Add lint against integer to pointer transmutes

compiler/rustc_lint/messages.ftl

@@ -476,6 +476,14 @@ lint_invalid_reference_casting_note_book = for more information, visit <https://
 
 lint_invalid_reference_casting_note_ty_has_interior_mutability = even for types with interior mutability, the only legal way to obtain a mutable pointer from a shared reference is through `UnsafeCell::get`
 
+lint_int_to_ptr_transmutes = transmuting an integer to a pointer creates a pointer without provenance
+    .note = this is dangerous because dereferencing the resulting pointer is undefined behavior
+    .note_exposed_provenance = exposed provenance semantics can be used to create a pointer based on some previously exposed provenance
+    .help_transmute = for more information about transmute, see <https://doc.rust-lang.org/std/mem/fn.transmute.html#transmutation-between-pointers-and-integers>
+    .help_exposed_provenance = for more information about exposed provenance, see <https://doc.rust-lang.org/std/ptr/index.html#exposed-provenance>
+    .suggestion_as = use `as` cast instead to use a previously exposed provenance
+    .suggestion_without_provenance_mut = if you truly mean to create a pointer without provenance, use `std::ptr::without_provenance_mut`
+
 lint_legacy_derive_helpers = derive helper attribute is used before it is introduced
     .label = the attribute is introduced here
 

compiler/rustc_lint/src/lints.rs

@@ -1603,6 +1603,63 @@ impl<'a> LintDiagnostic<'a, ()> for DropGlue<'_> {
     }
 }
 
+// transmute.rs
+#[derive(LintDiagnostic)]
+#[diag(lint_int_to_ptr_transmutes)]
+#[note]
+#[note(lint_note_exposed_provenance)]
+#[help(lint_suggestion_without_provenance_mut)]
+#[help(lint_help_transmute)]
+#[help(lint_help_exposed_provenance)]
+pub(crate) struct IntegerToPtrTransmutes<'tcx> {
+    #[subdiagnostic]
+    pub suggestion: IntegerToPtrTransmutesSuggestion<'tcx>,
+}
+
+#[derive(Subdiagnostic)]
+// FIXME: always recommend `with_exposed_provenance` when it's const-stable
+pub(crate) enum IntegerToPtrTransmutesSuggestion<'tcx> {
+    #[multipart_suggestion(lint_suggestion_as, applicability = "machine-applicable")]
+    ToPtr {
+        dst: Ty<'tcx>,
+        suffix: &'static str,
+        #[suggestion_part(code = "std::ptr::with_exposed_provenance{suffix}::<{dst}>(")]
+        start_call: Span,
+    },
+    #[multipart_suggestion(lint_suggestion_as, applicability = "machine-applicable")]
+    ToRef {
+        dst: Ty<'tcx>,
+        suffix: &'static str,
+        ref_mutbl: &'static str,
+        #[suggestion_part(
+            code = "&{ref_mutbl}*std::ptr::with_exposed_provenance{suffix}::<{dst}>("
+        )]
+        start_call: Span,
+    },
+    #[multipart_suggestion(lint_suggestion_as, applicability = "machine-applicable")]
+    ToPtrInConst {
+        dst: Ty<'tcx>,
+        paren_left: &'static str,
+        paren_right: &'static str,
+        #[suggestion_part(code = "{paren_left}")]
+        start_call: Span,
+        #[suggestion_part(code = "{paren_right} as {dst}")]
+        end_call: Span,
+    },
+    #[multipart_suggestion(lint_suggestion_as, applicability = "machine-applicable")]
+    ToRefInConst {
+        dst: Ty<'tcx>,
+        ptr_mutbl: &'static str,
+        ref_mutbl: &'static str,
+        paren_left: &'static str,
+        paren_right: &'static str,
+        #[suggestion_part(code = "&{ref_mutbl}*({paren_left}")]
+        start_call: Span,
+        #[suggestion_part(code = "{paren_right} as *{ptr_mutbl} {dst})")]
+        end_call: Span,
+    },
+}
+
 // types.rs
 #[derive(LintDiagnostic)]
 #[diag(lint_range_endpoint_out_of_range)]

compiler/rustc_lint/src/transmute.rs

@@ -1,3 +1,4 @@
+use rustc_ast::LitKind;
 use rustc_errors::Applicability;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::LocalDefId;
@@ -7,6 +8,7 @@ use rustc_middle::ty::{self, Ty};
 use rustc_session::{declare_lint, impl_lint_pass};
 use rustc_span::sym;
 
+use crate::lints::{IntegerToPtrTransmutes, IntegerToPtrTransmutesSuggestion};
 use crate::{LateContext, LateLintPass};
 
 declare_lint! {
@@ -67,9 +69,44 @@ declare_lint! {
     "detects transmutes that can also be achieved by other operations"
 }
 
+declare_lint! {
+    /// The `integer_to_ptr_transmutes` lint detects integer to pointer
+    /// transmutes where the resulting pointers are undefined behavior to dereference.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// fn foo(a: usize) -> *const u8 {
+    ///    unsafe {
+    ///        std::mem::transmute::<usize, *const u8>(a)
+    ///    }
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Any attempt to use the resulting pointers are undefined behavior as the resulting
+    /// pointers won't have any provenance.
+    ///
+    /// Alternatively, [`std::ptr::with_exposed_provenance`] or `as` casts should be used,
+    /// as they do not carry the provenance requirement. If the wanting to create pointers
+    /// without provenance [`std::ptr::without_provenance`] should be used instead.
+    ///
+    /// See [`std::mem::transmute`] in the reference for more details.
+    ///
+    /// [`std::mem::transmute`]: https://doc.rust-lang.org/std/mem/fn.transmute.html
+    /// [`std::ptr::with_exposed_provenance`]: https://doc.rust-lang.org/std/ptr/fn.with_exposed_provenance.html
+    /// [`std::ptr::without_provenance`]: https://doc.rust-lang.org/std/ptr/fn.without_provenance.html
+    pub INTEGER_TO_PTR_TRANSMUTES,
+    Warn,
+    "detects integer to pointer transmutes",
+}
+
 pub(crate) struct CheckTransmutes;
 
-impl_lint_pass!(CheckTransmutes => [PTR_TO_INTEGER_TRANSMUTE_IN_CONSTS, UNNECESSARY_TRANSMUTES]);
+impl_lint_pass!(CheckTransmutes => [PTR_TO_INTEGER_TRANSMUTE_IN_CONSTS, UNNECESSARY_TRANSMUTES, INTEGER_TO_PTR_TRANSMUTES]);
 
 impl<'tcx> LateLintPass<'tcx> for CheckTransmutes {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
@@ -94,6 +131,83 @@ impl<'tcx> LateLintPass<'tcx> for CheckTransmutes {
 
         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_int_to_ptr_transmute(cx, expr, arg, src, dst);
+    }
+}
+
+/// Check for transmutes from integer to pointers (*const/*mut, &/&mut and fn()).
+///
+/// Using the resulting pointers would be undefined behavior.
+fn check_int_to_ptr_transmute<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx hir::Expr<'tcx>,
+    arg: &'tcx hir::Expr<'tcx>,
+    src: Ty<'tcx>,
+    dst: Ty<'tcx>,
+) {
+    if matches!(src.kind(), ty::Uint(_) | ty::Int(_))
+        && let ty::Ref(_, inner_ty, mutbl) | ty::RawPtr(inner_ty, mutbl) = dst.kind()
+        // bail-out if the argument is literal 0 as we have other lints for those cases
+        && !matches!(arg.kind, hir::ExprKind::Lit(hir::Lit { node: LitKind::Int(v, _), .. }) if v == 0)
+        // bail-out if the inner type if a ZST
+        && cx.tcx
+            .layout_of(cx.typing_env().as_query_input(*inner_ty))
+            .is_ok_and(|layout| !layout.is_1zst())
+    {
+        cx.tcx.emit_node_span_lint(
+            INTEGER_TO_PTR_TRANSMUTES,
+            expr.hir_id,
+            expr.span,
+            IntegerToPtrTransmutes {
+                // FIXME: once https://github.com/rust-lang/rust/issues/144538 finishes,
+                // we can recommend the method in const context too.
+                suggestion: if !cx.tcx.hir_is_inside_const_context(expr.hir_id) {
+                    let suffix = if mutbl.is_mut() { "_mut" } else { "" };
+                    if dst.is_ref() {
+                        IntegerToPtrTransmutesSuggestion::ToRef {
+                            dst: *inner_ty,
+                            suffix,
+                            ref_mutbl: mutbl.prefix_str(),
+                            start_call: expr.span.shrink_to_lo().until(arg.span),
+                        }
+                    } else {
+                        IntegerToPtrTransmutesSuggestion::ToPtr {
+                            dst: *inner_ty,
+                            suffix,
+                            start_call: expr.span.shrink_to_lo().until(arg.span),
+                        }
+                    }
+                } else {
+                    // does the argument needs parenthesis
+                    let mut paren_left = "";
+                    let mut paren_right = "";
+                    if matches!(arg.kind, hir::ExprKind::Binary(..)) {
+                        paren_left = "(";
+                        paren_right = ")";
+                    }
+
+                    if dst.is_ref() {
+                        IntegerToPtrTransmutesSuggestion::ToRefInConst {
+                            dst: *inner_ty,
+                            ref_mutbl: mutbl.prefix_str(),
+                            ptr_mutbl: mutbl.ptr_str(),
+                            paren_left,
+                            paren_right,
+                            start_call: expr.span.shrink_to_lo().until(arg.span),
+                            end_call: arg.span.shrink_to_hi().until(expr.span.shrink_to_hi()),
+                        }
+                    } else {
+                        IntegerToPtrTransmutesSuggestion::ToPtrInConst {
+                            dst,
+                            paren_left,
+                            paren_right,
+                            start_call: expr.span.shrink_to_lo().until(arg.span),
+                            end_call: arg.span.shrink_to_hi().until(expr.span.shrink_to_hi()),
+                        }
+                    }
+                },
+            },
+        );
     }
 }
 

library/core/src/ptr/mod.rs

@@ -914,6 +914,7 @@ pub const fn dangling<T>() -> *const T {
 #[must_use]
 #[stable(feature = "strict_provenance", since = "1.84.0")]
 #[rustc_const_stable(feature = "strict_provenance", since = "1.84.0")]
+#[allow(integer_to_ptr_transmutes)] // Expected semantics here.
 pub const fn without_provenance_mut<T>(addr: usize) -> *mut T {
     // An int-to-pointer transmute currently has exactly the intended semantics: it creates a
     // pointer without provenance. Note that this is *not* a stable guarantee about transmute

src/tools/clippy/clippy_lints/src/transmute/useless_transmute.rs

@@ -49,17 +49,7 @@ pub(super) fn check<'tcx>(
             true
         },
         (ty::Int(_) | ty::Uint(_), ty::RawPtr(_, _)) => {
-            span_lint_and_then(
-                cx,
-                USELESS_TRANSMUTE,
-                e.span,
-                "transmute from an integer to a pointer",
-                |diag| {
-                    if let Some(arg) = sugg::Sugg::hir_opt(cx, arg) {
-                        diag.span_suggestion(e.span, "try", arg.as_ty(to_ty.to_string()), Applicability::Unspecified);
-                    }
-                },
-            );
+            // Handled by the upstream rustc `integer_to_ptr_transmutes` lint
             true
         },
         _ => false,

src/tools/clippy/tests/ui/transmute.rs

@@ -4,6 +4,7 @@
     dead_code,
     clippy::borrow_as_ptr,
     unnecessary_transmutes,
+    integer_to_ptr_transmutes,
     clippy::needless_lifetimes,
     clippy::missing_transmute_annotations
 )]
@@ -60,12 +61,10 @@ fn useless() {
         //~^ useless_transmute
 
         let _: *const usize = std::mem::transmute(5_isize);
-        //~^ useless_transmute
 
         let _ = std::ptr::dangling::<usize>();
 
         let _: *const usize = std::mem::transmute(1 + 1usize);
-        //~^ useless_transmute
 
         let _ = (1 + 1_usize) as *const usize;
     }