[kixelated]

Problem

rust2-ts refused panic::Location with a span (left by #538), so Location::caller, its accessors, and #[track_caller] didn't translate.

Approach

• Location is a new Std::Location, an immutable rt.Location { file, line, col } (runtime/rt/location.ts) with std's Display, Debug, ==, Ord, and Hash, usable as a key in std's collections. file, line, and column read its fields.
• A crate function rustc says requires its caller's location (InstanceKind::requires_caller_location) gets a trailing caller: rt.Location parameter (ts/src/lower/caller.rs). As in rustc's codegen, each call passes the location of the call's fn_span: the outermost macro call, with the file named as span_as_caller_location names it. Inside a tracked function, a call passes its own caller instead. Location::caller evaluates to that same location.
• Closures, chains, trait methods (direct, generic, and Index), and trait objects are handled. A dyn method takes caller when the trait declares #[track_caller]. When only the impl does, the wrapper passes the impl's own location, as rustc's vtable shim does.
• A function pointer to a tracked function (a reify cast, or a pointer constant) passes the function's definition span, as rustc's reify shim does.
• A const or static of &Location is read out of its allocation.
• Refused with a span, since natively the location is in std's source: a tracked function used as a value that std's Fn impls call (opt.map(tracked), iter.map(tracked), a generic impl Fn called with one), a crate call that reaches a tracked function only through std's code, a tracked method std calls by name (an iterator's next and other own methods, a Drop::drop), and Location::caller as a pointer. file_as_c_str falls to the usual refusal.
• Only the crate's functions take a location. Std's tracked functions, such as unwrap, indexing, borrow_mut, and the from-MIR unwrap_or_else, take none, because nothing reads it: translated panics carry no location, and the hook is ts-panic-hook. So map(Option::unwrap) still translates.

Native results that set the rules above, checked with rustc: a fn pointer reports the definition span, Fn-trait calls report core/src/ops/function.rs, and an impl-only tracked method called through dyn reports the impl's span.

Proof: ts/fixtures/lang/panics/src/located.rs runs differentially against native wasm32. It covers every case above, plus {:>60} (which std doesn't pad), {:#?}, {:x?}, ordering, dedup, BTreeSet and HashSet, a Location held in a struct and an Option, and a panic message carrying the location. Refusals are in fixtures/reject/src/panics.rs.

Cost

Measured by output, not timing: the moq-net codec (just ts moq) and both bench fixtures (fixtures/codec, fixtures/task) contain no tracked function, and their translations are byte-identical to main (no rt.Location in the output, snapshots unchanged). So just ts bench times the same code, and hot paths pay nothing. The cost of passing a location into std's runtime calls (unwrap, indexing) is left to the hook quest, the first place one is read.

Impact

• Translator output: a crate's tracked function, and a dyn method its trait tracks, takes an extra last rt.Location argument. A TS caller or implementer of such a trait object passes or accepts it. No existing output changes otherwise.
• Runtime: new rt.Location, locationEq, locationCmp, locationHash, fmtLocationDisplay, fmtLocationDebug.
• Not breaking: nothing that translated before changes shape.

Alternatives

• Passing a location to every runtime panic too (rt.unwrap(o, loc), rt.index(..., loc)): that is what the hook needs, but nothing reads it yet, and it would cost every hot call. Left to ts-panic-hook, whose plan now says so.
• Passing std's Fn-impl location for a tracked fn item called through Fn: it names the native build's sysroot path, which translation can't know. Refused instead.

Follow-ups

• ts-panic-hook, updated: std's runtime-mapped tracked functions and std-internal panic paths.

Review

quest review, Codex gpt-6.1-sol, medium effort. Round 2 on head 88604e185 reported no findings. Round 1 (head c95095cfb) had three findings:

• A #[track_caller] Iterator::next or other own iterator method is called by the runtime's protocol with no location. Fixed: a tracked crate function that std's code calls by symbol (iterator protocol, collect, drop glue's Drop::drop) is refused at its definition (caller::refuse_std_call). Covered in fixtures/reject.
• A reified pointer to a () function was typed void where values are undefined. Fixed: cast as fn_value does, covered by a [fn(bool); 2] in the fixture.
• Display should write in parts, as std's write! does. Fixed: it now goes through fmtWriteFmt. Refuted the request for a capped-writer differential case: translation already refuses placeholders written into a crate's own fmt::Write (reproduced while adding the case), and the runtime's writers never fail, so translated code can't observe it.

Since then: a conflict-free merge of origin/main (head 7117ecc02).

🤖 Generated with Claude Code

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Commits

Reviewing files that changed from the base of the PR and between 1b0fe64 and 7117ecc.

Files selected for processing: 36

• quest/m3/README.md
• quest/m3/ts-panic-hook.md
• quest/m3/ts-panic-location.md
• ts/README.md
• ts/fixtures/lang/panics/expected/core/bool.ts
• ts/fixtures/lang/panics/expected/core/cmp.ts
• ts/fixtures/lang/panics/expected/core/option.ts
• ts/fixtures/lang/panics/expected/lib.ts
• ts/fixtures/lang/panics/expected/located.Held$read.ts
• ts/fixtures/lang/panics/expected/located.Inherits.ts
• ts/fixtures/lang/panics/expected/located.Own.ts
• ts/fixtures/lang/panics/expected/located.Places.ts
• ts/fixtures/lang/panics/expected/located.Tracked.ts
• ts/fixtures/lang/panics/expected/located.Untracked.ts
• ts/fixtures/lang/panics/expected/located.ts
• ts/fixtures/lang/panics/src/lib.rs
• ts/fixtures/lang/panics/src/located.rs
• ts/fixtures/reject/census.txt
• ts/fixtures/reject/expected.txt
• ts/fixtures/reject/src/panics.rs
• ts/fixtures/reject/ts.toml
• ts/runtime/rt.ts
• ts/runtime/rt/location.ts
• ts/src/lower/body.rs
• ts/src/lower/caller.rs
• ts/src/lower/collections.rs
• ts/src/lower/drop.rs
• ts/src/lower/dynamic.rs
• ts/src/lower/fmt.rs
• ts/src/lower/iter.rs
• ts/src/lower/library.rs
• ts/src/lower/mir/mod.rs
• ts/src/lower/mir/place.rs
• ts/src/lower/mir/rvalue.rs
• ts/src/lower/mod.rs
• ts/src/program.rs

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