Modify the way line breaks are added when printing unions/intersections (#1047)

Adds an option `compositeTypesSingleLineLimit` to control when
unions/intersections are broken up onto multiple lines when printed.
Simple unions/intersections will remain on one line. Default value is
set to `5`.

I took the liberty to always expand overloaded function intersections on
multiple lines, as I believe they are best viewed that way, even if
theres only 2 functions.

Closes #963

Analysis/include/Luau/ToString.h

@@ -11,6 +11,7 @@
 
 LUAU_FASTINT(LuauTableTypeMaximumStringifierLength)
 LUAU_FASTINT(LuauTypeMaximumStringifierLength)
+LUAU_FASTFLAG(LuauToStringSimpleCompositeTypesSingleLine)
 
 namespace Luau
 {
@@ -39,6 +40,12 @@ struct ToStringNameMap
 
 struct ToStringOptions
 {
+    ToStringOptions(bool exhaustive = false)
+        : exhaustive(exhaustive)
+        , compositeTypesSingleLineLimit(FFlag::LuauToStringSimpleCompositeTypesSingleLine ? 5 : 0)
+    {
+    }
+
     bool exhaustive = false;                      // If true, we produce complete output rather than comprehensible output
     bool useLineBreaks = false;                   // If true, we insert new lines to separate long results such as table entries/metatable.
     bool functionTypeArguments = false;           // If true, output function type argument names when they are available
@@ -47,6 +54,7 @@ struct ToStringOptions
     bool hideFunctionSelfArgument = false;        // If true, `self: X` will be omitted from the function signature if the function has self
     size_t maxTableLength = size_t(FInt::LuauTableTypeMaximumStringifierLength); // Only applied to TableTypes
     size_t maxTypeLength = size_t(FInt::LuauTypeMaximumStringifierLength);
+    size_t compositeTypesSingleLineLimit = 5; // The number of type elements permitted on a single line when printing type unions/intersections
     ToStringNameMap nameMap;
     std::shared_ptr<Scope> scope; // If present, module names will be added and types that are not available in scope will be marked as 'invalid'
     std::vector<std::string> namedFunctionOverrideArgNames; // If present, named function argument names will be overridden

Analysis/src/ToString.cpp

@@ -17,6 +17,7 @@
 
 LUAU_FASTFLAG(DebugLuauDeferredConstraintResolution)
 LUAU_FASTFLAGVARIABLE(LuauToStringPrettifyLocation, false)
+LUAU_FASTFLAGVARIABLE(LuauToStringSimpleCompositeTypesSingleLine, false)
 
 /*
  * Enables increasing levels of verbosity for Luau type names when stringifying.
@@ -878,11 +879,15 @@ struct TypeStringifier
             state.emit("(");
 
         bool first = true;
+        bool shouldPlaceOnNewlines = results.size() > state.opts.compositeTypesSingleLineLimit;
         for (std::string& ss : results)
         {
             if (!first)
             {
+                if (shouldPlaceOnNewlines)
                     state.newline();
+                else
+                    state.emit(" ");
                 state.emit("| ");
             }
             state.emit(ss);
@@ -902,7 +907,7 @@ struct TypeStringifier
         }
     }
 
-    void operator()(TypeId, const IntersectionType& uv)
+    void operator()(TypeId ty, const IntersectionType& uv)
     {
         if (state.hasSeen(&uv))
         {
@@ -938,11 +943,15 @@ struct TypeStringifier
             std::sort(results.begin(), results.end());
 
         bool first = true;
+        bool shouldPlaceOnNewlines = results.size() > state.opts.compositeTypesSingleLineLimit || isOverloadedFunction(ty);
         for (std::string& ss : results)
         {
             if (!first)
             {
+                if (shouldPlaceOnNewlines)
                     state.newline();
+                else
+                    state.emit(" ");
                 state.emit("& ");
             }
             state.emit(ss);

tests/ToString.test.cpp

@@ -234,8 +234,41 @@ TEST_CASE_FIXTURE(Fixture, "functions_are_always_parenthesized_in_unions_or_inte
     CHECK_EQ(toString(&itv), "((number, string) -> (string, number)) & ((string, number) -> (number, string))");
 }
 
-TEST_CASE_FIXTURE(Fixture, "intersections_respects_use_line_breaks")
+TEST_CASE_FIXTURE(Fixture, "simple_intersections_printed_on_one_line")
 {
+    ScopedFastFlag sff{"LuauToStringSimpleCompositeTypesSingleLine", true};
+    CheckResult result = check(R"(
+        local a: string & number
+    )");
+
+    ToStringOptions opts;
+    opts.useLineBreaks = true;
+
+    CHECK_EQ("number & string", toString(requireType("a"), opts));
+}
+
+TEST_CASE_FIXTURE(Fixture, "complex_intersections_printed_on_multiple_lines")
+{
+    ScopedFastFlag sff{"LuauToStringSimpleCompositeTypesSingleLine", true};
+    CheckResult result = check(R"(
+        local a: string & number & boolean
+    )");
+
+    ToStringOptions opts;
+    opts.useLineBreaks = true;
+    opts.compositeTypesSingleLineLimit = 2;
+
+    //clang-format off
+    CHECK_EQ("boolean\n"
+             "& number\n"
+             "& string",
+        toString(requireType("a"), opts));
+    //clang-format on
+}
+
+TEST_CASE_FIXTURE(Fixture, "overloaded_functions_always_printed_on_multiple_lines")
+{
+    ScopedFastFlag sff{"LuauToStringSimpleCompositeTypesSingleLine", true};
     CheckResult result = check(R"(
         local a: ((string) -> string) & ((number) -> number)
     )");
@@ -250,13 +283,28 @@ TEST_CASE_FIXTURE(Fixture, "intersections_respects_use_line_breaks")
     //clang-format on
 }
 
-TEST_CASE_FIXTURE(Fixture, "unions_respects_use_line_breaks")
+TEST_CASE_FIXTURE(Fixture, "simple_unions_printed_on_one_line")
 {
+    ScopedFastFlag sff{"LuauToStringSimpleCompositeTypesSingleLine", true};
+    CheckResult result = check(R"(
+        local a: number | boolean
+    )");
+
+    ToStringOptions opts;
+    opts.useLineBreaks = true;
+
+    CHECK_EQ("boolean | number", toString(requireType("a"), opts));
+}
+
+TEST_CASE_FIXTURE(Fixture, "complex_unions_printed_on_multiple_lines")
+{
+    ScopedFastFlag sff{"LuauToStringSimpleCompositeTypesSingleLine", true};
     CheckResult result = check(R"(
         local a: string | number | boolean
     )");
 
     ToStringOptions opts;
+    opts.compositeTypesSingleLineLimit = 2;
     opts.useLineBreaks = true;
 
     //clang-format off