Support Control Flow type Refinements for "break" and "continue" stat…

…ements (#1004)

Fixes: #913

This PR adds support for type refinements around guard clauses that use
`break` and `continue` statements inside a loop, similar to how guard
clauses with `return` is supported.

I had some free time today, so I figure I'd give a shot at a naïve fix
for this at the very least.

---

## Resulting Change:

Luau now supports type refinements within loops where a `continue` or
`break` guard clause was used.
For example:
```lua
for _, object in objects :: {{value: string?}} do
    if not object.value then
        continue
    end
    local x: string = object.value -- OK; Used to emit "Type 'string?' could not be converted into 'string'"
end
```

---------

Co-authored-by: Alexander McCord <[email protected]>

Analysis/include/Luau/ControlFlow.h

@@ -14,8 +14,8 @@ enum class ControlFlow
     None = 0b00001,
     Returns = 0b00010,
     Throws = 0b00100,
-    Break = 0b01000,    // Currently unused.
-    Continue = 0b10000, // Currently unused.
+    Breaks = 0b01000,
+    Continues = 0b10000,
 };
 
 inline ControlFlow operator&(ControlFlow a, ControlFlow b)

Analysis/src/ConstraintGraphBuilder.cpp

@@ -24,6 +24,7 @@ LUAU_FASTINT(LuauCheckRecursionLimit);
 LUAU_FASTFLAG(DebugLuauLogSolverToJson);
 LUAU_FASTFLAG(DebugLuauMagicTypes);
 LUAU_FASTFLAG(LuauParseDeclareClassIndexer);
+LUAU_FASTFLAG(LuauLoopControlFlowAnalysis);
 LUAU_FASTFLAG(LuauFloorDivision);
 
 namespace Luau
@@ -537,11 +538,10 @@ ControlFlow ConstraintGraphBuilder::visit(const ScopePtr& scope, AstStat* stat)
         return visit(scope, s);
     else if (auto s = stat->as<AstStatRepeat>())
         return visit(scope, s);
-    else if (stat->is<AstStatBreak>() || stat->is<AstStatContinue>())
-    {
-        // Nothing
-        return ControlFlow::None; // TODO: ControlFlow::Break/Continue
-    }
+    else if (stat->is<AstStatBreak>())
+        return FFlag::LuauLoopControlFlowAnalysis ? ControlFlow::Breaks : ControlFlow::None;
+    else if (stat->is<AstStatContinue>())
+        return FFlag::LuauLoopControlFlowAnalysis ? ControlFlow::Continues : ControlFlow::None;
     else if (auto r = stat->as<AstStatReturn>())
         return visit(scope, r);
     else if (auto e = stat->as<AstStatExpr>())
@@ -1072,12 +1072,14 @@ ControlFlow ConstraintGraphBuilder::visit(const ScopePtr& scope, AstStatIf* ifSt
     if (ifStatement->elsebody)
         elsecf = visit(elseScope, ifStatement->elsebody);
 
-    if (matches(thencf, ControlFlow::Returns | ControlFlow::Throws) && elsecf == ControlFlow::None)
+    if (thencf != ControlFlow::None && elsecf == ControlFlow::None)
         scope->inheritRefinements(elseScope);
-    else if (thencf == ControlFlow::None && matches(elsecf, ControlFlow::Returns | ControlFlow::Throws))
+    else if (thencf == ControlFlow::None && elsecf != ControlFlow::None)
         scope->inheritRefinements(thenScope);
 
-    if (matches(thencf, ControlFlow::Returns | ControlFlow::Throws) && matches(elsecf, ControlFlow::Returns | ControlFlow::Throws))
+    if (FFlag::LuauLoopControlFlowAnalysis && thencf == elsecf)
+        return thencf;
+    else if (matches(thencf, ControlFlow::Returns | ControlFlow::Throws) && matches(elsecf, ControlFlow::Returns | ControlFlow::Throws))
         return ControlFlow::Returns;
     else
         return ControlFlow::None;

Analysis/src/TypeInfer.cpp

@@ -38,6 +38,7 @@ LUAU_FASTFLAG(LuauInstantiateInSubtyping)
 LUAU_FASTFLAGVARIABLE(LuauAllowIndexClassParameters, false)
 LUAU_FASTFLAG(LuauOccursIsntAlwaysFailure)
 LUAU_FASTFLAGVARIABLE(LuauTinyControlFlowAnalysis, false)
+LUAU_FASTFLAGVARIABLE(LuauLoopControlFlowAnalysis, false)
 LUAU_FASTFLAGVARIABLE(LuauVariadicOverloadFix, false)
 LUAU_FASTFLAGVARIABLE(LuauAlwaysCommitInferencesOfFunctionCalls, false)
 LUAU_FASTFLAG(LuauParseDeclareClassIndexer)
@@ -350,11 +351,10 @@ ControlFlow TypeChecker::check(const ScopePtr& scope, const AstStat& program)
         return check(scope, *while_);
     else if (auto repeat = program.as<AstStatRepeat>())
         return check(scope, *repeat);
-    else if (program.is<AstStatBreak>() || program.is<AstStatContinue>())
-    {
-        // Nothing to do
-        return ControlFlow::None;
-    }
+    else if (program.is<AstStatBreak>())
+        return FFlag::LuauLoopControlFlowAnalysis ? ControlFlow::Breaks : ControlFlow::None;
+    else if (program.is<AstStatContinue>())
+        return FFlag::LuauLoopControlFlowAnalysis ? ControlFlow::Continues : ControlFlow::None;
     else if (auto return_ = program.as<AstStatReturn>())
         return check(scope, *return_);
     else if (auto expr = program.as<AstStatExpr>())
@@ -752,12 +752,14 @@ ControlFlow TypeChecker::check(const ScopePtr& scope, const AstStatIf& statement
         if (statement.elsebody)
             elsecf = check(elseScope, *statement.elsebody);
 
-        if (matches(thencf, ControlFlow::Returns | ControlFlow::Throws) && elsecf == ControlFlow::None)
+        if (thencf != ControlFlow::None && elsecf == ControlFlow::None)
             scope->inheritRefinements(elseScope);
-        else if (thencf == ControlFlow::None && matches(elsecf, ControlFlow::Returns | ControlFlow::Throws))
+        else if (thencf == ControlFlow::None && elsecf != ControlFlow::None)
             scope->inheritRefinements(thenScope);
 
-        if (matches(thencf, ControlFlow::Returns | ControlFlow::Throws) && matches(elsecf, ControlFlow::Returns | ControlFlow::Throws))
+        if (FFlag::LuauLoopControlFlowAnalysis && thencf == elsecf)
+            return thencf;
+        else if (matches(thencf, ControlFlow::Returns | ControlFlow::Throws) && matches(elsecf, ControlFlow::Returns | ControlFlow::Throws))
             return ControlFlow::Returns;
         else
             return ControlFlow::None;