[13.x] Skip allocation in mergeFillable/Appends/Hidden/Visible when input is empty

[olivier-zenchef]

Problem

PR #59404 (released v13.3.0, fixing trait initializer collision) replaced if (empty($this->fillable)) { $this->fillable = ... } (and the analogous empty-checks for appends, hidden, visible) with calls to $this->mergeFillable(...) / mergeAppends(...) / mergeHidden(...) / mergeVisible(...).

The collision fix is correct and necessary.

However, those merge* methods are now invoked on every model construction, including when the input array is empty (the common case — most models don't declare #[Appends], so static::resolveClassAttribute(Appends::class, 'columns') ?? [] evaluates to []).

The current implementation:

public function mergeFillable(array $fillable)
{
    $this->fillable = array_values(array_unique(array_merge($this->fillable, $fillable)));
    return $this;
}

For empty $fillable, this still allocates three transient arrays per call (array_merge, array_unique, array_values) — and the same for mergeAppends, mergeHidden, mergeVisible. At high construction rates the cumulative peak-memory cost is significant.

Reproducible benchmark

laravel-eloquent-bench — pinned point-release skeletons, identical synthetic 120-fillable / 13-cast / 50-relation / 4-trait model, N=100,000 constructions on PHP 8.4.14:

Version	Peak memory	Δ vs v13.1.1
v13.1.1	148 MB	—
v13.2.0	148 MB	0
v13.3.0 (#59404 lands)	400 MB	+252 MB (+170%)
v13.6.0	400 MB	+252 MB

Memory recovers to the v13.1.1 baseline (148 MB) when the four merge* calls short-circuit on empty input. At scale on applications that hydrate many models per request, this delta is enough to exceed PHP's default memory_limit.

Fix

Add an early return when the input is empty. Four guards:

public function mergeFillable(array $fillable)
{
    if ($fillable === []) {
        return $this;
    }
    $this->fillable = array_values(array_unique(array_merge($this->fillable, $fillable)));
    return $this;
}

…and the same pattern for mergeAppends, mergeHidden, mergeVisible.

Behavior is preserved: array_values(array_unique(array_merge($x, []))) re-normalizes $x. Skipping that re-normalization when there's nothing to merge is a no-op for any developer-set $fillable / $appends / $hidden / $visible (which are conventionally already clean arrays). The trait-initializer-collision fix that motivated #59404 is preserved — it only matters when there's something to merge, which the guard explicitly skips.

Tests

Added four test_merge_*_with_empty_array_is_noop cases in tests/Database/DatabaseEloquentModelAttributesTest.php asserting:

• Calling each merge* method with [] returns $this.
• The underlying property is unchanged (same array reference).

Existing tests for the trait-initializer-collision fix from #59404 continue to pass.

References

• Regression introduced by: [13.x] Fix trait initializer collision with Attribute parsing #59404 (v13.3.0)
• Original bug correctly fixed by [13.x] Fix trait initializer collision with Attribute parsing #59404: [13.x] Model trait initializers collide with Attribute parsing #59381 (preserved)
• Benchmark: https://github.com/zenchef/laravel-eloquent-bench
• Sibling regressions in v13.x: Add symmetrical, expressive attributes #59284, [13.x] Allow Table Attribute on child to override parent #59701 (tracked separately)

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