Fix issue 22306 - scope array variable should be stack allocated

changelog/scope-array-on-stack.dd

@@ -0,0 +1,26 @@
+Array literals assigned to `scope` array variables are now allocated on the stack
+
+Formerly, they were always allocated with the Garbage Collector, making it unavailable in `@nogc` or `-betterC` code.
+This led to frequent use of the following workaround:
+
+---
+void main() @nogc
+{
+    int[3] buffer = [10, 20, 30];
+    int[] arr = buffer[];
+}
+---
+
+This can now be written in a single line:
+
+---
+void main() @nogc
+{
+    scope int[] arr = [10, 20, 30];
+}
+---
+
+This only applies to array elements that are Plain Old Data, proper destruction cannot be ensured when the array is not managed by the GC.
+
+Note: checking that a `scope` variable actually doesn't escape is only done in `@safe` code.
+In a `@system` function, it's your own risk when you corrupt memory by returning a `scope int[]`.

compiler/src/dmd/dsymbolsem.d

@@ -1059,6 +1059,12 @@ private extern(C++) final class DsymbolSemanticVisitor : Visitor
                             if (f.tookAddressOf)
                                 f.tookAddressOf--;
                         }
+                        else if (auto ale = ex.isArrayLiteralExp())
+                        {
+                            // or an array literal assigned to a `scope` variable
+                            if (!dsym.type.nextOf().needsDestruction())
+                                ale.onstack = true;
+                        }
                     }
 
                     Expression exp = ei.exp;

compiler/src/dmd/e2ir.d

@@ -3868,7 +3868,7 @@ elem* toElem(Expression e, IRState *irs)
         elem *e;
         if (dim > 0)
         {
-            if (tb.ty == Tsarray ||
+            if (ale.onstack || tb.ty == Tsarray ||
                 irs.Cfile && tb.ty == Tpointer)
             {
                 Symbol *stmp = null;

compiler/src/dmd/expression.d

@@ -2949,7 +2949,7 @@ extern (C++) final class ArrayLiteralExp : Expression
 
     Expressions* elements;
     OwnedBy ownedByCtfe = OwnedBy.code;
-
+    bool onstack = false;
 
     extern (D) this(const ref Loc loc, Type type, Expressions* elements)
     {

compiler/src/dmd/expression.h

@@ -419,6 +419,7 @@ class ArrayLiteralExp final : public Expression
     Expression *basis;
     Expressions *elements;
     OwnedBy ownedByCtfe;
+    bool onstack;
 
     static ArrayLiteralExp *create(const Loc &loc, Expressions *elements);
     static void emplace(UnionExp *pue, const Loc &loc, Expressions *elements);

compiler/src/dmd/frontend.h

@@ -6701,7 +6701,7 @@ struct UnionExp final
         char complexexp[80LLU];
         char symoffexp[72LLU];
         char stringexp[60LLU];
-        char arrayliteralexp[57LLU];
+        char arrayliteralexp[58LLU];
         char assocarrayliteralexp[57LLU];
         char structliteralexp[95LLU];
         char compoundliteralexp[48LLU];
@@ -6896,6 +6896,7 @@ class ArrayLiteralExp final : public Expression
     Expression* basis;
     Array<Expression* >* elements;
     OwnedBy ownedByCtfe;
+    bool onstack;
     static ArrayLiteralExp* create(const Loc& loc, Array<Expression* >* elements);
     static void emplace(UnionExp* pue, const Loc& loc, Array<Expression* >* elements);
     ArrayLiteralExp* syntaxCopy() override;

compiler/src/dmd/nogc.d

@@ -99,7 +99,7 @@ public:
 
     override void visit(ArrayLiteralExp e)
     {
-        if (e.type.ty != Tarray || !e.elements || !e.elements.dim)
+        if (e.type.ty != Tarray || !e.elements || !e.elements.dim || e.onstack)
             return;
         if (f.setGC())
         {

compiler/test/runnable/test20734.d

@@ -17,6 +17,14 @@ extern(C) int main() nothrow @nogc @safe
     takeScopeSlice([ S(1), S(2) ]); // @nogc => no GC allocation
     (() @trusted { assert(numDtor == 2); })(); // stack-allocated array literal properly destructed
     assert23100([]);
+
+    // https://issues.dlang.org/show_bug.cgi?id=22306
+    // scope array variable should be stack allocated
+    scope int[] sa = [10, 20];
+    assert(sa[0] == 10);
+    assert(sa[1] == 20);
+    assert(sa.length == 2);
+
     return 0;
 }