…ocation (#23927)

## Summary

`SemanticBuilder::hoisting_variables` is a nested map
`FxHashMap<ScopeId, IdentHashMap<'a, SymbolId>>`. Each
`entry(scope_id).or_default()`
on a vacant scope heap-allocates a fresh inner `IdentHashMap` via the
**system**
allocator, and subsequent inserts grow it. On `antd.js` this is the
single largest
system-allocation site in semantic analysis (`VariableDeclarator::bind`
/
`Function::bind` in `binder.rs`).

This flattens it to a single `FxHashMap<(ScopeId, Ident<'a>),
SymbolId>`:

- one insert / one point-probe per hoisted variable (was: outer probe →
inner-map
  allocation → inner probe);
- no per-scope inner map, so the per-scope system allocations disappear;
- one hash instead of two.

Behaviour is identical: `hoisting_variables` is only ever inserted into
(`binder.rs`)
and point-queried in `check_redeclaration` (`builder.rs`) — it is never
iterated, so a
flat `(scope, name)` key is a drop-in.

## Results — `cargo allocs` (system allocations)

| fixture | phase | before | after |
| --- | --- | ---: | ---: |
| antd.js | semantic | 1020 | **28** (−97%) |
| antd.js | minifier | 3041 | **1040** (−66%, rebuilds semantic
internally) |
| pdf.mjs | semantic | 25 | 16 |
| kitchen-sink.tsx | semantic | 41 | 36 |

`allocs_semantic.snap` / `allocs_minifier.snap` are updated accordingly.
This is on the
hot path for tools that run `SemanticBuilder` repeatedly per module
(e.g. Rolldown).

## Relation to #19134

#19134 previously tried to move this temporary state (including
`hoisting_variables`)
to a scratch arena; CodSpeed reported a ~3.6% regression and it was
closed. This PR
takes a different, minimal approach: it does **not** introduce an arena,
does **not**
change `SemanticBuilder::new`'s signature, and touches no call sites. It
removes a hash
and an allocation per hoisted variable (rather than adding arena
threading), so it
should be CodSpeed-neutral-or-better — please let CI confirm.

## Tests

- `cargo test -p oxc_semantic --lib --tests` — 60 passed (incl.
redeclaration / var-hoist tests)
- `cargo test -p oxc_minifier --lib --tests` — 538 passed

## AI disclosure

Prepared with AI assistance (Claude). I reviewed the change, located the
hotspot with a
per-call-site allocation profile, verified the reduction with `cargo
allocs`, and ran
the test suites above.

…ocation (#23927)

## Summary

`SemanticBuilder::hoisting_variables` is a nested map
`FxHashMap<ScopeId, IdentHashMap<'a, SymbolId>>`. Each
`entry(scope_id).or_default()`
on a vacant scope heap-allocates a fresh inner `IdentHashMap` via the
**system**
allocator, and subsequent inserts grow it. On `antd.js` this is the
single largest
system-allocation site in semantic analysis (`VariableDeclarator::bind`
/
`Function::bind` in `binder.rs`).

This flattens it to a single `FxHashMap<(ScopeId, Ident<'a>),
SymbolId>`:

- one insert / one point-probe per hoisted variable (was: outer probe →
inner-map
  allocation → inner probe);
- no per-scope inner map, so the per-scope system allocations disappear;
- one hash instead of two.

Behaviour is identical: `hoisting_variables` is only ever inserted into
(`binder.rs`)
and point-queried in `check_redeclaration` (`builder.rs`) — it is never
iterated, so a
flat `(scope, name)` key is a drop-in.

## Results — `cargo allocs` (system allocations)

| fixture | phase | before | after |
| --- | --- | ---: | ---: |
| antd.js | semantic | 1020 | **28** (−97%) |
| antd.js | minifier | 3041 | **1040** (−66%, rebuilds semantic
internally) |
| pdf.mjs | semantic | 25 | 16 |
| kitchen-sink.tsx | semantic | 41 | 36 |

`allocs_semantic.snap` / `allocs_minifier.snap` are updated accordingly.
This is on the
hot path for tools that run `SemanticBuilder` repeatedly per module
(e.g. Rolldown).

## Relation to #19134

#19134 previously tried to move this temporary state (including
`hoisting_variables`)
to a scratch arena; CodSpeed reported a ~3.6% regression and it was
closed. This PR
takes a different, minimal approach: it does **not** introduce an arena,
does **not**
change `SemanticBuilder::new`'s signature, and touches no call sites. It
removes a hash
and an allocation per hoisted variable (rather than adding arena
threading), so it
should be CodSpeed-neutral-or-better — please let CI confirm.

## Tests

- `cargo test -p oxc_semantic --lib --tests` — 60 passed (incl.
redeclaration / var-hoist tests)
- `cargo test -p oxc_minifier --lib --tests` — 538 passed

## AI disclosure

Prepared with AI assistance (Claude). I reviewed the change, located the
hotspot with a
per-call-site allocation profile, verified the reduction with `cargo
allocs`, and ran
the test suites above.