Loom 0.2.7 and higher support.

build.gradle

@@ -7,7 +7,7 @@ buildscript {
 		}
 	}
 	dependencies {
-		classpath "net.fabricmc:fabric-loom:0.2.6-SNAPSHOT"
+		classpath "net.fabricmc:fabric-loom:0.4.3"
 	}
 }
 

gradle.properties

@@ -3,9 +3,9 @@ org.gradle.jvmargs=-Xmx1G
 
 minecraft_version=1.15.2
 yarn_mappings=1.15.2+build.14
-loader_version=0.7.8+build.189
+loader_version=0.8.2+build.194
 
-fabric_version=0.5.1+build.294-1.15
+fabric_version=0.10.1+build.307-1.15
 fabric_asm_version=v2.0
 zt_zip_version=1.14
 slf4j_version=1.7.30

src/main/java/me/modmuss50/optifabric/mod/OptifineSetup.java

@@ -235,13 +235,28 @@ Path getMinecraftJar() throws FileNotFoundException {
 			Path path = entrypointResult.get().getParent();
 			Path minecraftJar = path.resolve(String.format("minecraft-%s-client.jar", OptifineVersion.minecraftVersion)); //Lets hope you are using loom in dev
 			if (!Files.exists(minecraftJar)) {
-				throw new FileNotFoundException("Could not find minecraft jar!");
+				return getNewMinecraftDevJar();
 			}
 			return minecraftJar;
 		}
 		return entrypointResult.get();
 	}
 
+	//Loom 0.2.7 fallback
+	Path getNewMinecraftDevJar() throws FileNotFoundException {
+		Optional<Path> entrypointResult = getSource(Knot.class.getClassLoader(), "mappings/mappings.tiny");
+
+		if (entrypointResult.isPresent()) {
+			Path path = entrypointResult.get().getParent();
+			Path minecraftJar = path.resolve(String.format("minecraft-%s-client.jar", OptifineVersion.minecraftVersion)); //Lets hope you are using loom in dev
+			if (Files.exists(minecraftJar)) {
+				return minecraftJar;
+			}
+		}
+
+		throw new FileNotFoundException("Could not find minecraft jar!");
+	}
+
 	//Stolen from fabric loader
 	static Optional<Path> findFirstClass(ClassLoader loader, List<String> classNames) {
 		List<String> entrypointFilenames = classNames.stream().map((ep) -> ep.replace('.', '/') + ".class").collect(Collectors.toList());