Rollup merge of #104363 - WaffleLapkin:bonk_box_new, r=Nilstrieb

Make `unused_allocation` lint against `Box::new` too

Previously it only linted against `box` syntax, which likely won't ever be stabilized, which is pretty useless. Even now I'm not sure if it's a meaningful lint, but it's at least something 🤷

This means that code like the following will be linted against:
```rust
Box::new([1, 2, 3]).len();
f(&Box::new(1)); // where f : &i32 -> ()
```
The lint works by checking if a `Box::new` (or `box`) expression has an a borrow adjustment, meaning that the code that first stores the box in a variable won't be linted against:
```rust
let boxed = Box::new([1, 2, 3]); // no lint
boxed.len();
```

Author: matthiaskrgr

compiler/rustc_lint/src/unused.rs

@@ -1349,9 +1349,8 @@ declare_lint! {
     /// ### Example
     ///
     /// ```rust
-    /// #![feature(box_syntax)]
     /// fn main() {
-    ///     let a = (box [1, 2, 3]).len();
+    ///     let a = Box::new([1, 2, 3]).len();
     /// }
     /// ```
     ///
@@ -1373,6 +1372,11 @@ impl<'tcx> LateLintPass<'tcx> for UnusedAllocation {
     fn check_expr(&mut self, cx: &LateContext<'_>, e: &hir::Expr<'_>) {
         match e.kind {
             hir::ExprKind::Box(_) => {}
+            hir::ExprKind::Call(path_expr, [_])
+                if let hir::ExprKind::Path(qpath) = &path_expr.kind
+                && let Some(did) = cx.qpath_res(qpath, path_expr.hir_id).opt_def_id()
+                && cx.tcx.is_diagnostic_item(sym::box_new, did)
+                => {}
             _ => return,
         }
 

compiler/rustc_span/src/symbol.rs

@@ -429,6 +429,7 @@ symbols! {
         borrowck_graphviz_format,
         borrowck_graphviz_postflow,
         box_free,
+        box_new,
         box_patterns,
         box_syntax,
         bpf_target_feature,

library/alloc/src/boxed.rs

@@ -214,6 +214,7 @@ impl<T> Box<T> {
     #[inline(always)]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use]
+    #[rustc_diagnostic_item = "box_new"]
     pub fn new(x: T) -> Self {
         #[rustc_box]
         Box::new(x)

library/alloc/src/tests.rs

@@ -4,7 +4,6 @@ use core::any::Any;
 use core::clone::Clone;
 use core::convert::TryInto;
 use core::ops::Deref;
-use core::result::Result::{Err, Ok};
 
 use std::boxed::Box;
 
@@ -15,32 +14,25 @@ fn test_owned_clone() {
     assert!(a == b);
 }
 
-#[derive(PartialEq, Eq)]
+#[derive(Debug, PartialEq, Eq)]
 struct Test;
 
 #[test]
 fn any_move() {
     let a = Box::new(8) as Box<dyn Any>;
     let b = Box::new(Test) as Box<dyn Any>;
 
-    match a.downcast::<i32>() {
-        Ok(a) => {
-            assert!(a == Box::new(8));
-        }
-        Err(..) => panic!(),
-    }
-    match b.downcast::<Test>() {
-        Ok(a) => {
-            assert!(a == Box::new(Test));
-        }
-        Err(..) => panic!(),
-    }
+    let a: Box<i32> = a.downcast::<i32>().unwrap();
+    assert_eq!(*a, 8);
+
+    let b: Box<Test> = b.downcast::<Test>().unwrap();
+    assert_eq!(*b, Test);
 
     let a = Box::new(8) as Box<dyn Any>;
     let b = Box::new(Test) as Box<dyn Any>;
 
-    assert!(a.downcast::<Box<Test>>().is_err());
-    assert!(b.downcast::<Box<i32>>().is_err());
+    assert!(a.downcast::<Box<i32>>().is_err());
+    assert!(b.downcast::<Box<Test>>().is_err());
 }
 
 #[test]

tests/ui/issues/issue-3029.rs

@@ -2,9 +2,7 @@
 // error-pattern:so long
 // ignore-emscripten no processes
 
-#![allow(unused_allocation)]
 #![allow(unreachable_code)]
-#![allow(unused_variables)]
 
 fn main() {
     let mut x = Vec::new();

tests/ui/iterators/into-iter-on-arrays-lint.fixed

@@ -2,7 +2,7 @@
 // run-rustfix
 // rustfix-only-machine-applicable
 
-#[allow(unused_must_use)]
+#[allow(unused_must_use, unused_allocation)]
 fn main() {
     let small = [1, 2];
     let big = [0u8; 33];

tests/ui/iterators/into-iter-on-arrays-lint.rs

@@ -2,7 +2,7 @@
 // run-rustfix
 // rustfix-only-machine-applicable
 
-#[allow(unused_must_use)]
+#[allow(unused_must_use, unused_allocation)]
 fn main() {
     let small = [1, 2];
     let big = [0u8; 33];

tests/ui/lint/unused/unused-allocation.rs

@@ -0,0 +1,7 @@
+#![feature(rustc_attrs, stmt_expr_attributes)]
+#![deny(unused_allocation)]
+
+fn main() {
+    _ = (#[rustc_box] Box::new([1])).len(); //~ error: unnecessary allocation, use `&` instead
+    _ = Box::new([1]).len(); //~ error: unnecessary allocation, use `&` instead
+}

tests/ui/lint/unused/unused-allocation.stderr

@@ -0,0 +1,20 @@
+error: unnecessary allocation, use `&` instead
+  --> $DIR/unused-allocation.rs:5:9
+   |
+LL |     _ = (#[rustc_box] Box::new([1])).len();
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-allocation.rs:2:9
+   |
+LL | #![deny(unused_allocation)]
+   |         ^^^^^^^^^^^^^^^^^
+
+error: unnecessary allocation, use `&` instead
+  --> $DIR/unused-allocation.rs:6:9
+   |
+LL |     _ = Box::new([1]).len();
+   |         ^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+

tests/ui/self/arbitrary_self_types_trait.rs

@@ -1,4 +1,5 @@
 // run-pass
+#![allow(unused_allocation)]
 
 use std::rc::Rc;
 
@@ -13,7 +14,7 @@ impl Trait for Vec<i32> {
 }
 
 fn main() {
-    let v = vec![1,2,3];
+    let v = vec![1, 2, 3];
 
-    assert_eq!(&[1,2,3], Box::new(Rc::new(v)).trait_method());
+    assert_eq!(&[1, 2, 3], Box::new(Rc::new(v)).trait_method());
 }