Rollup merge of rust-lang#139059 - RalfJung:uses_power_alignment, r=nagisa

uses_power_alignment: wording tweaks

Slightly improves the wording introduced with rust-lang#135552.

Author: Zalathar

compiler/rustc_lint/src/types.rs

@@ -755,10 +755,10 @@ declare_lint! {
     /// *subsequent* fields of the associated structs to use an alignment value
     /// where the floating-point type is aligned on a 4-byte boundary.
     ///
-    /// The power alignment rule for structs needed for C compatibility is
-    /// unimplementable within `repr(C)` in the compiler without building in
-    /// handling of references to packed fields and infectious nested layouts,
-    /// so a warning is produced in these situations.
+    /// Effectively, subsequent floating-point fields act as-if they are `repr(packed(4))`. This
+    /// would be unsound to do in a `repr(C)` type without all the restrictions that come with
+    /// `repr(packed)`. Rust instead chooses a layout that maintains soundness of Rust code, at the
+    /// expense of incompatibility with C code.
     ///
     /// ### Example
     ///
@@ -790,8 +790,10 @@ declare_lint! {
     ///  - offset_of!(Floats, a) == 0
     ///  - offset_of!(Floats, b) == 8
     ///  - offset_of!(Floats, c) == 12
-    /// However, rust currently aligns `c` at offset_of!(Floats, c) == 16.
-    /// Thus, a warning should be produced for the above struct in this case.
+    ///
+    /// However, Rust currently aligns `c` at `offset_of!(Floats, c) == 16`.
+    /// Using offset 12 would be unsound since `f64` generally must be 8-aligned on this target.
+    /// Thus, a warning is produced for the above struct.
     USES_POWER_ALIGNMENT,
     Warn,
     "Structs do not follow the power alignment rule under repr(C)"
@@ -1655,15 +1657,13 @@ impl ImproperCTypesDefinitions {
         cx: &LateContext<'tcx>,
         ty: Ty<'tcx>,
     ) -> bool {
+        assert!(cx.tcx.sess.target.os == "aix");
         // Structs (under repr(C)) follow the power alignment rule if:
         //   - the first field of the struct is a floating-point type that
         //     is greater than 4-bytes, or
         //   - the first field of the struct is an aggregate whose
         //     recursively first field is a floating-point type greater than
         //     4 bytes.
-        if cx.tcx.sess.target.os != "aix" {
-            return false;
-        }
         if ty.is_floating_point() && ty.primitive_size(cx.tcx).bytes() > 4 {
             return true;
         } else if let Adt(adt_def, _) = ty.kind()
@@ -1701,21 +1701,14 @@ impl ImproperCTypesDefinitions {
             && !adt_def.all_fields().next().is_none()
         {
             let struct_variant_data = item.expect_struct().1;
-            for (index, ..) in struct_variant_data.fields().iter().enumerate() {
+            for field_def in struct_variant_data.fields().iter().skip(1) {
                 // Struct fields (after the first field) are checked for the
                 // power alignment rule, as fields after the first are likely
                 // to be the fields that are misaligned.
-                if index != 0 {
-                    let first_field_def = struct_variant_data.fields()[index];
-                    let def_id = first_field_def.def_id;
-                    let ty = cx.tcx.type_of(def_id).instantiate_identity();
-                    if self.check_arg_for_power_alignment(cx, ty) {
-                        cx.emit_span_lint(
-                            USES_POWER_ALIGNMENT,
-                            first_field_def.span,
-                            UsesPowerAlignment,
-                        );
-                    }
+                let def_id = field_def.def_id;
+                let ty = cx.tcx.type_of(def_id).instantiate_identity();
+                if self.check_arg_for_power_alignment(cx, ty) {
+                    cx.emit_span_lint(USES_POWER_ALIGNMENT, field_def.span, UsesPowerAlignment);
                 }
             }
         }