Initial implementation of optionals-in-string-interpolation warnings

Basic extension of the optional-to-any AST walker to incorporate
warnings for the as-of-now up and coming Swift evolution proposal.

Author: CodaFi

include/swift/AST/DiagnosticsSema.def

@@ -2405,12 +2405,16 @@ WARNING(optional_pattern_match_promotion,none,
         (Type, Type))
 WARNING(optional_to_any_coercion,none,
         "expression implicitly coerced from %0 to Any", (Type))
+WARNING(optional_in_string_interpolation_segment,none,
+        "string interpolation of optional can have unintended effects", ())
 NOTE(default_optional_to_any,none,
      "provide a default value to avoid this warning", ())
 NOTE(force_optional_to_any,none,
      "force-unwrap the value to avoid this warning", ())
 NOTE(silence_optional_to_any,none,
      "explicitly cast to Any with 'as Any' to silence this warning", ())
+NOTE(silence_optional_in_interpolation_segment,none,
+      "explicitly use '.debugDescription' to silence this warning", ())
 
 ERROR(invalid_noescape_use,none,
       "non-escaping %select{value|parameter}1 %0 may only be called",

lib/Sema/MiscDiagnostics.cpp

@@ -3707,8 +3707,33 @@ static void diagnoseOptionalToAnyCoercion(TypeChecker &TC, const Expr *E,
               .highlight(subExpr->getSourceRange())
               .fixItInsertAfter(subExpr->getEndLoc(), " as Any");
         }
-      }
+      } else if (auto *literal = dyn_cast<InterpolatedStringLiteralExpr>(E)) {
+        // Warn about interpolated segments that contain optionals.
+        for (auto &segment : literal->getSegments()) {
+          // Allow explicit casts.
+          if (isa<ExplicitCastExpr>(segment)) {
+            continue;
+          }
 
+          auto segmentTy = segment->getType();
+          if (segmentTy && !segmentTy->getOptionalObjectType().isNull()) {
+            TC.diagnose(segment->getStartLoc(),
+                        diag::optional_in_string_interpolation_segment)
+                .highlight(segment->getSourceRange());
+
+            TC.diagnose(segment->getLoc(),
+                        diag::silence_optional_in_interpolation_segment)
+              .highlight(segment->getSourceRange())
+              .fixItInsertAfter(segment->getEndLoc(), ".debugDescription");
+            TC.diagnose(segment->getLoc(), diag::default_optional_to_any)
+              .highlight(segment->getSourceRange())
+            	.fixItInsertAfter(segment->getEndLoc(), " ?? <#default value#>");
+            TC.diagnose(segment->getLoc(), diag::force_optional_to_any)
+              .highlight(segment->getSourceRange())
+              .fixItInsertAfter(segment->getEndLoc(), "!");
+          }
+        }
+      }
       return { true, E };
     }