From 698f8c2f01ea549d77d7dc3338a12e04c11057b9 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Wed, 17 Apr 2024 14:02:58 +0200 Subject: Adding upstream version 1.64.0+dfsg1. Signed-off-by: Daniel Baumann --- .../rustc_error_codes/src/error_codes/E0690.md | 33 ++++++++++++++++++++++ 1 file changed, 33 insertions(+) create mode 100644 compiler/rustc_error_codes/src/error_codes/E0690.md (limited to 'compiler/rustc_error_codes/src/error_codes/E0690.md') diff --git a/compiler/rustc_error_codes/src/error_codes/E0690.md b/compiler/rustc_error_codes/src/error_codes/E0690.md new file mode 100644 index 000000000..ba706ad2b --- /dev/null +++ b/compiler/rustc_error_codes/src/error_codes/E0690.md @@ -0,0 +1,33 @@ +A struct with the representation hint `repr(transparent)` had two or more fields +that were not guaranteed to be zero-sized. + +Erroneous code example: + +```compile_fail,E0690 +#[repr(transparent)] +struct LengthWithUnit { // error: transparent struct needs at most one + value: f32, // non-zero-sized field, but has 2 + unit: U, +} +``` + +Because transparent structs are represented exactly like one of their fields at +run time, said field must be uniquely determined. If there are multiple fields, +it is not clear how the struct should be represented. +Note that fields of zero-sized types (e.g., `PhantomData`) can also exist +alongside the field that contains the actual data, they do not count for this +error. When generic types are involved (as in the above example), an error is +reported because the type parameter could be non-zero-sized. + +To combine `repr(transparent)` with type parameters, `PhantomData` may be +useful: + +``` +use std::marker::PhantomData; + +#[repr(transparent)] +struct LengthWithUnit { + value: f32, + unit: PhantomData, +} +``` -- cgit v1.2.3