fix: let-and-return suggests invalid cast (#16135)

changelog: [`let_and_return`]: fix lint suggestion for invalid cast to
raw pointer.

Author: ErinvanderVeen

clippy_lints/src/returns/let_and_return.rs

@@ -45,7 +45,10 @@ pub(super) fn check_block<'tcx>(cx: &LateContext<'tcx>, block: &'tcx Block<'_>)
                         } else {
                             format!("({src})")
                         }
-                    } else if !cx.typeck_results().expr_adjustments(retexpr).is_empty() {
+                    } else if !cx.typeck_results().expr_adjustments(retexpr).is_empty()
+                        // Do not suggest 'as _' for raw pointers; it's an invalid cast
+                        && !cx.typeck_results().expr_ty_adjusted(retexpr).is_raw_ptr()
+                    {
                         if has_enclosing_paren(&src) {
                             format!("{src} as _")
                         } else {

tests/ui/let_and_return.edition2021.fixed

@@ -271,4 +271,12 @@ fn issue15987() -> i32 {
     r
 }
 
+// https://github.com/rust-lang/rust-clippy/issues/16135
+fn issue16135() -> *const u8 {
+    let boxed_value = Box::new(42u8);
+    
+    Box::into_raw(boxed_value)
+    //~^ let_and_return
+}
+
 fn main() {}

tests/ui/let_and_return.edition2021.stderr

@@ -148,5 +148,19 @@ LL ~
 LL ~             ({ true } || { false } && { 2 <= 3 })
    |
 
-error: aborting due to 10 previous errors
+error: returning the result of a `let` binding from a block
+  --> tests/ui/let_and_return.rs:278:5
+   |
+LL |     let ptr = Box::into_raw(boxed_value);
+   |     ------------------------------------- unnecessary `let` binding
+LL |     ptr
+   |     ^^^
+   |
+help: return the expression directly
+   |
+LL ~     
+LL ~     Box::into_raw(boxed_value)
+   |
+
+error: aborting due to 11 previous errors
 

tests/ui/let_and_return.edition2024.fixed

@@ -271,4 +271,12 @@ fn issue15987() -> i32 {
     r
 }
 
+// https://github.com/rust-lang/rust-clippy/issues/16135
+fn issue16135() -> *const u8 {
+    let boxed_value = Box::new(42u8);
+    
+    Box::into_raw(boxed_value)
+    //~^ let_and_return
+}
+
 fn main() {}

tests/ui/let_and_return.edition2024.stderr

@@ -224,5 +224,19 @@ LL +         None => Ok(Ok(0)),
 LL +     }?
    |
 
-error: aborting due to 15 previous errors
+error: returning the result of a `let` binding from a block
+  --> tests/ui/let_and_return.rs:278:5
+   |
+LL |     let ptr = Box::into_raw(boxed_value);
+   |     ------------------------------------- unnecessary `let` binding
+LL |     ptr
+   |     ^^^
+   |
+help: return the expression directly
+   |
+LL ~     
+LL ~     Box::into_raw(boxed_value)
+   |
+
+error: aborting due to 16 previous errors
 

tests/ui/let_and_return.rs

@@ -271,4 +271,12 @@ fn issue15987() -> i32 {
     r
 }
 
+// https://github.com/rust-lang/rust-clippy/issues/16135
+fn issue16135() -> *const u8 {
+    let boxed_value = Box::new(42u8);
+    let ptr = Box::into_raw(boxed_value);
+    ptr
+    //~^ let_and_return
+}
+
 fn main() {}