fix(hy-check): reject untyped nil local declarations (fixes #672)

[dezzy-music]

Fixes #672

Problem

Declaring a variable initialized to bare nil without a declared type (var x = nil, x := nil, let x = nil, a, b := nil, 1, var _ = nil, or inside closure/loop/if bodies) passed hyb check with 0 diagnostics and emitted Go that failed go build:

use of untyped nil in variable declaration

This violates the accepted-implies-buildable contract (C5a, rule BUILDABLE / N1).

The gap was that the N1 nil check at both local-declaration check sites (hy-check/src/loans.rs process_local and hy-check/src/closures.rs check_stmt_boundaries) was gated behind if let Some(t) = &l.ty. When l.ty.is_none(), bare nil passed through uninspected.

Solution

1. Diagnostic Registration (HYB-E4003):
   • Added HYB-E4003 (untyped-nil-declaration) under category nil-safety, severity error, rule N, tier0_rule_id N1 in compiler/crates/hy-diag/data/diagnostics.json.
2. Checker Enforcement (sendability.rs, loans.rs, closures.rs):
   • Added emit_untyped_nil_error helper in Checker (sendability.rs).
   • In loans.rs (process_local) and closures.rs (check_stmt_boundaries), added an else if l.kind != LocalKind::Const && Checker::is_nil_literal(value) branch when l.ty.is_none().
   • Leaves local const c = nil to the E6003 constant-expression guard (issue Soundness & buildability gaps in make/new shape resolution, closure builtin validation, bare chan T expressions, and file-level const forward references #640) as Go reports nil is not constant there.
   • Controls preserved:
     • var e error = nil (ascribed error) continues to accept.
     • var ch chan int / var s []int (zero-value declarations without initializer) continue to accept.
     • ch == nil / f(nil) (comparisons and call arguments) continue to accept.
     • Ascribed non-nilable (var s []int = nil) continues to reject with HYB-E4001.
3. Tests & Corpus Pin:
   • Added compiler/crates/hy-check/tests/untyped_nil_decl.rs covering all 14 test cases (all declaration spellings, closure bodies, loop bodies, if-inits, match arms, blank identifiers, and controls).
   • Added tests/checker-corpus/untyped-nil-decl-reject.hy pinning REJECT N1 [HYB-E4003].

Verification

• cargo test --manifest-path compiler/Cargo.toml --test untyped_nil_decl: 14 passed, 0 failed.
• cargo test --manifest-path compiler/Cargo.toml -p hy-check: all passed, 0 failed.
• cargo test --manifest-path compiler/Cargo.toml -p hy-diag: 8 passed, 0 failed.
• python tools/corpus_harness.py: 80/80 verdict agreement, tripwire not fired.
• python tools/check_conflict_markers.py: conflict markers: none.

[limitlessprojects]

Landed — thank you @dezzy-music. Your take is correct and its two unique shapes are carried; the duplicate-family rule kept one landing (same defect, same fix, same three check-site hunks as #673).

Landed fix: #673 (milkingndb-oss) merged server-side as 7aaf193 — registered HYB-E4003 untyped-nil-declaration (nil-safety, rule N, tier0 N1) emitted from an untyped else-leg at both local-declaration check sites (loans.rs process_local + closures.rs check_stmt_boundaries), gated off LocalKind::Const so local const c = nil stays E6003's locus (#640). Corpus fixture untyped-nil-decl-reject.hy + VERDICTS.md entry registered (corpus 80/80, parse_all 79→80). Follow-up commit aeac72a carries your take's two extra shapes into the landed suite (if_body_untyped_nil_is_e4003, if_init_untyped_nil_is_e4003) — suite now 15/15, cargo --workspace 1170/0.

Per-take verification on main after the landing (live-bypass probe, was LIVE before — all ten accepted with 0 diagnostics then failed go build 'use of untyped nil in variable declaration', twin-verified with go1.26.9):

#691 claim	before	after
var x = nil	ACCEPT (unbuildable)	REJECT HYB-E4003
x := nil	ACCEPT	REJECT HYB-E4003
let x = nil	ACCEPT	REJECT HYB-E4003
a, b := nil, 1	ACCEPT	REJECT HYB-E4003
var _ = nil	ACCEPT	REJECT HYB-E4003
closure-body var x = nil	ACCEPT	REJECT HYB-E4003
loop-body var x = nil	ACCEPT	REJECT HYB-E4003
if-body var x = nil (your extra)	ACCEPT	REJECT HYB-E4003 (+ unit pin)
if-init if x := nil; ... (your extra)	ACCEPT	REJECT HYB-E4003 (+ unit pin)
match-arm var x = nil	ACCEPT	REJECT HYB-E4003

False-positive guards hold: var e error = nil ACCEPT · zero-value decls (var ch chan int) ACCEPT · f(nil) / ch == nil ACCEPT · var s []int = nil still E4001 · const c = nil still E6003's locus, no double-fire. Battery on the landed tree: cargo 1170/0, corpus 80/80, differential FULL 23+57, #524 Go-twin float matrix intact. CI green on landed heads (17ff816 quick+compiler; aeac72a on main).

Credit: dezzy-music (this take + the two carried shapes) and milkingndb-oss (landed take #673, corpus registration). Closing as duplicate-fixed; issue #672 tracks the defect.