Improves error emission of switch blocks, adds new pragma

Content.Tests/DMProject/Tests/Statements/Switch/weird_when_clause.dm

@@ -0,0 +1,15 @@
+
+//Issue OD#996, kinda: https://github.com/OpenDreamProject/OpenDream/issues/996
+
+/proc/RunTest()
+	var/x = 5
+	switch(x)
+		if(1)
+			CRASH("Strange branch chosen in switch statement")
+		if(4)
+			CRASH("Strange branch chosen in switch statement")
+		else if(x == 3)
+			CRASH("Parser failed to understand 'else if' in switch block")
+		else
+			return
+	CRASH("Parser failed to understand 'else if' in switch block")

Content.Tests/DMProject/Tests/Statements/Switch/weird_when_clause_pragma.dm

@@ -0,0 +1,16 @@
+//COMPILE ERROR
+//test to make sure SuspiciousSwitchCase is working
+
+#pragma SuspiciousSwitchCase error
+
+/proc/RunTest()
+	var/x = 5
+	switch(x)
+		if(1)
+			return
+		if(4)
+			return
+		else if(x == 3)
+			return
+		else
+			return

DMCompiler/Compiler/DM/DMParser.cs

@@ -1290,7 +1290,7 @@ DMASTProcBlockInner GetForBody() {
 
                 DMASTProcStatementSwitch.SwitchCase[]? switchCases = SwitchCases();
 
-                if (switchCases == null) Error("Expected switch cases");
+                if (switchCases == null) Error(WarningCode.BadExpression, "Expected switch cases");
                 return new DMASTProcStatementSwitch(loc, value, switchCases);
             }
 
@@ -1341,7 +1341,7 @@ public DMASTProcStatementSwitch.SwitchCase[] SwitchInner() {
             List<DMASTProcStatementSwitch.SwitchCase> switchCases = new();
             DMASTProcStatementSwitch.SwitchCase? switchCase = SwitchCase();
 
-            while (switchCase != null) {
+            while(switchCase is not null) {
                 switchCases.Add(switchCase);
                 Newline();
                 Whitespace();
@@ -1364,7 +1364,7 @@ public DMASTProcStatementSwitch.SwitchCase[] SwitchInner() {
                     DMASTExpression? expression = Expression();
                     if (expression == null) {
                         if (expressions.Count == 0) {
-                            Error("Expected an expression");
+                            Error(WarningCode.BadExpression, "Expected an expression");
                         } else {
                             //Eat a trailing comma if there's at least 1 expression
                             break;
@@ -1375,7 +1375,7 @@ public DMASTProcStatementSwitch.SwitchCase[] SwitchInner() {
                         var loc = Current().Location;
                         Whitespace();
                         DMASTExpression? rangeEnd = Expression();
-                        if (rangeEnd == null) Error("Expected an upper limit");
+                        if (rangeEnd == null) Error(WarningCode.BadExpression, "Expected an upper limit");
 
                         expressions.Add(new DMASTSwitchCaseRange(loc, expression, rangeEnd));
                     } else {
@@ -1405,7 +1405,9 @@ public DMASTProcStatementSwitch.SwitchCase[] SwitchInner() {
                 var loc = Current().Location;
                 Whitespace();
                 if (Current().Type == TokenType.DM_If) {
-                    Error("Expected \"if\" or \"else\", \"else if\" is not permitted as a switch case");
+                    //From now on, all ifs/elseifs/elses are actually part of this if's chain, not the switch's.
+                    //Ambiguous, but that is parity behaviour. Ergo, the following emission.
+                    Error(WarningCode.SuspiciousSwitchCase, "Expected \"if\" or \"else\" - \"else if\" is ambiguous as a switch case and may cause unintended flow");
                 }
                 DMASTProcBlockInner? body = ProcBlock();
 

DMCompiler/Compiler/DM/DMParserHelper.cs

@@ -23,6 +23,12 @@ protected bool Error(WarningCode code, string message) {
             return level == ErrorLevel.Error;
         }
 
+        /// <inheritdoc cref="Parser{SourceType}.Error(string, bool)"/>
+        [Obsolete("This is not a desirable way for DMParser to emit an error, as errors should emit an error code and not cause unnecessary throws. Use DMParser's overrides of this method, instead.")]
+        new protected void Error(string message, bool throwException = true) {
+            base.Error(message, throwException);
+        }
+
         protected bool PeekDelimiter() {
             return Current().Type == TokenType.Newline || Current().Type == TokenType.DM_Semicolon;
         }

DMCompiler/DMStandard/DefaultPragmaConfig.dm

@@ -30,3 +30,4 @@
 //3000-3999
 #pragma EmptyBlock notice
 #pragma EmptyProc disabled // NOTE: If you enable this in OD's default pragma config file, it will emit for OD's DMStandard. Put it in your codebase's pragma config file.
+#pragma SuspiciousSwitchCase warning

OpenDreamShared/Compiler/CompilerError.cs

@@ -54,7 +54,8 @@ public enum WarningCode {
 
         // 3000 - 3999 are reserved for stylistic configuration.
         EmptyBlock = 3100,
-        EmptyProc = 3101
+        EmptyProc = 3101,
+        SuspiciousSwitchCase = 3201, // "else if" cases are actually valid DM, they just spontaneously end the switch context and begin an if-else ladder within the else case of the switch
 
         // 4000 - 4999 are reserved for runtime configuration. (TODO: Runtime doesn't know about configs yet!)
     }