Improve diagnostic in cases like 'if let foo.bar'

Author: calda

include/swift/AST/DiagnosticsParse.def

@@ -1075,6 +1075,8 @@ ERROR(conditional_var_initializer_required,none,
 ERROR(wrong_condition_case_location,none,
       "pattern matching binding is spelled with 'case %0', not '%0 case'",
       (StringRef))
+ERROR(conditional_var_valid_identifiers_only,none,
+      "unwrap condition requires a valid identifier", ())
 
 // If Statement
 ERROR(expected_condition_if,PointsToFirstBadToken,

lib/Parse/ParseStmt.cpp

@@ -1635,6 +1635,13 @@ Parser::parseStmtConditionElement(SmallVectorImpl<StmtConditionElement> &result,
 
     declRefExpr->setImplicit();
     Init = makeParserResult(declRefExpr);
+  } else if (!ThePattern.isNull()) {
+    // If the pattern is present but isn't an identifier, the user wrote
+    // something invalid like `let foo.bar`. Emit a special diagnostic for this,
+    // with a fix-it prepending "<#identifier#> = "
+    auto diagLoc = ThePattern.get()->getSemanticsProvidingPattern()->getStartLoc();
+    diagnose(diagLoc, diag::conditional_var_valid_identifiers_only)
+      .fixItInsert(diagLoc, "<#identifier#> = ");
   } else {
     diagnose(Tok, diag::conditional_var_initializer_required);
   }

test/stmt/if_while_var.swift

@@ -42,8 +42,8 @@ if var nonOptional { nonOptional = nonOptionalStruct(); _ = nonOptional } // exp
 guard let nonOptional else { _ = nonOptional; fatalError() } // expected-error{{initializer for conditional binding must have Optional type, not 'NonOptionalStruct'}}
 guard var nonOptional else { _ = nonOptional; fatalError() } // expected-error{{initializer for conditional binding must have Optional type, not 'NonOptionalStruct'}}
 
-if let nonOptional.property { } // expected-error{{variable binding in a condition requires an initializer}} expected-error{{pattern matching in a condition requires the 'case' keyword}}
-if var nonOptional.property { } // expected-error{{variable binding in a condition requires an initializer}} expected-error{{pattern matching in a condition requires the 'case' keyword}}
+if let nonOptional.property { } // expected-error{{unwrap condition requires a valid identifier}} expected-error{{pattern matching in a condition requires the 'case' keyword}}
+if var nonOptional.property { } // expected-error{{unwrap condition requires a valid identifier}} expected-error{{pattern matching in a condition requires the 'case' keyword}}
 
 guard let _ = nonOptionalStruct() else { fatalError() } // expected-error{{initializer for conditional binding must have Optional type, not 'NonOptionalStruct'}}
 guard let _ = nonOptionalEnum() else { fatalError() } // expected-error{{initializer for conditional binding must have Optional type, not 'NonOptionalEnum'}}