Auto merge of rust-lang#8617 - Alexendoo:relax-needless-late-init, r=giraffate

`needless_late_init`: ignore `if let`, `let mut` and significant drops

No longer lints `if let`, personal taste on this one is pretty split, so it probably shouldn't be warning by default. Fixes rust-lang#8613

```rust
let x = if let Some(n) = y {
    n
} else {
    1
}
```

No longer lints `let mut`, things like the following are not uncommon and look fine as they are

https://github.com/shepmaster/twox-hash/blob/b169c16d86eb8ea4a296b0acb9d00ca7e3c3005f/src/sixty_four.rs#L88-L93

Avoids changing the drop order in an observable way, where the type of `x` has a drop with side effects and something between `x` and the first use also does, e.g.

https://github.com/denoland/rusty_v8/blob/48cc6cb791cac57d22fab1a2feaa92da8ddc9a68/tests/test_api.rs#L159-L167

The implementation of `type_needs_ordered_drop_inner` was changed a bit, it now uses `Ty::has_significant_drop` and reordered the ifs to check diagnostic name before checking the implicit drop impl

changelog: [`needless_late_init`]: No longer lints `if let` statements, `let mut` bindings and no longer significantly changes drop order

Author: bors

clippy_lints/src/matches/redundant_pattern_match.rs

@@ -2,17 +2,16 @@ use super::REDUNDANT_PATTERN_MATCHING;
 use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::source::snippet;
 use clippy_utils::sugg::Sugg;
-use clippy_utils::ty::{implements_trait, is_type_diagnostic_item, is_type_lang_item, match_type};
+use clippy_utils::ty::needs_ordered_drop;
 use clippy_utils::{higher, match_def_path};
 use clippy_utils::{is_lang_ctor, is_trait_method, paths};
 use if_chain::if_chain;
 use rustc_ast::ast::LitKind;
-use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::Applicability;
 use rustc_hir::LangItem::{OptionNone, PollPending};
 use rustc_hir::{
     intravisit::{walk_expr, Visitor},
-    Arm, Block, Expr, ExprKind, LangItem, Node, Pat, PatKind, QPath, UnOp,
+    Arm, Block, Expr, ExprKind, Node, Pat, PatKind, QPath, UnOp,
 };
 use rustc_lint::LateContext;
 use rustc_middle::ty::{self, subst::GenericArgKind, DefIdTree, Ty};
@@ -32,59 +31,6 @@ pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
     }
 }
 
-/// Checks if the drop order for a type matters. Some std types implement drop solely to
-/// deallocate memory. For these types, and composites containing them, changing the drop order
-/// won't result in any observable side effects.
-fn type_needs_ordered_drop<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
-    type_needs_ordered_drop_inner(cx, ty, &mut FxHashSet::default())
-}
-
-fn type_needs_ordered_drop_inner<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, seen: &mut FxHashSet<Ty<'tcx>>) -> bool {
-    if !seen.insert(ty) {
-        return false;
-    }
-    if !ty.needs_drop(cx.tcx, cx.param_env) {
-        false
-    } else if !cx
-        .tcx
-        .lang_items()
-        .drop_trait()
-        .map_or(false, |id| implements_trait(cx, ty, id, &[]))
-    {
-        // This type doesn't implement drop, so no side effects here.
-        // Check if any component type has any.
-        match ty.kind() {
-            ty::Tuple(fields) => fields.iter().any(|ty| type_needs_ordered_drop_inner(cx, ty, seen)),
-            ty::Array(ty, _) => type_needs_ordered_drop_inner(cx, *ty, seen),
-            ty::Adt(adt, subs) => adt
-                .all_fields()
-                .map(|f| f.ty(cx.tcx, subs))
-                .any(|ty| type_needs_ordered_drop_inner(cx, ty, seen)),
-            _ => true,
-        }
-    }
-    // Check for std types which implement drop, but only for memory allocation.
-    else if is_type_diagnostic_item(cx, ty, sym::Vec)
-        || is_type_lang_item(cx, ty, LangItem::OwnedBox)
-        || is_type_diagnostic_item(cx, ty, sym::Rc)
-        || is_type_diagnostic_item(cx, ty, sym::Arc)
-        || is_type_diagnostic_item(cx, ty, sym::cstring_type)
-        || is_type_diagnostic_item(cx, ty, sym::BTreeMap)
-        || is_type_diagnostic_item(cx, ty, sym::LinkedList)
-        || match_type(cx, ty, &paths::WEAK_RC)
-        || match_type(cx, ty, &paths::WEAK_ARC)
-    {
-        // Check all of the generic arguments.
-        if let ty::Adt(_, subs) = ty.kind() {
-            subs.types().any(|ty| type_needs_ordered_drop_inner(cx, ty, seen))
-        } else {
-            true
-        }
-    } else {
-        true
-    }
-}
-
 // Extract the generic arguments out of a type
 fn try_get_generic_ty(ty: Ty<'_>, index: usize) -> Option<Ty<'_>> {
     if_chain! {
@@ -115,7 +61,7 @@ fn temporaries_need_ordered_drop<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<
                 // e.g. In `(String::new(), 0).1` the string is a temporary value.
                 ExprKind::AddrOf(_, _, expr) | ExprKind::Field(expr, _) => {
                     if !matches!(expr.kind, ExprKind::Path(_)) {
-                        if type_needs_ordered_drop(self.cx, self.cx.typeck_results().expr_ty(expr)) {
+                        if needs_ordered_drop(self.cx, self.cx.typeck_results().expr_ty(expr)) {
                             self.res = true;
                         } else {
                             self.visit_expr(expr);
@@ -126,7 +72,7 @@ fn temporaries_need_ordered_drop<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<
                 // e.g. In `(vec![0])[0]` the vector is a temporary value.
                 ExprKind::Index(base, index) => {
                     if !matches!(base.kind, ExprKind::Path(_)) {
-                        if type_needs_ordered_drop(self.cx, self.cx.typeck_results().expr_ty(base)) {
+                        if needs_ordered_drop(self.cx, self.cx.typeck_results().expr_ty(base)) {
                             self.res = true;
                         } else {
                             self.visit_expr(base);
@@ -143,7 +89,7 @@ fn temporaries_need_ordered_drop<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<
                             .typeck_results()
                             .type_dependent_def_id(expr.hir_id)
                             .map_or(false, |id| self.cx.tcx.fn_sig(id).skip_binder().inputs()[0].is_ref());
-                        if self_by_ref && type_needs_ordered_drop(self.cx, self.cx.typeck_results().expr_ty(self_arg)) {
+                        if self_by_ref && needs_ordered_drop(self.cx, self.cx.typeck_results().expr_ty(self_arg)) {
                             self.res = true;
                         } else {
                             self.visit_expr(self_arg);
@@ -243,7 +189,7 @@ fn find_sugg_for_if_let<'tcx>(
     // scrutinee would be, so they have to be considered as well.
     // e.g. in `if let Some(x) = foo.lock().unwrap().baz.as_ref() { .. }` the lock will be held
     // for the duration if body.
-    let needs_drop = type_needs_ordered_drop(cx, check_ty) || temporaries_need_ordered_drop(cx, let_expr);
+    let needs_drop = needs_ordered_drop(cx, check_ty) || temporaries_need_ordered_drop(cx, let_expr);
 
     // check that `while_let_on_iterator` lint does not trigger
     if_chain! {

clippy_lints/src/needless_late_init.rs

@@ -1,10 +1,14 @@
 use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::path_to_local;
 use clippy_utils::source::snippet_opt;
-use clippy_utils::visitors::{expr_visitor, is_local_used};
+use clippy_utils::ty::needs_ordered_drop;
+use clippy_utils::visitors::{expr_visitor, expr_visitor_no_bodies, is_local_used};
 use rustc_errors::Applicability;
 use rustc_hir::intravisit::Visitor;
-use rustc_hir::{Block, Expr, ExprKind, HirId, Local, LocalSource, MatchSource, Node, Pat, PatKind, Stmt, StmtKind};
+use rustc_hir::{
+    BindingAnnotation, Block, Expr, ExprKind, HirId, Local, LocalSource, MatchSource, Node, Pat, PatKind, Stmt,
+    StmtKind,
+};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::Span;
@@ -73,6 +77,31 @@ fn contains_assign_expr<'tcx>(cx: &LateContext<'tcx>, stmt: &'tcx Stmt<'tcx>) ->
     seen
 }
 
+fn contains_let(cond: &Expr<'_>) -> bool {
+    let mut seen = false;
+    expr_visitor_no_bodies(|expr| {
+        if let ExprKind::Let(_) = expr.kind {
+            seen = true;
+        }
+
+        !seen
+    })
+    .visit_expr(cond);
+
+    seen
+}
+
+fn stmt_needs_ordered_drop(cx: &LateContext<'_>, stmt: &Stmt<'_>) -> bool {
+    let StmtKind::Local(local) = stmt.kind else { return false };
+    !local.pat.walk_short(|pat| {
+        if let PatKind::Binding(.., None) = pat.kind {
+            !needs_ordered_drop(cx, cx.typeck_results().pat_ty(pat))
+        } else {
+            true
+        }
+    })
+}
+
 #[derive(Debug)]
 struct LocalAssign {
     lhs_id: HirId,
@@ -187,11 +216,14 @@ fn first_usage<'tcx>(
     local_stmt_id: HirId,
     block: &'tcx Block<'tcx>,
 ) -> Option<Usage<'tcx>> {
+    let significant_drop = needs_ordered_drop(cx, cx.typeck_results().node_type(binding_id));
+
     block
         .stmts
         .iter()
         .skip_while(|stmt| stmt.hir_id != local_stmt_id)
         .skip(1)
+        .take_while(|stmt| !significant_drop || !stmt_needs_ordered_drop(cx, stmt))
         .find(|&stmt| is_local_used(cx, stmt, binding_id))
         .and_then(|stmt| match stmt.kind {
             StmtKind::Expr(expr) => Some(Usage {
@@ -258,7 +290,7 @@ fn check<'tcx>(
                 },
             );
         },
-        ExprKind::If(_, then_expr, Some(else_expr)) => {
+        ExprKind::If(cond, then_expr, Some(else_expr)) if !contains_let(cond) => {
             let (applicability, suggestions) = assignment_suggestions(cx, binding_id, [then_expr, else_expr])?;
 
             span_lint_and_then(
@@ -338,7 +370,7 @@ impl<'tcx> LateLintPass<'tcx> for NeedlessLateInit {
             if let Local {
                 init: None,
                 pat: &Pat {
-                    kind: PatKind::Binding(_, binding_id, _, None),
+                    kind: PatKind::Binding(BindingAnnotation::Unannotated, binding_id, _, None),
                     ..
                 },
                 source: LocalSource::Normal,

clippy_utils/src/ty.rs

@@ -3,11 +3,11 @@
 #![allow(clippy::module_name_repetitions)]
 
 use rustc_ast::ast::Mutability;
-use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, DefKind, Res};
 use rustc_hir::def_id::DefId;
-use rustc_hir::{Expr, TyKind, Unsafety};
+use rustc_hir::{Expr, LangItem, TyKind, Unsafety};
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_lint::LateContext;
 use rustc_middle::mir::interpret::{ConstValue, Scalar};
@@ -22,7 +22,7 @@ use rustc_trait_selection::infer::InferCtxtExt;
 use rustc_trait_selection::traits::query::normalize::AtExt;
 use std::iter;
 
-use crate::{match_def_path, must_use_attr, path_res};
+use crate::{match_def_path, must_use_attr, path_res, paths};
 
 // Checks if the given type implements copy.
 pub fn is_copy<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
@@ -83,6 +83,20 @@ pub fn get_associated_type<'tcx>(
         })
 }
 
+/// Get the diagnostic name of a type, e.g. `sym::HashMap`. To check if a type
+/// implements a trait marked with a diagnostic item use [`implements_trait`].
+///
+/// For a further exploitation what diagnostic items are see [diagnostic items] in
+/// rustc-dev-guide.
+///
+/// [Diagnostic Items]: https://rustc-dev-guide.rust-lang.org/diagnostics/diagnostic-items.html
+pub fn get_type_diagnostic_name(cx: &LateContext<'_>, ty: Ty<'_>) -> Option<Symbol> {
+    match ty.kind() {
+        ty::Adt(adt, _) => cx.tcx.get_diagnostic_name(adt.did()),
+        _ => None,
+    }
+}
+
 /// Returns true if ty has `iter` or `iter_mut` methods
 pub fn has_iter_method(cx: &LateContext<'_>, probably_ref_ty: Ty<'_>) -> Option<Symbol> {
     // FIXME: instead of this hard-coded list, we should check if `<adt>::iter`
@@ -319,6 +333,57 @@ pub fn match_type(cx: &LateContext<'_>, ty: Ty<'_>, path: &[&str]) -> bool {
     }
 }
 
+/// Checks if the drop order for a type matters. Some std types implement drop solely to
+/// deallocate memory. For these types, and composites containing them, changing the drop order
+/// won't result in any observable side effects.
+pub fn needs_ordered_drop<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
+    fn needs_ordered_drop_inner<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>, seen: &mut FxHashSet<Ty<'tcx>>) -> bool {
+        if !seen.insert(ty) {
+            return false;
+        }
+        if !ty.has_significant_drop(cx.tcx, cx.param_env) {
+            false
+        }
+        // Check for std types which implement drop, but only for memory allocation.
+        else if is_type_lang_item(cx, ty, LangItem::OwnedBox)
+            || matches!(
+                get_type_diagnostic_name(cx, ty),
+                Some(sym::HashSet | sym::Rc | sym::Arc | sym::cstring_type)
+            )
+            || match_type(cx, ty, &paths::WEAK_RC)
+            || match_type(cx, ty, &paths::WEAK_ARC)
+        {
+            // Check all of the generic arguments.
+            if let ty::Adt(_, subs) = ty.kind() {
+                subs.types().any(|ty| needs_ordered_drop_inner(cx, ty, seen))
+            } else {
+                true
+            }
+        } else if !cx
+            .tcx
+            .lang_items()
+            .drop_trait()
+            .map_or(false, |id| implements_trait(cx, ty, id, &[]))
+        {
+            // This type doesn't implement drop, so no side effects here.
+            // Check if any component type has any.
+            match ty.kind() {
+                ty::Tuple(fields) => fields.iter().any(|ty| needs_ordered_drop_inner(cx, ty, seen)),
+                ty::Array(ty, _) => needs_ordered_drop_inner(cx, *ty, seen),
+                ty::Adt(adt, subs) => adt
+                    .all_fields()
+                    .map(|f| f.ty(cx.tcx, subs))
+                    .any(|ty| needs_ordered_drop_inner(cx, ty, seen)),
+                _ => true,
+            }
+        } else {
+            true
+        }
+    }
+
+    needs_ordered_drop_inner(cx, ty, &mut FxHashSet::default())
+}
+
 /// Peels off all references on the type. Returns the underlying type and the number of references
 /// removed.
 pub fn peel_mid_ty_refs(ty: Ty<'_>) -> (Ty<'_>, usize) {