Struct FunctionDebugMeta 

#[repr(C)]
pub struct FunctionDebugMeta {
    pub func_name: *const u8,
    pub func_name_len: u32,
    pub local_count: u32,
    pub locals: *const DebugLocalMeta,
    pub span_start: u32,
    pub span_end: u32,
}

Everything the crash debugger needs about a function that does not vary per call: its name, its source extent, and the metadata for its Gc locals.

One of these exists per lowered function, interned by content in the JIT generation arena (ADR-043), so a debugger session that recompiles the same function on every p EXPR (DBG-05) pays for it once. A generated prologue stores its address into a DebugFrameEntry — one immediate, one store.

#[repr(C)] because generated code writes its address and crate::crash_snapshot reads its fields across the ABI boundary.

Fields

func_name: *const u8

The function’s source name (a 'static embedded string).

func_name_len: u32

The function name’s byte length.

local_count: u32

How many Gc locals this function has — the length of both locals and the run of value slots a call of it claims.

locals: *const DebugLocalMeta

local_count entries, in debug-slot order: the local’s position among this function’s Gc locals, in MIR local order. Entry i describes the word at displacement i of the run a call claims, which is what crate::crash_snapshot and DebugFrameStackHeader::clear_reclaimed rely on when they zip the two.

Debug-slot order is not shadow-slot order (ADR-128 decision 3): a shadow slot index is a colour — is_prime’s shadow indices are {0} while its debug indices are 0..33 — so the two stacks are not index-parallel with each other.

span_start: u32

The function’s source span [start, end) as byte offsets into the program source (§9.3 “current source span”, ADR-035 decision 3). (0, 0) means “no span recorded” — the __fnvalue_* adapter and __p_expr, whose bodies nobody wrote. A closure’s span is its literal’s.

span_end: u32