Warn write-only fields

compiler/rustc_passes/src/dead.rs

@@ -37,6 +37,19 @@ fn should_explore(tcx: TyCtxt<'_>, hir_id: hir::HirId) -> bool {
     )
 }
 
+fn base_expr<'a>(expr: &'a hir::Expr<'a>) -> &'a hir::Expr<'a> {
+    let mut current = expr;
+    loop {
+        match current.kind {
+            hir::ExprKind::Field(base, ..) => {
+                current = base;
+            }
+            _ => break,
+        }
+    }
+    current
+}
+
 struct MarkSymbolVisitor<'tcx> {
     worklist: Vec<hir::HirId>,
     tcx: TyCtxt<'tcx>,
@@ -263,6 +276,12 @@ impl<'tcx> Visitor<'tcx> for MarkSymbolVisitor<'tcx> {
             hir::ExprKind::MethodCall(..) => {
                 self.lookup_and_handle_method(expr.hir_id);
             }
+            hir::ExprKind::Assign(ref left, ref right, ..) => {
+                // Ignore write to field
+                self.visit_expr(base_expr(left));
+                self.visit_expr(right);
+                return;
+            }
             hir::ExprKind::Field(ref lhs, ..) => {
                 self.handle_field_access(&lhs, expr.hir_id);
             }

src/test/ui/borrowck/borrowck-assign-to-subfield.rs

@@ -1,5 +1,6 @@
 // run-pass
 // pretty-expanded FIXME #23616
+#![allow(dead_code)]
 
 pub fn main() {
     struct A {

src/test/ui/lint/dead-code/write-only-field.rs

@@ -0,0 +1,20 @@
+#![deny(dead_code)]
+
+struct S {
+    f: i32, //~ ERROR: field is never read
+    sub: Sub, //~ ERROR: field is never read
+}
+
+struct Sub {
+    f: i32, //~ ERROR: field is never read
+}
+
+fn field_write(s: &mut S) {
+    s.f = 1;
+    s.sub.f = 2;
+}
+
+fn main() {
+    let mut s = S { f: 0, sub: Sub { f: 0 } };
+    field_write(&mut s);
+}

src/test/ui/lint/dead-code/write-only-field.stderr

@@ -0,0 +1,26 @@
+error: field is never read: `f`
+  --> $DIR/write-only-field.rs:4:5
+   |
+LL |     f: i32,
+   |     ^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/write-only-field.rs:1:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: field is never read: `sub`
+  --> $DIR/write-only-field.rs:5:5
+   |
+LL |     sub: Sub,
+   |     ^^^^^^^^
+
+error: field is never read: `f`
+  --> $DIR/write-only-field.rs:9:5
+   |
+LL |     f: i32,
+   |     ^^^^^^
+
+error: aborting due to 3 previous errors
+