refactor(infer): delete the it/this text-scan ladder; broken syntax gets CST brace-repair

[Hessesian]

Summary

Task 6 of the CST lambda-triad collapse: the last text fallback — the backward text-scan ladder in find_it_element_type_in_lines_impl — is deleted. The it gate now routes through Indexer::live_doc_or_parse, making the CST path universal for it/this exactly like the this (Task 2b), named-param (Task 5), and completion-scope (Task 7) paths before it.

Broken syntax: repair the source, don't scan it

The deletion surfaced a real capability the text scan silently provided: for items.forEach { it.name (unclosed brace — the mid-typing state), tree-sitter forms no lambda_literal at all (… → statements → ERROR → source_file), so no parse can answer. Instead of keeping a text heuristic, the ERROR case now gets append-only brace repair: a typed LambdaTreeGate distinguishes ERROR-at/above-cursor (repair permitted) from a well-formed tree with no lambda (authoritative None); the repair appends } at EOF (zero offset shift — the cursor position stays valid), transient-parses (never cached into live_trees), self-verifies the cursor now sits in a lambda_literal (capped at 8 attempts), and runs the same resolver on the repaired tree. ItResolutionDoc::{Live, Repaired} keeps a repaired-tree answer from masquerading as an authoritative one. hints_survive_syntax_error now passes via the repair branch.

Root-fix found en route

extract_collection_element_type was stripping type arguments from the element (List<RegularAccount> → List) via dotted_ident_prefix(), silently breaking nested chains (collect { it.firstOrNull()?.account?.accountId?.also { … } } — inner it resolved to List instead of String). It now validates via the dotted prefix but returns the full parameterized type; inline_lambda_also_nested_in_outer_lambda pins it, assertion untouched.

Also deleted

• CstParamResult + into_option() — after the two-state collapse every consumer mapped both variants to None; the type stopped carrying a decision, so it's now plain Option<String> at all three producers.
• IT_SCAN_BACK_LINES and the ladder's helpers (re-homed to #[cfg(test)] where unit tests still exercise them directly).

Plus: a tempfile-based regression test pinning the scope.rs ungating fix from the s5 stack (named-param resolution from a disk-only file that is neither open nor indexed).

Verification

• cargo test --bin kmp-lsp — 1442 passed, 0 failed (baseline 1441 + 1 new regression test); clippy clean.
• Decoy evidence (RED→GREEN) for the repair branch and the generics fix in the task report; 13 former text-path tests converted to full-document setups and renamed to describe the transient-parse reality.
• Task-scoped review: Approved, no Critical/Important findings.

Stacked on #202 (refactor/cst-resolution-s7). Next: Task 8 — with this landed, lambda_receiver_type_from_context/lambda_receiver_type_named_arg_ml are down to two cst_lambda call sites; closing those deletes receiver.rs (~480 lines).

🤖 Generated with Claude Code

https://claude.ai/code/session_01EJsyQ1UgF8uJYR4HgpxEBB

[Copilot]

Pull request overview

This PR continues the CST lambda-triad refactor by removing the last production it/this text-scan fallback and routing it resolution through Indexer::live_doc_or_parse, making CST-based inference universal. It also addresses a real regression uncovered by the refactor (loss of element type arguments) and adds targeted regression coverage for disk-only (not live, not indexed) CST parsing.

Changes:

• Remove the backward text-scan ladder for it inference; always resolve via CST from live_doc_or_parse.
• Add bounded, append-only brace repair for broken syntax cases where tree-sitter produces no lambda_literal.
• Fix extract_collection_element_type to preserve the element type’s own type arguments (e.g. List<RegularAccount>), and simplify CST param-type plumbing by collapsing CstParamResult to Option<String>.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/resolver/infer_lines.rs	Preserve element type arguments when extracting collection element types to enable correct chained member substitution.
src/indexer/scope_tests.rs	Update it inference tests to use CST-backed find_it_element_type_in_lines, and add regression coverage for disk-only transient parsing.
src/indexer/infer/it_this.rs	Delete the production text-scan ladder; introduce CST gating + bounded brace-repair transient parses for broken syntax.
src/indexer/infer/it_this_tests.rs	Convert text-based it tests to full-document CST setups with explicit cursor positions; update regression test expectations to transient-parse reality.
src/indexer/infer/cst_lambda.rs	Remove CstParamResult tri-state and streamline CST parameter inference APIs to Option<String>; keep text-scan only under #[cfg(test)].
src/indexer/infer/args.rs	Mark has_named_params_not_it as test-only now that production named-param detection is CST-based.
src/indexer.rs	Adjust re-exports so has_named_params_not_it is only exposed under #[cfg(test)].