Lint against &T to &mut T and &T to &UnsafeCell<T> transmutes

compiler/rustc_lint/messages.ftl

@@ -95,6 +95,7 @@ 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
@@ -140,6 +141,10 @@ 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_impl = implementation of an `unsafe` trait
 
 lint_builtin_unsafe_trait = declaration of an `unsafe` trait

compiler/rustc_lint/src/builtin.rs

@@ -29,11 +29,10 @@ use crate::{
         BuiltinDerefNullptr, BuiltinEllipsisInclusiveRangePatternsLint, BuiltinExplicitOutlives,
         BuiltinExplicitOutlivesSuggestion, BuiltinFeatureIssueNote, BuiltinIncompleteFeatures,
         BuiltinIncompleteFeaturesHelp, BuiltinInternalFeatures, BuiltinKeywordIdents,
-        BuiltinMissingCopyImpl, BuiltinMissingDebugImpl, BuiltinMissingDoc,
-        BuiltinMutablesTransmutes, BuiltinNoMangleGeneric, BuiltinNonShorthandFieldPatterns,
-        BuiltinSpecialModuleNameUsed, BuiltinTrivialBounds, BuiltinTypeAliasGenericBounds,
-        BuiltinTypeAliasGenericBoundsSuggestion, BuiltinTypeAliasWhereClause,
-        BuiltinUngatedAsyncFnTrackCaller, BuiltinUnpermittedTypeInit,
+        BuiltinMissingCopyImpl, BuiltinMissingDebugImpl, BuiltinMissingDoc, BuiltinNoMangleGeneric,
+        BuiltinNonShorthandFieldPatterns, BuiltinSpecialModuleNameUsed, BuiltinTrivialBounds,
+        BuiltinTypeAliasGenericBounds, BuiltinTypeAliasGenericBoundsSuggestion,
+        BuiltinTypeAliasWhereClause, BuiltinUngatedAsyncFnTrackCaller, BuiltinUnpermittedTypeInit,
         BuiltinUnpermittedTypeInitSub, BuiltinUnreachablePub, BuiltinUnsafe,
         BuiltinUnstableFeatures, BuiltinUnusedDocComment, BuiltinUnusedDocCommentSub,
         BuiltinWhileTrue, SuggestChangingAssocTypes,
@@ -1165,72 +1164,6 @@ 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_spanned_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) && cx.tcx.item_name(def_id) == sym::transmute
-        }
-    }
-}
-
 declare_lint! {
     /// The `unstable_features` is deprecated and should no longer be used.
     UNSTABLE_FEATURES,
@@ -1641,7 +1574,6 @@ 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

@@ -74,6 +74,7 @@ mod lints;
 mod map_unit_fn;
 mod methods;
 mod multiple_supertrait_upcastable;
+mod mutable_transmutes;
 mod non_ascii_idents;
 mod non_fmt_panic;
 mod nonstandard_style;
@@ -112,6 +113,7 @@ use let_underscore::*;
 use map_unit_fn::*;
 use methods::*;
 use multiple_supertrait_upcastable::*;
+use mutable_transmutes::MutableTransmutes;
 use non_ascii_idents::*;
 use non_fmt_panic::NonPanicFmt;
 use nonstandard_style::*;

compiler/rustc_lint/src/lints.rs

@@ -230,7 +230,19 @@ pub struct BuiltinConstNoMangle {
 
 #[derive(LintDiagnostic)]
 #[diag(lint_builtin_mutable_transmutes)]
-pub struct BuiltinMutablesTransmutes;
+#[note]
+pub struct BuiltinMutablesTransmutes {
+    pub from: String,
+    pub to: String,
+}
+
+#[derive(LintDiagnostic)]
+#[diag(lint_builtin_unsafe_cell_transmutes)]
+#[note]
+pub struct BuiltinUnsafeCellTransmutes {
+    pub from: String,
+    pub to: String,
+}
 
 #[derive(LintDiagnostic)]
 #[diag(lint_builtin_unstable_features)]