chore(quest): plan a name or an operator argument at its own Rust span

quest/m3/README.md

@@ -36,3 +36,4 @@ generic std gap is planned here even when moq never calls it.
 - [Python's wake on the loop's own thread](/quest/m3/ffi-python-wake.md) - a wake on the event loop's thread resumes the future with no socket pair
 - [Python's call cost](/quest/m3/ffi-python-calls.md) - a call on an object costs what BoltFFI's does, or the gap is explained
 - [A std map with a crate's own BuildHasher](/quest/m3/ts-map-own-hasher.md) - std's and hashbrown's maps built with a crate's own hasher translate for keys that are JS objects
+- [A name or an operator argument at its own Rust span](/quest/m3/ts-argument-spans.md) - a `tsc` error on a plain-name or operator argument names that argument's Rust, not the method it is passed to

quest/m3/ts-argument-spans.md

@@ -0,0 +1,22 @@
+# [S] A name or an operator argument at its own Rust span
+
+## Goal
+
+When `tsc` rejects an argument of a translated call that is a plain name or an operator,
+[rust2-ts](/ts/README.md) reports the error at that argument's Rust, not at the method name of
+the call it is passed to, proven by the `spans` fixture's snapshot.
+
+## Plan
+
+Left out of [#520](https://github.com/moq-dev/rust2/pull/520), which maps an argument made by
+a call to its own Rust. In `Meter.scaled(m, rt.u32WrappingAdd(a, 1), ...)`, the receiver `m`
+still maps to `scaled`, as does an argument such as `(a ^ b) >>> 0`.
+
+MIR gives each argument its own span (`Spanned<Operand>`). Locating names and operators the
+way calls are located needs the lowering passes that match on expression shapes, such as
+`Expr::Ident`, `Expr::Member`, or `Expr::Binary`, to look through the location wrapper first.
+Wrapping members and binaries naively changed what the formatting code spills into a variable
+before its closure reads it, so the generated code changed. Prove the output text unchanged by
+the snapshots, as #520 did, and measure the source map's growth on moq's translation.
+
+Public API: the translator's error spans.