Auto merge of #6557 - rail-rain:extending_cast_ptr_alignment, r=phansch

Catch `pointer::cast` too in `cast_ptr_alignment`

Fixes #4708

Although there were some discussion in the issue, this PR implements the original feature. I think `cast_ptr_alignment` should exist as it is, separated from `ptr_as_ptr`.

---

changelog: Extend `cast_ptr_alignment` lint for the `pointer::cast` method

clippy_lints/src/types.rs

@@ -8,7 +8,6 @@ use if_chain::if_chain;
 use rustc_ast::{FloatTy, IntTy, LitFloatType, LitIntType, LitKind, UintTy};
 use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
-use rustc_hir::def::Res;
 use rustc_hir::intravisit::{walk_body, walk_expr, walk_ty, FnKind, NestedVisitorMap, Visitor};
 use rustc_hir::{
     BinOpKind, Block, Body, Expr, ExprKind, FnDecl, FnRetTy, FnSig, GenericArg, GenericBounds, GenericParamKind, HirId,
@@ -33,9 +32,9 @@ use crate::consts::{constant, Constant};
 use crate::utils::paths;
 use crate::utils::sugg::Sugg;
 use crate::utils::{
-    clip, comparisons, differing_macro_contexts, higher, in_constant, indent_of, int_bits, is_type_diagnostic_item,
-    last_path_segment, match_def_path, match_path, meets_msrv, method_chain_args, multispan_sugg,
-    numeric_literal::NumericLiteral, qpath_res, reindent_multiline, sext, snippet, snippet_opt,
+    clip, comparisons, differing_macro_contexts, higher, in_constant, indent_of, int_bits, is_hir_ty_cfg_dependant,
+    is_type_diagnostic_item, last_path_segment, match_def_path, match_path, meets_msrv, method_chain_args,
+    multispan_sugg, numeric_literal::NumericLiteral, qpath_res, reindent_multiline, sext, snippet, snippet_opt,
     snippet_with_applicability, snippet_with_macro_callsite, span_lint, span_lint_and_help, span_lint_and_sugg,
     span_lint_and_then, unsext,
 };
@@ -1282,8 +1281,8 @@ declare_clippy_lint! {
 }
 
 declare_clippy_lint! {
-    /// **What it does:** Checks for casts from a less-strictly-aligned pointer to a
-    /// more-strictly-aligned pointer
+    /// **What it does:** Checks for casts, using `as` or `pointer::cast`,
+    /// from a less-strictly-aligned pointer to a more-strictly-aligned pointer
     ///
     /// **Why is this bad?** Dereferencing the resulting pointer may be undefined
     /// behavior.
@@ -1296,6 +1295,9 @@ declare_clippy_lint! {
     /// ```rust
     /// let _ = (&1u8 as *const u8) as *const u16;
     /// let _ = (&mut 1u8 as *mut u8) as *mut u16;
+    ///
+    /// (&1u8 as *const u8).cast::<u16>();
+    /// (&mut 1u8 as *mut u8).cast::<u16>();
     /// ```
     pub CAST_PTR_ALIGNMENT,
     pedantic,
@@ -1637,12 +1639,8 @@ impl<'tcx> LateLintPass<'tcx> for Casts {
             return;
         }
         if let ExprKind::Cast(ref ex, cast_to) = expr.kind {
-            if let TyKind::Path(QPath::Resolved(_, path)) = cast_to.kind {
-                if let Res::Def(_, def_id) = path.res {
-                    if cx.tcx.has_attr(def_id, sym::cfg) || cx.tcx.has_attr(def_id, sym::cfg_attr) {
-                        return;
-                    }
-                }
+            if is_hir_ty_cfg_dependant(cx, cast_to) {
+                return;
             }
             let (cast_from, cast_to) = (cx.typeck_results().expr_ty(ex), cx.typeck_results().expr_ty(expr));
             lint_fn_to_numeric_cast(cx, expr, ex, cast_from, cast_to);
@@ -1692,6 +1690,19 @@ impl<'tcx> LateLintPass<'tcx> for Casts {
             }
 
             lint_cast_ptr_alignment(cx, expr, cast_from, cast_to);
+        } else if let ExprKind::MethodCall(method_path, _, args, _) = 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;
+            // 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(&args[0]), cx.typeck_results().expr_ty(expr));
+                lint_cast_ptr_alignment(cx, expr, cast_from, cast_to);
+            }
+            }
         }
     }
 }

clippy_lints/src/utils/mod.rs

@@ -1686,6 +1686,18 @@ macro_rules! unwrap_cargo_metadata {
     }};
 }
 
+pub fn is_hir_ty_cfg_dependant(cx: &LateContext<'_>, ty: &hir::Ty<'_>) -> bool {
+    if_chain! {
+        if let TyKind::Path(QPath::Resolved(_, path)) = ty.kind;
+        if let Res::Def(_, def_id) = path.res;
+        then {
+            cx.tcx.has_attr(def_id, sym::cfg) || cx.tcx.has_attr(def_id, sym::cfg_attr)
+        } else {
+            false
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::{reindent_multiline, without_block_comments};

tests/ui/cast_alignment.rs

@@ -12,6 +12,10 @@ fn main() {
     (&1u8 as *const u8) as *const u16;
     (&mut 1u8 as *mut u8) as *mut u16;
 
+    // cast to more-strictly-aligned type, but with the `pointer::cast` function.
+    (&1u8 as *const u8).cast::<u16>();
+    (&mut 1u8 as *mut u8).cast::<u16>();
+
     /* These should be ok */
 
     // not a pointer type

tests/ui/cast_alignment.stderr

@@ -12,5 +12,17 @@ error: casting from `*mut u8` to a more-strictly-aligned pointer (`*mut u16`) (1
 LL |     (&mut 1u8 as *mut u8) as *mut u16;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: casting from `*const u8` to a more-strictly-aligned pointer (`*const u16`) (1 < 2 bytes)
+  --> $DIR/cast_alignment.rs:16:5
+   |
+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:17:5
+   |
+LL |     (&mut 1u8 as *mut u8).cast::<u16>();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors