[significant_drop_tightening] Add MVP

Author: c410-f3r

CHANGELOG.md

@@ -4734,6 +4734,7 @@ Released 2018-09-13
 [`should_assert_eq`]: https://rust-lang.github.io/rust-clippy/master/index.html#should_assert_eq
 [`should_implement_trait`]: https://rust-lang.github.io/rust-clippy/master/index.html#should_implement_trait
 [`significant_drop_in_scrutinee`]: https://rust-lang.github.io/rust-clippy/master/index.html#significant_drop_in_scrutinee
+[`significant_drop_tightening`]: https://rust-lang.github.io/rust-clippy/master/index.html#significant_drop_tightening
 [`similar_names`]: https://rust-lang.github.io/rust-clippy/master/index.html#similar_names
 [`single_char_add_str`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_char_add_str
 [`single_char_lifetime_names`]: https://rust-lang.github.io/rust-clippy/master/index.html#single_char_lifetime_names

clippy_lints/src/declared_lints.rs

@@ -536,6 +536,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::shadow::SHADOW_REUSE_INFO,
     crate::shadow::SHADOW_SAME_INFO,
     crate::shadow::SHADOW_UNRELATED_INFO,
+    crate::significant_drop_tightening::SIGNIFICANT_DROP_TIGHTENING_INFO,
     crate::single_char_lifetime_names::SINGLE_CHAR_LIFETIME_NAMES_INFO,
     crate::single_component_path_imports::SINGLE_COMPONENT_PATH_IMPORTS_INFO,
     crate::size_of_in_element_count::SIZE_OF_IN_ELEMENT_COUNT_INFO,

clippy_lints/src/lib.rs

@@ -3,6 +3,7 @@
 #![feature(box_patterns)]
 #![feature(control_flow_enum)]
 #![feature(drain_filter)]
+#![feature(if_let_guard)]
 #![feature(iter_intersperse)]
 #![feature(let_chains)]
 #![feature(lint_reasons)]
@@ -264,6 +265,8 @@ mod semicolon_block;
 mod semicolon_if_nothing_returned;
 mod serde_api;
 mod shadow;
+mod sig_drop_aux;
+mod significant_drop_tightening;
 mod single_char_lifetime_names;
 mod single_component_path_imports;
 mod size_of_in_element_count;
@@ -559,6 +562,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|_| Box::new(eta_reduction::EtaReduction));
     store.register_late_pass(|_| Box::new(mut_mut::MutMut));
     store.register_late_pass(|_| Box::new(mut_reference::UnnecessaryMutPassed));
+    store.register_late_pass(|_| Box::new(significant_drop_tightening::SignificantDropTightening::new()));
     store.register_late_pass(|_| Box::new(len_zero::LenZero));
     store.register_late_pass(|_| Box::new(attrs::Attributes));
     store.register_late_pass(|_| Box::new(blocks_in_if_conditions::BlocksInIfConditions));

clippy_lints/src/sig_drop_aux.rs

@@ -0,0 +1,58 @@
+// Auxiliary structures for lints involving `#[clippy::has_significant_drop]`.
+
+use crate::FxHashSet;
+use clippy_utils::get_attr;
+use rustc_lint::{LateContext, LintContext};
+use rustc_middle::ty::{subst::GenericArgKind, Ty, TypeAndMut};
+
+pub(crate) struct SigDropChecker<'cx, 'sdt, 'tcx> {
+    cx: &'cx LateContext<'tcx>,
+    seen_types: &'sdt mut FxHashSet<Ty<'tcx>>,
+}
+
+impl<'cx, 'sdt, 'tcx> SigDropChecker<'cx, 'sdt, 'tcx> {
+    pub(crate) fn new(cx: &'cx LateContext<'tcx>, seen_types: &'sdt mut FxHashSet<Ty<'tcx>>) -> Self {
+        seen_types.clear();
+        Self { cx, seen_types }
+    }
+
+    pub(crate) fn has_sig_drop_attr(&mut self, ty: Ty<'tcx>) -> bool {
+        if let Some(adt) = ty.ty_adt_def() {
+            let iter = get_attr(
+                self.cx.sess(),
+                self.cx.tcx.get_attrs_unchecked(adt.did()),
+                "has_significant_drop",
+            );
+            if iter.count() > 0 {
+                return true;
+            }
+        }
+        match ty.kind() {
+            rustc_middle::ty::Adt(a, b) => {
+                for f in a.all_fields() {
+                    let ty = f.ty(self.cx.tcx, b);
+                    if !self.has_seen_ty(ty) && self.has_sig_drop_attr(ty) {
+                        return true;
+                    }
+                }
+                for generic_arg in b.iter() {
+                    if let GenericArgKind::Type(ty) = generic_arg.unpack() {
+                        if self.has_sig_drop_attr(ty) {
+                            return true;
+                        }
+                    }
+                }
+                false
+            },
+            rustc_middle::ty::Array(ty, _)
+            | rustc_middle::ty::RawPtr(TypeAndMut { ty, .. })
+            | rustc_middle::ty::Ref(_, ty, _)
+            | rustc_middle::ty::Slice(ty) => self.has_sig_drop_attr(*ty),
+            _ => false,
+        }
+    }
+
+    fn has_seen_ty(&mut self, ty: Ty<'tcx>) -> bool {
+        !self.seen_types.insert(ty)
+    }
+}