Address review comments: clarify documentation in code, add extra AIX check.

Author: amy-kwan

compiler/rustc_lint/messages.ftl

@@ -915,8 +915,6 @@ lint_unused_closure =
     }{$post} that must be used
     .note = closures are lazy and do nothing unless called
 
-lint_uses_power_alignment = repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
-
 lint_unused_comparisons = comparison is useless due to type limits
 
 lint_unused_coroutine =
@@ -972,6 +970,8 @@ lint_unused_result = unused result of type `{$ty}`
 
 lint_use_let_underscore_ignore_suggestion = use `let _ = ...` to ignore the expression or result
 
+lint_uses_power_alignment = repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
+
 lint_variant_size_differences =
     enum variant is more than three times larger ({$largest} bytes) than the next largest
 

compiler/rustc_lint/src/types.rs

@@ -729,23 +729,18 @@ declare_lint! {
 }
 
 declare_lint! {
-    /// In it's platform C ABI, the AIX target follows the power alignment rule
-    /// for structs. This rule applies the natural alignment of the first struct
-    /// field if it is a floating-point type that is greater than 4-bytes, or
-    /// is an aggregate whose recursively "first" field is a floating-point
-    /// type greater than 4-bytes.
-    /// The alignment for subsequent fields of the associated structs use an
-    /// alignment value where the floating-point type is aligned on a 4-byte
-    /// boundary.
-    /// In this case, "natural alignment" (detailed in
-    /// https://www.ibm.com/docs/en/xl-c-and-cpp-aix/16.1?topic=data-using-alignment-modes#alignment)
-    /// refers to the floating-point type being aligned to a multiple of its
-    /// size (for example, the size of a double is 8-bytes, and so is 8-byte
-    /// aligned).
-    ///
-    /// The power alignment rule for structs need for C compatibility is
-    /// currently unimplemented within repr(C) in the compiler, so a warning is
-    /// produced in these situations.
+    /// In it's platform C ABI, AIX uses the "power" (as in PowerPC) alignment
+    /// rule (detailed in https://www.ibm.com/docs/en/xl-c-and-cpp-aix/16.1?topic=data-using-alignment-modes#alignment),
+    /// which can also be set for XLC by `#pragma align(power)` or
+    /// `-qalign=power`. Aggregates with a floating-point type as the
+    /// recursively first field (as in "at offset 0") modify the layout of
+    /// *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.
     ///
     /// ### Example
     ///
@@ -1593,6 +1588,9 @@ impl ImproperCTypesDefinitions {
         //   - 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()

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

@@ -29,79 +29,79 @@ 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:58:5
+  --> $DIR/reprc-power-alignment.rs:59:5
    |
 LL |     y: FloatAgg2,
    |     ^^^^^^^^^^^^
 
 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:59:5
+  --> $DIR/reprc-power-alignment.rs:60:5
    |
 LL |     z: FloatAgg2,
    |     ^^^^^^^^^^^^
 
 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:65:5
+  --> $DIR/reprc-power-alignment.rs:66:5
    |
 LL |     y: FloatAgg2,
    |     ^^^^^^^^^^^^
 
 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:71:5
+  --> $DIR/reprc-power-alignment.rs:72:5
    |
 LL |     y: FloatAgg2,
    |     ^^^^^^^^^^^^
 
 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:72:5
+  --> $DIR/reprc-power-alignment.rs:73:5
    |
 LL |     z: FloatAgg3,
    |     ^^^^^^^^^^^^
 
 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:78:5
+  --> $DIR/reprc-power-alignment.rs:79:5
    |
 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:85:5
+  --> $DIR/reprc-power-alignment.rs:86:5
    |
 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:98:3
+  --> $DIR/reprc-power-alignment.rs:99:3
    |
 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:103:3
+  --> $DIR/reprc-power-alignment.rs:104:3
    |
 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:112:3
+  --> $DIR/reprc-power-alignment.rs:113:3
    |
 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:117:3
+  --> $DIR/reprc-power-alignment.rs:118:3
    |
 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:123:5
+  --> $DIR/reprc-power-alignment.rs:124:5
    |
 LL |     c: 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:125:5
+  --> $DIR/reprc-power-alignment.rs:126:5
    |
 LL |     e: f64,
    |     ^^^^^^

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

@@ -55,6 +55,7 @@ pub struct FloatAgg2 {
 #[repr(C)]
 pub struct FloatAgg3 {
     x: FloatAgg1,
+    // NOTE: the "power" alignment rule is infectious to nested struct fields.
     y: FloatAgg2, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
     z: FloatAgg2, //~ WARNING repr(C) does not follow the power alignment rule. This may affect platform C ABI compatibility for this type
 }