Auto merge of rust-lang#8632 - Jarcho:cast_ptr_alignment, r=llogiq

Don't lint `cast_ptr_alignment` when used for unaligned reads and writes

fixes rust-lang#2881

Ideally this would trace the usage of the value rather than only looking at the parent expression, but that would require dataflow analysis. e.g.
```rust
let x = ptr as *const u16;
c.read_unaligned(x);
```

Arch specific intrinsic functions need to be checked for ones which could take an unaligned pointer. This can be another PR.

changelog: Don't lint `cast_ptr_alignment` when used for unaligned reads and writes

Author: bors

clippy_lints/src/casts/cast_ptr_alignment.rs

@@ -1,7 +1,6 @@
 use clippy_utils::diagnostics::span_lint;
-use clippy_utils::is_hir_ty_cfg_dependant;
 use clippy_utils::ty::is_c_void;
-use if_chain::if_chain;
+use clippy_utils::{get_parent_expr, is_hir_ty_cfg_dependant, match_any_def_paths, paths};
 use rustc_hir::{Expr, ExprKind, GenericArg};
 use rustc_lint::LateContext;
 use rustc_middle::ty::layout::LayoutOf;
@@ -20,45 +19,78 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>) {
         );
         lint_cast_ptr_alignment(cx, expr, cast_from, cast_to);
     } else if let ExprKind::MethodCall(method_path, [self_arg, ..], _) = &expr.kind {
-        if_chain! {
-            if method_path.ident.name == sym!(cast);
-            if let Some(generic_args) = method_path.args;
-            if let [GenericArg::Type(cast_to)] = generic_args.args;
+        if method_path.ident.name == sym!(cast)
+            && let Some(generic_args) = method_path.args
+            && let [GenericArg::Type(cast_to)] = generic_args.args
             // There probably is no obvious reason to do this, just to be consistent with `as` cases.
-            if !is_hir_ty_cfg_dependant(cx, cast_to);
-            then {
-                let (cast_from, cast_to) =
-                    (cx.typeck_results().expr_ty(self_arg), cx.typeck_results().expr_ty(expr));
-                lint_cast_ptr_alignment(cx, expr, cast_from, cast_to);
-            }
+            && !is_hir_ty_cfg_dependant(cx, cast_to)
+        {
+            let (cast_from, cast_to) =
+                (cx.typeck_results().expr_ty(self_arg), cx.typeck_results().expr_ty(expr));
+            lint_cast_ptr_alignment(cx, expr, cast_from, cast_to);
         }
     }
 }
 
 fn lint_cast_ptr_alignment<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>, cast_from: Ty<'tcx>, cast_to: Ty<'tcx>) {
-    if_chain! {
-        if let ty::RawPtr(from_ptr_ty) = &cast_from.kind();
-        if let ty::RawPtr(to_ptr_ty) = &cast_to.kind();
-        if let Ok(from_layout) = cx.layout_of(from_ptr_ty.ty);
-        if let Ok(to_layout) = cx.layout_of(to_ptr_ty.ty);
-        if from_layout.align.abi < to_layout.align.abi;
+    if let ty::RawPtr(from_ptr_ty) = &cast_from.kind()
+        && let ty::RawPtr(to_ptr_ty) = &cast_to.kind()
+        && let Ok(from_layout) = cx.layout_of(from_ptr_ty.ty)
+        && let Ok(to_layout) = cx.layout_of(to_ptr_ty.ty)
+        && from_layout.align.abi < to_layout.align.abi
         // with c_void, we inherently need to trust the user
-        if !is_c_void(cx, from_ptr_ty.ty);
+        && !is_c_void(cx, from_ptr_ty.ty)
         // when casting from a ZST, we don't know enough to properly lint
-        if !from_layout.is_zst();
-        then {
-            span_lint(
-                cx,
-                CAST_PTR_ALIGNMENT,
-                expr.span,
-                &format!(
-                    "casting from `{}` to a more-strictly-aligned pointer (`{}`) ({} < {} bytes)",
-                    cast_from,
-                    cast_to,
-                    from_layout.align.abi.bytes(),
-                    to_layout.align.abi.bytes(),
-                ),
-            );
-        }
+        && !from_layout.is_zst()
+        && !is_used_as_unaligned(cx, expr)
+    {
+        span_lint(
+            cx,
+            CAST_PTR_ALIGNMENT,
+            expr.span,
+            &format!(
+                "casting from `{}` to a more-strictly-aligned pointer (`{}`) ({} < {} bytes)",
+                cast_from,
+                cast_to,
+                from_layout.align.abi.bytes(),
+                to_layout.align.abi.bytes(),
+            ),
+        );
+    }
+}
+
+fn is_used_as_unaligned(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
+    let Some(parent) = get_parent_expr(cx, e) else {
+        return false;
+    };
+    match parent.kind {
+        ExprKind::MethodCall(name, [self_arg, ..], _) if self_arg.hir_id == e.hir_id => {
+            if matches!(name.ident.as_str(), "read_unaligned" | "write_unaligned")
+                && let Some(def_id) = cx.typeck_results().type_dependent_def_id(parent.hir_id)
+                && let Some(def_id) = cx.tcx.impl_of_method(def_id)
+                && cx.tcx.type_of(def_id).is_unsafe_ptr()
+            {
+                true
+            } else {
+                false
+            }
+        },
+        ExprKind::Call(func, [arg, ..]) if arg.hir_id == e.hir_id => {
+            static PATHS: &[&[&str]] = &[
+                paths::PTR_READ_UNALIGNED.as_slice(),
+                paths::PTR_WRITE_UNALIGNED.as_slice(),
+                paths::PTR_UNALIGNED_VOLATILE_LOAD.as_slice(),
+                paths::PTR_UNALIGNED_VOLATILE_STORE.as_slice(),
+            ];
+            if let ExprKind::Path(path) = &func.kind
+                && let Some(def_id) = cx.qpath_res(path, func.hir_id).opt_def_id()
+                && match_any_def_paths(cx, def_id, PATHS).is_some()
+            {
+                true
+            } else {
+                false
+            }
+        },
+        _ => false,
     }
 }

clippy_utils/src/paths.rs

@@ -105,6 +105,8 @@ pub const PTR_READ_UNALIGNED: [&str; 3] = ["core", "ptr", "read_unaligned"];
 pub const PTR_READ_VOLATILE: [&str; 3] = ["core", "ptr", "read_volatile"];
 pub const PTR_REPLACE: [&str; 3] = ["core", "ptr", "replace"];
 pub const PTR_SWAP: [&str; 3] = ["core", "ptr", "swap"];
+pub const PTR_UNALIGNED_VOLATILE_LOAD: [&str; 3] = ["core", "intrinsics", "unaligned_volatile_load"];
+pub const PTR_UNALIGNED_VOLATILE_STORE: [&str; 3] = ["core", "intrinsics", "unaligned_volatile_store"];
 pub const PTR_WRITE: [&str; 3] = ["core", "ptr", "write"];
 pub const PTR_WRITE_BYTES: [&str; 3] = ["core", "intrinsics", "write_bytes"];
 pub const PTR_WRITE_UNALIGNED: [&str; 3] = ["core", "ptr", "write_unaligned"];

tests/ui/cast_alignment.rs

@@ -1,6 +1,7 @@
 //! Test casts for alignment issues
 
 #![feature(rustc_private)]
+#![feature(core_intrinsics)]
 extern crate libc;
 
 #[warn(clippy::cast_ptr_alignment)]
@@ -34,4 +35,17 @@ fn main() {
     (&1u32 as *const u32 as *const libc::c_void) as *const u32;
     // For ZST, we should trust the user. See #4256
     (&1u32 as *const u32 as *const ()) as *const u32;
+
+    // Issue #2881
+    let mut data = [0u8, 0u8];
+    unsafe {
+        let ptr = &data as *const [u8; 2] as *const u8;
+        let _ = (ptr as *const u16).read_unaligned();
+        let _ = core::ptr::read_unaligned(ptr as *const u16);
+        let _ = core::intrinsics::unaligned_volatile_load(ptr as *const u16);
+        let ptr = &mut data as *mut [u8; 2] as *mut u8;
+        let _ = (ptr as *mut u16).write_unaligned(0);
+        let _ = core::ptr::write_unaligned(ptr as *mut u16, 0);
+        let _ = core::intrinsics::unaligned_volatile_store(ptr as *mut u16, 0);
+    }
 }

tests/ui/cast_alignment.stderr

@@ -1,25 +1,25 @@
 error: casting from `*const u8` to a more-strictly-aligned pointer (`*const u16`) (1 < 2 bytes)
-  --> $DIR/cast_alignment.rs:18:5
+  --> $DIR/cast_alignment.rs:19:5
    |
 LL |     (&1u8 as *const u8) as *const u16;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `-D clippy::cast-ptr-alignment` implied by `-D warnings`
 
 error: casting from `*mut u8` to a more-strictly-aligned pointer (`*mut u16`) (1 < 2 bytes)
-  --> $DIR/cast_alignment.rs:19:5
+  --> $DIR/cast_alignment.rs:20:5
    |
 LL |     (&mut 1u8 as *mut u8) as *mut u16;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: casting from `*const u8` to a more-strictly-aligned pointer (`*const u16`) (1 < 2 bytes)
-  --> $DIR/cast_alignment.rs:22:5
+  --> $DIR/cast_alignment.rs:23:5
    |
 LL |     (&1u8 as *const u8).cast::<u16>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: casting from `*mut u8` to a more-strictly-aligned pointer (`*mut u16`) (1 < 2 bytes)
-  --> $DIR/cast_alignment.rs:23:5
+  --> $DIR/cast_alignment.rs:24:5
    |
 LL |     (&mut 1u8 as *mut u8).cast::<u16>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^