error/E0691: include alignment in error message

Include the computed alignment of the violating field when rejecting
transparent types with non-trivially aligned ZSTs.

ZST member fields in transparent types must have an alignment of 1 (to
ensure it does not raise the layout requirements of the transparent
field). The current error message looks like this:

 LL | struct Foobar(u32, [u32; 0]);
    |                    ^^^^^^^^ has alignment larger than 1

This patch changes the report to include the alignment of the violating
field:

 LL | struct Foobar(u32, [u32; 0]);
    |                    ^^^^^^^^ has alignment of 4, which is larger than 1

In case of unknown alignments, it will yield:

 LL | struct Foobar<T>(u32, [T; 0]);
    |                       ^^^^^^ may have alignment larger than 1

This allows developers to get a better grasp why a specific field is
rejected. Knowing the alignment of the violating field makes it easier
to judge where that alignment-requirement originates, and thus hopefully
provide better hints on how to mitigate the problem.

This idea was proposed in 2022 in #98071 as part of a bigger change.
This commit simply extracts this error-message change, to decouple it
from the other diagnostic improvements.

1 files changed, 4 insertions, 4 deletions

diff --git a/tests/ui/repr/repr-transparent.stderr b/tests/ui/repr/repr-transparent.stderr
index cb1e2337776..028fc25db46 100644
--- a/tests/ui/repr/repr-transparent.stderr
+++ b/tests/ui/repr/repr-transparent.stderr
@@ -20,13 +20,13 @@ error[E0691]: zero-sized field in transparent struct has alignment larger than 1
   --> $DIR/repr-transparent.rs:36:32
    |
 LL | struct NontrivialAlignZst(u32, [u16; 0]);
-   |                                ^^^^^^^^ has alignment larger than 1
+   |                                ^^^^^^^^ has alignment of 2, which is larger than 1
 
 error[E0691]: zero-sized field in transparent struct has alignment larger than 1
   --> $DIR/repr-transparent.rs:42:24
    |
 LL | struct GenericAlign<T>(ZstAlign32<T>, u32);
-   |                        ^^^^^^^^^^^^^ has alignment larger than 1
+   |                        ^^^^^^^^^^^^^ has alignment of 32, which is larger than 1
 
 error[E0084]: unsupported representation for zero-variant enum
   --> $DIR/repr-transparent.rs:44:1
@@ -66,13 +66,13 @@ error[E0691]: zero-sized field in transparent enum has alignment larger than 1
   --> $DIR/repr-transparent.rs:71:14
    |
 LL |     Foo(u32, [u16; 0]),
-   |              ^^^^^^^^ has alignment larger than 1
+   |              ^^^^^^^^ has alignment of 2, which is larger than 1
 
 error[E0691]: zero-sized field in transparent enum has alignment larger than 1
   --> $DIR/repr-transparent.rs:76:11
    |
 LL |     Foo { bar: ZstAlign32<T>, baz: u32 }
-   |           ^^^^^^^^^^^^^^^^^^ has alignment larger than 1
+   |           ^^^^^^^^^^^^^^^^^^ has alignment of 32, which is larger than 1
 
 error[E0690]: transparent union needs at most one non-zero-sized field, but has 2
   --> $DIR/repr-transparent.rs:85:1