Auto merge of rust-lang#7978 - smoelius:master, r=llogiq

Add `unnecessary_to_owned` lint

This PR adds a lint to check for unnecessary calls to `ToOwned::to_owned` and other similar functions (e.g., `Cow::into_owned`, `ToString::to_string`, etc.).

The lint checks for expressions of the form `&receiver.to_owned_like()` used in a position requiring type `&T` where one of the following is true:
* `receiver`'s type is `T` exactly
* `receiver`'s type implements `Deref<Target = T>`
* `receiver`'s type implements `AsRef<T>`

The lint additionally checks for expressions of the form `receiver.to_owned_like()` used as arguments of type `impl AsRef<T>`.

It would be nice if the lint could also check for expressions used as arguments to functions like the following:
```
fn foo<T: AsRef<str>>(x: T) { ... }
```
However, I couldn't figure out how to determine whether a function input type was instantiated from a parameter with a trait bound.

If someone could offer me some guidance, I would be happy to add such functionality.

Closes rust-lang#7933

changelog: Add [`unnecessary_to_owned`] lint

Author: bors

CHANGELOG.md

@@ -3210,6 +3210,7 @@ Released 2018-09-13
 [`unnecessary_operation`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_operation
 [`unnecessary_self_imports`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_self_imports
 [`unnecessary_sort_by`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_sort_by
+[`unnecessary_to_owned`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_to_owned
 [`unnecessary_unwrap`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_unwrap
 [`unnecessary_wraps`]: https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_wraps
 [`unneeded_field_pattern`]: https://rust-lang.github.io/rust-clippy/master/index.html#unneeded_field_pattern

clippy_lints/src/lib.register_all.rs

@@ -181,6 +181,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(methods::UNNECESSARY_FILTER_MAP),
     LintId::of(methods::UNNECESSARY_FOLD),
     LintId::of(methods::UNNECESSARY_LAZY_EVALUATIONS),
+    LintId::of(methods::UNNECESSARY_TO_OWNED),
     LintId::of(methods::UNWRAP_OR_ELSE_DEFAULT),
     LintId::of(methods::USELESS_ASREF),
     LintId::of(methods::WRONG_SELF_CONVENTION),

clippy_lints/src/lib.register_lints.rs

@@ -315,6 +315,7 @@ store.register_lints(&[
     methods::UNNECESSARY_FILTER_MAP,
     methods::UNNECESSARY_FOLD,
     methods::UNNECESSARY_LAZY_EVALUATIONS,
+    methods::UNNECESSARY_TO_OWNED,
     methods::UNWRAP_OR_ELSE_DEFAULT,
     methods::UNWRAP_USED,
     methods::USELESS_ASREF,

clippy_lints/src/lib.register_perf.rs

@@ -17,6 +17,7 @@ store.register_group(true, "clippy::perf", Some("clippy_perf"), vec![
     LintId::of(methods::MANUAL_STR_REPEAT),
     LintId::of(methods::OR_FUN_CALL),
     LintId::of(methods::SINGLE_CHAR_PATTERN),
+    LintId::of(methods::UNNECESSARY_TO_OWNED),
     LintId::of(misc::CMP_OWNED),
     LintId::of(mutex_atomic::MUTEX_ATOMIC),
     LintId::of(redundant_clone::REDUNDANT_CLONE),

clippy_lints/src/methods/implicit_clone.rs

@@ -12,15 +12,7 @@ use super::IMPLICIT_CLONE;
 pub fn check(cx: &LateContext<'_>, method_name: &str, expr: &hir::Expr<'_>, recv: &hir::Expr<'_>, span: Span) {
     if_chain! {
         if let Some(method_def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id);
-        if match method_name {
-            "to_os_string" => is_diag_item_method(cx, method_def_id, sym::OsStr),
-            "to_owned" => is_diag_trait_item(cx, method_def_id, sym::ToOwned),
-            "to_path_buf" => is_diag_item_method(cx, method_def_id, sym::Path),
-            "to_vec" => cx.tcx.impl_of_method(method_def_id)
-                .map(|impl_did| Some(impl_did) == cx.tcx.lang_items().slice_alloc_impl())
-                == Some(true),
-            _ => false,
-        };
+        if is_clone_like(cx, method_name, method_def_id);
         let return_type = cx.typeck_results().expr_ty(expr);
         let input_type = cx.typeck_results().expr_ty(recv).peel_refs();
         if let Some(ty_name) = input_type.ty_adt_def().map(|adt_def| cx.tcx.item_name(adt_def.did));
@@ -38,3 +30,22 @@ pub fn check(cx: &LateContext<'_>, method_name: &str, expr: &hir::Expr<'_>, recv
         }
     }
 }
+
+/// Returns true if the named method can be used to clone the receiver.
+/// Note that `to_string` is not flagged by `implicit_clone`. So other lints that call
+/// `is_clone_like` and that do flag `to_string` must handle it separately. See, e.g.,
+/// `is_to_owned_like` in `unnecessary_to_owned.rs`.
+pub fn is_clone_like(cx: &LateContext<'_>, method_name: &str, method_def_id: hir::def_id::DefId) -> bool {
+    match method_name {
+        "to_os_string" => is_diag_item_method(cx, method_def_id, sym::OsStr),
+        "to_owned" => is_diag_trait_item(cx, method_def_id, sym::ToOwned),
+        "to_path_buf" => is_diag_item_method(cx, method_def_id, sym::Path),
+        "to_vec" => {
+            cx.tcx
+                .impl_of_method(method_def_id)
+                .map(|impl_did| Some(impl_did) == cx.tcx.lang_items().slice_alloc_impl())
+                == Some(true)
+        },
+        _ => false,
+    }
+}

clippy_lints/src/methods/mod.rs

@@ -56,7 +56,9 @@ mod suspicious_splitn;
 mod uninit_assumed_init;
 mod unnecessary_filter_map;
 mod unnecessary_fold;
+mod unnecessary_iter_cloned;
 mod unnecessary_lazy_eval;
+mod unnecessary_to_owned;
 mod unwrap_or_else_default;
 mod unwrap_used;
 mod useless_asref;
@@ -1885,6 +1887,32 @@ declare_clippy_lint! {
     "usages of `str::splitn` that can be replaced with `str::split`"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for unnecessary calls to [`ToOwned::to_owned`](https://doc.rust-lang.org/std/borrow/trait.ToOwned.html#tymethod.to_owned)
+    /// and other `to_owned`-like functions.
+    ///
+    /// ### Why is this bad?
+    /// The unnecessary calls result in useless allocations.
+    ///
+    /// ### Example
+    /// ```rust
+    /// let path = std::path::Path::new("x");
+    /// foo(&path.to_string_lossy().to_string());
+    /// fn foo(s: &str) {}
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// let path = std::path::Path::new("x");
+    /// foo(&path.to_string_lossy());
+    /// fn foo(s: &str) {}
+    /// ```
+    #[clippy::version = "1.58.0"]
+    pub UNNECESSARY_TO_OWNED,
+    perf,
+    "unnecessary calls to `to_owned`-like functions"
+}
+
 pub struct Methods {
     avoid_breaking_exported_api: bool,
     msrv: Option<RustcVersion>,
@@ -1964,7 +1992,8 @@ impl_lint_pass!(Methods => [
     MANUAL_STR_REPEAT,
     EXTEND_WITH_DRAIN,
     MANUAL_SPLIT_ONCE,
-    NEEDLESS_SPLITN
+    NEEDLESS_SPLITN,
+    UNNECESSARY_TO_OWNED,
 ]);
 
 /// Extracts a method call name, args, and `Span` of the method name.
@@ -2007,6 +2036,7 @@ impl<'tcx> LateLintPass<'tcx> for Methods {
                 single_char_add_str::check(cx, expr, args);
                 into_iter_on_ref::check(cx, expr, *method_span, method_call.ident.name, args);
                 single_char_pattern::check(cx, expr, method_call.ident.name, args);
+                unnecessary_to_owned::check(cx, expr, method_call.ident.name, args);
             },
             hir::ExprKind::Binary(op, lhs, rhs) if op.node == hir::BinOpKind::Eq || op.node == hir::BinOpKind::Ne => {
                 let mut info = BinaryExprInfo {

clippy_lints/src/methods/unnecessary_iter_cloned.rs

@@ -0,0 +1,177 @@
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::higher::ForLoop;
+use clippy_utils::source::snippet_opt;
+use clippy_utils::ty::{get_associated_type, get_iterator_item_ty, implements_trait};
+use clippy_utils::{fn_def_id, get_parent_expr, path_to_local_id, usage};
+use rustc_errors::Applicability;
+use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
+use rustc_hir::{def_id::DefId, BorrowKind, Expr, ExprKind, HirId, LangItem, Mutability, Pat};
+use rustc_lint::LateContext;
+use rustc_middle::{hir::map::Map, ty};
+use rustc_span::{sym, Symbol};
+
+use super::UNNECESSARY_TO_OWNED;
+
+pub fn check(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>, method_name: Symbol, receiver: &'tcx Expr<'tcx>) -> bool {
+    if_chain! {
+        if let Some(parent) = get_parent_expr(cx, expr);
+        if let Some(callee_def_id) = fn_def_id(cx, parent);
+        if is_into_iter(cx, callee_def_id);
+        then {
+            check_for_loop_iter(cx, parent, method_name, receiver)
+        } else {
+            false
+        }
+    }
+}
+
+/// Checks whether `expr` is an iterator in a `for` loop and, if so, determines whether the
+/// iterated-over items could be iterated over by reference. The reason why `check` above does not
+/// include this code directly is so that it can be called from
+/// `unnecessary_into_owned::check_into_iter_call_arg`.
+pub fn check_for_loop_iter(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx Expr<'tcx>,
+    method_name: Symbol,
+    receiver: &'tcx Expr<'tcx>,
+) -> bool {
+    if_chain! {
+        if let Some(grandparent) = get_parent_expr(cx, expr).and_then(|parent| get_parent_expr(cx, parent));
+        if let Some(ForLoop { pat, body, .. }) = ForLoop::hir(grandparent);
+        let (clone_or_copy_needed, addr_of_exprs) = clone_or_copy_needed(cx, pat, body);
+        if !clone_or_copy_needed;
+        if let Some(receiver_snippet) = snippet_opt(cx, receiver.span);
+        then {
+            let snippet = if_chain! {
+                if let ExprKind::MethodCall(maybe_iter_method_name, _, [collection], _) = receiver.kind;
+                if maybe_iter_method_name.ident.name == sym::iter;
+
+                if let Some(iterator_trait_id) = cx.tcx.get_diagnostic_item(sym::Iterator);
+                let receiver_ty = cx.typeck_results().expr_ty(receiver);
+                if implements_trait(cx, receiver_ty, iterator_trait_id, &[]);
+                if let Some(iter_item_ty) = get_iterator_item_ty(cx, receiver_ty);
+
+                if let Some(into_iterator_trait_id) = cx.tcx.get_diagnostic_item(sym::IntoIterator);
+                let collection_ty = cx.typeck_results().expr_ty(collection);
+                if implements_trait(cx, collection_ty, into_iterator_trait_id, &[]);
+                if let Some(into_iter_item_ty) = get_associated_type(cx, collection_ty, into_iterator_trait_id, "Item");
+
+                if iter_item_ty == into_iter_item_ty;
+                if let Some(collection_snippet) = snippet_opt(cx, collection.span);
+                then {
+                    collection_snippet
+                } else {
+                    receiver_snippet
+                }
+            };
+            span_lint_and_then(
+                cx,
+                UNNECESSARY_TO_OWNED,
+                expr.span,
+                &format!("unnecessary use of `{}`", method_name),
+                |diag| {
+                    diag.span_suggestion(expr.span, "use", snippet, Applicability::MachineApplicable);
+                    for addr_of_expr in addr_of_exprs {
+                        match addr_of_expr.kind {
+                            ExprKind::AddrOf(_, _, referent) => {
+                                let span = addr_of_expr.span.with_hi(referent.span.lo());
+                                diag.span_suggestion(span, "remove this `&`", String::new(), Applicability::MachineApplicable);
+                            }
+                            _ => unreachable!(),
+                        }
+                    }
+                }
+            );
+            return true;
+        }
+    }
+    false
+}
+
+/// The core logic of `check_for_loop_iter` above, this function wraps a use of
+/// `CloneOrCopyVisitor`.
+fn clone_or_copy_needed(
+    cx: &LateContext<'tcx>,
+    pat: &Pat<'tcx>,
+    body: &'tcx Expr<'tcx>,
+) -> (bool, Vec<&'tcx Expr<'tcx>>) {
+    let mut visitor = CloneOrCopyVisitor {
+        cx,
+        binding_hir_ids: pat_bindings(pat),
+        clone_or_copy_needed: false,
+        addr_of_exprs: Vec::new(),
+    };
+    visitor.visit_expr(body);
+    (visitor.clone_or_copy_needed, visitor.addr_of_exprs)
+}
+
+/// Returns a vector of all `HirId`s bound by the pattern.
+fn pat_bindings(pat: &Pat<'_>) -> Vec<HirId> {
+    let mut collector = usage::ParamBindingIdCollector {
+        binding_hir_ids: Vec::new(),
+    };
+    collector.visit_pat(pat);
+    collector.binding_hir_ids
+}
+
+/// `clone_or_copy_needed` will be false when `CloneOrCopyVisitor` is done visiting if the only
+/// operations performed on `binding_hir_ids` are:
+/// * to take non-mutable references to them
+/// * to use them as non-mutable `&self` in method calls
+/// If any of `binding_hir_ids` is used in any other way, then `clone_or_copy_needed` will be true
+/// when `CloneOrCopyVisitor` is done visiting.
+struct CloneOrCopyVisitor<'cx, 'tcx> {
+    cx: &'cx LateContext<'tcx>,
+    binding_hir_ids: Vec<HirId>,
+    clone_or_copy_needed: bool,
+    addr_of_exprs: Vec<&'tcx Expr<'tcx>>,
+}
+
+impl<'cx, 'tcx> Visitor<'tcx> for CloneOrCopyVisitor<'cx, 'tcx> {
+    type Map = Map<'tcx>;
+
+    fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
+        NestedVisitorMap::OnlyBodies(self.cx.tcx.hir())
+    }
+
+    fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
+        walk_expr(self, expr);
+        if self.is_binding(expr) {
+            if let Some(parent) = get_parent_expr(self.cx, expr) {
+                match parent.kind {
+                    ExprKind::AddrOf(BorrowKind::Ref, Mutability::Not, _) => {
+                        self.addr_of_exprs.push(parent);
+                        return;
+                    },
+                    ExprKind::MethodCall(_, _, args, _) => {
+                        if_chain! {
+                            if args.iter().skip(1).all(|arg| !self.is_binding(arg));
+                            if let Some(method_def_id) = self.cx.typeck_results().type_dependent_def_id(parent.hir_id);
+                            let method_ty = self.cx.tcx.type_of(method_def_id);
+                            let self_ty = method_ty.fn_sig(self.cx.tcx).input(0).skip_binder();
+                            if matches!(self_ty.kind(), ty::Ref(_, _, Mutability::Not));
+                            then {
+                                return;
+                            }
+                        }
+                    },
+                    _ => {},
+                }
+            }
+            self.clone_or_copy_needed = true;
+        }
+    }
+}
+
+impl<'cx, 'tcx> CloneOrCopyVisitor<'cx, 'tcx> {
+    fn is_binding(&self, expr: &Expr<'tcx>) -> bool {
+        self.binding_hir_ids
+            .iter()
+            .any(|hir_id| path_to_local_id(expr, *hir_id))
+    }
+}
+
+/// Returns true if the named method is `IntoIterator::into_iter`.
+pub fn is_into_iter(cx: &LateContext<'_>, callee_def_id: DefId) -> bool {
+    cx.tcx.lang_items().require(LangItem::IntoIterIntoIter) == Ok(callee_def_id)
+}