let qualify_min_const_fn deal with drop checks

Author: y21

clippy_lints/src/missing_const_for_fn.rs

@@ -1,7 +1,6 @@
 use clippy_config::msrvs::{self, Msrv};
 use clippy_utils::diagnostics::span_lint;
 use clippy_utils::qualify_min_const_fn::is_min_const_fn;
-use clippy_utils::ty::has_drop;
 use clippy_utils::{fn_has_unsatisfiable_preds, is_entrypoint_fn, is_from_proc_macro, trait_ref_of_method};
 use rustc_hir as hir;
 use rustc_hir::def_id::CRATE_DEF_ID;
@@ -121,10 +120,7 @@ impl<'tcx> LateLintPass<'tcx> for MissingConstForFn {
                 }
             },
             FnKind::Method(_, sig, ..) => {
-                if trait_ref_of_method(cx, def_id).is_some()
-                    || already_const(sig.header)
-                    || method_accepts_droppable(cx, def_id)
-                {
+                if trait_ref_of_method(cx, def_id).is_some() || already_const(sig.header) {
                     return;
                 }
             },
@@ -162,15 +158,6 @@ impl<'tcx> LateLintPass<'tcx> for MissingConstForFn {
     extract_msrv_attr!(LateContext);
 }
 
-/// Returns true if any of the method parameters is a type that implements `Drop`. The method
-/// can't be made const then, because `drop` can't be const-evaluated.
-fn method_accepts_droppable(cx: &LateContext<'_>, def_id: LocalDefId) -> bool {
-    let sig = cx.tcx.fn_sig(def_id).instantiate_identity().skip_binder();
-
-    // If any of the params are droppable, return true
-    sig.inputs().iter().any(|&ty| has_drop(cx, ty))
-}
-
 // We don't have to lint on something that's already `const`
 #[must_use]
 fn already_const(header: hir::FnHeader) -> bool {

tests/ui/missing_const_for_fn/could_be_const.rs

@@ -113,3 +113,21 @@ impl const Drop for D {
 // Lint this, since it can be dropped in const contexts
 // FIXME(effects)
 fn d(this: D) {}
+
+mod issue12677 {
+    pub struct Wrapper {
+        pub strings: Vec<String>,
+    }
+
+    impl Wrapper {
+        #[must_use]
+        pub fn new(strings: Vec<String>) -> Self {
+            Self { strings }
+        }
+
+        #[must_use]
+        pub fn empty() -> Self {
+            Self { strings: Vec::new() }
+        }
+    }
+}

tests/ui/missing_const_for_fn/could_be_const.stderr

@@ -102,5 +102,21 @@ LL | |     46
 LL | | }
    | |_^
 
-error: aborting due to 11 previous errors
+error: this could be a `const fn`
+  --> tests/ui/missing_const_for_fn/could_be_const.rs:124:9
+   |
+LL | /         pub fn new(strings: Vec<String>) -> Self {
+LL | |             Self { strings }
+LL | |         }
+   | |_________^
+
+error: this could be a `const fn`
+  --> tests/ui/missing_const_for_fn/could_be_const.rs:129:9
+   |
+LL | /         pub fn empty() -> Self {
+LL | |             Self { strings: Vec::new() }
+LL | |         }
+   | |_________^
+
+error: aborting due to 13 previous errors