active conditional compilations: show friendly error message

Author: Nonchalant

include/swift/AST/DiagnosticsFrontend.def

@@ -209,6 +209,10 @@ ERROR(symbol_in_ir_not_in_tbd,none,
       "symbol '%0' (%1) is in generated IR file, but not in TBD file",
       (StringRef, StringRef))
 
+ERROR(redundant_prefix_compilation_flag,none,
+      "did you provide a redundant '-D' in your build settings? (got '-D%0')", 
+      (StringRef))    
+
 ERROR(invalid_conditional_compilation_flag,none,
       "conditional compilation flags must be valid Swift identifiers (rather than '%0')",
       (StringRef))

lib/Driver/Driver.cpp

@@ -192,6 +192,9 @@ static void validateArgs(DiagnosticEngine &diags, const ArgList &Args) {
       diags.diagnose(SourceLoc(),
                      diag::cannot_assign_value_to_conditional_compilation_flag,
                      name);
+    else if (name.startswith("-D"))
+      diags.diagnose(SourceLoc(), diag::redundant_prefix_compilation_flag,
+                     name);
     else if (!Lexer::isIdentifier(name))
       diags.diagnose(SourceLoc(), diag::invalid_conditional_compilation_flag,
                      name);