From 968996fb3e1ee343da8754efa4e8b3b034edabf8 Mon Sep 17 00:00:00 2001
From: Nicholas Wilson <iamthewilsonator@hotmail.com>
Date: Sat, 21 Dec 2024 10:39:11 +0800
Subject: [PATCH 1/2] Fix #20583 - Initialisation of `@mustuse` member variable
 causes spurious error

---
 compiler/src/dmd/mustuse.d                 | 24 +++++++++++++++++++++-
 compiler/test/compilable/must_use_assign.d | 14 +++++++++++++
 2 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/compiler/src/dmd/mustuse.d b/compiler/src/dmd/mustuse.d
index fc7618bfc0ed..8ad89ebbad43 100644
--- a/compiler/src/dmd/mustuse.d
+++ b/compiler/src/dmd/mustuse.d
@@ -38,7 +38,10 @@ bool checkMustUse(Expression e, Scope* sc)
     {
         auto sd = sym.isStructDeclaration();
         // isStructDeclaration returns non-null for both structs and unions
-        if (sd && hasMustUseAttribute(sd, sc) && !isAssignment(e) && !isIncrementOrDecrement(e))
+        if (sd && hasMustUseAttribute(sd, sc)
+            && !isAssignment(e)
+            && !isCtorAssignment(e)
+            && !isIncrementOrDecrement(e))
         {
             error(e.loc, "ignored value of `@%s` type `%s`; prepend a `cast(void)` if intentional",
                 Id.udaMustUse.toChars(), e.type.toPrettyChars(true));
@@ -104,6 +107,25 @@ private bool isAssignment(Expression e)
     return false;
 }
 
+/**
+ * Returns: true if `e` is
+ *     `(this.field = typeof(this.field).init, this.field.__ctor(arg))`
+ */
+private bool isCtorAssignment(Expression e)
+{
+    import dmd.id : Id;
+
+    auto comma = e.isCommaExp();
+    if (!comma)
+        return false;
+    if (!isAssignment(comma.e1))
+        return false;
+    auto call = comma.e2.isCallExp();
+    if (!call)
+        return false;
+    return call.f.ident == Id.ctor;
+}
+
 /**
  * Returns: true if id is the identifier of an assignment operator overload.
  */
diff --git a/compiler/test/compilable/must_use_assign.d b/compiler/test/compilable/must_use_assign.d
index bd1983a40e63..6fae4a5e4238 100644
--- a/compiler/test/compilable/must_use_assign.d
+++ b/compiler/test/compilable/must_use_assign.d
@@ -7,3 +7,17 @@ void test()
     S a, b;
     a = b;
 }
+
+@mustuse struct Inner
+{
+    this(int n) {}
+}
+
+struct Outer
+{
+    Inner inner;
+    this(int n)
+    {
+        this.inner = n;
+    }
+}

From 848fcc541bf029b8c08fc000b78e5875205f58be Mon Sep 17 00:00:00 2001
From: Nicholas Wilson <thewilsonator@users.noreply.github.com>
Date: Sat, 21 Dec 2024 11:25:02 +0800
Subject: [PATCH 2/2] Update compiler/src/dmd/mustuse.d

Co-authored-by: Paul Backus <snarwin@gmail.com>
---
 compiler/src/dmd/mustuse.d | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/compiler/src/dmd/mustuse.d b/compiler/src/dmd/mustuse.d
index 8ad89ebbad43..2dfe2ed5b665 100644
--- a/compiler/src/dmd/mustuse.d
+++ b/compiler/src/dmd/mustuse.d
@@ -108,8 +108,8 @@ private bool isAssignment(Expression e)
 }
 
 /**
- * Returns: true if `e` is
- *     `(this.field = typeof(this.field).init, this.field.__ctor(arg))`
+ * Returns: true if `e` is an assignment that's been rewritten to a
+ * constructor call.
  */
 private bool isCtorAssignment(Expression e)
 {