Auto merge of rust-lang#8813 - evantypanski:redundant_alloc_fat_ptr, r=Alexendoo

Fix redundant_allocation warning for Rc<Box<str>>

changelog: [`redundant_allocation`] Fixes rust-lang#8604

Fixes false positives where a fat pointer with `str` type was made thin by another allocation, but that thinning allocation was marked as redundant

Author: bors

clippy_lints/src/types/redundant_allocation.rs

@@ -5,6 +5,7 @@ use rustc_errors::Applicability;
 use rustc_hir::{self as hir, def_id::DefId, QPath, TyKind};
 use rustc_lint::LateContext;
 use rustc_span::symbol::sym;
+use rustc_typeck::hir_ty_to_ty;
 
 use super::{utils, REDUNDANT_ALLOCATION};
 
@@ -54,8 +55,10 @@ pub(super) fn check(cx: &LateContext<'_>, hir_ty: &hir::Ty<'_>, qpath: &QPath<'_
     };
     let inner_span = match qpath_generic_tys(inner_qpath).next() {
         Some(ty) => {
-            // Box<Box<dyn T>> is smaller than Box<dyn T> because of wide pointers
-            if matches!(ty.kind, TyKind::TraitObject(..)) {
+            // Reallocation of a fat pointer causes it to become thin. `hir_ty_to_ty` is safe to use
+            // here because `mod.rs` guarantees this lint is only run on types outside of bodies and
+            // is not run on locals.
+            if !hir_ty_to_ty(cx.tcx, ty).is_sized(cx.tcx.at(ty.span), cx.param_env) {
                 return false;
             }
             ty.span

tests/ui/redundant_allocation.rs

@@ -97,4 +97,39 @@ mod box_dyn {
     pub fn test_rc_box(_: Rc<Box<Box<dyn T>>>) {}
 }
 
+// https://github.com/rust-lang/rust-clippy/issues/8604
+mod box_fat_ptr {
+    use std::boxed::Box;
+    use std::path::Path;
+    use std::rc::Rc;
+    use std::sync::Arc;
+
+    pub struct DynSized {
+        foo: [usize],
+    }
+
+    struct S {
+        a: Box<Box<str>>,
+        b: Rc<Box<str>>,
+        c: Arc<Box<str>>,
+
+        e: Box<Box<[usize]>>,
+        f: Box<Box<Path>>,
+        g: Box<Box<DynSized>>,
+    }
+
+    pub fn test_box_str(_: Box<Box<str>>) {}
+    pub fn test_rc_str(_: Rc<Box<str>>) {}
+    pub fn test_arc_str(_: Arc<Box<str>>) {}
+
+    pub fn test_box_slice(_: Box<Box<[usize]>>) {}
+    pub fn test_box_path(_: Box<Box<Path>>) {}
+    pub fn test_box_custom(_: Box<Box<DynSized>>) {}
+
+    pub fn test_rc_box_str(_: Rc<Box<Box<str>>>) {}
+    pub fn test_rc_box_slice(_: Rc<Box<Box<[usize]>>>) {}
+    pub fn test_rc_box_path(_: Rc<Box<Box<Path>>>) {}
+    pub fn test_rc_box_custom(_: Rc<Box<Box<DynSized>>>) {}
+}
+
 fn main() {}

tests/ui/redundant_allocation.stderr

@@ -143,5 +143,41 @@ LL |     pub fn test_rc_box(_: Rc<Box<Box<dyn T>>>) {}
    = note: `Box<Box<dyn T>>` is already on the heap, `Rc<Box<Box<dyn T>>>` makes an extra allocation
    = help: consider using just `Rc<Box<dyn T>>` or `Box<Box<dyn T>>`
 
-error: aborting due to 16 previous errors
+error: usage of `Rc<Box<Box<str>>>`
+  --> $DIR/redundant_allocation.rs:129:31
+   |
+LL |     pub fn test_rc_box_str(_: Rc<Box<Box<str>>>) {}
+   |                               ^^^^^^^^^^^^^^^^^
+   |
+   = note: `Box<Box<str>>` is already on the heap, `Rc<Box<Box<str>>>` makes an extra allocation
+   = help: consider using just `Rc<Box<str>>` or `Box<Box<str>>`
+
+error: usage of `Rc<Box<Box<[usize]>>>`
+  --> $DIR/redundant_allocation.rs:130:33
+   |
+LL |     pub fn test_rc_box_slice(_: Rc<Box<Box<[usize]>>>) {}
+   |                                 ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Box<Box<[usize]>>` is already on the heap, `Rc<Box<Box<[usize]>>>` makes an extra allocation
+   = help: consider using just `Rc<Box<[usize]>>` or `Box<Box<[usize]>>`
+
+error: usage of `Rc<Box<Box<Path>>>`
+  --> $DIR/redundant_allocation.rs:131:32
+   |
+LL |     pub fn test_rc_box_path(_: Rc<Box<Box<Path>>>) {}
+   |                                ^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Box<Box<Path>>` is already on the heap, `Rc<Box<Box<Path>>>` makes an extra allocation
+   = help: consider using just `Rc<Box<Path>>` or `Box<Box<Path>>`
+
+error: usage of `Rc<Box<Box<DynSized>>>`
+  --> $DIR/redundant_allocation.rs:132:34
+   |
+LL |     pub fn test_rc_box_custom(_: Rc<Box<Box<DynSized>>>) {}
+   |                                  ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Box<Box<DynSized>>` is already on the heap, `Rc<Box<Box<DynSized>>>` makes an extra allocation
+   = help: consider using just `Rc<Box<DynSized>>` or `Box<Box<DynSized>>`
+
+error: aborting due to 20 previous errors