…24007)

Improve the performance of creating a var name from an AST node.

## Perf

2 different optimizations:

#### 1. Avoid building a huge string

Previously we concatenated all identifiers into a string of potentially large length, then truncated it to 20 bytes at the end.

Instead, once the string reaches the maximum length, stop adding to it. This avoids pointless work, and avoids extra allocations to grow the `String`.

#### 2. Remove "needs to grow?" checks

The first optimization ensures that the `String` never exceeds 20 bytes. Utilize this invariant to remove "does the `String` need to grow?" checks on every push to the `String`.

This uses the `push_unchecked` and `push_str_unchecked` methods added to `String` in #24006.

Adverserial review by Claude found the implementation to be sound, and it performed fuzzing using 500,000 random cases to test the implementation. Tests which are added in this PR also cover all the edge cases.

## Fix

This PR also fixes incorrect truncation when some of the parts contain Unicode characters.

Previously the string would be truncated to be shorter than 20 bytes when the last char before the cut point is a complete multi-byte (Unicode) character. Now it checks whether to truncate to less than 20 bytes based on whether the 21st byte is a character sequence start byte, instead of whether the 20th byte is a continuation character.

The tests are altered to reflect this fix.

Introduce a `StringExt` trait which adds 2 unsafe methods to `String`:

- `push_unchecked`
- `push_str_unchecked`

Both push to the string without checking there's sufficient capacity for it. The caller guarantees the push cannot cause the `String`'s length to exceed its capacity.

Both methods call the corresponding safe methods of `String` internally, with `assert_unchecked!`s preceding those calls to allow compiler to remove the bounds check. The shape of these assertions matches the code in `String`'s methods exactly.

Examination of generated assembly confirms that the optimizations work.

…24007)

Improve the performance of creating a var name from an AST node.

## Perf

2 different optimizations:

#### 1. Avoid building a huge string

Previously we concatenated all identifiers into a string of potentially large length, then truncated it to 20 bytes at the end.

Instead, once the string reaches the maximum length, stop adding to it. This avoids pointless work, and avoids extra allocations to grow the `String`.

#### 2. Remove "needs to grow?" checks

The first optimization ensures that the `String` never exceeds 20 bytes. Utilize this invariant to remove "does the `String` need to grow?" checks on every push to the `String`.

This uses the `push_unchecked` and `push_str_unchecked` methods added to `String` in #24006.

Adverserial review by Claude found the implementation to be sound, and it performed fuzzing using 500,000 random cases to test the implementation. Tests which are added in this PR also cover all the edge cases.

## Fix

This PR also fixes incorrect truncation when some of the parts contain Unicode characters.

Previously the string would be truncated to be shorter than 20 bytes when the last char before the cut point is a complete multi-byte (Unicode) character. Now it checks whether to truncate to less than 20 bytes based on whether the 21st byte is a character sequence start byte, instead of whether the 20th byte is a continuation character.

The tests are altered to reflect this fix.

### 🚀 Features

- 260425f semantic/examples: Include unresolved references (#24214)
(camc314)
- 2d9b0b3 minifier: Fold boolean-literal ternary branches in value
contexts (#24110) (Dunqing)
- 61fbf10 ast: Implement `ReplaceWith` on all AST types (#24013)
(overlookmotel)
- 7db7a29 allocator: Add `ReplaceWith` trait (#24012) (overlookmotel)
- 4eb074e mangler: Add `reserved` option for names that must not be
mangled (#24041) (Dunqing)
- 2e62012 data_structures: Add `StringExt` trait (#24006)
(overlookmotel)
- 60e7160 minifier: Drop side-effect-free IIFEs whose result is unused
(#23967) (Dunqing)
- 26dd9e2 ast: Add method to widen inherited enum ref to parent ref
(#23961) (overlookmotel)

### 🐛 Bug Fixes

- e8b50ee transformer: Clean up semantics for stripped TypeScript syntax
(#24180) (camc314)
- d966d0b react_compiler: Remove clippy allows (#24168) (Boshen)
- 854ef8d react_compiler: Compile generic functions instead of
over-bailing on type-param hoisting (#24158) (Boshen)
- 093586c react_compiler: Align memoization cache-slot allocation with
Babel (#24157) (Boshen)
- 09c8f59 react_compiler: Normalize snapshot fixture paths (#24142)
(camc314)
- f13df97 react_compiler: Drop stray empty statement from catch bindings
(#24133) (Boshen)
- cb2a505 react_compiler: Codegen destructuring reassignment targets
(#24131) (Boshen)
- b82c394 react_compiler: Propagate codegen invariants instead of
emitting empty bodies (#24128) (Boshen)
- 5771982 react_compiler: Render unchanged programs as source in fixture
snapshots (#24129) (Boshen)
- 4b16e1a transformer/async-to-generator: Preserve direct eval scope
flags (#24136) (camc314)
- 4e9194f react_compiler: Lower `delete obj.prop` to
Property/ComputedDelete (#24123) (Boshen)
- 0b25582 ast: Type binding node `typeAnnotation` as `TSTypeAnnotation |
null` (#23113) (Boshen)
- 018c0e5 transformer: Hoist lowered async declarations (#22770)
(camc314)
- 652fbaf mangler: Keep names of destructured exported bindings (#24036)
(Dunqing)
- e274415 minifier: Don't drop global calls that throw despite pure
arguments (#23917) (Dunqing)
- 59abb30 minifier: Only merge string literals in `try_fold_add` when
the inner operator is `+` (#23622) (Jerry Zhao)

### ⚡ Performance

- c5ca77b transformer: Avoid cloning refresh options (#24191) (camc314)
- bf1a151 react_compiler: Compile out debug printers (#24184) (Boshen)
- abb44a0 transformer: Build fixed object-rest arguments (#24190)
(camc314)
- a4db731 isolated_declarations: Use `ReplaceWith` instead of `TakeIn`
(#24016) (overlookmotel)
- ff10855 transformer: Use `ReplaceWith` instead of `TakeIn` (#24015)
(overlookmotel)
- bd49aff ecmascript: Avoid heap-allocating Math.min/max/imul operands
(#23941) (Lawrence Lin)
- e4b708b react_compiler: Skip compiled files before prefilters (#24171)
(Boshen)
- c59f2fe rust: Return impl ExactSizeIterator from slice-backed
accessors (#24144) (Boshen)
- 5d6d04a codegen: SWAR-skip boring byte runs in sourcemap line/column
scan (#24023) (Boshen)
- a55e0be traverse: Reduce string operations in `get_var_name_from_node`
(#24007) (overlookmotel)
- e6d48e1 transformer/nullish_coalescing: Move cold path into separate
function (#23989) (overlookmotel)
- c4e35b5 transformer/object_rest_spread: Pre-allocate capacity in `Vec`
(#23988) (overlookmotel)
- 527b8e5 transformer/decorators: Narrow type earlier (#23987)
(overlookmotel)

### 📚 Documentation

- 30d17f5 allocator: Clarify docs for `TakeIn::take_in_box` (#24093)
(overlookmotel)
- 675e6a8 ast: Correct doc comment for `PrivateFieldExpression` (#24008)
(overlookmotel)
- e4c30e6 minifier: Explain what `dce` mode means (#23994) (Dunqing)
- 37cbf88 ast_macros: Document fields of `StructDetails` (#23959)
(overlookmotel)
- 4de3e54 ast: Correct doc comment (#23948) (overlookmotel)

Co-authored-by: Boshen <[email protected]>