[Types] Vec<u8> annotation rejects Vec::with_capacity and cascades into undefined-binding errors #50

Problem

Adding an explicit Vec<u8> annotation to a working bounded byte encoder makes Vec::with_capacity(52) fail type checking. This was reproduced while simplifying AgoraSeal's external-verifier context encoding.

The failure should either be resolved through expected-element-type propagation or have a precise documented constructor boundary and actionable diagnostic. It should not cascade into errors for every later use of the binding.

Reproduction

Tested on 2026-10-07 with the local cellc binary reporting 0.31.0, metadata schema 72; compiler checkout HEAD 35bf983. These are local binary results, not a fresh release-build attestation.

Use an isolated copy of AgoraSeal's vote package at 5b6106a. Replace its 36 OutPoint byte pushes with:

let mut context: Vec<u8> = Vec::with_capacity(52)
for i in 8..44 {
    context.push(ckb::cell_data_u8(ballot, i) as u8)
}

Keep the subsequent sixteen height-byte pushes, trusted verifier call, and package manifest unchanged. Repin only the isolated copy after this intentional source change, then compile:

cellc lock
cellc . --target riscv64-elf --target-profile ckb --entry-action cast --primitive-strict 0.16 -o /tmp/typed-context.elf

First diagnostic:

error: type mismatch: expected Named("Vec<u8>"), found Named("Vec")
 --> src/main.cell:53:9

The complete package reports 19 diagnostics, including subsequent undefined variable 'context' errors.

Removing only : Vec<u8> makes the same package compile successfully. Using unannotated Vec::new() also compiles, and produces the same ELF hash as unannotated Vec::with_capacity(52) in this reproduction.

The constructor type-checking branch should be inspected for loss of element-type information: the local ("Vec", "with_capacity") branch returns Type::Named("Vec".to_string()). This is an investigation lead, not a complete root-cause analysis.

Expected behavior and acceptance

• Define and document whether expected Vec<T> types propagate into Vec::new() and Vec::with_capacity(n) for supported fixed-width local elements.
• Support the annotated Vec<u8> case above, or provide an explicitly documented alternative with a targeted diagnostic if this combination is intentionally excluded.
• Preserve rejection of incompatible pushed element types and unsupported element layouts.
• Avoid the undefined-binding diagnostic cascade after a constructor type mismatch.
• Keep CLI, recovering diagnostics/LSP, metadata, and the collection support matrix consistent.
• Add focused constructor/contextual-typing regressions; run the applicable project gates for the eventual fix.

This issue does not request Rust heap allocation semantics. CellScript currently documents fixed stack-backed local Vec capacity (256 / element_width); with_capacity(52) is not a declaration of an exact 52-byte backing buffer. See the collection support matrix.

Related: #23 (generic authoring), #47 (allocation-backed collections, a separate scope).