Struct MetadataOnlyOptions 

#[non_exhaustive]
pub struct MetadataOnlyOptions {
    pub include_oa_location: bool,
}

What a metadata-only resolve may spend beyond its single round-trip.

One knob, and it exists because #517 removed the reason there was none. The DOI path is Crossref-first, and its rationale was that Crossref’s message.link[] supplied an OA URL for free. Measurement showed every entry is programme-scoped, so extract_crossref_publisher_url correctly returns None for every DOI – leaving oa_url permanently null while docs/MCP_TOOLS.md s11 told callers to act on it (#539).

Crossref-first is still the right default: it resolves nearly every DOI in one request. What was wrong was promising a field it cannot fill. So the caller says whether it wants the second request.

Fields (Non-exhaustive)

Non-exhaustive structs could have additional fields added in future. Therefore, non-exhaustive structs cannot be constructed in external crates using the traditional Struct { .. } syntax; cannot be matched against without a wildcard ..; and struct update syntax will not work.

include_oa_location: bool

Consult Unpaywall for a real OA location, filling oa_url, oa_status and license from its record.

Costs one extra request against Unpaywall. Unpaywall is a metadata source, so this does not weaken the “never fetches a PDF, never touches a publisher” guarantee: the URL is still reported and never followed.

Off by default. A caller that wants metadata should not pay for a location it will not use.

Reading the result

With this set, the oa_status/oa_url pair says which of three things happened:

oa_status	oa_url	meaning
"closed"	None	asked; this work has no OA location
"gold", "green", …	Some	asked; here it is
None	None	the lookup did not complete

The third row is why a failed Unpaywall call leaves both fields None rather than filling in a plausible-looking oa_status: a caller that paid for a lookup has to be able to tell “no OA location exists” from “I could not find out”, and that distinction is the whole of #539.

Implementations

impl MetadataOnlyOptions

pub const fn with_oa_location(self, include: bool) -> Self

Ask for the OA location (or not).

A builder rather than a struct literal because the struct is #[non_exhaustive]: a future option must not be a breaking change for doiget-mcp, doiget-cli, or anyone else downstream.

Trait Implementations

impl Clone for MetadataOnlyOptions

fn clone(&self) -> MetadataOnlyOptions

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for MetadataOnlyOptions

impl Debug for MetadataOnlyOptions

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for MetadataOnlyOptions

fn default() -> MetadataOnlyOptions

Returns the “default value” for a type.

impl Eq for MetadataOnlyOptions

impl PartialEq for MetadataOnlyOptions

fn eq(&self, other: &MetadataOnlyOptions) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl StructuralPartialEq for MetadataOnlyOptions