Auto merge of #147375 - folkertdev:power-align-union, r=RalfJung

ignore power alignment rule on unions

fixes https://github.com/rust-lang/rust/issues/147348

The power alignment rule only applies to the non-first field of a struct, and so should not apply to unions at all.

The current code also does not consider enums (whose fields might be, morally, structs). Given that C does not actually have ADTs like this it's probably fine from a compatibility perspective, but I'll leave that to the powerpc folks.

cc `@daltenty` `@gilamn5tr`
r? compiler

compiler/rustc_lint/src/types/improper_ctypes.rs

@@ -220,21 +220,21 @@ fn check_struct_for_power_alignment<'tcx>(
     adt_def: AdtDef<'tcx>,
 ) {
     let tcx = cx.tcx;
-    // repr(C) structs also with packed or aligned representation
-    // should be ignored.
-    if adt_def.repr().c()
-        && !adt_def.repr().packed()
-        && adt_def.repr().align.is_none()
-        && tcx.sess.target.os == "aix"
-        && !adt_def.all_fields().next().is_none()
-    {
+
+    // Only consider structs (not enums or unions) on AIX.
+    if tcx.sess.target.os != "aix" || !adt_def.is_struct() {
+        return;
+    }
+
+    // The struct must be repr(C), but ignore it if it explicitly specifies its alignment with
+    // either `align(N)` or `packed(N)`.
+    if adt_def.repr().c() && !adt_def.repr().packed() && adt_def.repr().align.is_none() {
         let struct_variant_data = item.expect_struct().2;
         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.
-            let def_id = field_def.def_id;
-            let ty = tcx.type_of(def_id).instantiate_identity();
+            let ty = tcx.type_of(field_def.def_id).instantiate_identity();
             if check_arg_for_power_alignment(cx, ty) {
                 cx.emit_span_lint(USES_POWER_ALIGNMENT, field_def.span, UsesPowerAlignment);
             }

tests/ui/layout/reprc-power-alignment.rs

@@ -5,12 +5,12 @@
 #![feature(no_core)]
 #![no_core]
 #![no_std]
+#![crate_type = "lib"]
 
 extern crate minicore;
 use minicore::*;
 
 #[warn(uses_power_alignment)]
-
 #[repr(C)]
 pub struct Floats {
     a: f64,
@@ -96,32 +96,32 @@ pub struct FloatAgg7 {
 
 #[repr(C)]
 pub struct A {
-  d: f64,
+    d: f64,
 }
 #[repr(C)]
 pub struct B {
-  a: A,
-  f: f32,
-  d: f64, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+    a: A,
+    f: f32,
+    d: f64, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
 }
 #[repr(C)]
 pub struct C {
-  c: u8,
-  b: B, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+    c: u8,
+    b: B, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
 }
 #[repr(C)]
 pub struct D {
-  x: f64,
+    x: f64,
 }
 #[repr(C)]
 pub struct E {
-  x: i32,
-  d: D, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+    x: i32,
+    d: D, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
 }
 #[repr(C)]
 pub struct F {
-  a: u8,
-  b: f64, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+    a: u8,
+    b: f64, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
 }
 #[repr(C)]
 pub struct G {
@@ -173,4 +173,19 @@ pub struct M {
     b: K,
     c: L,
 }
-fn main() { }
+
+// The lint ignores unions
+#[repr(C)]
+pub union Union {
+    a: f64,
+    b: u8,
+    c: f64,
+    d: f32,
+}
+
+// The lint ignores enums
+#[repr(C)]
+pub enum Enum {
+    A { a: f64, b: u8, c: f64, d: f32 },
+    B,
+}

tests/ui/layout/reprc-power-alignment.stderr

@@ -5,7 +5,7 @@ LL |     c: f64,
    |     ^^^^^^
    |
 note: the lint level is defined here
-  --> $DIR/reprc-power-alignment.rs:12:8
+  --> $DIR/reprc-power-alignment.rs:13:8
    |
 LL | #[warn(uses_power_alignment)]
    |        ^^^^^^^^^^^^^^^^^^^^
@@ -73,28 +73,28 @@ LL |     y: Floats,
    |     ^^^^^^^^^
 
 warning: repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
-  --> $DIR/reprc-power-alignment.rs:105:3
+  --> $DIR/reprc-power-alignment.rs:105:5
    |
-LL |   d: f64,
-   |   ^^^^^^
+LL |     d: f64,
+   |     ^^^^^^
 
 warning: repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
-  --> $DIR/reprc-power-alignment.rs:110:3
+  --> $DIR/reprc-power-alignment.rs:110:5
    |
-LL |   b: B,
-   |   ^^^^
+LL |     b: B,
+   |     ^^^^
 
 warning: repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
-  --> $DIR/reprc-power-alignment.rs:119:3
+  --> $DIR/reprc-power-alignment.rs:119:5
    |
-LL |   d: D,
-   |   ^^^^
+LL |     d: D,
+   |     ^^^^
 
 warning: repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
-  --> $DIR/reprc-power-alignment.rs:124:3
+  --> $DIR/reprc-power-alignment.rs:124:5
    |
-LL |   b: f64,
-   |   ^^^^^^
+LL |     b: f64,
+   |     ^^^^^^
 
 warning: repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
   --> $DIR/reprc-power-alignment.rs:130:5