From 068d87a036f0efd4c67e49878e76aecc15f250fb Mon Sep 17 00:00:00 2001
From: Walter Bright <walter@walterbright.com>
Date: Thu, 31 Oct 2024 18:58:51 -0700
Subject: [PATCH] add __traits getBitfieldOffset and getBitfieldWidth

---
 changelog/dmd.getBitfieldInfo.dd            | 16 ++++++++++
 compiler/src/dmd/frontend.h                 |  2 ++
 compiler/src/dmd/id.d                       |  2 ++
 compiler/src/dmd/traits.d                   | 35 +++++++++++++++++++++
 compiler/test/fail_compilation/biterrors5.d | 25 +++++++++++++++
 5 files changed, 80 insertions(+)
 create mode 100644 changelog/dmd.getBitfieldInfo.dd
 create mode 100644 compiler/test/fail_compilation/biterrors5.d

diff --git a/changelog/dmd.getBitfieldInfo.dd b/changelog/dmd.getBitfieldInfo.dd
new file mode 100644
index 000000000000..8d797184cdb8
--- /dev/null
+++ b/changelog/dmd.getBitfieldInfo.dd
@@ -0,0 +1,16 @@
+Add __traits getBitfieldOffset and getBitfieldWidth
+
+This completes the introspection capabilities of builtin bitfields. For example:
+
+---
+struct S
+{
+    int a,b;
+    int :2, c:3;
+}
+
+static assert(__traits(getBitfieldOffset, S.b) == 0);
+static assert(__traits(getBitfieldOffset, S.c) == 2);
+static assert(__traits(getBitfieldWidth, S.b) == 32);
+static assert(__traits(getBitfieldWidth, S.c) == 3);
+---
diff --git a/compiler/src/dmd/frontend.h b/compiler/src/dmd/frontend.h
index 4162052b111f..659bd5af576a 100644
--- a/compiler/src/dmd/frontend.h
+++ b/compiler/src/dmd/frontend.h
@@ -8923,6 +8923,8 @@ struct Id final
     static Identifier* hasMember;
     static Identifier* identifier;
     static Identifier* fullyQualifiedName;
+    static Identifier* getBitfieldOffset;
+    static Identifier* getBitfieldWidth;
     static Identifier* getProtection;
     static Identifier* getVisibility;
     static Identifier* parent;
diff --git a/compiler/src/dmd/id.d b/compiler/src/dmd/id.d
index f676361d958b..b60172d0518d 100644
--- a/compiler/src/dmd/id.d
+++ b/compiler/src/dmd/id.d
@@ -484,6 +484,8 @@ immutable Msgtable[] msgtable =
     { "hasMember" },
     { "identifier" },
     { "fullyQualifiedName" },
+    { "getBitfieldOffset" },
+    { "getBitfieldWidth" },
     { "getProtection" },
     { "getVisibility" },
     { "parent" },
diff --git a/compiler/src/dmd/traits.d b/compiler/src/dmd/traits.d
index 7b32209fd2ef..5bf7d7ce8007 100644
--- a/compiler/src/dmd/traits.d
+++ b/compiler/src/dmd/traits.d
@@ -776,6 +776,41 @@ Expression semanticTraits(TraitsExp e, Scope* sc)
         return se.expressionSemantic(sc);
 
     }
+    if (e.ident == Id.getBitfieldOffset || e.ident == Id.getBitfieldWidth)
+    {
+        if (dim != 1)
+            return dimError(1);
+
+        auto o = (*e.args)[0];
+        auto s = getDsymbolWithoutExpCtx(o);
+        if (!s)
+        {
+            error(e.loc, "bitfield symbol expected not `%s`", o.toChars());
+            return ErrorExp.get();
+        }
+
+        auto vd = s.toAlias.isVarDeclaration();
+        if (!vd || !(vd.storage_class & STC.field))
+        {
+            error(e.loc, "bitfield symbol expected not %s `%s`", s.kind, s.toPrettyChars);
+            return ErrorExp.get();
+        }
+
+        uint fieldWidth;
+        uint bitOffset;
+        if (auto bf = vd.isBitFieldDeclaration())
+        {
+            fieldWidth = bf.fieldWidth;
+            bitOffset  = bf.bitOffset;
+        }
+        else // just a regular field
+        {
+            fieldWidth = cast(uint)size(vd.type) * 8;
+            bitOffset  = 0;
+        }
+        uint value = e.ident == Id.getBitfieldOffset ? bitOffset : fieldWidth;
+        return new IntegerExp(e.loc, value, Type.tuns32);
+    }
     if (e.ident == Id.getProtection || e.ident == Id.getVisibility)
     {
         if (dim != 1)
diff --git a/compiler/test/fail_compilation/biterrors5.d b/compiler/test/fail_compilation/biterrors5.d
new file mode 100644
index 000000000000..2665d69cc98e
--- /dev/null
+++ b/compiler/test/fail_compilation/biterrors5.d
@@ -0,0 +1,25 @@
+/* REQUIRED_ARGS: -preview=bitfields
+ * TEST_OUTPUT:
+---
+fail_compilation/biterrors5.d(23): Error: bitfield symbol expected not struct `biterrors5.S`
+fail_compilation/biterrors5.d(24): Error: bitfield symbol expected not variable `biterrors5.test0.i`
+---
+*/
+
+struct S
+{
+    int a,b;
+    int :2, c:3;
+}
+
+static assert(__traits(getBitfieldOffset, S.b) == 0);
+static assert(__traits(getBitfieldWidth, S.b) == 32);
+static assert(__traits(getBitfieldOffset, S.c) == 2);
+static assert(__traits(getBitfieldWidth, S.c) == 3);
+
+void test0()
+{
+    int i;
+    i = __traits(getBitfieldOffset, S);
+    i = __traits(getBitfieldOffset, i);
+}