Make in a first-class storage class, not a const [scope] alias

For a long time, the 'in' storage class was only a second class citizen,
merely an alias for 'const', which is a type constructor.
While it was also documented for a long time as being 'scope',
this was removed when 'scope' actually started to have an effect
(when DIP1000 started to be implemented).
Currently, a switch (-preview=in) allows to get back this 'scope'.

However, the 'in' storage class does not really exists,
it gets lowered to 'const [scope]' at an early stage by the compiler,
which means that we expose what is essentially an implementation
detail to the user.

This led to a variety of problems for both developers and users.
The most obvious one was that functions involving `in` would show
up simply as `const` (or `const scope`), both in generated header
and error messagges, hindering the ability to do header generation
with `-preview=in` as the same value for this switch was needed
for both producer and consumer.
Another issue was that the result of `__traits(getParameterStorageClass)`
was inaccurate, giving either an empty tuple or `scope`.
Lastly, the lack of mangling for `in` means that stack trace would show
the wrong signature for a function, potentially confusing the users.

For compiler developers, the `in` storage class simply couldn't be relied on,
as it was replaced by `const [scope]` rather early in the semantic phase
(in TypeFunction's semantic), which lead to some dead code (e.g. in dmangle).

After this change, the class will show up properly in error message,
and in `getParameterStorageClass`, which can be a surprising change for users.
However, it is not expected that code can break as a result,
unless they break as a result of using the `-preview=in` switch.

src/dmd/dmangle.d

@@ -79,7 +79,7 @@ private immutable char[TMAX] mangleChar =
     Tfunction    : 'F', // D function
     Tsarray      : 'G',
     Taarray      : 'H',
-    Tident       : 'I',
+    //              I   // in
     //              J   // out
     //              K   // ref
     //              L   // lazy
@@ -99,6 +99,7 @@ private immutable char[TMAX] mangleChar =
     //              Z   // not variadic, end of parameters
 
     // '@' shouldn't appear anywhere in the deco'd names
+    Tident       : '@',
     Tinstance    : '@',
     Terror       : '@',
     Ttypeof      : '@',
@@ -1112,7 +1113,12 @@ public:
         switch (p.storageClass & (STC.IOR | STC.lazy_))
         {
         case 0:
+            break;
         case STC.in_:
+            buf.writeByte('I');
+            break;
+        case STC.in_ | STC.ref_:
+            buf.writestring("IK");
             break;
         case STC.out_:
             buf.writeByte('J');

src/dmd/dsymbolsem.d

@@ -1102,6 +1102,11 @@ private extern(C++) final class DsymbolSemanticVisitor : Visitor
             dsym.storage_class &= ~stc; // strip off
         }
 
+        // At this point we can add `scope` to the STC instead of `in`,
+        // because we are never going to use this variable's STC for user messages
+        if (dsym.storage_class & STC.in_ && global.params.previewIn)
+            dsym.storage_class |= STC.scope_;
+
         if (dsym.storage_class & STC.scope_)
         {
             StorageClass stc = dsym.storage_class & (STC.static_ | STC.extern_ | STC.manifest | STC.tls | STC.gshared);

src/dmd/hdrgen.d

@@ -3057,17 +3057,18 @@ private void parameterToBuffer(Parameter p, OutBuffer* buf, HdrGenState* hgs)
     if (p.storageClass & STC.return_)
         buf.writestring("return ");
 
-    if (p.storageClass & STC.out_)
-        buf.writestring("out ");
-    else if (p.storageClass & STC.ref_)
-        buf.writestring("ref ");
-    else if (p.storageClass & STC.in_)
+    if (p.storageClass & STC.in_)
         buf.writestring("in ");
+    else if (p.storageClass & STC.out_)
+        buf.writestring("out ");
     else if (p.storageClass & STC.lazy_)
         buf.writestring("lazy ");
     else if (p.storageClass & STC.alias_)
         buf.writestring("alias ");
 
+    if (p.storageClass & STC.ref_)
+        buf.writestring("ref ");
+
     StorageClass stc = p.storageClass;
     if (p.type && p.type.mod & MODFlags.shared_)
         stc &= ~STC.shared_;
@@ -3089,7 +3090,7 @@ private void parameterToBuffer(Parameter p, OutBuffer* buf, HdrGenState* hgs)
     }
     else
     {
-        typeToBuffer(p.type, p.ident, buf, hgs);
+        typeToBuffer(p.type, p.ident, buf, hgs, (stc & STC.in_) ? MODFlags.const_ : 0);
     }
 
     if (p.defaultArg)
@@ -3220,14 +3221,15 @@ private void expToBuffer(Expression e, PREC pr, OutBuffer* buf, HdrGenState* hgs
 /**************************************************
  * An entry point to pretty-print type.
  */
-private void typeToBuffer(Type t, const Identifier ident, OutBuffer* buf, HdrGenState* hgs)
+private void typeToBuffer(Type t, const Identifier ident, OutBuffer* buf, HdrGenState* hgs,
+                          ubyte modMask = 0)
 {
     if (auto tf = t.isTypeFunction())
     {
         visitFuncIdentWithPrefix(tf, ident, null, buf, hgs);
         return;
     }
-    visitWithMask(t, 0, buf, hgs);
+    visitWithMask(t, modMask, buf, hgs);
     if (ident)
     {
         buf.writeByte(' ');

src/dmd/mtype.d

@@ -4440,6 +4440,10 @@ extern (C++) final class TypeFunction : TypeNext
         if (!global.params.vsafe)
             return stc;
 
+        // When the preview switch is enable, `in` parameters are `scope`
+        if (stc & STC.in_ && global.params.previewIn)
+            return stc | STC.scope_;
+
         if (stc & (STC.scope_ | STC.return_ | STC.lazy_) || purity == PURE.impure)
             return stc;
 
@@ -6829,19 +6833,35 @@ extern (C++) final class Parameter : ASTNode
     /*********************************
      * Compute covariance of parameters `this` and `p`
      * as determined by the storage classes of both.
+     *
      * Params:
      *  returnByRef = true if the function returns by ref
      *  p = Parameter to compare with
+     *  previewIn = Whether `-previewIn` is being used, and thus if
+     *              `in` means `scope`.
+     *
      * Returns:
      *  true = `this` can be used in place of `p`
      *  false = nope
      */
-    bool isCovariant(bool returnByRef, const Parameter p) const pure nothrow @nogc @safe
+    bool isCovariant(bool returnByRef, const Parameter p, bool previewIn = global.params.previewIn)
+        const pure nothrow @nogc @safe
     {
-        enum stc = STC.IOR | STC.lazy_;
-        if ((this.storageClass & stc) != (p.storageClass & stc))
+        ulong thisSTC = this.storageClass;
+        ulong otherSTC = p.storageClass;
+
+        if (previewIn)
+        {
+            if (thisSTC & STC.in_)
+                thisSTC |= STC.scope_;
+            if (otherSTC & STC.in_)
+                otherSTC |= STC.scope_;
+        }
+
+        enum stc = STC.ref_ | STC.out_ | STC.lazy_;
+        if ((thisSTC & stc) != (otherSTC & stc))
             return false;
-        return isCovariantScope(returnByRef, this.storageClass, p.storageClass);
+        return isCovariantScope(returnByRef, thisSTC, otherSTC);
     }
 
     extern (D) private static bool isCovariantScope(bool returnByRef, StorageClass from, StorageClass to) pure nothrow @nogc @safe

src/dmd/typesem.d

@@ -1261,9 +1261,6 @@ extern(C++) Type typeSemantic(Type t, const ref Loc loc, Scope* sc)
             for (size_t i = 0; i < dim; i++)
             {
                 Parameter fparam = tf.parameterList[i];
-                // If `-preview=in` is on, set `scope` as well
-                if ((fparam.storageClass & STC.in_) && global.params.previewIn)
-                    fparam.storageClass |= STC.scope_;
                 fparam.storageClass |= STC.parameter;
                 mtype.inuse++;
                 fparam.type = fparam.type.typeSemantic(loc, argsc);
@@ -1520,7 +1517,7 @@ extern(C++) Type typeSemantic(Type t, const ref Loc loc, Scope* sc)
                 }
 
                 // Remove redundant storage classes for type, they are already applied
-                fparam.storageClass &= ~(STC.TYPECTOR | STC.in_);
+                fparam.storageClass &= ~(STC.TYPECTOR);
             }
             argsc.pop();
         }

test/compilable/previewin.d

@@ -3,5 +3,5 @@
 
 @safe:
 void fun(in int* inParam);
-static assert(__traits(getParameterStorageClasses, fun, 0)[0] == "scope");
+static assert([__traits(getParameterStorageClasses, fun, 0)] == ["in"]);
 static assert (is(typeof(fun) P == __parameters) && is(P[0] == const int*));

test/fail_compilation/diagin.d

@@ -0,0 +1,28 @@
+/*
+PERMUTE_ARGS: -preview=in
+TEST_OUTPUT:
+---
+fail_compilation/diagin.d(18): Error: function `diagin.foo(in string)` is not callable using argument types `()`
+fail_compilation/diagin.d(18):        missing argument for parameter #1: `in string`
+fail_compilation/diagin.d(19): Error: function `diagin.foo1(in ref string)` is not callable using argument types `()`
+fail_compilation/diagin.d(19):        missing argument for parameter #1: `in ref string`
+fail_compilation/diagin.d(20): Error: template `diagin.foo2` cannot deduce function from argument types `!()(int)`, candidates are:
+fail_compilation/diagin.d(27):        `foo2(T)(in T v, string)`
+fail_compilation/diagin.d(22): Error: template `diagin.foo3` cannot deduce function from argument types `!()(bool[])`, candidates are:
+fail_compilation/diagin.d(28):        `foo3(T)(in ref T v, string)`
+---
+ */
+
+void main ()
+{
+    foo();
+    foo1();
+    foo2(42);
+    bool[] lvalue;
+    foo3(lvalue);
+}
+
+void foo(in string) {}
+void foo1(in ref string) {}
+void foo2(T)(in T v, string) {}
+void foo3(T)(ref in T v, string) {}

test/fail_compilation/ice10922.d

@@ -1,8 +1,8 @@
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/ice10922.d(10): Error: function `ice10922.__lambda4(const(uint) n)` is not callable using argument types `()`
-fail_compilation/ice10922.d(10):        missing argument for parameter #1: `const(uint) n`
+fail_compilation/ice10922.d(10): Error: function `ice10922.__lambda4(in uint n)` is not callable using argument types `()`
+fail_compilation/ice10922.d(10):        missing argument for parameter #1: `in uint n`
 ---
 */
 

test/runnable/xtest46.d

@@ -4963,7 +4963,7 @@ void test6763()
     static assert(typeof(f6763).stringof == "void(int _param_0)");
     static assert(typeof(c6763).stringof == "void(const(int) _param_0)");
     static assert(typeof(r6763).stringof == "void(ref int _param_0)");
-    static assert(typeof(i6763).stringof == "void(const(int) _param_0)");
+    static assert(typeof(i6763).stringof == "void(in int _param_0)");
     static assert(typeof(o6763).stringof == "void(out int _param_0)");
 }