Auto merge of #6682 - camsteffen:let-underscore-ref, r=llogiq

Fix let_underscore_drop FP

changelog: Fix let_underscore_drop false positives and negatives

Fixes #6633

clippy_dev/src/serve.rs

@@ -34,7 +34,7 @@ pub fn run(port: u16, lint: Option<&str>) -> ! {
                 // Give some time for python to start
                 thread::sleep(Duration::from_millis(500));
                 // Launch browser after first export.py has completed and http.server is up
-                let _ = opener::open(url);
+                let _result = opener::open(url);
             });
         }
         thread::sleep(Duration::from_millis(1000));

clippy_lints/src/let_underscore.rs

@@ -5,7 +5,7 @@ use rustc_middle::lint::in_external_macro;
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 
-use crate::utils::{implements_trait, is_must_use_func_call, is_must_use_ty, match_type, paths, span_lint_and_help};
+use crate::utils::{is_must_use_func_call, is_must_use_ty, match_type, paths, span_lint_and_help};
 
 declare_clippy_lint! {
     /// **What it does:** Checks for `let _ = <expr>`
@@ -125,15 +125,6 @@ impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
 
                     GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => false,
                 });
-                let implements_drop = cx.tcx.lang_items().drop_trait().map_or(false, |drop_trait|
-                    init_ty.walk().any(|inner| match inner.unpack() {
-                        GenericArgKind::Type(inner_ty) => {
-                            implements_trait(cx, inner_ty, drop_trait, &[])
-                        },
-
-                        GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => false,
-                    })
-                );
                 if contains_sync_guard {
                     span_lint_and_help(
                         cx,
@@ -144,7 +135,7 @@ impl<'tcx> LateLintPass<'tcx> for LetUnderscore {
                         "consider using an underscore-prefixed named \
                             binding or dropping explicitly with `std::mem::drop`"
                     )
-                } else if implements_drop {
+                } else if init_ty.needs_drop(cx.tcx, cx.param_env) {
                     span_lint_and_help(
                         cx,
                         LET_UNDERSCORE_DROP,

clippy_lints/src/macro_use.rs

@@ -160,7 +160,7 @@ impl<'tcx> LateLintPass<'tcx> for MacroUseImports {
             let found_idx = self.mac_refs.iter().position(|mac| import.ends_with(&mac.name));
 
             if let Some(idx) = found_idx {
-                let _ = self.mac_refs.remove(idx);
+                self.mac_refs.remove(idx);
                 let seg = import.split("::").collect::<Vec<_>>();
 
                 match seg.as_slice() {

src/main.rs

@@ -195,7 +195,7 @@ mod tests {
     #[should_panic]
     fn fix_without_unstable() {
         let args = "cargo clippy --fix".split_whitespace().map(ToString::to_string);
-        let _ = ClippyCmd::new(args);
+        ClippyCmd::new(args);
     }
 
     #[test]

tests/ui/let_underscore_drop.rs

@@ -16,4 +16,12 @@ fn main() {
     let _ = Box::new(());
     let _ = Droppable;
     let _ = Some(Droppable);
+
+    // no lint for reference
+    let _ = droppable_ref();
+}
+
+#[must_use]
+fn droppable_ref() -> &'static mut Droppable {
+    unimplemented!()
 }