perf_lint: add PERF025 — redundant string() in string interpolation

[borisbat]

Summary

New daslib/perf_lint.das rule PERF025: flags a string(value) cast used as a direct string-interpolation element. The interpolation already stringifies every element via the string builder's DebugDataWalker, so the cast just allocates a throwaway string the builder then copies.

// Bad
return "{string(iv)}"      // PERF025
// Good
return "{iv}"

Behavior

Fires for stringify-equivalent value types — where "{x}" and string(x) produce identical text (verified empirically):

value type	action
signed int/int8/int16/int64, float, double, string, das_string	warn → drop the cast
uint/uint8/uint16/uint64	warn → drop, plus a {x:d} hint: unsigned interpolates as hex by default ("{42u}" → 0x2a) while string() gives decimal

Deliberately skipped (dropping the cast would change the value, not just the format — no format tag fixes it):

• string(array<uint8>) (bytes→text), string(uri), string(text_range)
• string() nested as an argument to another call, e.g. "{length(string(x))}" — only direct interpolation elements are flagged
• explicit hex request string(x, true)

Detection lives in the per-element preVisitExprStringBuilderElement hook; the kind is modeled as an enum (Perf025Kind { skip; equivalent; unsigned_hex }). A format spec ("{x:fmt}") lowers to _::fmt(":fmt", x), but fmt has no string-valued overload, so string(x) can never sit under a fmt wrapper — no unwrap needed.

Also drops one redundant string(das_string) the new rule found in perf_lint's own PERF024 message string.

Files

• daslib/perf_lint.das — the rule (enum + 3 helpers + 1 visitor override) and the self-hit fix
• utils/lint/tests/perf025_redundant_string_in_interp.das — fixture (expect 31208:8)
• doc/source/reference/language/lint.rst — PERF025 reference section
• skills/perf_lint.md — rules-table row

Test plan

• utils/lint/main.das on changed .das files → 0 warnings (PERF025 is lint-clean against its own code)
• Fixture emits exactly 8 error[31208] (5 plain-drop + 3 :d-hint); good cases produce 0; matches expect 31208:8
• das_string headline case ("{string(fn.name)}") confirmed firing; direct "{fn.name}" does not
• All 25 existing perf* lint fixtures still match their declared expect counts (no regression)
• detect-dupe on the new helpers → no duplicates
• Formatter-clean (das-fmt --verify passed on the full tree via pre-push hook)
• Clean Sphinx build for the lint.rst change (no warnings attributable to it)

AOT/JIT steps are N/A: this is a compile-time lint pass that touches no codegen and is required by no AOT/runtime test.

Follow-up (separate sweep, not this PR): the rule surfaces a handful of pre-existing hits elsewhere in the tree (utils/mcp/subtools/aot.das, tutorials/macros/function_macro_mod.das, modules/dasClangBind/cbind/cbind_boost.das).

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a new performance lint rule PERF025 to daslib/perf_lint.das that flags string(value) casts used as direct string-interpolation elements (e.g. "{string(iv)}"), since the string builder's DebugDataWalker already stringifies elements, making the cast a wasted allocation. The rule classifies the wrapped value to decide whether to warn plainly (signed ints, float, double, string, das_string) or warn with a :d format-tag hint (unsigned ints, which interpolate as hex by default), and intentionally skips value-shape-changing conversions (array<uint8>, uri, text_range), nested string(...) arguments, and explicit string(x, true) hex requests.

Changes:

• Add Perf025Kind enum, three helpers, and a preVisitExprStringBuilderElement override implementing the rule.
• Add fixture utils/lint/tests/perf025_redundant_string_in_interp.das with expect 31208:8 covering all equivalent and unsigned-hex cases plus negative cases.
• Drop a redundant string(das_string) cast inside PERF024's own warning message (a self-hit from the new rule), and update lint.rst and skills/perf_lint.md documentation.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
daslib/perf_lint.das	Implements PERF025 (enum, helpers, visitor hook) and removes a self-hit in PERF024's message.
utils/lint/tests/perf025_redundant_string_in_interp.das	New fixture exercising 8 bad cases (5 equivalent + 3 unsigned_hex) and 4 good cases.
doc/source/reference/language/lint.rst	Reference section describing PERF025 behavior, the :d hint, and skipped cases.
skills/perf_lint.md	Adds a PERF025 row summarizing detection shape and edge cases.

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