Fix suggestion in transmutes_expressible_as_ptr_casts when the source type is a borrow.

Author: Jarcho

clippy_lints/src/transmute/mod.rs

@@ -479,7 +479,10 @@ impl<'tcx> LateLintPass<'tcx> for Transmute {
                 // - char conversions (https://github.com/rust-lang/rust/issues/89259)
                 let const_context = in_constant(cx, e.hir_id);
 
-                let from_ty = cx.typeck_results().expr_ty_adjusted(arg);
+                let (from_ty, from_ty_adjusted) = match cx.typeck_results().expr_adjustments(arg) {
+                    [] => (cx.typeck_results().expr_ty(arg), false),
+                    [.., a] => (a.target, true),
+                };
                 // Adjustments for `to_ty` happen after the call to `transmute`, so don't use them.
                 let to_ty = cx.typeck_results().expr_ty(e);
 
@@ -506,7 +509,7 @@ impl<'tcx> LateLintPass<'tcx> for Transmute {
                     );
 
                 if !linted {
-                    transmutes_expressible_as_ptr_casts::check(cx, e, from_ty, to_ty, arg);
+                    transmutes_expressible_as_ptr_casts::check(cx, e, from_ty, from_ty_adjusted, to_ty, arg);
                 }
             }
         }

clippy_lints/src/transmute/transmutes_expressible_as_ptr_casts.rs

@@ -1,36 +1,52 @@
-use super::utils::can_be_expressed_as_pointer_cast;
+use super::utils::check_cast;
 use super::TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS;
-use clippy_utils::diagnostics::span_lint_and_then;
-use clippy_utils::sugg;
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::sugg::Sugg;
 use rustc_errors::Applicability;
 use rustc_hir::Expr;
 use rustc_lint::LateContext;
-use rustc_middle::ty::Ty;
+use rustc_middle::ty::{cast::CastKind, Ty};
 
 /// Checks for `transmutes_expressible_as_ptr_casts` lint.
 /// Returns `true` if it's triggered, otherwise returns `false`.
 pub(super) fn check<'tcx>(
     cx: &LateContext<'tcx>,
     e: &'tcx Expr<'_>,
     from_ty: Ty<'tcx>,
+    from_ty_adjusted: bool,
     to_ty: Ty<'tcx>,
     arg: &'tcx Expr<'_>,
 ) -> bool {
-    if can_be_expressed_as_pointer_cast(cx, e, from_ty, to_ty) {
-        span_lint_and_then(
-            cx,
-            TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS,
-            e.span,
-            &format!("transmute from `{from_ty}` to `{to_ty}` which could be expressed as a pointer cast instead"),
-            |diag| {
-                if let Some(arg) = sugg::Sugg::hir_opt(cx, arg) {
-                    let sugg = arg.as_ty(to_ty.to_string()).to_string();
-                    diag.span_suggestion(e.span, "try", sugg, Applicability::MachineApplicable);
-                }
-            },
-        );
-        true
-    } else {
-        false
-    }
+    use CastKind::{AddrPtrCast, ArrayPtrCast, FnPtrAddrCast, FnPtrPtrCast, PtrAddrCast, PtrPtrCast};
+    let mut app = Applicability::MachineApplicable;
+    let sugg = match check_cast(cx, e, from_ty, to_ty) {
+        Some(PtrPtrCast | AddrPtrCast | ArrayPtrCast | FnPtrPtrCast | FnPtrAddrCast) => {
+            Sugg::hir_with_context(cx, arg, e.span.ctxt(), "..", &mut app)
+                .as_ty(to_ty.to_string())
+                .to_string()
+        },
+        Some(PtrAddrCast) if !from_ty_adjusted => Sugg::hir_with_context(cx, arg, e.span.ctxt(), "..", &mut app)
+            .as_ty(to_ty.to_string())
+            .to_string(),
+
+        // The only adjustments here would be ref-to-ptr and unsize coercions. The result of an unsize coercions can't
+        // be transmuted to a usize. For ref-to-ptr coercions, borrows need to be cast to a pointer before being cast to
+        // a usize.
+        Some(PtrAddrCast) => format!(
+            "{} as {to_ty}",
+            Sugg::hir_with_context(cx, arg, e.span.ctxt(), "..", &mut app).as_ty(from_ty)
+        ),
+        _ => return false,
+    };
+
+    span_lint_and_sugg(
+        cx,
+        TRANSMUTES_EXPRESSIBLE_AS_PTR_CASTS,
+        e.span,
+        &format!("transmute from `{from_ty}` to `{to_ty}` which could be expressed as a pointer cast instead"),
+        "try",
+        sugg,
+        app,
+    );
+    true
 }

clippy_lints/src/transmute/utils.rs

@@ -20,28 +20,16 @@ pub(super) fn is_layout_incompatible<'tcx>(cx: &LateContext<'tcx>, from: Ty<'tcx
     }
 }
 
-/// Check if the type conversion can be expressed as a pointer cast, instead of
-/// a transmute. In certain cases, including some invalid casts from array
-/// references to pointers, this may cause additional errors to be emitted and/or
-/// ICE error messages. This function will panic if that occurs.
-pub(super) fn can_be_expressed_as_pointer_cast<'tcx>(
-    cx: &LateContext<'tcx>,
-    e: &'tcx Expr<'_>,
-    from_ty: Ty<'tcx>,
-    to_ty: Ty<'tcx>,
-) -> bool {
-    use CastKind::{AddrPtrCast, ArrayPtrCast, FnPtrAddrCast, FnPtrPtrCast, PtrAddrCast, PtrPtrCast};
-    matches!(
-        check_cast(cx, e, from_ty, to_ty),
-        Some(PtrPtrCast | PtrAddrCast | AddrPtrCast | ArrayPtrCast | FnPtrPtrCast | FnPtrAddrCast)
-    )
-}
-
 /// If a cast from `from_ty` to `to_ty` is valid, returns an Ok containing the kind of
 /// the cast. In certain cases, including some invalid casts from array references
 /// to pointers, this may cause additional errors to be emitted and/or ICE error
 /// messages. This function will panic if that occurs.
-fn check_cast<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, from_ty: Ty<'tcx>, to_ty: Ty<'tcx>) -> Option<CastKind> {
+pub(super) fn check_cast<'tcx>(
+    cx: &LateContext<'tcx>,
+    e: &'tcx Expr<'_>,
+    from_ty: Ty<'tcx>,
+    to_ty: Ty<'tcx>,
+) -> Option<CastKind> {
     let hir_id = e.hir_id;
     let local_def_id = hir_id.owner.def_id;
 

tests/ui/transmutes_expressible_as_ptr_casts.fixed

@@ -51,6 +51,8 @@ fn main() {
     // e is a function pointer type and U is an integer; fptr-addr-cast
     let _usize_from_fn_ptr_transmute = unsafe { foo as usize };
     let _usize_from_fn_ptr = foo as *const usize;
+
+    let _usize_from_ref = unsafe { &1u32 as *const u32 as usize };
 }
 
 // If a ref-to-ptr cast of this form where the pointer type points to a type other

tests/ui/transmutes_expressible_as_ptr_casts.rs

@@ -51,6 +51,8 @@ fn main() {
     // e is a function pointer type and U is an integer; fptr-addr-cast
     let _usize_from_fn_ptr_transmute = unsafe { transmute::<fn(usize) -> u8, usize>(foo) };
     let _usize_from_fn_ptr = foo as *const usize;
+
+    let _usize_from_ref = unsafe { transmute::<*const u32, usize>(&1u32) };
 }
 
 // If a ref-to-ptr cast of this form where the pointer type points to a type other

tests/ui/transmutes_expressible_as_ptr_casts.stderr

@@ -46,11 +46,17 @@ error: transmute from `fn(usize) -> u8` to `usize` which could be expressed as a
 LL |     let _usize_from_fn_ptr_transmute = unsafe { transmute::<fn(usize) -> u8, usize>(foo) };
    |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `foo as usize`
 
+error: transmute from `*const u32` to `usize` which could be expressed as a pointer cast instead
+  --> $DIR/transmutes_expressible_as_ptr_casts.rs:55:36
+   |
+LL |     let _usize_from_ref = unsafe { transmute::<*const u32, usize>(&1u32) };
+   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&1u32 as *const u32 as usize`
+
 error: transmute from a reference to a pointer
-  --> $DIR/transmutes_expressible_as_ptr_casts.rs:64:14
+  --> $DIR/transmutes_expressible_as_ptr_casts.rs:66:14
    |
 LL |     unsafe { transmute::<&[i32; 1], *const u8>(in_param) }
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `in_param as *const [i32; 1] as *const u8`
 
-error: aborting due to 8 previous errors
+error: aborting due to 9 previous errors