Do not allow defining labels outside sections (#2013)

Creating a `Symbol` with `type = SYM_LABEL` but `section = nullptr`
is inconsistent and dangerous. I was not able to cause any buggy
behavior so far, but it's safer and reasonable to not create such
a symbol in the first place.

The main consequence is that `DEF(LabelOutsideSection)` will now
evaluate as 0.

src/asm/rpn.cpp

@@ -227,9 +227,9 @@ static int32_t tryConstMask(Expression const &lhs, Expression const &rhs) {
 		return -1;
 	}
 
-	assume(sym.isNumeric());
+	assume(sym.type == SYM_LABEL);
+	// We can now safely use `expr.value()` and `sym.getSection()`
 
-	// We can now safely use `expr.value()`
 	int32_t mask = expr.value();
 
 	// The mask must not cover any unknown bits

src/asm/symbol.cpp

@@ -537,6 +537,12 @@ Symbol *sym_AddVar(InternedStr symName, int32_t value) {
 static Symbol *addLabel(InternedStr symName) {
 	assumeAlreadyExpanded(symName);
 
+	Section *section = sect_GetSymbolSection();
+	if (!section) {
+		error("Cannot define label `%s` outside of a `SECTION`", symName.c_str());
+		return nullptr;
+	}
+
 	Symbol *sym = sym_FindExactSymbol(symName);
 
 	if (!sym) {
@@ -547,6 +553,7 @@ static Symbol *addLabel(InternedStr symName) {
 	} else {
 		updateSymbolFilename(*sym);
 	}
+
 	// If the symbol already exists as a ref, just "take over" it
 	sym->type = SYM_LABEL;
 	sym->data = static_cast<int32_t>(sect_GetSymbolOffset());
@@ -554,11 +561,7 @@ static Symbol *addLabel(InternedStr symName) {
 	if (options.exportAll && !symName.str().starts_with('!')) {
 		sym->isExported = true;
 	}
-	sym->section = sect_GetSymbolSection();
+	sym->section = section;
-
-	if (sym && !sym->section) {
-		error("Label `%s` created outside of a `SECTION`", symName.c_str());
-	}
 
 	return sym;
 }

test/asm/anon-label-bad.err

@@ -1,4 +1,4 @@
-error: Label `!0` created outside of a `SECTION`
+error: Cannot define label `!0` outside of a `SECTION`
     at anon-label-bad.asm(2)
 error: Reference to anonymous label 2 before, when only 1 has been created so far
     at anon-label-bad.asm(6)

test/asm/label-outside-section.err

@@ -1,3 +1,3 @@
-error: Label `bad` created outside of a `SECTION`
+error: Cannot define label `bad` outside of a `SECTION`
     at label-outside-section.asm(1)
 Assembly aborted with 1 error

test/asm/line-continuation-whitespace.err

@@ -1,3 +1,3 @@
-error: Label `foo` created outside of a `SECTION`
+error: Cannot define label `foo` outside of a `SECTION`
     at line-continuation-whitespace.asm(7)
 Assembly aborted with 1 error

test/asm/line-continuation.err

@@ -2,6 +2,6 @@ error: Invalid character 's' after line continuation
     at line-continuation.asm(3)
 warning: spam [-Wuser]
     at line-continuation.asm::spam(4) <- line-continuation.asm(6)
-error: Label `foo` created outside of a `SECTION`
+error: Cannot define label `foo` outside of a `SECTION`
     at line-continuation.asm(14)
 Assembly aborted with 2 errors

test/asm/macro-@.err

@@ -1,4 +1,4 @@
-error: Label `foo` created outside of a `SECTION`
+error: Cannot define label `foo` outside of a `SECTION`
     at [email protected](1)
 error: `@` is not a macro
     at [email protected](1)

test/asm/macro-syntax.err

@@ -1,4 +1,4 @@
-error: Label `old` created outside of a `SECTION`
+error: Cannot define label `old` outside of a `SECTION`
     at macro-syntax.asm(7)
 error: syntax error, unexpected MACRO
     at macro-syntax.asm(7)
@@ -6,6 +6,6 @@ error: `\1` cannot be used outside of a macro
     at macro-syntax.asm(8)
 error: syntax error, unexpected ENDM
     at macro-syntax.asm(9)
-error: `old` is not a macro
+error: Undefined macro `old`
     at macro-syntax.asm(11)
 Assembly aborted with 5 errors

test/asm/pops-restore-no-section.err

@@ -1,4 +1,4 @@
-error: Label `DisallowedContent` created outside of a `SECTION`
+error: Cannot define label `DisallowedContent` outside of a `SECTION`
     at pops-restore-no-section.asm(9)
 error: Cannot output data outside of a `SECTION`
     at pops-restore-no-section.asm(10)

test/asm/use-label-outside-section.asm

@@ -1,2 +1,3 @@
 lab:
-	PRINTLN lab-lab
+	PRINTLN DEF(lab)
+	PRINTLN lab - lab

test/asm/use-label-outside-section.err

@@ -1,5 +1,5 @@
-error: Label `lab` created outside of a `SECTION`
+error: Cannot define label `lab` outside of a `SECTION`
     at use-label-outside-section.asm(1)
-error: Expected constant expression: `lab` is not constant at assembly time
+error: Expected constant expression: undefined symbol `lab`
-    at use-label-outside-section.asm(2)
+    at use-label-outside-section.asm(3)
 Assembly aborted with 2 errors

test/asm/use-label-outside-section.out

@@ -1 +1,2 @@
 $0
+$0