perf_lint: add PERF020 — redundant same-name workhorse cast

[borisbat]

Summary

Adds PERF020 to daslib/perf_lint. Flags T(x) where x is already of workhorse type T — a no-op ExprCall against the cast-named overload that the typer does not elide.

Fires for the 15 workhorse cast names registered by ADD_NUMERIC_CASTS / ADD_BITFIELD_CASTS + das_lexical_cast_*:

int, int8, int16, int64, uint, uint8, uint16, uint64, float, double, string, bitfield, bitfield8, bitfield16, bitfield64.

// Bad
def widen(a : int64) : int64 {
    return int64(a)                 // PERF020
}

// Good
def widen(a : int64) : int64 {
    return a
}

Implementation

• Match: call.func.fromGeneric?.name ?? call.func.name is in the workhorse-cast set AND arg._type.baseType equals the cast's target type. Const/ref/temp qualifiers on the argument are ignored — only baseType matters.
• Dispatch: hoisted above the closure guard in preVisitExprCall so the rule fires anywhere (a redundant cast inside a closure body is still redundant). perf_warning handles macro-generated / generic-instantiation suppression.
• Out of scope by design: user-named bitfield/enum ctors (MyBitfield(x), MyEnum(x)), vector ctors (int2(x,y) etc.), and string(das_string) (covered by PERF007 / PERF012). The single-argument gate excludes multi-arg ctors; user-named ctors don't match the workhorse name set.

Drive-by

• daslib/perf_lint.das:394 — dropped uint64(intptr(...)); intptr already returns uint64. Self-hit found by the new rule.
• utils/lint/tests/perf019_int_cast_collapse.das — good_nested_cast used int(int(m1)) deliberately to exercise PERF019's nested-call unwrap, but that shape is now a textbook PERF020 hit. Replaced inner cast with an int64 widen so the same PERF019 code path is exercised without tripping PERF020.

CI

The lint-on-changed-files gate at extended_checks.yml:225-238 already runs the unified lint runner on every changed .das file in PRs. No workflow edits needed — the new rule rides the existing pipeline.

A daslib sweep found ~30 PERF020 hits in 14 other files (aot_cpp.das, ast_boost.das, coverage.das, debug.das, profiler.das, others). Per the standard rule-then-sweep pattern, those go to a follow-up cleanup PR — they don't fail current PRs unless those files are touched.

Test plan

• dastest on utils/lint/tests/perf020_redundant_cast.das — 11 PERF020 fires, matches expect 31208:11
• dastest on all utils/lint/tests/ — 28/28 pass after PERF019 fixture restructure
• Unified lint runner on daslib/perf_lint.das (paranoid+perf+style) — PASS
• das-fmt --verify on the two changed .das files — Verified
• Sphinx clean build — no new warnings on lint.rst
• CI: extended_checks job will lint changed files; daslib sweep showed daslib/perf_lint.das (the only daslib file in this PR) is clean

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a new performance lint rule (PERF020) to catch redundant same-type “workhorse” casts (e.g., int64(x) where x : int64) that the typer does not elide and which therefore remain as real ExprCall nodes.

Changes:

• Implement PERF020 detection in daslib/perf_lint and ensure it runs even inside closures.
• Add a dedicated dastest fixture for PERF020 and adjust the PERF019 fixture to avoid the new rule while still exercising PERF019’s unwrap path.
• Document PERF020 in both the skills guide and the Sphinx reference docs.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
utils/lint/tests/perf020_redundant_cast.das	New dastest fixture covering redundant same-type casts across the 15 workhorse cast names.
utils/lint/tests/perf019_int_cast_collapse.das	Updates a “good” case to avoid triggering PERF020 while still testing PERF019’s nested-cast unwrap behavior.
skills/perf_lint.md	Adds PERF020 to the perf lint rules reference table with matching/exclusion notes.
doc/source/reference/language/lint.rst	Adds a new PERF020 section to the lint reference documentation.
daslib/perf_lint.das	Implements PERF020 (type mapping + call-site check) and wires it into preVisitExprCall before the closure guard; also removes a redundant uint64(intptr(...)) cast.
CLAUDE.md	Updates the “Don’t write / Write instead” migration table to include PERF020 guidance.

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[aleksisch]

string(a ? b : c) instead of a ? string(b) : string(c)? Can be added to linter too btw. It's not first time such code was written.