Description

close #3427

Supersedes #8218.

Root cause

#3405 stopped merging external named imports whose local binding is re-exported (export { a } from 'ext'), because eager merging happens before chunks exist and can move a symbol's canonical ref where a chunk can't render it: in ESM output a chunk renders external imports only from its own modules' named_imports (direct_imports_from_external_modules), and compute_chunk_imports intentionally skips external-owned symbols. That fixed the Nuxt crash, but it left every re-export of the same (external module, name) pair as its own symbol:

// tests/rolldown/function/external/export_external, before
import { a, a as a$1, a as a1, b, b as b$1, b as b1 } from "external";

Fix

Defer the merge until chunk assignment is known, then merge strictly per chunk (next to the existing namespace merger in merge_external_import_symbols):

• bind_imports_and_exports now collects re-exported external named imports into deferred_external_binding_merges, keyed by (external module, imported name), instead of skipping them.
• At chunk level, each group merges within its chunk. The canonical is the first used symbol by (exec_order, symbol index), preferring a member whose name equals the imported name so the output reads import { a } rather than import { a as a$1 }.
• The facade created by the eager merger (for plain, non-re-exported imports) is folded into that canonical only when every module owning those plain imports lives in the same chunk. A plain import is only referenced from its own module, so in that case all of the facade's references stay inside the chunk; otherwise the facade stays a root and each chunk keeps rendering its own import.

The per-chunk restriction preserves #3405's constraint by construction: a merged group's canonical always stays a symbol declared inside that chunk, so cross-chunk references keep flowing through the regular cross-chunk import/export machinery. (This is also where #8218 went wrong: it linked re-export symbols from every chunk into the external-owned facade, which reintroduces the #3405 scenario for indirect importers.)

// tests/rolldown/function/external/export_external, after
import { a, b } from "external";

Tests

Two node-executed fixtures assert runtime identity of the merged bindings:

• external/reexport_external_binding_dedup: single chunk; plain import + re-export + pass-through re-export (named and default) collapse to one specifier each.
• external/splitting_reexport_external_dedup: the fix(splitting): indirect external symbol #3405 splitting shape; the shared chunk collapses its duplicate re-exports into one import/one cross-chunk export, while the entry chunks' facade correctly does not fold across chunks.

Rollup compat suite: 1214 passed / 0 failed.