Replace NEXT_LOCATION with NO_LOCATION if there is no meaningful location to use. Assert NEXT_LOCATION never gets emitted

Author: markshannon

Python/assemble.c

@@ -292,6 +292,7 @@ write_location_info_entry(struct assembler* a, location loc, int isize)
         RETURN_IF_ERROR(_PyBytes_Resize(&a->a_linetable, len*2));
     }
     if (loc.lineno < 0) {
+        assert(loc.lineno == NO_LOCATION.lineno);
         write_location_info_none(a, isize);
         return SUCCESS;
     }
@@ -342,19 +343,19 @@ assemble_location_info(struct assembler *a, instr_sequence *instrs,
 {
     a->a_lineno = firstlineno;
     location loc = NO_LOCATION;
-    int size = 0;
-    // The last location should not be NEXT_LOCATION, but don't crash non-debug builds
-    if (same_location(instrs->s_instrs[instrs->s_used-1].i_loc, NEXT_LOCATION)) {
-        assert(0 && "last instruction has NEXT_LOCATION");
-        instrs->s_instrs[instrs->s_used-1].i_loc = NO_LOCATION;
-    }
-    for (int i = instrs->s_used-1; i > 0; i--) {
+    for (int i = instrs->s_used-1; i >= 0; i--) {
         instruction *instr = &instrs->s_instrs[i];
-        if (same_location(instr[-1].i_loc, NEXT_LOCATION)) {
-            assert(!IS_TERMINATOR_OPCODE(instr[-1].i_opcode));
-            instr[-1].i_loc = instr->i_loc;
+        if (same_location(instr->i_loc, NEXT_LOCATION)) {
+            if (IS_TERMINATOR_OPCODE(instr->i_opcode)) {
+                instr->i_loc = NO_LOCATION;
+            }
+            else {
+                assert(i < instrs->s_used-1);
+                instr->i_loc = instr[1].i_loc;
+            }
         }
     }
+    int size = 0;
     for (int i = 0; i < instrs->s_used; i++) {
         instruction *instr = &instrs->s_instrs[i];
         if (!same_location(loc, instr->i_loc)) {