Introduce a command-line option to limit the # of typo corrections.

Typo correction can be particularly expensive, so introduce a
command-line flag to limit the number of typo corrections we will
perform per type-checker instance. Default this limit to 10.

Addresses rdar://problem/28469270 to some extent.

Author: DougGregor

include/swift/Basic/LangOptions.h

@@ -79,8 +79,8 @@ namespace swift {
     /// \brief Disable API availability checking.
     bool DisableAvailabilityChecking = false;
 
-    /// \brief Disable typo correction.
-    bool DisableTypoCorrection = false;
+    /// \brief Maximum number of typo corrections we are allowed to perform.
+    unsigned TypoCorrectionLimit = 10;
 
     /// Should access control be respected?
     bool EnableAccessControl = true;

include/swift/Frontend/Frontend.h

@@ -271,7 +271,8 @@ class CompilerInvocation {
     CodeCompletionBuffer = Buf;
     CodeCompletionOffset = Offset;
     // We don't need typo-correction for code-completion.
-    LangOpts.DisableTypoCorrection = true;
+    // FIXME: This isn't really true, but is a performance issue.
+    LangOpts.TypoCorrectionLimit = 0;
   }
 
   std::pair<llvm::MemoryBuffer *, unsigned> getCodeCompletionPoint() const {

include/swift/Option/Options.td

@@ -295,6 +295,11 @@ def warn_swift3_objc_inference_minimal :
   Flags<[FrontendOption, DoesNotAffectIncrementalBuild]>,
   HelpText<"Warn about deprecated @objc inference in Swift 3 based on direct uses of the Objective-C entrypoint">;
 
+def typo_correction_limit : Separate<["-"], "typo-correction-limit">,
+  Flags<[FrontendOption, HelpHidden]>,
+  MetaVarName<"<n>">,
+  HelpText<"Limit the number of times the compiler will attempt typo correction to <n>">;
+
 def warn_swift3_objc_inference : Flag<["-"], "warn-swift3-objc-inference">,
   Alias<warn_swift3_objc_inference_complete>,
   Flags<[FrontendOption, DoesNotAffectIncrementalBuild, HelpHidden]>;

lib/Driver/ToolChains.cpp

@@ -129,6 +129,7 @@ static void addCommonFrontendArgs(const ToolChain &TC,
   inputArgs.AddLastArg(arguments,
                        options::OPT_warn_swift3_objc_inference_minimal,
                        options::OPT_warn_swift3_objc_inference_complete);
+  inputArgs.AddLastArg(arguments, options::OPT_typo_correction_limit);
   inputArgs.AddLastArg(arguments, options::OPT_enable_app_extension);
   inputArgs.AddLastArg(arguments, options::OPT_enable_testing);
   inputArgs.AddLastArg(arguments, options::OPT_g_Group);

lib/Frontend/CompilerInvocation.cpp

@@ -981,7 +981,21 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
       = A->getOption().matches(OPT_enable_access_control);
   }
 
-  Opts.DisableTypoCorrection |= Args.hasArg(OPT_disable_typo_correction);
+  if (auto A = Args.getLastArg(OPT_disable_typo_correction,
+                               OPT_typo_correction_limit)) {
+    if (A->getOption().matches(OPT_disable_typo_correction))
+      Opts.TypoCorrectionLimit = 0;
+    else {
+      unsigned limit;
+      if (StringRef(A->getValue()).getAsInteger(10, limit)) {
+        Diags.diagnose(SourceLoc(), diag::error_invalid_arg_value,
+                       A->getAsString(Args), A->getValue());
+        return true;
+      }
+
+      Opts.TypoCorrectionLimit = limit;
+    }
+  }
 
   Opts.CodeCompleteInitsInPostfixExpr |=
       Args.hasArg(OPT_code_complete_inits_in_postfix_expr);

lib/Sema/TypeCheckNameLookup.cpp

@@ -564,12 +564,15 @@ void TypeChecker::performTypoCorrection(DeclContext *DC, DeclRefKind refKind,
                                         LookupResult &result,
                                         GenericSignatureBuilder *gsb,
                                         unsigned maxResults) {
-  // Disable typo-correction if we won't show the diagnostic anyway.
-  if (getLangOpts().DisableTypoCorrection ||
+  // Disable typo-correction if we won't show the diagnostic anyway or if
+  // we've hit our typo correction limit.
+  if (NumTypoCorrections >= getLangOpts().TypoCorrectionLimit ||
       (Diags.hasFatalErrorOccurred() &&
        !Diags.getShowDiagnosticsAfterFatalError()))
     return;
 
+  ++NumTypoCorrections;
+
   // Fill in a collection of the most reasonable entries.
   TopCollection<unsigned, ValueDecl*> entries(maxResults);
   auto consumer = makeDeclConsumer([&](ValueDecl *decl,

lib/Sema/TypeChecker.h

@@ -747,6 +747,9 @@ class TypeChecker final : public LazyResolver {
   llvm::DenseMap<AbstractFunctionDecl *, llvm::DenseSet<ApplyExpr *>>
     FunctionAsEscapingArg;
 
+  /// The # of times we have performed typo correction.
+  unsigned NumTypoCorrections = 0;
+
 public:
   /// Record an occurrence of a function that captures inout values as an
   /// argument.

test/Sema/typo_correction.swift

@@ -1,5 +1,6 @@
-// RUN: %target-typecheck-verify-swift
+// RUN: %target-typecheck-verify-swift -typo-correction-limit 20
 // RUN: not %target-swift-frontend -typecheck -disable-typo-correction %s 2>&1 | %FileCheck %s -check-prefix=DISABLED
+// RUN: not %target-swift-frontend -typecheck -typo-correction-limit 0 %s 2>&1 | %FileCheck %s -check-prefix=DISABLED
 // RUN: not %target-swift-frontend -typecheck -DIMPORT_FAIL %s 2>&1 | %FileCheck %s -check-prefix=DISABLED
 // DISABLED-NOT: did you mean
 

test/Sema/typo_correction_limit.swift

@@ -0,0 +1,12 @@
+// RUN: %target-typecheck-verify-swift -typo-correction-limit 5
+
+// This is close enough to get typo-correction.
+func test_short_and_close() {
+  let foo = 4 // expected-note 5 {{did you mean 'foo'?}}
+  let _ = fob + 1 // expected-error {{use of unresolved identifier}}
+  let _ = fob + 1 // expected-error {{use of unresolved identifier}}
+  let _ = fob + 1 // expected-error {{use of unresolved identifier}}
+  let _ = fob + 1 // expected-error {{use of unresolved identifier}}
+  let _ = fob + 1 // expected-error {{use of unresolved identifier}}
+  let _ = fob + 1 // expected-error {{use of unresolved identifier}}
+}

tools/swift-ide-test/swift-ide-test.cpp

@@ -2762,7 +2762,7 @@ static int doPrintIndexedSymbols(const CompilerInvocation &InitInvok,
   CompilerInvocation Invocation(InitInvok);
   Invocation.addInputFilename(SourceFileName);
   Invocation.getLangOptions().DisableAvailabilityChecking = false;
-  Invocation.getLangOptions().DisableTypoCorrection = true;
+  Invocation.getLangOptions().TypoCorrectionLimit = 0;
 
   CompilerInstance CI;