From 90b9c6e8da8bafe4390e3d22877d3ce866c35706 Mon Sep 17 00:00:00 2001
From: Marc Sallin <marc.sallin@outlook.com>
Date: Sun, 6 Sep 2026 21:51:47 +0200
Subject: [PATCH] perf: stop allocating a warnings array for a clean compile

Every document allocated a string array even when the compiler reported no
warnings, which is the common case. Array.Empty covers it without allocating.
Array.AsReadOnly already returns the shared empty collection for a zero-length
array, so the wrapper it is passed to costs nothing either way.
---
 src/typstsharp.tests/Tests.cs   | 20 ++++++++++++++++++++
 src/typstsharp/TypstDocument.cs |  3 ++-
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/src/typstsharp.tests/Tests.cs b/src/typstsharp.tests/Tests.cs
index 4a5b967..1bf8aa6 100644
--- a/src/typstsharp.tests/Tests.cs
+++ b/src/typstsharp.tests/Tests.cs
@@ -910,6 +910,26 @@ public async Task WarningsFromADocumentCannotBeMutatedByCallers()
         await Assert.That(document.Warnings is string[]).IsFalse();
     }
 
+    /// <summary>
+    /// The warning-free case had no coverage. This pins the public contract rather than the
+    /// allocation, so it holds whichever way the empty list is produced: empty, and not a mutable
+    /// array handed out to callers.
+    /// </summary>
+    [Test]
+    public async Task WarningsFromACleanCompileAreEmptyAndStillImmutable()
+    {
+        using var compiler = TypstCompiler.FromSource("= No warnings here");
+        var document = compiler.CompileToDocument();
+
+        await Assert.That(document.Warnings.Count).IsEqualTo(0);
+        await Assert.That(document.Warnings is string[]).IsFalse();
+
+        document.Dispose();
+
+        // Warnings are copied out of native memory eagerly, so they outlive the document.
+        await Assert.That(document.Warnings.Count).IsEqualTo(0);
+    }
+
     private const string TwoPageSource = """
                                          First page
                                          #pagebreak()
diff --git a/src/typstsharp/TypstDocument.cs b/src/typstsharp/TypstDocument.cs
index 5813764..73326ce 100644
--- a/src/typstsharp/TypstDocument.cs
+++ b/src/typstsharp/TypstDocument.cs
@@ -53,7 +53,8 @@ internal unsafe TypstDocument(CsBindgen.CompileResult native)
         }
 
         // Warnings are small and are copied eagerly so that they stay usable after disposal.
-        var warnings = new string[warningCount];
+        // A clean compile is the common case, and Array.Empty spares it the only allocation here.
+        var warnings = warningCount == 0 ? Array.Empty<string>() : new string[warningCount];
         for (int i = 0; i < warnings.Length; i++)
         {
             var warning = native.warnings[i];