From b7a2034bcfbc33e5621a83c6ecf9784d1e112e09 Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Fri, 9 Oct 2026 02:27:22 +0000
Subject: [PATCH] fix: double the braces in a Rust compile-time assertion
 message [patch]

assert! reads its message as a format string, so a message such as
"size {N} fits" was parsed as a placeholder naming N and the output
failed with E0425, and "brace {} here" failed with a missing argument.
The Rust generator now doubles { and } after escaping the message, so
the text reaches the compiler's error unchanged.

Fixes #166

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01X8BPy6trRZiUxiwJcbUGzR
---
 .../Languages/RustGeneratedSourceCompilesTests.cs    |  5 +++++
 Coder.Test/Languages/RustGeneratorTests.cs           | 12 ++++++++++++
 Coder/Languages/RustGenerator.cs                     |  9 ++++++++-
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/Coder.Test/Languages/RustGeneratedSourceCompilesTests.cs b/Coder.Test/Languages/RustGeneratedSourceCompilesTests.cs
index ddc0fef..f9ee681 100644
--- a/Coder.Test/Languages/RustGeneratedSourceCompilesTests.cs
+++ b/Coder.Test/Languages/RustGeneratedSourceCompilesTests.cs
@@ -303,6 +303,11 @@ private static SourceFile Exemplar()
 			Condition = "std::mem::size_of::<i32>() == 4",
 			Message = "an i32 is four bytes",
 		});
+		geometry.Members.Add(new CompileTimeAssertion
+		{
+			Condition = "std::mem::size_of::<u8>() == 1",
+			Message = "size {N} fits, brace {} here",
+		});
 
 		file.Members.Add(geometry);
 		return file;
diff --git a/Coder.Test/Languages/RustGeneratorTests.cs b/Coder.Test/Languages/RustGeneratorTests.cs
index c7d2dc6..2acf980 100644
--- a/Coder.Test/Languages/RustGeneratorTests.cs
+++ b/Coder.Test/Languages/RustGeneratorTests.cs
@@ -692,6 +692,18 @@ public void CompileTimeAssertion_IsAnEvaluatedConstant()
 		StringAssert.Contains(code, "\"a Point is two i32s\");", StringComparison.Ordinal);
 	}
 
+	/// <summary>
+	/// Tests that braces in an assertion's message are doubled, because <c>assert!</c> reads its
+	/// message as a format string and a single brace would open a placeholder.
+	/// </summary>
+	[TestMethod]
+	public void CompileTimeAssertion_DoublesBracesInTheMessage()
+	{
+		CompileTimeAssertion assertion = new("true", "size {N} fits, brace {} here");
+
+		StringAssert.Contains(Generator.Generate(assertion), "\"size {{N}} fits, brace {{}} here\");", StringComparison.Ordinal);
+	}
+
 	/// <summary>
 	/// Tests that an alias is a type alias.
 	/// </summary>
diff --git a/Coder/Languages/RustGenerator.cs b/Coder/Languages/RustGenerator.cs
index b1fb54d..131cf7a 100644
--- a/Coder/Languages/RustGenerator.cs
+++ b/Coder/Languages/RustGenerator.cs
@@ -1383,6 +1383,10 @@ protected override void GenerateUsingAlias(UsingAlias usingAlias, CodeBlocker co
 	/// <c>const _: () = assert!(…)</c>, which is Rust's own compile-time assertion and needs no
 	/// macro crate: a constant nobody names still has to be evaluated for the program to build, and
 	/// a failed <c>assert!</c> in that position is a compile error naming the message.
+	/// <para>
+	/// The message is <c>assert!</c>'s format string rather than a plain literal, so its braces are
+	/// doubled: a single <c>{N}</c> would be read as a placeholder naming a value that does not exist.
+	/// </para>
 	/// </remarks>
 	protected override void GenerateCompileTimeAssertion(CompileTimeAssertion assertion, CodeBlocker code)
 	{
@@ -1397,7 +1401,10 @@ protected override void GenerateCompileTimeAssertion(CompileTimeAssertion assert
 		{
 			code.WriteLine(",");
 			code.Indent();
-			code.Write($"\"{EscapeString(assertion.Message)}\"");
+			string message = EscapeString(assertion.Message)
+				.Replace("{", "{{", StringComparison.Ordinal)
+				.Replace("}", "}}", StringComparison.Ordinal);
+			code.Write($"\"{message}\"");
 			code.Outdent();
 		}