From a6ab57ae211b8de6affdae604417a55dff3ca4e3 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?=E9=BB=91=E6=80=95?= Date: Tue, 5 Aug 2025 16:40:46 +0800 Subject: [PATCH] E0793: Clarify that it applies to unions as well --- .../src/error_codes/E0793.md | 29 ++++++++++++++++++- 1 file changed, 28 insertions(+), 1 deletion(-) diff --git a/compiler/rustc_error_codes/src/error_codes/E0793.md b/compiler/rustc_error_codes/src/error_codes/E0793.md index ccd1b43bd194a..0ebe9da006edc 100644 --- a/compiler/rustc_error_codes/src/error_codes/E0793.md +++ b/compiler/rustc_error_codes/src/error_codes/E0793.md @@ -1,4 +1,4 @@ -An unaligned reference to a field of a [packed] struct got created. +An unaligned reference to a field of a [packed] `struct` or `union` was created. Erroneous code example: @@ -45,9 +45,36 @@ unsafe { // For formatting, we can create a copy to avoid the direct reference. let copy = foo.field1; println!("{}", copy); + // Creating a copy can be written in a single line with curly braces. // (This is equivalent to the two lines above.) println!("{}", { foo.field1 }); + + // A reference to a field that will always be sufficiently aligned is safe: + println!("{}", foo.field2); +} +``` + +### Unions + +Although creating a reference to a `union` field is `unsafe`, this error +will still be triggered if the referenced field is not sufficiently +aligned. Use `addr_of!` and raw pointers in the same way as for struct fields. + +```compile_fail,E0793 +#[repr(packed)] +pub union Foo { + field1: u64, + field2: u8, +} + +unsafe { + let foo = Foo { field1: 0 }; + // Accessing the field directly is fine. + let val = foo.field1; + + // A reference to a packed union field causes an error. + let val = &foo.field1; // ERROR } ```