Silence sun.misc.unsafe warnings with separate compilation

Signed-off-by: Andrew Helwer <2n8rn1w1f@mozmail.com>

tlatools/org.lamport.tlatools/customBuild.xml

@@ -146,14 +146,50 @@
 ==================================================================
 = This code uses sun.misc.Unsafe for manual memory management in =
 = the class src/tlc2/tool/fp/LongArray.java. This unsafe API is  =
-= marked as internal meaning it can change without warning. Java =
-= emits a lot of warnings about this and makes it essentially    =
-= impossible to silence them, so just ignore them. The unsafe    =
-= API is in the process of being standardized in newer versions  =
-= of Java and we will switch to that as soon as possible.        =
+= marked as internal meaning it can change without warning.      =
+= Ordinarily this would result in a lot of compiler warnings,    =
+= but to reduce noise these have been explicitly silenced. If    =
+= the build fails after upgrading Java, it might be because this =
+= API has been changed.                                          =
 ==================================================================
 		</echo>
 		<mkdir dir="${class.dir}" />
+		<!--
+			Separate compilation step for files which must have their warnings
+			uniformly ignored. Java 11 removed the -XDignore.symbol.file compiler
+			parameter that worked in Java 8, so this is the new method. For some
+			reason Java *really, really* does not want you to silence the warnings
+			around sun.misc.Unsafe; every method that works when suppressing every
+			other warning does not work here (SuppressWarning annotations,
+			-Xlint:none compiler arg, even -nowarn). So, we are reduced to
+			compiling the files we want to ignore with an actual command line call,
+			then only printing the stderr output if the compilation fails.
+		-->
+		<exec executable="javac" errorproperty="PreCompileErrOutput" resultproperty="PreCompileResult" failonerror="false">
+			<arg line="--release ${java.release} -encoding UTF-8 -g:none -nowarn -Xlint:none"/>
+			<arg value="-sourcepath" />
+			<arg path="${src.dir}" />
+			<arg value="-d" />
+			<arg path="${class.dir}" />
+			<arg value="-classpath" />
+			<arg path="lib/javax.mail/mailapi-1.6.3.jar:lib/gson/gson-2.8.6.jar:lib/lsp/org.eclipse.lsp4j.jsonrpc_0.21.1.v20230829-0012.jar:lib/lsp/org.eclipse.lsp4j.debug_0.21.1.v20230829-0012.jar" />
+			<!--
+				Suppress sun.misc.Unsafe warnings. These warnings cannot be suppressed
+				using in-file annotations like ordinary warnings, so we must completely
+				ignore all warnings by compiling the file separately here.
+			-->
+			<arg file="${src.dir}/tlc2/tool/fp/LongArray.java" />
+			<!--
+				This file is generated by JavaCC and generates warnings, so we must
+				compile it separately here to silence them.
+			-->
+			<arg file="${src.dir}/tla2sany/parser/SimpleCharStream.java" />
+		</exec>
+		<fail message="${PreCompileErrOutput}">
+			<condition>
+				<isfailure code="${PreCompileResult}" />
+			</condition>
+		</fail>
 		<javac
 			includeantruntime="false"
 			srcdir="${src.dir}"
@@ -163,6 +199,11 @@
 			release="${java.release}"
 			encoding="UTF-8"
 		>
+			<!-- Exclude the files compiled separately up above to silence warnings. -->
+			<exclude name="tlc2/tool/fp/LongArray.java" />
+			<exclude name="tla2sany/parser/SimpleCharStream.java" />
+			<!-- Treat all warnings as errors. -->
+			<compilerarg value="-Werror" />
 			<classpath refid="project.classpath" />
 			<classpath>
 				<pathelement location="lib/javax.mail/mailapi-1.6.3.jar" />