fix unused assigment issue for variable with drop, issue 148418

Author: chenyukang

compiler/rustc_mir_transform/src/liveness.rs

@@ -981,8 +981,10 @@ impl<'a, 'tcx> AssignmentResult<'a, 'tcx> {
                     self.checked_places,
                     self.body,
                 ) {
-                    statements.clear();
-                    continue;
+                    statements.retain(|_, access| access.is_direct);
+                    if statements.is_empty() {
+                        continue;
+                    }
                 }
 
                 let typo = maybe_suggest_typo();

tests/ui/drop/or-pattern-drop-order.rs

@@ -33,15 +33,15 @@ fn main() {
         // Drops are right-to-left: `z`, `y`, `x`.
         let (x, Ok(y) | Err(y), z);
         // Assignment order doesn't matter.
-        z = LogDrop(o, 1);
-        y = LogDrop(o, 2);
-        x = LogDrop(o, 3);
+        z = LogDrop(o, 1); //~ WARN value assigned to `z` is never read
+        y = LogDrop(o, 2); //~ WARN value assigned to `y` is never read
+        x = LogDrop(o, 3); //~ WARN value assigned to `x` is never read
     });
     assert_drop_order(1..=2, |o| {
         // The first or-pattern alternative determines the bindings' drop order: `y`, `x`.
         let ((true, x, y) | (false, y, x));
-        x = LogDrop(o, 2);
-        y = LogDrop(o, 1);
+        x = LogDrop(o, 2); //~ WARN value assigned to `x` is never read
+        y = LogDrop(o, 1); //~ WARN value assigned to `y` is never read
     });
 
     // `let pat = expr;` should have the same drop order.
@@ -61,15 +61,21 @@ fn main() {
     // `match` should have the same drop order.
     assert_drop_order(1..=3, |o| {
         // Drops are right-to-left: `z`, `y` `x`.
-        match (LogDrop(o, 3), Ok(LogDrop(o, 2)), LogDrop(o, 1)) { (x, Ok(y) | Err(y), z) => {} }
+        match (LogDrop(o, 3), Ok(LogDrop(o, 2)), LogDrop(o, 1)) {
+            (x, Ok(y) | Err(y), z) => {}
+        }
     });
     assert_drop_order(1..=2, |o| {
         // The first or-pattern alternative determines the bindings' drop order: `y`, `x`.
-        match (true, LogDrop(o, 2), LogDrop(o, 1)) { (true, x, y) | (false, y, x) => {} }
+        match (true, LogDrop(o, 2), LogDrop(o, 1)) {
+            (true, x, y) | (false, y, x) => {}
+        }
     });
     assert_drop_order(1..=2, |o| {
         // That drop order is used regardless of which or-pattern alternative matches: `y`, `x`.
-        match (false, LogDrop(o, 1), LogDrop(o, 2)) { (true, x, y) | (false, y, x) => {} }
+        match (false, LogDrop(o, 1), LogDrop(o, 2)) {
+            (true, x, y) | (false, y, x) => {}
+        }
     });
 
     // Function params are visited one-by-one, and the order of bindings within a param's pattern is

tests/ui/drop/or-pattern-drop-order.stderr

@@ -0,0 +1,43 @@
+warning: value assigned to `x` is never read
+  --> $DIR/or-pattern-drop-order.rs:43:9
+   |
+LL |         x = LogDrop(o, 2);
+   |         ^
+   |
+   = help: maybe it is overwritten before being read?
+   = note: `#[warn(unused_assignments)]` (part of `#[warn(unused)]`) on by default
+
+warning: value assigned to `y` is never read
+  --> $DIR/or-pattern-drop-order.rs:44:9
+   |
+LL |         y = LogDrop(o, 1);
+   |         ^
+   |
+   = help: maybe it is overwritten before being read?
+
+warning: value assigned to `x` is never read
+  --> $DIR/or-pattern-drop-order.rs:38:9
+   |
+LL |         x = LogDrop(o, 3);
+   |         ^
+   |
+   = help: maybe it is overwritten before being read?
+
+warning: value assigned to `y` is never read
+  --> $DIR/or-pattern-drop-order.rs:37:9
+   |
+LL |         y = LogDrop(o, 2);
+   |         ^
+   |
+   = help: maybe it is overwritten before being read?
+
+warning: value assigned to `z` is never read
+  --> $DIR/or-pattern-drop-order.rs:36:9
+   |
+LL |         z = LogDrop(o, 1);
+   |         ^
+   |
+   = help: maybe it is overwritten before being read?
+
+warning: 5 warnings emitted
+

tests/ui/lint/unused/unused-assign-148960.rs

@@ -1,5 +1,6 @@
 //@ check-fail
 #![deny(unused)]
+#![allow(dead_code)]
 
 fn test_one_extra_assign() {
     let mut value = b"0".to_vec(); //~ ERROR value assigned to `value` is never read
@@ -26,8 +27,16 @@ fn test_indirect_assign() {
     println!("{}", p.y);
 }
 
-fn main() {
-    test_one_extra_assign();
-    test_two_extra_assign();
-    test_indirect_assign();
+struct Foo;
+
+impl Drop for Foo {
+    fn drop(&mut self) {}
+}
+
+// testcase for issue #148418
+fn test_unused_variable() {
+    let mut foo = Foo; //~ ERROR variable `foo` is assigned to, but never used
+    foo = Foo; //~ ERROR value assigned to `foo` is never read
 }
+
+fn main() {}

tests/ui/lint/unused/unused-assign-148960.stderr

@@ -1,5 +1,5 @@
 error: value assigned to `value` is never read
-  --> $DIR/unused-assign-148960.rs:5:21
+  --> $DIR/unused-assign-148960.rs:6:21
    |
 LL |     let mut value = b"0".to_vec();
    |                     ^^^^^^^^^^^^^
@@ -13,28 +13,50 @@ LL | #![deny(unused)]
    = note: `#[deny(unused_assignments)]` implied by `#[deny(unused)]`
 
 error: value assigned to `x` is never read
-  --> $DIR/unused-assign-148960.rs:11:17
+  --> $DIR/unused-assign-148960.rs:12:17
    |
 LL |     let mut x = 1;
    |                 ^
    |
    = help: maybe it is overwritten before being read?
 
 error: value assigned to `x` is never read
-  --> $DIR/unused-assign-148960.rs:12:5
+  --> $DIR/unused-assign-148960.rs:13:5
    |
 LL |     x = 2;
    |     ^^^^^
    |
    = help: maybe it is overwritten before being read?
 
 error: value assigned to `p` is never read
-  --> $DIR/unused-assign-148960.rs:23:17
+  --> $DIR/unused-assign-148960.rs:24:17
    |
 LL |     let mut p = Point { x: 1, y: 1 };
    |                 ^^^^^^^^^^^^^^^^^^^^
    |
    = help: maybe it is overwritten before being read?
 
-error: aborting due to 4 previous errors
+error: variable `foo` is assigned to, but never used
+  --> $DIR/unused-assign-148960.rs:38:9
+   |
+LL |     let mut foo = Foo;
+   |         ^^^^^^^
+   |
+   = note: consider using `_foo` instead
+   = note: `#[deny(unused_variables)]` implied by `#[deny(unused)]`
+help: you might have meant to pattern match on the similarly named struct `Foo`
+   |
+LL -     let mut foo = Foo;
+LL +     let Foo = Foo;
+   |
+
+error: value assigned to `foo` is never read
+  --> $DIR/unused-assign-148960.rs:39:5
+   |
+LL |     foo = Foo;
+   |     ^^^
+   |
+   = help: maybe it is overwritten before being read?
+
+error: aborting due to 6 previous errors
 

tests/ui/liveness/liveness-unused.rs

@@ -244,8 +244,8 @@ fn f10<T>(mut a: T, b: T) {
     //~^ ERROR value assigned to `a` is never read
 }
 
-fn f10b<T>(mut a: Box<T>, b: Box<T>) {
-    a = b;
+fn f10b<T>(mut a: Box<T>, b: Box<T>) { //~ ERROR variable `a` is assigned to, but never used
+    a = b; //~ ERROR value assigned to `a` is never read
 }
 
 // unused params warnings are not needed for intrinsic functions without bodies

tests/ui/liveness/liveness-unused.stderr

@@ -283,5 +283,21 @@ LL |     a = b;
    |
    = help: maybe it is overwritten before being read?
 
-error: aborting due to 34 previous errors; 1 warning emitted
+error: variable `a` is assigned to, but never used
+  --> $DIR/liveness-unused.rs:247:12
+   |
+LL | fn f10b<T>(mut a: Box<T>, b: Box<T>) {
+   |            ^^^^^
+   |
+   = note: consider using `_a` instead
+
+error: value assigned to `a` is never read
+  --> $DIR/liveness-unused.rs:248:5
+   |
+LL |     a = b;
+   |     ^
+   |
+   = help: maybe it is overwritten before being read?
+
+error: aborting due to 36 previous errors; 1 warning emitted