@mark(c) marker (ADR-0085)

The @mark(c) marker applies to function, struct, and enum declarations. (ADR-0086 widened the applicability set to include enums; the enum case is documented in section 10.4.)

On a function declaration, @mark(c) selects the platform C calling convention and suppresses Gruel name mangling on the emitted symbol. Such a function is callable from C using its Gruel identifier as the symbol name unless overridden by @link_name("…").

On a struct declaration, @mark(c) selects C-compatible field layout: field order matches declaration order, each field is placed at the lowest offset satisfying its natural alignment, and niche optimisation is disabled. 
@mark(c) fn my_callback(x: i32) -> i32 { x + 1 }

@mark(c) struct Point { x: i32, y: i32 }

A @mark(c) struct may not declare an inline fn __drop destructor: cross-boundary destructor semantics are not defined in this revision.