 @@ impl<'tcx> MutVisitor<'tcx> for Patch<'tcx> {
         if let PlaceElem::Index(local) = elem {
             let offset = self.before_effect.get(&(location, local.into()))?;
             let offset = offset.try_to_scalar()?;
-            let offset = offset.to_target_usize(&self.tcx).ok()?;
+            let offset = offset.to_target_usize(&self.tcx).discard_err()?;
             let min_length = offset.checked_add(1)?;
             Some(PlaceElem::ConstantIndex { offset, min_length, from_end: false })
         } else {
diff --git a/compiler/rustc_mir_transform/src/dump_mir.rs b/compiler/rustc_mir_transform/src/dump_mir.rs
index d7c47360419..5dd84975b88 100644
--- a/compiler/rustc_mir_transform/src/dump_mir.rs
+++ b/compiler/rustc_mir_transform/src/dump_mir.rs
@@ -24,7 +24,7 @@ pub fn emit_mir(tcx: TyCtxt<'_>) -> io::Result<()> {
             write_mir_pretty(tcx, None, &mut f)?;
         }
         OutFileName::Real(path) => {
-            let mut f = io::BufWriter::new(File::create(&path)?);
+            let mut f = File::create_buffered(&path)?;
             write_mir_pretty(tcx, None, &mut f)?;
             if tcx.sess.opts.json_artifact_notifications {
                 tcx.dcx().emit_artifact_notification(&path, "mir");
diff --git a/compiler/rustc_mir_transform/src/errors.rs b/compiler/rustc_mir_transform/src/errors.rs
index 84d44c2ab4c..8b309147c64 100644
--- a/compiler/rustc_mir_transform/src/errors.rs
+++ b/compiler/rustc_mir_transform/src/errors.rs
@@ -89,6 +89,14 @@ pub(crate) struct FnItemRef {
     pub ident: String,
 }
 
+#[derive(Diagnostic)]
+#[diag(mir_transform_exceeds_mcdc_test_vector_limit)]
+pub(crate) struct MCDCExceedsTestVectorLimit {
+    #[primary_span]
+    pub(crate) span: Span,
+    pub(crate) max_num_test_vectors: usize,
+}
+
 pub(crate) struct MustNotSupend<'a, 'tcx> {
     pub tcx: TyCtxt<'tcx>,
     pub yield_sp: Span,
@@ -121,3 +129,10 @@ pub(crate) struct MustNotSuspendReason {
     pub span: Span,
     pub reason: String,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(mir_transform_undefined_transmute)]
+#[note]
+#[note(mir_transform_note2)]
+#[help]
+pub(crate) struct UndefinedTransmute;
diff --git a/compiler/rustc_mir_transform/src/gvn.rs b/compiler/rustc_mir_transform/src/gvn.rs
index f735d08fca5..daf868559bc 100644
--- a/compiler/rustc_mir_transform/src/gvn.rs
+++ b/compiler/rustc_mir_transform/src/gvn.rs
@@ -103,7 +103,7 @@ use rustc_middle::ty::layout::{HasParamEnv, LayoutOf};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::DUMMY_SP;
 use rustc_span::def_id::DefId;
-use rustc_target::abi::{self, Abi, FIRST_VARIANT, FieldIdx, Size, VariantIdx};
+use rustc_target::abi::{self, Abi, FIRST_VARIANT, FieldIdx, Primitive, Size, VariantIdx};
 use smallvec::SmallVec;
 use tracing::{debug, instrument, trace};
 
@@ -393,7 +393,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             Repeat(..) => return None,
 
             Constant { ref value, disambiguator: _ } => {
-                self.ecx.eval_mir_constant(value, DUMMY_SP, None).ok()?
+                self.ecx.eval_mir_constant(value, DUMMY_SP, None).discard_err()?
             }
             Aggregate(kind, variant, ref fields) => {
                 let fields = fields
@@ -419,29 +419,32 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                     ImmTy::uninit(ty).into()
                 } else if matches!(kind, AggregateTy::RawPtr { .. }) {
                     // Pointers don't have fields, so don't `project_field` them.
-                    let data = self.ecx.read_pointer(fields[0]).ok()?;
+                    let data = self.ecx.read_pointer(fields[0]).discard_err()?;
                     let meta = if fields[1].layout.is_zst() {
                         MemPlaceMeta::None
                     } else {
-                        MemPlaceMeta::Meta(self.ecx.read_scalar(fields[1]).ok()?)
+                        MemPlaceMeta::Meta(self.ecx.read_scalar(fields[1]).discard_err()?)
                     };
                     let ptr_imm = Immediate::new_pointer_with_meta(data, meta, &self.ecx);
                     ImmTy::from_immediate(ptr_imm, ty).into()
                 } else if matches!(ty.abi, Abi::Scalar(..) | Abi::ScalarPair(..)) {
-                    let dest = self.ecx.allocate(ty, MemoryKind::Stack).ok()?;
+                    let dest = self.ecx.allocate(ty, MemoryKind::Stack).discard_err()?;
                     let variant_dest = if let Some(variant) = variant {
-                        self.ecx.project_downcast(&dest, variant).ok()?
+                        self.ecx.project_downcast(&dest, variant).discard_err()?
                     } else {
                         dest.clone()
                     };
                     for (field_index, op) in fields.into_iter().enumerate() {
-                        let field_dest = self.ecx.project_field(&variant_dest, field_index).ok()?;
-                        self.ecx.copy_op(op, &field_dest).ok()?;
+                        let field_dest =
+                            self.ecx.project_field(&variant_dest, field_index).discard_err()?;
+                        self.ecx.copy_op(op, &field_dest).discard_err()?;
                     }
-                    self.ecx.write_discriminant(variant.unwrap_or(FIRST_VARIANT), &dest).ok()?;
+                    self.ecx
+                        .write_discriminant(variant.unwrap_or(FIRST_VARIANT), &dest)
+                        .discard_err()?;
                     self.ecx
                         .alloc_mark_immutable(dest.ptr().provenance.unwrap().alloc_id())
-                        .ok()?;
+                        .discard_err()?;
                     dest.into()
                 } else {
                     return None;
@@ -467,7 +470,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                     // This should have been replaced by a `ConstantIndex` earlier.
                     ProjectionElem::Index(_) => return None,
                 };
-                self.ecx.project(value, elem).ok()?
+                self.ecx.project(value, elem).discard_err()?
             }
             Address { place, kind, provenance: _ } => {
                 if !place.is_indirect_first_projection() {
@@ -475,14 +478,14 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                 }
                 let local = self.locals[place.local]?;
                 let pointer = self.evaluated[local].as_ref()?;
-                let mut mplace = self.ecx.deref_pointer(pointer).ok()?;
+                let mut mplace = self.ecx.deref_pointer(pointer).discard_err()?;
                 for proj in place.projection.iter().skip(1) {
                     // We have no call stack to associate a local with a value, so we cannot
                     // interpret indexing.
                     if matches!(proj, ProjectionElem::Index(_)) {
                         return None;
                     }
-                    mplace = self.ecx.project(&mplace, proj).ok()?;
+                    mplace = self.ecx.project(&mplace, proj).discard_err()?;
                 }
                 let pointer = mplace.to_ref(&self.ecx);
                 let ty = match kind {
@@ -500,15 +503,15 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
 
             Discriminant(base) => {
                 let base = self.evaluated[base].as_ref()?;
-                let variant = self.ecx.read_discriminant(base).ok()?;
+                let variant = self.ecx.read_discriminant(base).discard_err()?;
                 let discr_value =
-                    self.ecx.discriminant_for_variant(base.layout.ty, variant).ok()?;
+                    self.ecx.discriminant_for_variant(base.layout.ty, variant).discard_err()?;
                 discr_value.into()
             }
             Len(slice) => {
                 let slice = self.evaluated[slice].as_ref()?;
                 let usize_layout = self.ecx.layout_of(self.tcx.types.usize).unwrap();
-                let len = slice.len(&self.ecx).ok()?;
+                let len = slice.len(&self.ecx).discard_err()?;
                 let imm = ImmTy::from_uint(len, usize_layout);
                 imm.into()
             }
@@ -535,67 +538,83 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             }
             UnaryOp(un_op, operand) => {
                 let operand = self.evaluated[operand].as_ref()?;
-                let operand = self.ecx.read_immediate(operand).ok()?;
-                let val = self.ecx.unary_op(un_op, &operand).ok()?;
+                let operand = self.ecx.read_immediate(operand).discard_err()?;
+                let val = self.ecx.unary_op(un_op, &operand).discard_err()?;
                 val.into()
             }
             BinaryOp(bin_op, lhs, rhs) => {
                 let lhs = self.evaluated[lhs].as_ref()?;
-                let lhs = self.ecx.read_immediate(lhs).ok()?;
+                let lhs = self.ecx.read_immediate(lhs).discard_err()?;
                 let rhs = self.evaluated[rhs].as_ref()?;
-                let rhs = self.ecx.read_immediate(rhs).ok()?;
-                let val = self.ecx.binary_op(bin_op, &lhs, &rhs).ok()?;
+                let rhs = self.ecx.read_immediate(rhs).discard_err()?;
+                let val = self.ecx.binary_op(bin_op, &lhs, &rhs).discard_err()?;
                 val.into()
             }
             Cast { kind, value, from: _, to } => match kind {
                 CastKind::IntToInt | CastKind::IntToFloat => {
                     let value = self.evaluated[value].as_ref()?;
-                    let value = self.ecx.read_immediate(value).ok()?;
+                    let value = self.ecx.read_immediate(value).discard_err()?;
                     let to = self.ecx.layout_of(to).ok()?;
-                    let res = self.ecx.int_to_int_or_float(&value, to).ok()?;
+                    let res = self.ecx.int_to_int_or_float(&value, to).discard_err()?;
                     res.into()
                 }
                 CastKind::FloatToFloat | CastKind::FloatToInt => {
                     let value = self.evaluated[value].as_ref()?;
-                    let value = self.ecx.read_immediate(value).ok()?;
+                    let value = self.ecx.read_immediate(value).discard_err()?;
                     let to = self.ecx.layout_of(to).ok()?;
-                    let res = self.ecx.float_to_float_or_int(&value, to).ok()?;
+                    let res = self.ecx.float_to_float_or_int(&value, to).discard_err()?;
                     res.into()
                 }
                 CastKind::Transmute => {
                     let value = self.evaluated[value].as_ref()?;
                     let to = self.ecx.layout_of(to).ok()?;
-                    // `offset` for immediates only supports scalar/scalar-pair ABIs,
-                    // so bail out if the target is not one.
+                    // `offset` for immediates generally only supports projections that match the
+                    // type of the immediate. However, as a HACK, we exploit that it can also do
+                    // limited transmutes: it only works between types with the same layout, and
+                    // cannot transmute pointers to integers.
                     if value.as_mplace_or_imm().is_right() {
-                        match (value.layout.abi, to.abi) {
-                            (Abi::Scalar(..), Abi::Scalar(..)) => {}
-                            (Abi::ScalarPair(..), Abi::ScalarPair(..)) => {}
-                            _ => return None,
+                        let can_transmute = match (value.layout.abi, to.abi) {
+                            (Abi::Scalar(s1), Abi::Scalar(s2)) => {
+                                s1.size(&self.ecx) == s2.size(&self.ecx)
+                                    && !matches!(s1.primitive(), Primitive::Pointer(..))
+                            }
+                            (Abi::ScalarPair(a1, b1), Abi::ScalarPair(a2, b2)) => {
+                                a1.size(&self.ecx) == a2.size(&self.ecx) &&
+                                b1.size(&self.ecx) == b2.size(&self.ecx) &&
+                                // The alignment of the second component determines its offset, so that also needs to match.
+                                b1.align(&self.ecx) == b2.align(&self.ecx) &&
+                                // None of the inputs may be a pointer.
+                                !matches!(a1.primitive(), Primitive::Pointer(..))
+                                    && !matches!(b1.primitive(), Primitive::Pointer(..))
+                            }
+                            _ => false,
+                        };
+                        if !can_transmute {
+                            return None;
                         }
                     }
-                    value.offset(Size::ZERO, to, &self.ecx).ok()?
+                    value.offset(Size::ZERO, to, &self.ecx).discard_err()?
                 }
                 CastKind::PointerCoercion(ty::adjustment::PointerCoercion::Unsize, _) => {
                     let src = self.evaluated[value].as_ref()?;
                     let to = self.ecx.layout_of(to).ok()?;
-                    let dest = self.ecx.allocate(to, MemoryKind::Stack).ok()?;
-                    self.ecx.unsize_into(src, to, &dest.clone().into()).ok()?;
+                    let dest = self.ecx.allocate(to, MemoryKind::Stack).discard_err()?;
+                    self.ecx.unsize_into(src, to, &dest.clone().into()).discard_err()?;
                     self.ecx
                         .alloc_mark_immutable(dest.ptr().provenance.unwrap().alloc_id())
-                        .ok()?;
+                        .discard_err()?;
                     dest.into()
                 }
                 CastKind::FnPtrToPtr | CastKind::PtrToPtr => {
                     let src = self.evaluated[value].as_ref()?;
-                    let src = self.ecx.read_immediate(src).ok()?;
+                    let src = self.ecx.read_immediate(src).discard_err()?;
                     let to = self.ecx.layout_of(to).ok()?;
-                    let ret = self.ecx.ptr_to_ptr(&src, to).ok()?;
+                    let ret = self.ecx.ptr_to_ptr(&src, to).discard_err()?;
                     ret.into()
                 }
                 CastKind::PointerCoercion(ty::adjustment::PointerCoercion::UnsafeFnPointer, _) => {
                     let src = self.evaluated[value].as_ref()?;
-                    let src = self.ecx.read_immediate(src).ok()?;
+                    let src = self.ecx.read_immediate(src).discard_err()?;
                     let to = self.ecx.layout_of(to).ok()?;
                     ImmTy::from_immediate(*src, to).into()
                 }
@@ -708,7 +727,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                 && let Some(idx) = self.locals[idx_local]
             {
                 if let Some(offset) = self.evaluated[idx].as_ref()
-                    && let Ok(offset) = self.ecx.read_target_usize(offset)
+                    && let Some(offset) = self.ecx.read_target_usize(offset).discard_err()
                     && let Some(min_length) = offset.checked_add(1)
                 {
                     projection.to_mut()[i] =
@@ -868,7 +887,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             && let DefKind::Enum = self.tcx.def_kind(enum_did)
         {
             let enum_ty = self.tcx.type_of(enum_did).instantiate(self.tcx, enum_args);
-            let discr = self.ecx.discriminant_for_variant(enum_ty, variant).ok()?;
+            let discr = self.ecx.discriminant_for_variant(enum_ty, variant).discard_err()?;
             return Some(self.insert_scalar(discr.to_scalar(), discr.layout.ty));
         }
 
@@ -1134,7 +1153,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             (UnOp::PtrMetadata, Value::Aggregate(AggregateTy::RawPtr { .. }, _, fields)) => {
                 return Some(fields[1]);
             }
-            // We have an unsizing cast, which assigns the length to fat pointer metadata.
+            // We have an unsizing cast, which assigns the length to wide pointer metadata.
             (
                 UnOp::PtrMetadata,
                 Value::Cast {
@@ -1223,8 +1242,8 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
         let as_bits = |value| {
             let constant = self.evaluated[value].as_ref()?;
             if layout.abi.is_scalar() {
-                let scalar = self.ecx.read_scalar(constant).ok()?;
-                scalar.to_bits(constant.layout.size).ok()
+                let scalar = self.ecx.read_scalar(constant).discard_err()?;
+                scalar.to_bits(constant.layout.size).discard_err()
             } else {
                 // `constant` is a wide pointer. Do not evaluate to bits.
                 None
@@ -1418,7 +1437,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
 
         let mut inner = self.simplify_place_value(place, location)?;
 
-        // The length information is stored in the fat pointer.
+        // The length information is stored in the wide pointer.
         // Reborrowing copies length information from one pointer to the other.
         while let Value::Address { place: borrowed, .. } = self.get(inner)
             && let [PlaceElem::Deref] = borrowed.projection[..]
@@ -1427,7 +1446,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             inner = borrowed;
         }
 
-        // We have an unsizing cast, which assigns the length to fat pointer metadata.
+        // We have an unsizing cast, which assigns the length to wide pointer metadata.
         if let Value::Cast { kind, from, to, .. } = self.get(inner)
             && let CastKind::PointerCoercion(ty::adjustment::PointerCoercion::Unsize, _) = kind
             && let Some(from) = from.builtin_deref(true)
@@ -1484,7 +1503,7 @@ fn op_to_prop_const<'tcx>(
 
     // If this constant has scalar ABI, return it as a `ConstValue::Scalar`.
     if let Abi::Scalar(abi::Scalar::Initialized { .. }) = op.layout.abi
-        && let Ok(scalar) = ecx.read_scalar(op)
+        && let Some(scalar) = ecx.read_scalar(op).discard_err()
     {
         if !scalar.try_to_scalar_int().is_ok() {
             // Check that we do not leak a pointer.
@@ -1498,12 +1517,12 @@ fn op_to_prop_const<'tcx>(
     // If this constant is already represented as an `Allocation`,
     // try putting it into global memory to return it.
     if let Either::Left(mplace) = op.as_mplace_or_imm() {
-        let (size, _align) = ecx.size_and_align_of_mplace(&mplace).ok()??;
+        let (size, _align) = ecx.size_and_align_of_mplace(&mplace).discard_err()??;
 
         // Do not try interning a value that contains provenance.
         // Due to https://github.com/rust-lang/rust/issues/79738, doing so could lead to bugs.
         // FIXME: remove this hack once that issue is fixed.
-        let alloc_ref = ecx.get_ptr_alloc(mplace.ptr(), size).ok()??;
+        let alloc_ref = ecx.get_ptr_alloc(mplace.ptr(), size).discard_err()??;
         if alloc_ref.has_provenance() {
             return None;
         }
@@ -1511,7 +1530,7 @@ fn op_to_prop_const<'tcx>(
         let pointer = mplace.ptr().into_pointer_or_addr().ok()?;
         let (prov, offset) = pointer.into_parts();
         let alloc_id = prov.alloc_id();
-        intern_const_alloc_for_constprop(ecx, alloc_id).ok()?;
+        intern_const_alloc_for_constprop(ecx, alloc_id).discard_err()?;
 
         // `alloc_id` may point to a static. Codegen will choke on an `Indirect` with anything
         // by `GlobalAlloc::Memory`, so do fall through to copying if needed.
@@ -1526,7 +1545,8 @@ fn op_to_prop_const<'tcx>(
     }
 
     // Everything failed: create a new allocation to hold the data.
-    let alloc_id = ecx.intern_with_temp_alloc(op.layout, |ecx, dest| ecx.copy_op(op, dest)).ok()?;
+    let alloc_id =
+        ecx.intern_with_temp_alloc(op.layout, |ecx, dest| ecx.copy_op(op, dest)).discard_err()?;
     let value = ConstValue::Indirect { alloc_id, offset: Size::ZERO };
 
     // Check that we do not leak a pointer.
diff --git a/compiler/rustc_mir_transform/src/jump_threading.rs b/compiler/rustc_mir_transform/src/jump_threading.rs
index 9d85b5ba5a7..9b9b0b705bf 100644
--- a/compiler/rustc_mir_transform/src/jump_threading.rs
+++ b/compiler/rustc_mir_transform/src/jump_threading.rs
@@ -200,7 +200,9 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
         debug!(?discr, ?bb);
 
         let discr_ty = discr.ty(self.body, self.tcx).ty;
-        let Ok(discr_layout) = self.ecx.layout_of(discr_ty) else { return };
+        let Ok(discr_layout) = self.ecx.layout_of(discr_ty) else {
+            return;
+        };
 
         let Some(discr) = self.map.find(discr.as_ref()) else { return };
         debug!(?discr);
@@ -388,24 +390,24 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
             lhs,
             constant,
             &mut |elem, op| match elem {
-                TrackElem::Field(idx) => self.ecx.project_field(op, idx.as_usize()).ok(),
-                TrackElem::Variant(idx) => self.ecx.project_downcast(op, idx).ok(),
+                TrackElem::Field(idx) => self.ecx.project_field(op, idx.as_usize()).discard_err(),
+                TrackElem::Variant(idx) => self.ecx.project_downcast(op, idx).discard_err(),
                 TrackElem::Discriminant => {
-                    let variant = self.ecx.read_discriminant(op).ok()?;
+                    let variant = self.ecx.read_discriminant(op).discard_err()?;
                     let discr_value =
-                        self.ecx.discriminant_for_variant(op.layout.ty, variant).ok()?;
+                        self.ecx.discriminant_for_variant(op.layout.ty, variant).discard_err()?;
                     Some(discr_value.into())
                 }
                 TrackElem::DerefLen => {
-                    let op: OpTy<'_> = self.ecx.deref_pointer(op).ok()?.into();
-                    let len_usize = op.len(&self.ecx).ok()?;
+                    let op: OpTy<'_> = self.ecx.deref_pointer(op).discard_err()?.into();
+                    let len_usize = op.len(&self.ecx).discard_err()?;
                     let layout = self.ecx.layout_of(self.tcx.types.usize).unwrap();
                     Some(ImmTy::from_uint(len_usize, layout).into())
                 }
             },
             &mut |place, op| {
                 if let Some(conditions) = state.try_get_idx(place, &self.map)
-                    && let Ok(imm) = self.ecx.read_immediate_raw(op)
+                    && let Some(imm) = self.ecx.read_immediate_raw(op).discard_err()
                     && let Some(imm) = imm.right()
                     && let Immediate::Scalar(Scalar::Int(int)) = *imm
                 {
@@ -429,8 +431,8 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
         match rhs {
             // If we expect `lhs ?= A`, we have an opportunity if we assume `constant == A`.
             Operand::Constant(constant) => {
-                let Ok(constant) =
-                    self.ecx.eval_mir_constant(&constant.const_, constant.span, None)
+                let Some(constant) =
+                    self.ecx.eval_mir_constant(&constant.const_, constant.span, None).discard_err()
                 else {
                     return;
                 };
@@ -469,8 +471,10 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
                     AggregateKind::Adt(.., Some(_)) => return,
                     AggregateKind::Adt(_, variant_index, ..) if agg_ty.is_enum() => {
                         if let Some(discr_target) = self.map.apply(lhs, TrackElem::Discriminant)
-                            && let Ok(discr_value) =
-                                self.ecx.discriminant_for_variant(agg_ty, *variant_index)
+                            && let Some(discr_value) = self
+                                .ecx
+                                .discriminant_for_variant(agg_ty, *variant_index)
+                                .discard_err()
                         {
                             self.process_immediate(bb, discr_target, discr_value, state);
                         }
@@ -490,8 +494,16 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
             }
             // Transfer the conditions on the copy rhs, after inversing polarity.
             Rvalue::UnaryOp(UnOp::Not, Operand::Move(place) | Operand::Copy(place)) => {
+                if !place.ty(self.body, self.tcx).ty.is_bool() {
+                    // Constructing the conditions by inverting the polarity
+                    // of equality is only correct for bools. That is to say,
+                    // `!a == b` is not `a != b` for integers greater than 1 bit.
+                    return;
+                }
                 let Some(conditions) = state.try_get_idx(lhs, &self.map) else { return };
                 let Some(place) = self.map.find(place.as_ref()) else { return };
+                // FIXME: I think This could be generalized to not bool if we
+                // actually perform a logical not on the condition's value.
                 let conds = conditions.map(self.arena, Condition::inv);
                 state.insert_value_idx(place, conds, &self.map);
             }
@@ -516,9 +528,7 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
                     // Avoid handling them, though this could be extended in the future.
                     return;
                 }
-                let Some(value) =
-                    value.const_.normalize(self.tcx, self.param_env).try_to_scalar_int()
-                else {
+                let Some(value) = value.const_.try_eval_scalar_int(self.tcx, self.param_env) else {
                     return;
                 };
                 let conds = conditions.map(self.arena, |c| Condition {
@@ -557,7 +567,9 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
                 // `SetDiscriminant` may be a no-op if the assigned variant is the untagged variant
                 // of a niche encoding. If we cannot ensure that we write to the discriminant, do
                 // nothing.
-                let Ok(enum_layout) = self.ecx.layout_of(enum_ty) else { return };
+                let Ok(enum_layout) = self.ecx.layout_of(enum_ty) else {
+                    return;
+                };
                 let writes_discriminant = match enum_layout.variants {
                     Variants::Single { index } => {
                         assert_eq!(index, *variant_index);
@@ -570,7 +582,8 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
                     } => *variant_index != untagged_variant,
                 };
                 if writes_discriminant {
-                    let Ok(discr) = self.ecx.discriminant_for_variant(enum_ty, *variant_index)
+                    let Some(discr) =
+                        self.ecx.discriminant_for_variant(enum_ty, *variant_index).discard_err()
                     else {
                         return;
                     };
@@ -647,7 +660,9 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
 
         let Some(discr) = discr.place() else { return };
         let discr_ty = discr.ty(self.body, self.tcx).ty;
-        let Ok(discr_layout) = self.ecx.layout_of(discr_ty) else { return };
+        let Ok(discr_layout) = self.ecx.layout_of(discr_ty) else {
+            return;
+        };
         let Some(conditions) = state.try_get(discr.as_ref(), &self.map) else { return };
 
         if let Some((value, _)) = targets.iter().find(|&(_, target)| target == target_bb) {
diff --git a/compiler/rustc_mir_transform/src/known_panics_lint.rs b/compiler/rustc_mir_transform/src/known_panics_lint.rs
index 783e7aabe85..8f490094d60 100644
--- a/compiler/rustc_mir_transform/src/known_panics_lint.rs
+++ b/compiler/rustc_mir_transform/src/known_panics_lint.rs
@@ -6,7 +6,7 @@ use std::fmt::Debug;
 
 use rustc_const_eval::const_eval::DummyMachine;
 use rustc_const_eval::interpret::{
-    ImmTy, InterpCx, InterpResult, Projectable, Scalar, format_interp_error,
+    ImmTy, InterpCx, InterpResult, Projectable, Scalar, format_interp_error, interp_ok,
 };
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir::HirId;
@@ -101,7 +101,7 @@ impl<'tcx> Value<'tcx> {
                 }
                 (PlaceElem::Index(idx), Value::Aggregate { fields, .. }) => {
                     let idx = prop.get_const(idx.into())?.immediate()?;
-                    let idx = prop.ecx.read_target_usize(idx).ok()?.try_into().ok()?;
+                    let idx = prop.ecx.read_target_usize(idx).discard_err()?.try_into().ok()?;
                     if idx <= FieldIdx::MAX_AS_U32 {
                         fields.get(FieldIdx::from_u32(idx)).unwrap_or(&Value::Uninit)
                     } else {
@@ -231,21 +231,20 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
     where
         F: FnOnce(&mut Self) -> InterpResult<'tcx, T>,
     {
-        match f(self) {
-            Ok(val) => Some(val),
-            Err(error) => {
-                trace!("InterpCx operation failed: {:?}", error);
+        f(self)
+            .map_err(|err| {
+                trace!("InterpCx operation failed: {:?}", err);
                 // Some errors shouldn't come up because creating them causes
                 // an allocation, which we should avoid. When that happens,
                 // dedicated error variants should be introduced instead.
                 assert!(
-                    !error.kind().formatted_string(),
+                    !err.kind().formatted_string(),
                     "known panics lint encountered formatting error: {}",
-                    format_interp_error(self.ecx.tcx.dcx(), error),
+                    format_interp_error(self.ecx.tcx.dcx(), err),
                 );
-                None
-            }
-        }
+                err
+            })
+            .discard_err()
     }
 
     /// Returns the value, if any, of evaluating `c`.
@@ -315,7 +314,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
                     .ecx
                     .binary_op(BinOp::SubWithOverflow, &ImmTy::from_int(0, arg.layout), &arg)?
                     .to_scalar_pair();
-                Ok((arg, overflow.to_bool()?))
+                interp_ok((arg, overflow.to_bool()?))
             })?;
             if overflow {
                 self.report_assert_as_lint(
@@ -349,7 +348,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
             let left_ty = left.ty(self.local_decls(), self.tcx);
             let left_size = self.ecx.layout_of(left_ty).ok()?.size;
             let right_size = r.layout.size;
-            let r_bits = r.to_scalar().to_bits(right_size).ok();
+            let r_bits = r.to_scalar().to_bits(right_size).discard_err();
             if r_bits.is_some_and(|b| b >= left_size.bits() as u128) {
                 debug!("check_binary_op: reporting assert for {:?}", location);
                 let panic = AssertKind::Overflow(
@@ -496,7 +495,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
                 // This can be `None` if the lhs wasn't const propagated and we just
                 // triggered the assert on the value of the rhs.
                 self.eval_operand(op)
-                    .and_then(|op| self.ecx.read_immediate(&op).ok())
+                    .and_then(|op| self.ecx.read_immediate(&op).discard_err())
                     .map_or(DbgVal::Underscore, |op| DbgVal::Val(op.to_const_int()))
             };
             let msg = match msg {
@@ -601,13 +600,15 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
             }
 
             Len(place) => {
-                let len = match self.get_const(place)? {
-                    Value::Immediate(src) => src.len(&self.ecx).ok()?,
-                    Value::Aggregate { fields, .. } => fields.len() as u64,
-                    Value::Uninit => match place.ty(self.local_decls(), self.tcx).ty.kind() {
-                        ty::Array(_, n) => n.try_eval_target_usize(self.tcx, self.param_env)?,
-                        _ => return None,
-                    },
+                let len = if let ty::Array(_, n) = place.ty(self.local_decls(), self.tcx).ty.kind()
+                {
+                    n.try_eval_target_usize(self.tcx, self.param_env)?
+                } else {
+                    match self.get_const(place)? {
+                        Value::Immediate(src) => src.len(&self.ecx).discard_err()?,
+                        Value::Aggregate { fields, .. } => fields.len() as u64,
+                        Value::Uninit => return None,
+                    }
                 };
                 ImmTy::from_scalar(Scalar::from_target_usize(len, self), layout).into()
             }
@@ -615,7 +616,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
             Ref(..) | RawPtr(..) => return None,
 
             NullaryOp(ref null_op, ty) => {
-                let op_layout = self.use_ecx(|this| this.ecx.layout_of(ty))?;
+                let op_layout = self.ecx.layout_of(ty).ok()?;
                 let val = match null_op {
                     NullOp::SizeOf => op_layout.size.bytes(),
                     NullOp::AlignOf => op_layout.align.abi.bytes(),
@@ -633,16 +634,16 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
             Cast(ref kind, ref value, to) => match kind {
                 CastKind::IntToInt | CastKind::IntToFloat => {
                     let value = self.eval_operand(value)?;
-                    let value = self.ecx.read_immediate(&value).ok()?;
+                    let value = self.ecx.read_immediate(&value).discard_err()?;
                     let to = self.ecx.layout_of(to).ok()?;
-                    let res = self.ecx.int_to_int_or_float(&value, to).ok()?;
+                    let res = self.ecx.int_to_int_or_float(&value, to).discard_err()?;
                     res.into()
                 }
                 CastKind::FloatToFloat | CastKind::FloatToInt => {
                     let value = self.eval_operand(value)?;
-                    let value = self.ecx.read_immediate(&value).ok()?;
+                    let value = self.ecx.read_immediate(&value).discard_err()?;
                     let to = self.ecx.layout_of(to).ok()?;
-                    let res = self.ecx.float_to_float_or_int(&value, to).ok()?;
+                    let res = self.ecx.float_to_float_or_int(&value, to).discard_err()?;
                     res.into()
                 }
                 CastKind::Transmute => {
@@ -656,7 +657,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
                         _ => return None,
                     }
 
-                    value.offset(Size::ZERO, to, &self.ecx).ok()?.into()
+                    value.offset(Size::ZERO, to, &self.ecx).discard_err()?.into()
                 }
                 _ => return None,
             },
@@ -781,7 +782,7 @@ impl<'tcx> Visitor<'tcx> for ConstPropagator<'_, 'tcx> {
             TerminatorKind::SwitchInt { ref discr, ref targets } => {
                 if let Some(ref value) = self.eval_operand(discr)
                     && let Some(value_const) = self.use_ecx(|this| this.ecx.read_scalar(value))
-                    && let Ok(constant) = value_const.to_bits(value_const.size())
+                    && let Some(constant) = value_const.to_bits(value_const.size()).discard_err()
                 {
                     // We managed to evaluate the discriminant, so we know we only need to visit
                     // one target.
diff --git a/compiler/rustc_mir_transform/src/lib.rs b/compiler/rustc_mir_transform/src/lib.rs
index 56268c1d201..d184328748f 100644
--- a/compiler/rustc_mir_transform/src/lib.rs
+++ b/compiler/rustc_mir_transform/src/lib.rs
@@ -3,6 +3,7 @@
 #![feature(box_patterns)]
 #![feature(const_type_name)]
 #![feature(cow_is_borrowed)]
+#![feature(file_buffered)]
 #![feature(if_let_guard)]
 #![feature(impl_trait_in_assoc_type)]
 #![feature(let_chains)]
@@ -50,6 +51,7 @@ mod add_subtyping_projections;
 mod check_alignment;
 mod check_const_item_mutation;
 mod check_packed_ref;
+mod check_undefined_transmutes;
 // This pass is public to allow external drivers to perform MIR cleanup
 pub mod cleanup_post_borrowck;
 mod copy_prop;
@@ -297,6 +299,7 @@ fn mir_built(tcx: TyCtxt<'_>, def: LocalDefId) -> &Steal<Body<'_>> {
             &Lint(check_packed_ref::CheckPackedRef),
             &Lint(check_const_item_mutation::CheckConstItemMutation),
             &Lint(function_item_references::FunctionItemReferences),
+            &Lint(check_undefined_transmutes::CheckUndefinedTransmutes),
             // What we need to do constant evaluation.
             &simplify::SimplifyCfg::Initial,
             &Lint(sanity_check::SanityCheck),
diff --git a/compiler/rustc_mir_transform/src/single_use_consts.rs b/compiler/rustc_mir_transform/src/single_use_consts.rs
index 9884b6dd1c3..277a33c0311 100644
--- a/compiler/rustc_mir_transform/src/single_use_consts.rs
+++ b/compiler/rustc_mir_transform/src/single_use_consts.rs
@@ -185,15 +185,14 @@ impl<'tcx> MutVisitor<'tcx> for LocalReplacer<'tcx> {
             && let Some(local) = place.as_local()
             && local == self.local
         {
-            let const_op = self
+            let const_op = *self
                 .operand
                 .as_ref()
                 .unwrap_or_else(|| {
                     bug!("the operand was already stolen");
                 })
                 .constant()
-                .unwrap()
-                .clone();
+                .unwrap();
             var_debug_info.value = VarDebugInfoContents::Const(const_op);
         }
     }
diff --git a/compiler/rustc_mir_transform/src/validate.rs b/compiler/rustc_mir_transform/src/validate.rs
index eda0b8c75f3..e353be6a105 100644
--- a/compiler/rustc_mir_transform/src/validate.rs
+++ b/compiler/rustc_mir_transform/src/validate.rs
@@ -4,7 +4,8 @@ use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_hir::LangItem;
 use rustc_index::IndexVec;
 use rustc_index::bit_set::BitSet;
-use rustc_infer::traits::Reveal;
+use rustc_infer::infer::TyCtxtInferExt;
+use rustc_infer::traits::{Obligation, ObligationCause, Reveal};
 use rustc_middle::mir::coverage::CoverageKind;
 use rustc_middle::mir::visit::{NonUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
@@ -16,6 +17,8 @@ use rustc_middle::ty::{
 use rustc_middle::{bug, span_bug};
 use rustc_target::abi::{FIRST_VARIANT, Size};
 use rustc_target::spec::abi::Abi;
+use rustc_trait_selection::traits::ObligationCtxt;
+use rustc_type_ir::Upcast;
 
 use crate::util::{is_within_packed, relate_types};
 
@@ -586,6 +589,22 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
         crate::util::relate_types(self.tcx, self.param_env, variance, src, dest)
     }
+
+    /// Check that the given predicate definitely holds in the param-env of this MIR body.
+    fn predicate_must_hold_modulo_regions(
+        &self,
+        pred: impl Upcast<TyCtxt<'tcx>, ty::Predicate<'tcx>>,
+    ) -> bool {
+        let infcx = self.tcx.infer_ctxt().build();
+        let ocx = ObligationCtxt::new(&infcx);
+        ocx.register_obligation(Obligation::new(
+            self.tcx,
+            ObligationCause::dummy(),
+            self.param_env,
+            pred,
+        ));
+        ocx.select_all_or_error().is_empty()
+    }
 }
 
 impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
@@ -1202,8 +1221,18 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                         }
                     }
                     CastKind::PointerCoercion(PointerCoercion::Unsize, _) => {
-                        // This is used for all `CoerceUnsized` types,
-                        // not just pointers/references, so is hard to check.
+                        // Pointers being unsize coerced should at least implement
+                        // `CoerceUnsized`.
+                        if !self.predicate_must_hold_modulo_regions(ty::TraitRef::new(
+                            self.tcx,
+                            self.tcx.require_lang_item(
+                                LangItem::CoerceUnsized,
+                                Some(self.body.source_info(location).span),
+                            ),
+                            [op_ty, *target_type],
+                        )) {
+                            self.fail(location, format!("Unsize coercion, but `{op_ty}` isn't coercible to `{target_type}`"));
+                        }
                     }
                     CastKind::PointerCoercion(PointerCoercion::DynStar, _) => {
                         // FIXME(dyn-star): make sure nothing needs to be done here.