ptr_cast_constness: check for implicit mut to const pointer coercion

clippy_lints/src/casts/mod.rs

@@ -919,5 +919,6 @@ impl<'tcx> LateLintPass<'tcx> for Casts {
         cast_ptr_alignment::check_cast_method(cx, expr);
         cast_slice_different_sizes::check(cx, expr, self.msrv);
         ptr_cast_constness::check_null_ptr_cast_method(cx, expr);
+        ptr_cast_constness::check_implicit_cast_from_mut(cx, expr, self.msrv);
     }
 }

clippy_lints/src/casts/ptr_cast_constness.rs

@@ -2,10 +2,11 @@ use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::msrvs::{self, Msrv};
 use clippy_utils::source::snippet_with_applicability;
 use clippy_utils::sugg::Sugg;
-use clippy_utils::{std_or_core, sym};
+use clippy_utils::{get_parent_expr, std_or_core, sym};
 use rustc_errors::Applicability;
 use rustc_hir::{self as hir, Expr, ExprKind, QPath};
 use rustc_lint::LateContext;
+use rustc_middle::ty::adjustment::{Adjust, PointerCoercion};
 use rustc_middle::ty::{self, Ty, TypeVisitableExt};
 
 use super::PTR_CAST_CONSTNESS;
@@ -102,3 +103,25 @@ pub(super) fn check_null_ptr_cast_method(cx: &LateContext<'_>, expr: &Expr<'_>)
         );
     }
 }
+
+pub(super) fn check_implicit_cast_from_mut(cx: &LateContext<'_>, expr: &Expr<'_>, msrv: Msrv) {
+    if msrv.meets(cx, msrvs::POINTER_CAST_CONSTNESS)
+        && let ty::RawPtr(..) = cx.typeck_results().expr_ty(expr).kind()
+        && !matches!(get_parent_expr(cx, expr).map(|e| e.kind), Some(ExprKind::Cast(..)))
+        && let [coercion] = cx.typeck_results().expr_adjustments(expr)
+        && let Adjust::Pointer(PointerCoercion::MutToConstPointer) = coercion.kind
+        && !expr.span.from_expansion()
+    {
+        let mut app = Applicability::MachineApplicable;
+        let sugg = Sugg::hir_with_applicability(cx, expr, "_", &mut app);
+        span_lint_and_sugg(
+            cx,
+            PTR_CAST_CONSTNESS,
+            expr.span,
+            "implicit casting from mut pointer to const pointer",
+            "try `pointer::cast_const`, a safer alternative",
+            format!("{}.cast_const()", sugg.maybe_paren()),
+            app,
+        );
+    }
+}

tests/ui/ptr_cast_constness.fixed

@@ -112,3 +112,16 @@ fn issue11317() {
     let _ptr: *mut u32 = (r as *const u32).cast_mut();
     //~^ ptr_cast_constness
 }
+
+/// Checks for implicit mut pointer to const pointer coercion
+fn issue13667() {
+    fn expect_const_ptr<T>(_: *const T) {}
+
+    let p = &mut 0 as *mut i32;
+    let _: *const i32 = p.cast_const();
+    //~^ ptr_cast_constness
+    expect_const_ptr(p.cast_const());
+    //~^ ptr_cast_constness
+    let _: *const u8 = (p as *mut u8).cast_const();
+    //~^ ptr_cast_constness
+}

tests/ui/ptr_cast_constness.rs

@@ -112,3 +112,16 @@ fn issue11317() {
     let _ptr: *mut u32 = r as *const _ as *mut _;
     //~^ ptr_cast_constness
 }
+
+/// Checks for implicit mut pointer to const pointer coercion
+fn issue13667() {
+    fn expect_const_ptr<T>(_: *const T) {}
+
+    let p = &mut 0 as *mut i32;
+    let _: *const i32 = p;
+    //~^ ptr_cast_constness
+    expect_const_ptr(p);
+    //~^ ptr_cast_constness
+    let _: *const u8 = p as *mut u8;
+    //~^ ptr_cast_constness
+}

tests/ui/ptr_cast_constness.stderr

@@ -95,5 +95,23 @@ error: `as` casting between raw pointers while changing only its constness
 LL |     let _ptr: *mut u32 = r as *const _ as *mut _;
    |                          ^^^^^^^^^^^^^^^^^^^^^^^ help: try `pointer::cast_mut`, a safer alternative: `(r as *const u32).cast_mut()`
 
-error: aborting due to 15 previous errors
+error: implicit casting from mut pointer to const pointer
+  --> tests/ui/ptr_cast_constness.rs:121:25
+   |
+LL |     let _: *const i32 = p;
+   |                         ^ help: try `pointer::cast_const`, a safer alternative: `p.cast_const()`
+
+error: implicit casting from mut pointer to const pointer
+  --> tests/ui/ptr_cast_constness.rs:123:22
+   |
+LL |     expect_const_ptr(p);
+   |                      ^ help: try `pointer::cast_const`, a safer alternative: `p.cast_const()`
+
+error: implicit casting from mut pointer to const pointer
+  --> tests/ui/ptr_cast_constness.rs:125:24
+   |
+LL |     let _: *const u8 = p as *mut u8;
+   |                        ^^^^^^^^^^^^ help: try `pointer::cast_const`, a safer alternative: `(p as *mut u8).cast_const()`
+
+error: aborting due to 18 previous errors