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[Diagnostics] Add fix-it to @main struct without main static function. #82989

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Jul 31, 2025
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[Diagnostics] Add fix-it to @main struct without main static function. #82989

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8 changes: 8 additions & 0 deletions include/swift/AST/DiagnosticsSema.def
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Original file line number Diff line number Diff line change
Expand Up @@ -4264,6 +4264,14 @@ ERROR(attr_MainType_without_main,none,
"%0 is annotated with '@main' and must provide a main static function "
"of type %" SELECT_APPLICATION_MAIN_TYPES "1",
(const ValueDecl *, bool))
NOTE(note_add_main_sync,none,
"add 'static func main()'", ())
NOTE(note_add_main_sync_throws,none,
"add 'static func main() throws'", ())
NOTE(note_add_main_async,none,
"add 'static func main() async'", ())
NOTE(note_add_main_async_throws,none,
"add 'static func main() async throws'", ())

#undef SELECT_APPLICATION_MAIN_TYPES
#undef SELECT_APPLICATION_MAIN
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71 changes: 68 additions & 3 deletions lib/Sema/TypeCheckAttr.cpp
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Expand Up @@ -22,6 +22,7 @@
#include "TypeCheckObjC.h"
#include "TypeCheckType.h"
#include "TypeChecker.h"
#include "swift/AST/ASTPrinter.h"
#include "swift/AST/ASTVisitor.h"
#include "swift/AST/AvailabilityInference.h"
#include "swift/AST/ClangModuleLoader.h"
Expand Down Expand Up @@ -3081,6 +3082,36 @@ synthesizeMainBody(AbstractFunctionDecl *fn, void *arg) {
return std::make_pair(body, /*typechecked=*/false);
}

static llvm::SmallString<128>
generateMainFunctionText(ASTContext &C, NominalTypeDecl *parentDecl,
bool isThrows, bool isAsync) {
StringRef ExtraIndent;
StringRef CurrentIndent = Lexer::getIndentationForLine(
C.SourceMgr, parentDecl->getStartLoc(), &ExtraIndent);
std::string MethodIndent = (CurrentIndent + ExtraIndent).str();

llvm::SmallString<128> Text;
llvm::raw_svector_ostream OS(Text);
ExtraIndentStreamPrinter Printer(OS, MethodIndent);

Printer.printNewline();

Printer << "static func main() ";
if (isAsync)
Printer << "async ";
if (isThrows)
Printer << "throws ";

// Print the "{ <#code#> }" placeholder body.
Printer << "{\n";
Printer.printIndent();
Printer << ExtraIndent << getCodePlaceholder();
Printer.printNewline();
Printer << "}\n";
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@ahoppen ahoppen Jul 16, 2025

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Nitpick: Should we add two newlines here to separate the generated main function from the remaining body using a newline?

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@AnthonyLatsis AnthonyLatsis Jul 16, 2025 •
edited
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I don’t have a clear preference. Ideally (not in this PR!), I imagine a formatCodeForInsertion function that parses a given string into a SwiftSyntax tree and formats it using SwiftBasicFormat (indentation, separation) for insertion at a given location.

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@ahoppen ahoppen Jul 16, 2025

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The way that Fix-Its and refactorings are designed to work is that the editor applies its own indentation after applying the Fix-It refactoring because it can really know about the user’s preferences (the compiler can only guess based on existing lines). But whether to add a newline is currently an opinionated decision by the compiler.

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@CrazyFanFan CrazyFanFan Jul 18, 2025 •
edited
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Thank you for the discussion.

I've looked at other fix-it instances in TypeCheckProtocol.cpp and TypeCheckDistributed.cpp, and found that they only add a single \n at the end of inserted code, without additional blank lines.

Considering this, and editors applying indentation based on user preferences, I'm inclined to stay consistent with existing practices and not insert extra blank lines.

Of course, if you have any other thoughts, I'm open to further discussion.

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Thanks for finding those examples. In that case let’s stick with the existing practice of only adding one newline. 👍🏽


return Text;
}

FuncDecl *
SynthesizeMainFunctionRequest::evaluate(Evaluator &evaluator,
Decl *D) const {
Expand Down Expand Up @@ -3210,9 +3241,43 @@ SynthesizeMainFunctionRequest::evaluate(Evaluator &evaluator,
const bool hasAsyncSupport =
AvailabilityRange::forDeploymentTarget(context).isContainedIn(
context.getBackDeployedConcurrencyAvailability());
context.Diags.diagnose(attr->getLocation(),
diag::attr_MainType_without_main,
nominal, hasAsyncSupport);

auto location = attr->getLocation();
auto fixLocation = braces.Start;

context.Diags.diagnose(location, diag::attr_MainType_without_main, nominal,
hasAsyncSupport);

// Offer fix-its to add the `main` function for different combinations of
// effects, starting with no effects.

context.Diags.diagnose(location, diag::note_add_main_sync)
.fixItInsertAfter(fixLocation, generateMainFunctionText(
context, nominal, /*isThrows*/ false,
/*isAsync*/ false)
.str());

context.Diags.diagnose(location, diag::note_add_main_sync_throws)
.fixItInsertAfter(fixLocation, generateMainFunctionText(
context, nominal, /*isThrows*/ true,
/*isAsync*/ false)
.str());

if (hasAsyncSupport) {
context.Diags.diagnose(location, diag::note_add_main_async)
.fixItInsertAfter(fixLocation,
generateMainFunctionText(context, nominal,
/*isThrows*/ false,
/*isAsync*/ true)
.str());

context.Diags.diagnose(location, diag::note_add_main_async_throws)
.fixItInsertAfter(fixLocation,
generateMainFunctionText(context, nominal,
/*isThrows*/ true,
/*isAsync*/ true)
.str());
}
attr->setInvalid();
return nullptr;
}
Expand Down
5 changes: 4 additions & 1 deletion test/attr/ApplicationMain/attr_main_arguments.swift
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@@ -1,8 +1,11 @@
// RUN: %target-swift-frontend -typecheck -parse-as-library -verify %s

@main // expected-error{{'MyBase' is annotated with '@main' and must provide a main static function}}
// expected-note@-1{{add 'static func main()'}} {{8:16-16=\n static func main() {\n <#code#>\n }\n}}
// expected-note@-2{{add 'static func main() throws'}} {{8:16-16=\n static func main() throws {\n <#code#>\n }\n}}
// expected-note@-3{{add 'static func main() async'}} {{8:16-16=\n static func main() async {\n <#code#>\n }\n}}
// expected-note@-4{{add 'static func main() async throws'}} {{8:16-16=\n static func main() async throws {\n <#code#>\n }\n}}
struct MyBase {
static func main(_ argc: Int, _ argv: [String]) {
}
}

4 changes: 4 additions & 0 deletions test/attr/ApplicationMain/attr_main_dynamicCallable.swift
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@@ -1,6 +1,10 @@
// RUN: %target-swift-frontend -typecheck -parse-as-library -verify %s

@main // expected-error{{'Foo' is annotated with '@main' and must provide a main static function}}
// expected-note@-1{{add 'static func main()'}} {{8:13-13=\n static func main() {\n <#code#>\n }\n}}
// expected-note@-2{{add 'static func main() throws'}} {{8:13-13=\n static func main() throws {\n <#code#>\n }\n}}
// expected-note@-3{{add 'static func main() async'}} {{8:13-13=\n static func main() async {\n <#code#>\n }\n}}
// expected-note@-4{{add 'static func main() async throws'}} {{8:13-13=\n static func main() async throws {\n <#code#>\n }\n}}
struct Foo {
@dynamicCallable
struct main {
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5 changes: 4 additions & 1 deletion test/attr/ApplicationMain/attr_main_dynamicMemberLookup.swift
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@@ -1,10 +1,13 @@
// RUN: %target-swift-frontend -typecheck -parse-as-library -verify %s

@main @dynamicMemberLookup // expected-error{{'Main' is annotated with '@main' and must provide a main static function}}
// expected-note@-1{{add 'static func main()'}} {{8:14-14=\n static func main() {\n <#code#>\n }\n}}
// expected-note@-2{{add 'static func main() throws'}} {{8:14-14=\n static func main() throws {\n <#code#>\n }\n}}
// expected-note@-3{{add 'static func main() async'}} {{8:14-14=\n static func main() async {\n <#code#>\n }\n}}
// expected-note@-4{{add 'static func main() async throws'}} {{8:14-14=\n static func main() async throws {\n <#code#>\n }\n}}
struct Main {
subscript(dynamicMember member: String) -> () -> Void {
return {
}
}
}

6 changes: 4 additions & 2 deletions test/attr/ApplicationMain/attr_main_extension_nofunc.swift
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Expand Up @@ -4,7 +4,9 @@ class EntryPoint {
}

@main // expected-error{{'EntryPoint' is annotated with '@main' and must provide a main static function}}
// expected-note@-1{{add 'static func main()'}} {{11:23-23=\n static func main() {\n <#code#>\n }\n}}
// expected-note@-2{{add 'static func main() throws'}} {{11:23-23=\n static func main() throws {\n <#code#>\n }\n}}
// expected-note@-3{{add 'static func main() async'}} {{11:23-23=\n static func main() async {\n <#code#>\n }\n}}
// expected-note@-4{{add 'static func main() async throws'}} {{11:23-23=\n static func main() async throws {\n <#code#>\n }\n}}
extension EntryPoint {
}


15 changes: 15 additions & 0 deletions test/attr/ApplicationMain/attr_main_instance.swift
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@@ -1,8 +1,23 @@
// RUN: %target-swift-frontend -typecheck -parse-as-library -verify %s

@main // expected-error{{'MyBase' is annotated with '@main' and must provide a main static function}}
// expected-note@-1{{add 'static func main()'}} {{8:15-15=\n static func main() {\n <#code#>\n }\n}}
// expected-note@-2{{add 'static func main() throws'}} {{8:15-15=\n static func main() throws {\n <#code#>\n }\n}}
// expected-note@-3{{add 'static func main() async'}} {{8:15-15=\n static func main() async {\n <#code#>\n }\n}}
// expected-note@-4{{add 'static func main() async throws'}} {{8:15-15=\n static func main() async throws {\n <#code#>\n }\n}}
class MyBase {
func main() {
}
}

enum Nested {
@main // expected-error{{'MyBase' is annotated with '@main' and must provide a main static function}}
// expected-note@-1{{add 'static func main()'}} {{19:17-17=\n static func main() {\n <#code#>\n }\n}}
// expected-note@-2{{add 'static func main() throws'}} {{19:17-17=\n static func main() throws {\n <#code#>\n }\n}}
// expected-note@-3{{add 'static func main() async'}} {{19:17-17=\n static func main() async {\n <#code#>\n }\n}}
// expected-note@-4{{add 'static func main() async throws'}} {{19:17-17=\n static func main() async throws {\n <#code#>\n }\n}}
class MyBase {
func main() {
}
}
}
10 changes: 10 additions & 0 deletions test/attr/ApplicationMain/attr_main_instance_without_async_support.swift
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@@ -0,0 +1,10 @@
// RUN: %target-swift-frontend -typecheck -parse-as-library -target %target-swift-5.0-abi-triple -verify %s
// REQUIRES: OS=macosx && CPU=x86_64

@main // expected-error{{'MyBaseWithoutAsyncSupport' is annotated with '@main' and must provide a main static function}}
// expected-note@-1{{add 'static func main()'}} {{7:34-34=\n static func main() {\n <#code#>\n }\n}}
// expected-note@-2{{add 'static func main() throws'}} {{7:34-34=\n static func main() throws {\n <#code#>\n }\n}}
class MyBaseWithoutAsyncSupport {
func main() {
}
}
5 changes: 4 additions & 1 deletion test/attr/ApplicationMain/attr_main_return.swift
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@@ -1,8 +1,11 @@
// RUN: %target-swift-frontend -typecheck -parse-as-library -verify %s

@main // expected-error{{'MyBase' is annotated with '@main' and must provide a main static function}}
// expected-note@-1{{add 'static func main()'}} {{8:16-16=\n static func main() {\n <#code#>\n }\n}}
// expected-note@-2{{add 'static func main() throws'}} {{8:16-16=\n static func main() throws {\n <#code#>\n }\n}}
// expected-note@-3{{add 'static func main() async'}} {{8:16-16=\n static func main() async {\n <#code#>\n }\n}}
// expected-note@-4{{add 'static func main() async throws'}} {{8:16-16=\n static func main() async throws {\n <#code#>\n }\n}}
struct MyBase {
static func main() -> Int {
}
}

5 changes: 4 additions & 1 deletion test/attr/ApplicationMain/attr_main_struct_from_two_protocols_one_missing.swift
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Expand Up @@ -16,8 +16,11 @@ extension Runnable where Self : OtherThing {
}

@main //expected-error{{'EntryPoint' is annotated with '@main' and must provide a main static function}}
// expected-note@-1{{add 'static func main()'}} {{23:31-31=\n static func main() {\n <#code#>\n }\n}}
// expected-note@-2{{add 'static func main() throws'}} {{23:31-31=\n static func main() throws {\n <#code#>\n }\n}}
// expected-note@-3{{add 'static func main() async'}} {{23:31-31=\n static func main() async {\n <#code#>\n }\n}}
// expected-note@-4{{add 'static func main() async throws'}} {{23:31-31=\n static func main() async throws {\n <#code#>\n }\n}}
struct EntryPoint : Runnable {
func run() {
}
}