Sync to upstream/release/645 (#1440)

In this update, we continue to improve the overall stability of the new
type solver. We're also shipping some early bits of two new features,
one of the language and one of the analysis API: user-defined type
functions and an incremental typechecking API.

If you use the new solver and want to use all new fixes included in this
release, you have to reference an additional Luau flag:
```c++
LUAU_DYNAMIC_FASTINT(LuauTypeSolverRelease)
```
And set its value to `645`:
```c++
DFInt::LuauTypeSolverRelease.value = 645; // Or a higher value for future updates
```

## New Solver

* Fix a crash where scopes are incorrectly accessed cross-module after
they've been deallocated by appropriately zeroing out associated scope
pointers for free types, generic types, table types, etc.
* Fix a crash where we were incorrectly caching results for bound types
in generalization.
* Eliminated some unnecessary intermediate allocations in the constraint
solver and type function infrastructure.
* Built some initial groundwork for an incremental typecheck API for use
by language servers.
* Built an initial technical preview for [user-defined type
functions](https://rfcs.luau-lang.org/user-defined-type-functions.html),
more work still to come (including calling type functions from other
type functions), but adventurous folks wanting to experiment with it can
try it out by enabling `FFlag::LuauUserDefinedTypeFunctionsSyntax` and
`FFlag::LuauUserDefinedTypeFunction` in their local environment. Special
thanks to @joonyoo181 who built up all the initial infrastructure for
this during his internship!

## Miscellaneous changes

* Fix a compilation error on Ubuntu (fixes #1437)

---

Internal Contributors:

Co-authored-by: Aaron Weiss <[email protected]>
Co-authored-by: Hunter Goldstein <[email protected]>
Co-authored-by: Jeremy Yoo <[email protected]>
Co-authored-by: Vighnesh Vijay <[email protected]>
Co-authored-by: Vyacheslav Egorov <[email protected]>

---------

Co-authored-by: Alexander McCord <[email protected]>
Co-authored-by: Andy Friesen <[email protected]>
Co-authored-by: Vighnesh <[email protected]>
Co-authored-by: Aviral Goel <[email protected]>
Co-authored-by: David Cope <[email protected]>
Co-authored-by: Lily Brown <[email protected]>
Co-authored-by: Vyacheslav Egorov <[email protected]>
Co-authored-by: Junseo Yoo <[email protected]>

Analysis/include/Luau/ConstraintSolver.h

@@ -12,6 +12,7 @@
 #include "Luau/ToString.h"
 #include "Luau/Type.h"
 #include "Luau/TypeCheckLimits.h"
+#include "Luau/TypeFunction.h"
 #include "Luau/TypeFwd.h"
 #include "Luau/Variant.h"
 
@@ -62,6 +63,7 @@ struct ConstraintSolver
     NotNull<BuiltinTypes> builtinTypes;
     InternalErrorReporter iceReporter;
     NotNull<Normalizer> normalizer;
+    NotNull<TypeFunctionRuntime> typeFunctionRuntime;
     // The entire set of constraints that the solver is trying to resolve.
     std::vector<NotNull<Constraint>> constraints;
     NotNull<Scope> rootScope;
@@ -111,6 +113,7 @@ struct ConstraintSolver
 
     explicit ConstraintSolver(
         NotNull<Normalizer> normalizer,
+        NotNull<TypeFunctionRuntime> typeFunctionRuntime,
         NotNull<Scope> rootScope,
         std::vector<NotNull<Constraint>> constraints,
         ModuleName moduleName,
@@ -278,18 +281,18 @@ struct ConstraintSolver
     /**
      * @returns true if the TypeId is in a blocked state.
      */
-    bool isBlocked(TypeId ty);
+    bool isBlocked(TypeId ty) const;
 
     /**
      * @returns true if the TypePackId is in a blocked state.
      */
-    bool isBlocked(TypePackId tp);
+    bool isBlocked(TypePackId tp) const;
 
     /**
      * Returns whether the constraint is blocked on anything.
      * @param constraint the constraint to check.
      */
-    bool isBlocked(NotNull<const Constraint> constraint);
+    bool isBlocked(NotNull<const Constraint> constraint) const;
 
     /** Pushes a new solver constraint to the solver.
      * @param cv the body of the constraint.
@@ -381,8 +384,8 @@ struct ConstraintSolver
 
     TypePackId anyifyModuleReturnTypePackGenerics(TypePackId tp);
 
-    void throwTimeLimitError();
-    void throwUserCancelError();
+    void throwTimeLimitError() const;
+    void throwUserCancelError() const;
 
     ToStringOptions opts;
 };

Analysis/include/Luau/Error.h

@@ -448,6 +448,13 @@ struct UnexpectedTypePackInSubtyping
     bool operator==(const UnexpectedTypePackInSubtyping& rhs) const;
 };
 
+struct UserDefinedTypeFunctionError
+{
+    std::string message;
+
+    bool operator==(const UserDefinedTypeFunctionError& rhs) const;
+};
+
 using TypeErrorData = Variant<
     TypeMismatch,
     UnknownSymbol,
@@ -496,7 +503,8 @@ using TypeErrorData = Variant<
     CheckedFunctionIncorrectArgs,
     UnexpectedTypeInSubtyping,
     UnexpectedTypePackInSubtyping,
-    ExplicitFunctionAnnotationRecommended>;
+    ExplicitFunctionAnnotationRecommended,
+    UserDefinedTypeFunctionError>;
 
 struct TypeErrorSummary
 {

Analysis/include/Luau/FragmentAutocomplete.h

@@ -0,0 +1,23 @@
+// This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
+#pragma once
+
+#include "Luau/DenseHash.h"
+#include "Luau/Ast.h"
+
+#include <vector>
+
+
+namespace Luau
+{
+
+struct FragmentAutocompleteAncestryResult
+{
+    DenseHashMap<AstName, AstLocal*> localMap{AstName()};
+    std::vector<AstLocal*> localStack;
+    std::vector<AstNode*> ancestry;
+    AstStat* nearestStatement;
+};
+
+FragmentAutocompleteAncestryResult findAncestryForFragmentParse(AstStatBlock* root, const Position& cursorPos);
+
+} // namespace Luau

Analysis/include/Luau/OverloadResolution.h

@@ -35,6 +35,7 @@ struct OverloadResolver
         NotNull<BuiltinTypes> builtinTypes,
         NotNull<TypeArena> arena,
         NotNull<Normalizer> normalizer,
+        NotNull<TypeFunctionRuntime> typeFunctionRuntime,
         NotNull<Scope> scope,
         NotNull<InternalErrorReporter> reporter,
         NotNull<TypeCheckLimits> limits,
@@ -44,6 +45,7 @@ struct OverloadResolver
     NotNull<BuiltinTypes> builtinTypes;
     NotNull<TypeArena> arena;
     NotNull<Normalizer> normalizer;
+    NotNull<TypeFunctionRuntime> typeFunctionRuntime;
     NotNull<Scope> scope;
     NotNull<InternalErrorReporter> ice;
     NotNull<TypeCheckLimits> limits;
@@ -109,6 +111,7 @@ SolveResult solveFunctionCall(
     NotNull<TypeArena> arena,
     NotNull<BuiltinTypes> builtinTypes,
     NotNull<Normalizer> normalizer,
+    NotNull<TypeFunctionRuntime> typeFunctionRuntime,
     NotNull<InternalErrorReporter> iceReporter,
     NotNull<TypeCheckLimits> limits,
     NotNull<Scope> scope,

Analysis/include/Luau/Subtyping.h

@@ -135,6 +135,7 @@ struct Subtyping
     NotNull<BuiltinTypes> builtinTypes;
     NotNull<TypeArena> arena;
     NotNull<Normalizer> normalizer;
+    NotNull<TypeFunctionRuntime> typeFunctionRuntime;
     NotNull<InternalErrorReporter> iceReporter;
 
     TypeCheckLimits limits;
@@ -155,6 +156,7 @@ struct Subtyping
         NotNull<BuiltinTypes> builtinTypes,
         NotNull<TypeArena> typeArena,
         NotNull<Normalizer> normalizer,
+        NotNull<TypeFunctionRuntime> typeFunctionRuntime,
         NotNull<InternalErrorReporter> iceReporter
     );
 

Analysis/include/Luau/TypeChecker2.h

@@ -83,6 +83,7 @@ struct TypeChecker2
     DenseHashSet<TypeId> seenTypeFunctionInstances{nullptr};
 
     Normalizer normalizer;
+    TypeFunctionRuntime typeFunctionRuntime;
     Subtyping _subtyping;
     NotNull<Subtyping> subtyping;
 

Analysis/include/Luau/TypeFunction.h

@@ -1,10 +1,11 @@
 // This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
 #pragma once
 
-#include "Luau/ConstraintSolver.h"
+#include "Luau/Constraint.h"
 #include "Luau/Error.h"
 #include "Luau/NotNull.h"
 #include "Luau/TypeCheckLimits.h"
+#include "Luau/TypeFunctionRuntime.h"
 #include "Luau/TypeFwd.h"
 
 #include <functional>
@@ -16,14 +17,23 @@ namespace Luau
 
 struct TypeArena;
 struct TxnLog;
+struct ConstraintSolver;
 class Normalizer;
 
+struct TypeFunctionRuntime
+{
+    // For user-defined type functions, we store all generated types and packs for the duration of the typecheck
+    TypedAllocator<TypeFunctionType> typeArena;
+    TypedAllocator<TypeFunctionTypePackVar> typePackArena;
+};
+
 struct TypeFunctionContext
 {
     NotNull<TypeArena> arena;
     NotNull<BuiltinTypes> builtins;
     NotNull<Scope> scope;
     NotNull<Normalizer> normalizer;
+    NotNull<TypeFunctionRuntime> typeFunctionRuntime;
     NotNull<InternalErrorReporter> ice;
     NotNull<TypeCheckLimits> limits;
 
@@ -35,38 +45,30 @@ struct TypeFunctionContext
     std::optional<AstName> userFuncName;          // Name of the user-defined type function; only available for UDTFs
     std::optional<AstExprFunction*> userFuncBody; // Body of the user-defined type function; only available for UDTFs
 
-    TypeFunctionContext(NotNull<ConstraintSolver> cs, NotNull<Scope> scope, NotNull<const Constraint> constraint)
-        : arena(cs->arena)
-        , builtins(cs->builtinTypes)
-        , scope(scope)
-        , normalizer(cs->normalizer)
-        , ice(NotNull{&cs->iceReporter})
-        , limits(NotNull{&cs->limits})
-        , solver(cs.get())
-        , constraint(constraint.get())
-    {
-    }
+    TypeFunctionContext(NotNull<ConstraintSolver> cs, NotNull<Scope> scope, NotNull<const Constraint> constraint);
 
     TypeFunctionContext(
         NotNull<TypeArena> arena,
         NotNull<BuiltinTypes> builtins,
         NotNull<Scope> scope,
         NotNull<Normalizer> normalizer,
+        NotNull<TypeFunctionRuntime> typeFunctionRuntime,
         NotNull<InternalErrorReporter> ice,
         NotNull<TypeCheckLimits> limits
     )
         : arena(arena)
         , builtins(builtins)
         , scope(scope)
         , normalizer(normalizer)
+        , typeFunctionRuntime(typeFunctionRuntime)
         , ice(ice)
         , limits(limits)
         , solver(nullptr)
         , constraint(nullptr)
     {
     }
 
-    NotNull<Constraint> pushConstraint(ConstraintV&& c);
+    NotNull<Constraint> pushConstraint(ConstraintV&& c) const;
 };
 
 /// Represents a reduction result, which may have successfully reduced the type,
@@ -88,6 +90,8 @@ struct TypeFunctionReductionResult
     /// Any type packs that need to be progressed or mutated before the
     /// reduction may proceed.
     std::vector<TypePackId> blockedPacks;
+    /// A runtime error message from user-defined type functions
+    std::optional<std::string> error;
 };
 
 template<typename T>