From c9438c5e280eb3ae6045197a3dcd1637b6fa1cdd Mon Sep 17 00:00:00 2001
From: tranquillity-codes
 <120999089+tranquillity-codes@users.noreply.github.com>
Date: Wed, 18 Dec 2024 18:38:35 +0100
Subject: [PATCH] Make LineNumberRemapper stop trying to guess line numbers

Usually the guesses were wrong, so instead let's not generate a
LineNumberTable for lines that lack a mapping in the linemap. This makes
the behavior on decompiler bugs/issues more predictable.
---
 .../loom/decompilers/LineNumberRemapper.java  | 14 ++++-------
 .../test/unit/LineNumberRemapperTests.groovy  | 25 +++++++++++++++++++
 2 files changed, 30 insertions(+), 9 deletions(-)

diff --git a/src/main/java/net/fabricmc/loom/decompilers/LineNumberRemapper.java b/src/main/java/net/fabricmc/loom/decompilers/LineNumberRemapper.java
index 9b3ab78f7..705be2564 100644
--- a/src/main/java/net/fabricmc/loom/decompilers/LineNumberRemapper.java
+++ b/src/main/java/net/fabricmc/loom/decompilers/LineNumberRemapper.java
@@ -106,20 +106,16 @@ public MethodVisitor visitMethod(int access, String name, String descriptor, Str
 			return new MethodVisitor(api, super.visitMethod(access, name, descriptor, signature, exceptions)) {
 				@Override
 				public void visitLineNumber(int line, Label start) {
-					int tLine = line;
-
-					if (tLine <= 0) {
+					if (line <= 0) {
 						super.visitLineNumber(line, start);
-					} else if (tLine >= lineNumbers.maxLine()) {
+					} else if (line >= lineNumbers.maxLine()) {
 						super.visitLineNumber(lineNumbers.maxLineDest(), start);
 					} else {
-						Integer matchedLine = null;
+						Integer matchedLine = lineNumbers.lineMap().get(line);
 
-						while (tLine <= lineNumbers.maxLine() && ((matchedLine = lineNumbers.lineMap().get(tLine)) == null)) {
-							tLine++;
+						if(matchedLine != null) {
+							super.visitLineNumber(matchedLine, start);
 						}
-
-						super.visitLineNumber(matchedLine != null ? matchedLine : lineNumbers.maxLineDest(), start);
 					}
 				}
 			};
diff --git a/src/test/groovy/net/fabricmc/loom/test/unit/LineNumberRemapperTests.groovy b/src/test/groovy/net/fabricmc/loom/test/unit/LineNumberRemapperTests.groovy
index e07bfc705..c9ef451f2 100644
--- a/src/test/groovy/net/fabricmc/loom/test/unit/LineNumberRemapperTests.groovy
+++ b/src/test/groovy/net/fabricmc/loom/test/unit/LineNumberRemapperTests.groovy
@@ -65,6 +65,31 @@ class LineNumberRemapperTests extends Specification {
 		readLineNumbers(unpacked) == [37, 39, 40]
 	}
 
+	def "remapLinenumbersExclude"() {
+		given:
+		def className = LineNumberSource.class.name.replace('.', '/')
+		def input = ZipTestUtils.createZipFromBytes([(className + ".class"): getClassBytes(LineNumberSource.class)])
+
+		// + 10 to each line number
+		def entry = new ClassLineNumbers.Entry(className, 30, 40, [
+			27: 37,
+			30: 40
+		])
+		def lineNumbers = new ClassLineNumbers([(className): entry])
+
+		def outputJar = Files.createTempDirectory("loom").resolve("output.jar")
+
+		when:
+		def remapper = new LineNumberRemapper(lineNumbers)
+		remapper.process(input, outputJar)
+
+		def unpacked = ZipUtils.unpack(outputJar, className + ".class")
+
+		then:
+		readLineNumbers(getClassBytes(LineNumberSource.class)) == [27, 29, 30]
+		readLineNumbers(unpacked) == [37, 40]
+	}
+
 	static byte[] getClassBytes(Class<?> clazz) {
 		return clazz.classLoader.getResourceAsStream(clazz.name.replace('.', '/') + ".class").withCloseable {
 			it.bytes