Auto merge of #128351 - ChayimFriedman2:lint-transmute-unsafe-cell, r=<try>

[crater] Lint against &T to &mut T and &T to &UnsafeCell<T> transmutes

Needs a (check-only) crater run as per https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Lint.20against.20.60.26.60-.3E.60.26UnsafeCell.60.20transmutes/near/454868964.

r? ghost

Author: bors

compiler/rustc_lint/messages.ftl

@@ -119,6 +119,7 @@ lint_builtin_missing_doc = missing documentation for {$article} {$desc}
 
 lint_builtin_mutable_transmutes =
     transmuting &T to &mut T is undefined behavior, even if the reference is unused, consider instead using an UnsafeCell
+    .note = transmute from `{$from}` to `{$to}`
 
 lint_builtin_no_mangle_fn = declaration of a `no_mangle` function
 lint_builtin_no_mangle_generic = functions generic over types or consts must be mangled
@@ -169,6 +170,10 @@ lint_builtin_unreachable_pub = unreachable `pub` {$what}
 
 lint_builtin_unsafe_block = usage of an `unsafe` block
 
+lint_builtin_unsafe_cell_transmutes =
+    transmuting &T to &UnsafeCell<T> is undefined behavior, even if the reference is unused, consider using UnsafeCell on the original data
+    .note = transmute from `{$from}` to `{$to}`
+
 lint_builtin_unsafe_extern_block = usage of an `unsafe extern` block
 
 lint_builtin_unsafe_impl = implementation of an `unsafe` trait
@@ -862,6 +867,11 @@ lint_unsafe_attr_outside_unsafe = unsafe attribute used without unsafe
     .label = usage of unsafe attribute
 lint_unsafe_attr_outside_unsafe_suggestion = wrap the attribute in `unsafe(...)`
 
+lint_unsafe_cell_reference_casting =
+    casting `&T` to `&UnsafeCell<T>` is undefined behavior, even if the reference is unused, consider using an `UnsafeCell` on the original data
+    .label = casting happend here
+    .note = cast from `{$from}` to `{$to}`
+
 lint_unsupported_group = `{$lint_group}` lint group is not supported with ´--force-warn´
 
 lint_untranslatable_diag = diagnostics should be created using translatable messages

compiler/rustc_lint/src/builtin.rs

@@ -61,12 +61,11 @@ use crate::lints::{
     BuiltinEllipsisInclusiveRangePatternsLint, BuiltinExplicitOutlives,
     BuiltinExplicitOutlivesSuggestion, BuiltinFeatureIssueNote, BuiltinIncompleteFeatures,
     BuiltinIncompleteFeaturesHelp, BuiltinInternalFeatures, BuiltinKeywordIdents,
-    BuiltinMissingCopyImpl, BuiltinMissingDebugImpl, BuiltinMissingDoc, BuiltinMutablesTransmutes,
-    BuiltinNoMangleGeneric, BuiltinNonShorthandFieldPatterns, BuiltinSpecialModuleNameUsed,
-    BuiltinTrivialBounds, BuiltinTypeAliasBounds, BuiltinUngatedAsyncFnTrackCaller,
-    BuiltinUnpermittedTypeInit, BuiltinUnpermittedTypeInitSub, BuiltinUnreachablePub,
-    BuiltinUnsafe, BuiltinUnstableFeatures, BuiltinUnusedDocComment, BuiltinUnusedDocCommentSub,
-    BuiltinWhileTrue, InvalidAsmLabel,
+    BuiltinMissingCopyImpl, BuiltinMissingDebugImpl, BuiltinMissingDoc, BuiltinNoMangleGeneric,
+    BuiltinNonShorthandFieldPatterns, BuiltinSpecialModuleNameUsed, BuiltinTrivialBounds,
+    BuiltinTypeAliasBounds, BuiltinUngatedAsyncFnTrackCaller, BuiltinUnpermittedTypeInit,
+    BuiltinUnpermittedTypeInitSub, BuiltinUnreachablePub, BuiltinUnsafe, BuiltinUnstableFeatures,
+    BuiltinUnusedDocComment, BuiltinUnusedDocCommentSub, BuiltinWhileTrue, InvalidAsmLabel,
 };
 use crate::nonstandard_style::{method_context, MethodLateContext};
 use crate::{
@@ -1100,72 +1099,6 @@ impl<'tcx> LateLintPass<'tcx> for InvalidNoMangleItems {
     }
 }
 
-declare_lint! {
-    /// The `mutable_transmutes` lint catches transmuting from `&T` to `&mut
-    /// T` because it is [undefined behavior].
-    ///
-    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
-    ///
-    /// ### Example
-    ///
-    /// ```rust,compile_fail
-    /// unsafe {
-    ///     let y = std::mem::transmute::<&i32, &mut i32>(&5);
-    /// }
-    /// ```
-    ///
-    /// {{produces}}
-    ///
-    /// ### Explanation
-    ///
-    /// Certain assumptions are made about aliasing of data, and this transmute
-    /// violates those assumptions. Consider using [`UnsafeCell`] instead.
-    ///
-    /// [`UnsafeCell`]: https://doc.rust-lang.org/std/cell/struct.UnsafeCell.html
-    MUTABLE_TRANSMUTES,
-    Deny,
-    "transmuting &T to &mut T is undefined behavior, even if the reference is unused"
-}
-
-declare_lint_pass!(MutableTransmutes => [MUTABLE_TRANSMUTES]);
-
-impl<'tcx> LateLintPass<'tcx> for MutableTransmutes {
-    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &hir::Expr<'_>) {
-        if let Some((&ty::Ref(_, _, from_mutbl), &ty::Ref(_, _, to_mutbl))) =
-            get_transmute_from_to(cx, expr).map(|(ty1, ty2)| (ty1.kind(), ty2.kind()))
-        {
-            if from_mutbl < to_mutbl {
-                cx.emit_span_lint(MUTABLE_TRANSMUTES, expr.span, BuiltinMutablesTransmutes);
-            }
-        }
-
-        fn get_transmute_from_to<'tcx>(
-            cx: &LateContext<'tcx>,
-            expr: &hir::Expr<'_>,
-        ) -> Option<(Ty<'tcx>, Ty<'tcx>)> {
-            let def = if let hir::ExprKind::Path(ref qpath) = expr.kind {
-                cx.qpath_res(qpath, expr.hir_id)
-            } else {
-                return None;
-            };
-            if let Res::Def(DefKind::Fn, did) = def {
-                if !def_id_is_transmute(cx, did) {
-                    return None;
-                }
-                let sig = cx.typeck_results().node_type(expr.hir_id).fn_sig(cx.tcx);
-                let from = sig.inputs().skip_binder()[0];
-                let to = sig.output().skip_binder();
-                return Some((from, to));
-            }
-            None
-        }
-
-        fn def_id_is_transmute(cx: &LateContext<'_>, def_id: DefId) -> bool {
-            cx.tcx.is_intrinsic(def_id, sym::transmute)
-        }
-    }
-}
-
 declare_lint! {
     /// The `unstable_features` lint detects uses of `#![feature]`.
     ///
@@ -1612,7 +1545,6 @@ declare_lint_pass!(
         UNUSED_DOC_COMMENTS,
         NO_MANGLE_CONST_ITEMS,
         NO_MANGLE_GENERIC_ITEMS,
-        MUTABLE_TRANSMUTES,
         UNSTABLE_FEATURES,
         UNREACHABLE_PUB,
         TYPE_ALIAS_BOUNDS,

compiler/rustc_lint/src/lib.rs

@@ -65,6 +65,7 @@ mod macro_expr_fragment_specifier_2024_migration;
 mod map_unit_fn;
 mod methods;
 mod multiple_supertrait_upcastable;
+mod mutable_transmutes;
 mod non_ascii_idents;
 mod non_fmt_panic;
 mod non_local_def;
@@ -99,6 +100,7 @@ use macro_expr_fragment_specifier_2024_migration::*;
 use map_unit_fn::*;
 use methods::*;
 use multiple_supertrait_upcastable::*;
+use mutable_transmutes::*;
 use non_ascii_idents::*;
 use non_fmt_panic::NonPanicFmt;
 use non_local_def::*;
@@ -209,6 +211,7 @@ late_lint_methods!(
             // Depends on referenced function signatures in expressions
             PtrNullChecks: PtrNullChecks,
             MutableTransmutes: MutableTransmutes,
+            UnsafeCellReferenceCasting: UnsafeCellReferenceCasting,
             TypeAliasBounds: TypeAliasBounds,
             TrivialConstraints: TrivialConstraints,
             TypeLimits: TypeLimits::new(),

compiler/rustc_lint/src/lints.rs

@@ -224,9 +224,34 @@ pub struct BuiltinConstNoMangle {
     pub suggestion: Span,
 }
 
+// mutable_transmutes.rs
 #[derive(LintDiagnostic)]
 #[diag(lint_builtin_mutable_transmutes)]
-pub struct BuiltinMutablesTransmutes;
+#[note]
+pub struct BuiltinMutablesTransmutes {
+    pub from: String,
+    pub to: String,
+}
+
+// mutable_transmutes.rs
+#[derive(LintDiagnostic)]
+#[diag(lint_builtin_unsafe_cell_transmutes)]
+#[note]
+pub struct BuiltinUnsafeCellTransmutes {
+    pub from: String,
+    pub to: String,
+}
+
+// mutable_transmutes.rs
+#[derive(LintDiagnostic)]
+#[diag(lint_unsafe_cell_reference_casting)]
+#[note]
+pub struct UnsafeCellReferenceCastingDiag {
+    #[label]
+    pub orig_cast: Option<Span>,
+    pub from: String,
+    pub to: String,
+}
 
 #[derive(LintDiagnostic)]
 #[diag(lint_builtin_unstable_features)]