Rollup merge of #141551 - compiler-errors:hir-lints, r=BoxyUwU

Make two transmute-related MIR lints into HIR lint

Make `PTR_TO_INTEGER_TRANSMUTE_IN_CONSTS` (rust-lang/rust#130540) and `UNNECESSARY_TRANSMUTES` (rust-lang/rust#136083) into "normal" HIR-based lints.

Funny enough this came up in the review of the latter (https://github.com/rust-lang/rust/pull/136083#issuecomment-2614301413), but I guess it just was overlooked.

But anywyas, there's no reason for these to be MIR lints; in fact, it makes the suggestions for them a bit more complicated than necessary.

Note that there's probably a few more simplifications and improvements to be done here. Follow-ups can be done in a separate PR, especially if they're about the messaging and suggestions themselves, which I didn't write.

11 files changed, 292 insertions, 312 deletions

diff --git a/compiler/rustc_lint/messages.ftl b/compiler/rustc_lint/messages.ftl
index b470dea5e82..6913fa3e60b 100644
--- a/compiler/rustc_lint/messages.ftl
+++ b/compiler/rustc_lint/messages.ftl
@@ -807,6 +807,11 @@ lint_type_ir_inherent_usage = do not use `rustc_type_ir::inherent` unless you're
 lint_type_ir_trait_usage = do not use `rustc_type_ir::Interner` or `rustc_type_ir::InferCtxtLike` unless you're inside of the trait solver
     .note = the method or struct you're looking for is likely defined somewhere else downstream in the compiler
 
+lint_undefined_transmute = pointers cannot be transmuted to integers during const eval
+    .note = at compile-time, pointers do not have an integer value
+    .note2 = avoiding this restriction via `union` or raw pointers leads to compile-time undefined behavior
+    .help = for more information, see https://doc.rust-lang.org/std/mem/fn.transmute.html
+
 lint_undropped_manually_drops = calls to `std::mem::drop` with `std::mem::ManuallyDrop` instead of the inner value does nothing
     .label = argument has type `{$arg_ty}`
     .suggestion = use `std::mem::ManuallyDrop::into_inner` to get the inner value
diff --git a/compiler/rustc_lint/src/lib.rs b/compiler/rustc_lint/src/lib.rs
index 0de4c43a659..ce290eab8e9 100644
--- a/compiler/rustc_lint/src/lib.rs
+++ b/compiler/rustc_lint/src/lib.rs
@@ -75,6 +75,7 @@ mod reference_casting;
 mod shadowed_into_iter;
 mod static_mut_refs;
 mod traits;
+mod transmute;
 mod types;
 mod unit_bindings;
 mod unqualified_local_imports;
@@ -118,6 +119,7 @@ use shadowed_into_iter::ShadowedIntoIter;
 pub use shadowed_into_iter::{ARRAY_INTO_ITER, BOXED_SLICE_INTO_ITER};
 use static_mut_refs::*;
 use traits::*;
+use transmute::CheckTransmutes;
 use types::*;
 use unit_bindings::*;
 use unqualified_local_imports::*;
@@ -246,6 +248,7 @@ late_lint_methods!(
             IfLetRescope: IfLetRescope::default(),
             StaticMutRefs: StaticMutRefs,
             UnqualifiedLocalImports: UnqualifiedLocalImports,
+            CheckTransmutes: CheckTransmutes,
         ]
     ]
 );
diff --git a/compiler/rustc_lint/src/transmute.rs b/compiler/rustc_lint/src/transmute.rs
new file mode 100644
index 00000000000..bc1d4587d07
--- /dev/null
+++ b/compiler/rustc_lint/src/transmute.rs
@@ -0,0 +1,278 @@
+use rustc_errors::Applicability;
+use rustc_hir::def::{DefKind, Res};
+use rustc_hir::def_id::LocalDefId;
+use rustc_hir::{self as hir};
+use rustc_macros::LintDiagnostic;
+use rustc_middle::ty::{self, Ty};
+use rustc_session::{declare_lint, impl_lint_pass};
+use rustc_span::sym;
+
+use crate::{LateContext, LateLintPass};
+
+declare_lint! {
+    /// The `ptr_to_integer_transmute_in_consts` lint detects pointer to integer
+    /// transmute in const functions and associated constants.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// const fn foo(ptr: *const u8) -> usize {
+    ///    unsafe {
+    ///        std::mem::transmute::<*const u8, usize>(ptr)
+    ///    }
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Transmuting pointers to integers in a `const` context is undefined behavior.
+    /// Any attempt to use the resulting integer will abort const-evaluation.
+    ///
+    /// But sometimes the compiler might not emit an error for pointer to integer transmutes
+    /// inside const functions and associated consts because they are evaluated only when referenced.
+    /// Therefore, this lint serves as an extra layer of defense to prevent any undefined behavior
+    /// from compiling without any warnings or errors.
+    ///
+    /// See [std::mem::transmute] in the reference for more details.
+    ///
+    /// [std::mem::transmute]: https://doc.rust-lang.org/std/mem/fn.transmute.html
+    pub PTR_TO_INTEGER_TRANSMUTE_IN_CONSTS,
+    Warn,
+    "detects pointer to integer transmutes in const functions and associated constants",
+}
+
+declare_lint! {
+    /// The `unnecessary_transmutes` lint detects transmutations that have safer alternatives.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// fn bytes_at_home(x: [u8; 4]) -> u32 {
+    ///   unsafe { std::mem::transmute(x) }
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Using an explicit method is preferable over calls to
+    /// [`transmute`](https://doc.rust-lang.org/std/mem/fn.transmute.html) as
+    /// they more clearly communicate the intent, are easier to review, and
+    /// are less likely to accidentally result in unsoundness.
+    pub UNNECESSARY_TRANSMUTES,
+    Warn,
+    "detects transmutes that can also be achieved by other operations"
+}
+
+pub(crate) struct CheckTransmutes;
+
+impl_lint_pass!(CheckTransmutes => [PTR_TO_INTEGER_TRANSMUTE_IN_CONSTS, UNNECESSARY_TRANSMUTES]);
+
+impl<'tcx> LateLintPass<'tcx> for CheckTransmutes {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
+        let hir::ExprKind::Call(callee, [arg]) = expr.kind else {
+            return;
+        };
+        let hir::ExprKind::Path(qpath) = callee.kind else {
+            return;
+        };
+        let Res::Def(DefKind::Fn, def_id) = cx.qpath_res(&qpath, callee.hir_id) else {
+            return;
+        };
+        if !cx.tcx.is_intrinsic(def_id, sym::transmute) {
+            return;
+        };
+        let body_owner_def_id = cx.tcx.hir_enclosing_body_owner(expr.hir_id);
+        let const_context = cx.tcx.hir_body_const_context(body_owner_def_id);
+        let args = cx.typeck_results().node_args(callee.hir_id);
+
+        let src = args.type_at(0);
+        let dst = args.type_at(1);
+
+        check_ptr_transmute_in_const(cx, expr, body_owner_def_id, const_context, src, dst);
+        check_unnecessary_transmute(cx, expr, callee, arg, const_context, src, dst);
+    }
+}
+
+/// Check for transmutes that exhibit undefined behavior.
+/// For example, transmuting pointers to integers in a const context.
+///
+/// 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 check_ptr_transmute_in_const<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx hir::Expr<'tcx>,
+    body_owner_def_id: LocalDefId,
+    const_context: Option<hir::ConstContext>,
+    src: Ty<'tcx>,
+    dst: Ty<'tcx>,
+) {
+    if matches!(const_context, Some(hir::ConstContext::ConstFn))
+        || matches!(cx.tcx.def_kind(body_owner_def_id), DefKind::AssocConst)
+    {
+        if src.is_raw_ptr() && dst.is_integral() {
+            cx.tcx.emit_node_span_lint(
+                PTR_TO_INTEGER_TRANSMUTE_IN_CONSTS,
+                expr.hir_id,
+                expr.span,
+                UndefinedTransmuteLint,
+            );
+        }
+    }
+}
+
+/// Check for transmutes that overlap with stdlib methods.
+/// For example, transmuting `[u8; 4]` to `u32`.
+///
+/// We chose not to lint u8 -> bool transmutes, see #140431.
+fn check_unnecessary_transmute<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx hir::Expr<'tcx>,
+    callee: &'tcx hir::Expr<'tcx>,
+    arg: &'tcx hir::Expr<'tcx>,
+    const_context: Option<hir::ConstContext>,
+    src: Ty<'tcx>,
+    dst: Ty<'tcx>,
+) {
+    let callee_span = callee.span.find_ancestor_inside(expr.span).unwrap_or(callee.span);
+    let (sugg, help) = match (src.kind(), dst.kind()) {
+        // dont check the length; transmute does that for us.
+        // [u8; _] => primitive
+        (ty::Array(t, _), ty::Uint(_) | ty::Float(_) | ty::Int(_))
+            if *t.kind() == ty::Uint(ty::UintTy::U8) =>
+        {
+            (
+                Some(vec![(callee_span, format!("{dst}::from_ne_bytes"))]),
+                Some(
+                    "there's also `from_le_bytes` and `from_be_bytes` if you expect a particular byte order",
+                ),
+            )
+        }
+        // primitive => [u8; _]
+        (ty::Uint(_) | ty::Float(_) | ty::Int(_), ty::Array(t, _))
+            if *t.kind() == ty::Uint(ty::UintTy::U8) =>
+        {
+            (
+                Some(vec![(callee_span, format!("{src}::to_ne_bytes"))]),
+                Some(
+                    "there's also `to_le_bytes` and `to_be_bytes` if you expect a particular byte order",
+                ),
+            )
+        }
+        // char → u32
+        (ty::Char, ty::Uint(ty::UintTy::U32)) => {
+            (Some(vec![(callee_span, "u32::from".to_string())]), None)
+        }
+        // char (→ u32) → i32
+        (ty::Char, ty::Int(ty::IntTy::I32)) => (
+            Some(vec![
+                (callee_span, "u32::from".to_string()),
+                (expr.span.shrink_to_hi(), ".cast_signed()".to_string()),
+            ]),
+            None,
+        ),
+        // u32 → char
+        (ty::Uint(ty::UintTy::U32), ty::Char) => (
+            Some(vec![(callee_span, "char::from_u32_unchecked".to_string())]),
+            Some("consider using `char::from_u32(…).unwrap()`"),
+        ),
+        // i32 → char
+        (ty::Int(ty::IntTy::I32), ty::Char) => (
+            Some(vec![
+                (callee_span, "char::from_u32_unchecked(i32::cast_unsigned".to_string()),
+                (expr.span.shrink_to_hi(), ")".to_string()),
+            ]),
+            Some("consider using `char::from_u32(i32::cast_unsigned(…)).unwrap()`"),
+        ),
+        // uNN → iNN
+        (ty::Uint(_), ty::Int(_)) => {
+            (Some(vec![(callee_span, format!("{src}::cast_signed"))]), None)
+        }
+        // iNN → uNN
+        (ty::Int(_), ty::Uint(_)) => {
+            (Some(vec![(callee_span, format!("{src}::cast_unsigned"))]), None)
+        }
+        // fNN → usize, isize
+        (ty::Float(_), ty::Uint(ty::UintTy::Usize) | ty::Int(ty::IntTy::Isize)) => (
+            Some(vec![
+                (callee_span, format!("{src}::to_bits")),
+                (expr.span.shrink_to_hi(), format!(" as {dst}")),
+            ]),
+            None,
+        ),
+        // fNN (→ uNN) → iNN
+        (ty::Float(_), ty::Int(..)) => (
+            Some(vec![
+                (callee_span, format!("{src}::to_bits")),
+                (expr.span.shrink_to_hi(), ".cast_signed()".to_string()),
+            ]),
+            None,
+        ),
+        // fNN → uNN
+        (ty::Float(_), ty::Uint(..)) => {
+            (Some(vec![(callee_span, format!("{src}::to_bits"))]), None)
+        }
+        // xsize → fNN
+        (ty::Uint(ty::UintTy::Usize) | ty::Int(ty::IntTy::Isize), ty::Float(_)) => (
+            Some(vec![
+                (callee_span, format!("{dst}::from_bits")),
+                (arg.span.shrink_to_hi(), " as _".to_string()),
+            ]),
+            None,
+        ),
+        // iNN (→ uNN) → fNN
+        (ty::Int(_), ty::Float(_)) => (
+            Some(vec![
+                (callee_span, format!("{dst}::from_bits({src}::cast_unsigned")),
+                (expr.span.shrink_to_hi(), ")".to_string()),
+            ]),
+            None,
+        ),
+        // uNN → fNN
+        (ty::Uint(_), ty::Float(_)) => {
+            (Some(vec![(callee_span, format!("{dst}::from_bits"))]), None)
+        }
+        // bool → x8 in const context since `From::from` is not const yet
+        // FIXME: Consider arg expr's precedence to avoid parentheses.
+        // FIXME(const_traits): Remove this when `From::from` is constified.
+        (ty::Bool, ty::Int(..) | ty::Uint(..)) if const_context.is_some() => (
+            Some(vec![
+                (callee_span, "".to_string()),
+                (expr.span.shrink_to_hi(), format!(" as {dst}")),
+            ]),
+            None,
+        ),
+        // bool → x8 using `x8::from`
+        (ty::Bool, ty::Int(..) | ty::Uint(..)) => {
+            (Some(vec![(callee_span, format!("{dst}::from"))]), None)
+        }
+        _ => return,
+    };
+
+    cx.tcx.node_span_lint(UNNECESSARY_TRANSMUTES, expr.hir_id, expr.span, |diag| {
+        diag.primary_message("unnecessary transmute");
+        if let Some(sugg) = sugg {
+            diag.multipart_suggestion("replace this with", sugg, Applicability::MachineApplicable);
+        }
+        if let Some(help) = help {
+            diag.help(help);
+        }
+    });
+}
+
+#[derive(LintDiagnostic)]
+#[diag(lint_undefined_transmute)]
+#[note]
+#[note(lint_note2)]
+#[help]
+pub(crate) struct UndefinedTransmuteLint;
diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
index b8d242bad86..abf4840a026 100644
--- a/compiler/rustc_lint_defs/src/builtin.rs
+++ b/compiler/rustc_lint_defs/src/builtin.rs
@@ -79,7 +79,6 @@ declare_lint_pass! {
         PRIVATE_BOUNDS,
         PRIVATE_INTERFACES,
         PROC_MACRO_DERIVE_RESOLUTION_FALLBACK,
-        PTR_TO_INTEGER_TRANSMUTE_IN_CONSTS,
         PUB_USE_OF_PRIVATE_EXTERN_CRATE,
         REDUNDANT_IMPORTS,
         REDUNDANT_LIFETIMES,
@@ -118,7 +117,6 @@ declare_lint_pass! {
         UNKNOWN_OR_MALFORMED_DIAGNOSTIC_ATTRIBUTES,
         UNNAMEABLE_TEST_ITEMS,
         UNNAMEABLE_TYPES,
-        UNNECESSARY_TRANSMUTES,
         UNREACHABLE_CODE,
         UNREACHABLE_PATTERNS,
         UNSAFE_ATTR_OUTSIDE_UNSAFE,
@@ -4852,64 +4850,6 @@ declare_lint! {
 }
 
 declare_lint! {
-    /// The `ptr_to_integer_transmute_in_consts` lint detects pointer to integer
-    /// transmute in const functions and associated constants.
-    ///
-    /// ### Example
-    ///
-    /// ```rust
-    /// const fn foo(ptr: *const u8) -> usize {
-    ///    unsafe {
-    ///        std::mem::transmute::<*const u8, usize>(ptr)
-    ///    }
-    /// }
-    /// ```
-    ///
-    /// {{produces}}
-    ///
-    /// ### Explanation
-    ///
-    /// Transmuting pointers to integers in a `const` context is undefined behavior.
-    /// Any attempt to use the resulting integer will abort const-evaluation.
-    ///
-    /// But sometimes the compiler might not emit an error for pointer to integer transmutes
-    /// inside const functions and associated consts because they are evaluated only when referenced.
-    /// Therefore, this lint serves as an extra layer of defense to prevent any undefined behavior
-    /// from compiling without any warnings or errors.
-    ///
-    /// See [std::mem::transmute] in the reference for more details.
-    ///
-    /// [std::mem::transmute]: https://doc.rust-lang.org/std/mem/fn.transmute.html
-    pub PTR_TO_INTEGER_TRANSMUTE_IN_CONSTS,
-    Warn,
-    "detects pointer to integer transmutes in const functions and associated constants",
-}
-
-declare_lint! {
-    /// The `unnecessary_transmutes` lint detects transmutations that have safer alternatives.
-    ///
-    /// ### Example
-    ///
-    /// ```rust
-    /// fn bytes_at_home(x: [u8; 4]) -> u32 {
-    ///   unsafe { std::mem::transmute(x) }
-    /// }
-    /// ```
-    ///
-    /// {{produces}}
-    ///
-    /// ### Explanation
-    ///
-    /// Using an explicit method is preferable over calls to
-    /// [`transmute`](https://doc.rust-lang.org/std/mem/fn.transmute.html) as
-    /// they more clearly communicate the intent, are easier to review, and
-    /// are less likely to accidentally result in unsoundness.
-    pub UNNECESSARY_TRANSMUTES,
-    Warn,
-    "detects transmutes that are shadowed by std methods"
-}
-
-declare_lint! {
     /// The `tail_expr_drop_order` lint looks for those values generated at the tail expression location,
     /// that runs a custom `Drop` destructor.
     /// Some of them may be dropped earlier in Edition 2024 that they used to in Edition 2021 and prior.
diff --git a/compiler/rustc_middle/src/hir/map.rs b/compiler/rustc_middle/src/hir/map.rs
index fee707f7b4c..9c8f1c9eccf 100644
--- a/compiler/rustc_middle/src/hir/map.rs
+++ b/compiler/rustc_middle/src/hir/map.rs
@@ -310,7 +310,7 @@ impl<'tcx> TyCtxt<'tcx> {
     /// This should only be used for determining the context of a body, a return
     /// value of `Some` does not always suggest that the owner of the body is `const`,
     /// just that it has to be checked as if it were.
-    pub fn hir_body_const_context(self, def_id: impl Into<DefId>) -> Option<ConstContext> {
+    pub fn hir_body_const_context(self, def_id: LocalDefId) -> Option<ConstContext> {
         let def_id = def_id.into();
         let ccx = match self.hir_body_owner_kind(def_id) {
             BodyOwnerKind::Const { inline } => ConstContext::Const { inline },
diff --git a/compiler/rustc_middle/src/mir/pretty.rs b/compiler/rustc_middle/src/mir/pretty.rs
index 57ae7dc55c5..6b262a27500 100644
--- a/compiler/rustc_middle/src/mir/pretty.rs
+++ b/compiler/rustc_middle/src/mir/pretty.rs
@@ -1624,7 +1624,11 @@ pub fn write_allocations<'tcx>(
             Some(GlobalAlloc::Static(did)) if !tcx.is_foreign_item(did) => {
                 write!(w, " (static: {}", tcx.def_path_str(did))?;
                 if body.phase <= MirPhase::Runtime(RuntimePhase::PostCleanup)
-                    && tcx.hir_body_const_context(body.source.def_id()).is_some()
+                    && body
+                        .source
+                        .def_id()
+                        .as_local()
+                        .is_some_and(|def_id| tcx.hir_body_const_context(def_id).is_some())
                 {
                     // Statics may be cyclic and evaluating them too early
                     // in the MIR pipeline may cause cycle errors even though
diff --git a/compiler/rustc_mir_transform/messages.ftl b/compiler/rustc_mir_transform/messages.ftl
index a1264471a2d..ae3062f07de 100644
--- a/compiler/rustc_mir_transform/messages.ftl
+++ b/compiler/rustc_mir_transform/messages.ftl
@@ -78,10 +78,4 @@ mir_transform_unconditional_recursion = function cannot return without recursing
 
 mir_transform_unconditional_recursion_call_site_label = recursive call site
 
-mir_transform_undefined_transmute = pointers cannot be transmuted to integers during const eval
-    .note = at compile-time, pointers do not have an integer value
-    .note2 = avoiding this restriction via `union` or raw pointers leads to compile-time undefined behavior
-    .help = for more information, see https://doc.rust-lang.org/std/mem/fn.transmute.html
-
 mir_transform_unknown_pass_name = MIR pass `{$name}` is unknown and will be ignored
-mir_transform_unnecessary_transmute = unnecessary transmute
diff --git a/compiler/rustc_mir_transform/src/check_undefined_transmutes.rs b/compiler/rustc_mir_transform/src/check_undefined_transmutes.rs
deleted file mode 100644
index daddb5dedbc..00000000000
--- a/compiler/rustc_mir_transform/src/check_undefined_transmutes.rs
+++ /dev/null
@@ -1,77 +0,0 @@
-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_raw_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/check_unnecessary_transmutes.rs b/compiler/rustc_mir_transform/src/check_unnecessary_transmutes.rs
deleted file mode 100644
index 1a3715465ad..00000000000
--- a/compiler/rustc_mir_transform/src/check_unnecessary_transmutes.rs
+++ /dev/null
@@ -1,136 +0,0 @@
-use rustc_middle::mir::visit::Visitor;
-use rustc_middle::mir::{Body, Location, Operand, Terminator, TerminatorKind};
-use rustc_middle::ty::*;
-use rustc_session::lint::builtin::UNNECESSARY_TRANSMUTES;
-use rustc_span::source_map::Spanned;
-use rustc_span::{Span, sym};
-
-use crate::errors::UnnecessaryTransmute as Error;
-
-/// Check for transmutes that overlap with stdlib methods.
-/// For example, transmuting `[u8; 4]` to `u32`.
-/// We chose not to lint u8 -> bool transmutes, see #140431
-pub(super) struct CheckUnnecessaryTransmutes;
-
-impl<'tcx> crate::MirLint<'tcx> for CheckUnnecessaryTransmutes {
-    fn run_lint(&self, tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
-        let mut checker = UnnecessaryTransmuteChecker { body, tcx };
-        checker.visit_body(body);
-    }
-}
-
-struct UnnecessaryTransmuteChecker<'a, 'tcx> {
-    body: &'a Body<'tcx>,
-    tcx: TyCtxt<'tcx>,
-}
-
-impl<'a, 'tcx> UnnecessaryTransmuteChecker<'a, 'tcx> {
-    fn is_unnecessary_transmute(
-        &self,
-        function: &Operand<'tcx>,
-        arg: String,
-        span: Span,
-        is_in_const: bool,
-    ) -> Option<Error> {
-        let fn_sig = function.ty(self.body, self.tcx).fn_sig(self.tcx).skip_binder();
-        let [input] = fn_sig.inputs() else { return None };
-
-        let err = |sugg| Error { span, sugg, help: None };
-
-        Some(match (input.kind(), fn_sig.output().kind()) {
-            // dont check the length; transmute does that for us.
-            // [u8; _] => primitive
-            (Array(t, _), Uint(_) | Float(_) | Int(_)) if *t.kind() == Uint(UintTy::U8) => Error {
-                sugg: format!("{}::from_ne_bytes({arg})", fn_sig.output()),
-                help: Some(
-                    "there's also `from_le_bytes` and `from_be_bytes` if you expect a particular byte order",
-                ),
-                span,
-            },
-            // primitive => [u8; _]
-            (Uint(_) | Float(_) | Int(_), Array(t, _)) if *t.kind() == Uint(UintTy::U8) => Error {
-                sugg: format!("{input}::to_ne_bytes({arg})"),
-                help: Some(
-                    "there's also `to_le_bytes` and `to_be_bytes` if you expect a particular byte order",
-                ),
-                span,
-            },
-            // char → u32
-            (Char, Uint(UintTy::U32)) => err(format!("u32::from({arg})")),
-            // char (→ u32) → i32
-            (Char, Int(IntTy::I32)) => err(format!("u32::from({arg}).cast_signed()")),
-            // u32 → char
-            (Uint(UintTy::U32), Char) => Error {
-                sugg: format!("char::from_u32_unchecked({arg})"),
-                help: Some("consider `char::from_u32(…).unwrap()`"),
-                span,
-            },
-            // i32 → char
-            (Int(IntTy::I32), Char) => Error {
-                sugg: format!("char::from_u32_unchecked(i32::cast_unsigned({arg}))"),
-                help: Some("consider `char::from_u32(i32::cast_unsigned(…)).unwrap()`"),
-                span,
-            },
-            // uNN → iNN
-            (Uint(ty), Int(_)) => err(format!("{}::cast_signed({arg})", ty.name_str())),
-            // iNN → uNN
-            (Int(ty), Uint(_)) => err(format!("{}::cast_unsigned({arg})", ty.name_str())),
-            // fNN → xsize
-            (Float(ty), Uint(UintTy::Usize)) => {
-                err(format!("{}::to_bits({arg}) as usize", ty.name_str()))
-            }
-            (Float(ty), Int(IntTy::Isize)) => {
-                err(format!("{}::to_bits({arg}) as isize", ty.name_str()))
-            }
-            // fNN (→ uNN) → iNN
-            (Float(ty), Int(..)) => err(format!("{}::to_bits({arg}).cast_signed()", ty.name_str())),
-            // fNN → uNN
-            (Float(ty), Uint(..)) => err(format!("{}::to_bits({arg})", ty.name_str())),
-            // xsize → fNN
-            (Uint(UintTy::Usize) | Int(IntTy::Isize), Float(ty)) => {
-                err(format!("{}::from_bits({arg} as _)", ty.name_str(),))
-            }
-            // iNN (→ uNN) → fNN
-            (Int(int_ty), Float(ty)) => err(format!(
-                "{}::from_bits({}::cast_unsigned({arg}))",
-                ty.name_str(),
-                int_ty.name_str()
-            )),
-            // uNN → fNN
-            (Uint(_), Float(ty)) => err(format!("{}::from_bits({arg})", ty.name_str())),
-            // bool → { x8 } in const context since `From::from` is not const yet
-            // FIXME: is it possible to know when the parentheses arent necessary?
-            // FIXME(const_traits): Remove this when From::from is constified?
-            (Bool, Int(..) | Uint(..)) if is_in_const => {
-                err(format!("({arg}) as {}", fn_sig.output()))
-            }
-            // " using `x8::from`
-            (Bool, Int(..) | Uint(..)) => err(format!("{}::from({arg})", fn_sig.output())),
-            _ => return None,
-        })
-    }
-}
-
-impl<'tcx> Visitor<'tcx> for UnnecessaryTransmuteChecker<'_, '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, args, .. } = &terminator.kind
-            && let [Spanned { span: arg, .. }] = **args
-            && let Some((func_def_id, _)) = func.const_fn_def()
-            && self.tcx.is_intrinsic(func_def_id, sym::transmute)
-            && let span = self.body.source_info(location).span
-            && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(arg)
-            && let def_id = self.body.source.def_id()
-            && let Some(lint) = self.is_unnecessary_transmute(
-                func,
-                snippet,
-                span,
-                self.tcx.hir_body_const_context(def_id.expect_local()).is_some(),
-            )
-            && let Some(hir_id) = terminator.source_info.scope.lint_root(&self.body.source_scopes)
-        {
-            self.tcx.emit_node_span_lint(UNNECESSARY_TRANSMUTES, hir_id, span, lint);
-        }
-    }
-}
diff --git a/compiler/rustc_mir_transform/src/errors.rs b/compiler/rustc_mir_transform/src/errors.rs
index 5b03a4987ed..cffa0183fa7 100644
--- a/compiler/rustc_mir_transform/src/errors.rs
+++ b/compiler/rustc_mir_transform/src/errors.rs
@@ -158,33 +158,6 @@ pub(crate) struct MustNotSuspendReason {
     pub reason: String,
 }
 
-pub(crate) struct UnnecessaryTransmute {
-    pub span: Span,
-    pub sugg: String,
-    pub help: Option<&'static str>,
-}
-
-// Needed for def_path_str
-impl<'a> LintDiagnostic<'a, ()> for UnnecessaryTransmute {
-    fn decorate_lint<'b>(self, diag: &'b mut rustc_errors::Diag<'a, ()>) {
-        diag.primary_message(fluent::mir_transform_unnecessary_transmute);
-        diag.span_suggestion(
-            self.span,
-            "replace this with",
-            self.sugg,
-            lint::Applicability::MachineApplicable,
-        );
-        self.help.map(|help| diag.help(help));
-    }
-}
-
-#[derive(LintDiagnostic)]
-#[diag(mir_transform_undefined_transmute)]
-#[note]
-#[note(mir_transform_note2)]
-#[help]
-pub(crate) struct UndefinedTransmute;
-
 #[derive(Diagnostic)]
 #[diag(mir_transform_force_inline)]
 #[note]
diff --git a/compiler/rustc_mir_transform/src/lib.rs b/compiler/rustc_mir_transform/src/lib.rs
index 10dbb3437dc..d26e4468715 100644
--- a/compiler/rustc_mir_transform/src/lib.rs
+++ b/compiler/rustc_mir_transform/src/lib.rs
@@ -123,8 +123,6 @@ declare_passes! {
     mod check_const_item_mutation : CheckConstItemMutation;
     mod check_null : CheckNull;
     mod check_packed_ref : CheckPackedRef;
-    mod check_undefined_transmutes : CheckUndefinedTransmutes;
-    mod check_unnecessary_transmutes: CheckUnnecessaryTransmutes;
     // This pass is public to allow external drivers to perform MIR cleanup
     pub mod cleanup_post_borrowck : CleanupPostBorrowck;
 
@@ -390,8 +388,6 @@ 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),
-            &Lint(check_unnecessary_transmutes::CheckUnnecessaryTransmutes),
             // What we need to do constant evaluation.
             &simplify::SimplifyCfg::Initial,
             &Lint(sanity_check::SanityCheck),