Macro fetch_glue 

macro_rules! fetch_glue {
    (no_extensions $name:ident, $feat:literal, $draft:ident) => { ... };
    (count_extensions $name:ident, $feat:literal, $draft:ident) => { ... };
    (length_extensions $name:ident, $feat:literal, $draft:ident) => { ... };
}

Generates the conversion glue for one draft’s fetch objects.

The leading keyword selects the extension-block shape, as for legacy_subgroup_glue!. Unlike subgroup objects, the fetch extension block is unconditional on drafts 09-13 — it is never gated on the stream type.