Struct AltrepAttrs 

struct AltrepAttrs {
    len_field: Option<Ident>,
    elt_field: Option<Ident>,
    elt_delegate: Option<Ident>,
    generate_lowlevel: bool,
    lowlevel_options: Vec<Ident>,
    guard: Option<Ident>,
    class_name: Option<String>,
    manual: bool,
}

Parsed #[altrep(...)] attributes controlling ALTREP derive code generation.

These attributes are placed on the struct and parsed by all ALTREP derive macros (AltrepInteger, AltrepReal, etc.) to customize the generated trait implementations.

Supported #[altrep(...)] keys

Key	Type	Description
len = "field"	String	Name of the struct field that holds the vector length. Auto-detected if a field is named len or length.
elt = "field"	String	Name of the struct field to return as the element value (produces a constant-value vector). If omitted, the default elt() returns NA / NaN / 0 / None depending on the family.
manual	Flag	Skip automatic AltrepLen and Alt*Data trait generation. Use when you want to write those trait impls by hand (e.g., for custom elt logic, no_na, sum, etc.). The low-level trait impls (Altrep, AltVec, family methods, InferBase) are still emitted automatically — you do not need to write them yourself. Use no_lowlevel as an additional escape hatch if you also want to suppress those.
no_lowlevel	Flag	Suppress the automatic low-level trait impls. Use this when you want to provide your own Altrep, AltVec, and family-specific trait implementations.
dataptr	Flag	Enable Dataptr method registration, allowing R to get a direct pointer to the underlying data. Mutually exclusive with subset. Not supported for List.
serialize	Flag	Enable Serialized_state and Unserialize method registration for ALTREP serialization support.
subset	Flag	Enable Extract_subset method registration. Mutually exclusive with dataptr. Supported on all atomic families. Not supported for List.
unsafe	Flag	Set guard mode to Unsafe – no panic protection on ALTREP callbacks.
rust_unwind	Flag	Set guard mode to RustUnwind – uses catch_unwind only (unsafe if callbacks call R APIs).
r_unwind	Flag	Set guard mode to RUnwind (default) – uses with_r_unwind_protect for safe R API calls.
class = "name"	String	Override the ALTREP class name (default: struct name).

Fields

len_field: Option<Ident>

Field name containing the vector length, set via #[altrep(len = "field")]. If None, auto-detection looks for fields named len or length.

elt_field: Option<Ident>

Field name for constant-value element access, set via #[altrep(elt = "field")]. When set, elt() returns self.{field} for every index.

elt_delegate: Option<Ident>

Field name for delegated element access, set via #[altrep(elt_delegate = "field")]. When set, elt() calls self.{field}.elt(i), delegating to the inner type’s AltIntegerData/AltRealData/etc. implementation. Useful for wrapper types around StreamingIntData, StreamingRealData, etc.

generate_lowlevel: bool

Whether to generate the low-level trait impls (Altrep, AltVec, family methods, InferBase). Defaults to true. Set to false by #[altrep(no_lowlevel)].

lowlevel_options: Vec<Ident>

Collected option flags (dataptr, serialize, subset) passed to the runtime macro.

guard: Option<Ident>

Guard mode override for ALTREP trampoline callbacks. Maps to AltrepGuard variants:

• Unsafe – no protection
• RustUnwind – catch_unwind only
• RUnwind – with_r_unwind_protect (default)

class_name: Option<String>

Override the ALTREP class name. Default: struct name.

manual: bool

Manual mode: skip AltrepLen and Alt*Data generation. User provides their own. Set by #[altrep(manual)]. Lowlevel traits + registration still generated.

Implementations

impl AltrepAttrs

fn parse(input: &DeriveInput) -> Result<Self>

Parses all #[altrep(...)] attributes from a derive input struct.

Multiple #[altrep(...)] attributes are supported and their contents are merged. Unknown keys produce a compile error.

Errors

Returns Err if an #[altrep(...)] attribute has malformed syntax or contains an unknown key.

fn get_len_field(&self, input: &DeriveInput) -> Result<Ident>

Returns the length field identifier, either from the explicit len = "..." attribute or by auto-detecting a field named len or length on the struct.

Errors

Returns Err if the input is not a struct, or if no length field was specified and auto-detection fails.

fn has_non_default_guard(&self) -> bool

Returns true if a non-default guard mode is set (i.e., Unsafe or RustUnwind).

The default guard is RUnwind. Non-default guards (e.g., RustUnwind, Unsafe) suppress the materializing-dataptr AltVec impl on the no-option arm (behaviour preserved from the pre-#711 expanded path).

fn validate_options(&self, family: &str, supports_subset: bool) -> Result<()>

Validates that the requested #[altrep(...)] option flags are compatible with the given ALTREP type family.

Enforces two rules:

1. subset is only valid for families where supports_subset is true.
2. dataptr and subset are mutually exclusive.

Arguments

• family – A human-readable family name used in error messages (e.g., "AltrepList").
• supports_subset – Whether this family supports the Extract_subset method.

Errors

Returns Err with a span pointing to the offending option identifier.

fn generate_lowlevel( &self, name: &Ident, generics: &Generics, family: &AltrepFamilyConfig<'_>, ) -> Result<TokenStream>

Generates low-level ALTREP trait implementation code for a given type family.

Emits the underlying trait-impl macros directly (Path (a) from #682, landed via #711/#933), reproducing the impl_alt<family>_from_data! arm expansions from the proc-macro with no declarative-macro hop. The four items every arm produces are:

1. __impl_altrep_base!(Ty, <guard>[, with_serialize]) — impl Altrep.
2. an AltVec impl, one of:
   • materializing (materializing_dataptr_macro) — default/serialize arms,
   • direct dataptr (__impl_altvec_dataptr!(Ty, <elem>); string routes through __impl_altvec_string_dataptr!) — dataptr arm,
   • subset (__impl_altvec_extract_subset!) — subset arm,
3. methods_macro!(Ty) — the family Alt<Family> impl,
4. inferbase_macro!(Ty) — impl InferBase.

The guard is honoured uniformly (the retired declarative-macro delegation only handled the default RUnwind guard; non-default guards used a separate expanded path) and option combinations are validated here at the derive call site rather than 7 hops deep in macro expansion.

Arguments

• name – The struct identifier.
• family – The family-specific configuration controlling which macros to emit.

Returns

A token stream containing the macro invocations, or an empty stream if no_lowlevel was specified.

Errors

Returns Err if option validation fails (e.g., subset on an unsupported family, or dataptr combined with subset).

Layout

Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain repr(...) attributes. Please see the Rust Reference's “Type Layout” chapter for details on type layout guarantees.

Size: 184 bytes