Auto merge of rust-lang#7105 - Jarcho:needless_borrow_pat, r=camsteffen

fix `needless_borrow` suggestion

fixes: rust-lang#2610

While I'm working on this, should needless_borrow be split into two? One lint for expressions and another for patterns. In expression it only lints when the compiler inserts a dereference, but for patterns it's whenever a double reference is created. I think at least the case where a double reference is needed should be split into a new lint as it's not 'needless', it can just be done without a ref binding.

For illustration:

```rust
fn foo(x: &&str) {}

match Some("test") {
    // ref binding is useless here
    Some(ref x) => *x,
    _ => (),
}

match Some("test") {
    // ref binding is useless here
    Some(ref x) => x.len(),
    _ => (),
}

match Some("test") {
    // double reference is needed, but could be `Some(x) => foo(&x)`
    Some(ref x) => foo(x),
    _ => (),
}
```

changelog: Improve the suggestion for `needless_borrow` in patterns to change all usage sites as needed.
changelog: Add lint `ref_binding_to_reference`

Author: bors

CHANGELOG.md

@@ -2622,6 +2622,7 @@ Released 2018-09-13
 [`redundant_pub_crate`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_pub_crate
 [`redundant_slicing`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_slicing
 [`redundant_static_lifetimes`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_static_lifetimes
+[`ref_binding_to_reference`]: https://rust-lang.github.io/rust-clippy/master/index.html#ref_binding_to_reference
 [`ref_in_deref`]: https://rust-lang.github.io/rust-clippy/master/index.html#ref_in_deref
 [`ref_option_ref`]: https://rust-lang.github.io/rust-clippy/master/index.html#ref_option_ref
 [`regex_macro`]: https://rust-lang.github.io/rust-clippy/master/index.html#regex_macro

clippy_lints/src/lib.rs

@@ -841,6 +841,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         needless_bool::BOOL_COMPARISON,
         needless_bool::NEEDLESS_BOOL,
         needless_borrow::NEEDLESS_BORROW,
+        needless_borrow::REF_BINDING_TO_REFERENCE,
         needless_borrowed_ref::NEEDLESS_BORROWED_REFERENCE,
         needless_continue::NEEDLESS_CONTINUE,
         needless_for_each::NEEDLESS_FOR_EACH,
@@ -1116,6 +1117,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(misc_early::UNSEPARATED_LITERAL_SUFFIX),
         LintId::of(mut_mut::MUT_MUT),
         LintId::of(needless_bitwise_bool::NEEDLESS_BITWISE_BOOL),
+        LintId::of(needless_borrow::REF_BINDING_TO_REFERENCE),
         LintId::of(needless_continue::NEEDLESS_CONTINUE),
         LintId::of(needless_for_each::NEEDLESS_FOR_EACH),
         LintId::of(needless_pass_by_value::NEEDLESS_PASS_BY_VALUE),

clippy_lints/src/needless_borrow.rs

@@ -3,16 +3,18 @@
 //! This lint is **warn** by default
 
 use clippy_utils::diagnostics::span_lint_and_then;
-use clippy_utils::is_automatically_derived;
-use clippy_utils::source::snippet_opt;
+use clippy_utils::source::{snippet_opt, snippet_with_applicability, snippet_with_context};
+use clippy_utils::{get_parent_expr, in_macro, path_to_local};
 use if_chain::if_chain;
+use rustc_ast::util::parser::PREC_POSTFIX;
+use rustc_data_structures::fx::FxIndexMap;
 use rustc_errors::Applicability;
-use rustc_hir::{BindingAnnotation, BorrowKind, Expr, ExprKind, Item, Mutability, Pat, PatKind};
+use rustc_hir::{BindingAnnotation, Body, BodyId, BorrowKind, Expr, ExprKind, HirId, Mutability, Pat, PatKind, UnOp};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty;
 use rustc_middle::ty::adjustment::{Adjust, Adjustment};
 use rustc_session::{declare_tool_lint, impl_lint_pass};
-use rustc_span::def_id::LocalDefId;
+use rustc_span::Span;
 
 declare_clippy_lint! {
     /// **What it does:** Checks for address of operations (`&`) that are going to
@@ -36,16 +38,66 @@ declare_clippy_lint! {
     "taking a reference that is going to be automatically dereferenced"
 }
 
+declare_clippy_lint! {
+    /// **What it does:** Checks for `ref` bindings which create a reference to a reference.
+    ///
+    /// **Why is this bad?** The address-of operator at the use site is clearer about the need for a reference.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// // Bad
+    /// let x = Some("");
+    /// if let Some(ref x) = x {
+    ///     // use `x` here
+    /// }
+    ///
+    /// // Good
+    /// let x = Some("");
+    /// if let Some(x) = x {
+    ///     // use `&x` here
+    /// }
+    /// ```
+    pub REF_BINDING_TO_REFERENCE,
+    pedantic,
+    "`ref` binding to a reference"
+}
+
+impl_lint_pass!(NeedlessBorrow => [NEEDLESS_BORROW, REF_BINDING_TO_REFERENCE]);
 #[derive(Default)]
 pub struct NeedlessBorrow {
-    derived_item: Option<LocalDefId>,
+    /// The body the first local was found in. Used to emit lints when the traversal of the body has
+    /// been finished. Note we can't lint at the end of every body as they can be nested within each
+    /// other.
+    current_body: Option<BodyId>,
+    /// The list of locals currently being checked by the lint.
+    /// If the value is `None`, then the binding has been seen as a ref pattern, but is not linted.
+    /// This is needed for or patterns where one of the branches can be linted, but another can not
+    /// be.
+    ///
+    /// e.g. `m!(x) | Foo::Bar(ref x)`
+    ref_locals: FxIndexMap<HirId, Option<RefPat>>,
 }
 
-impl_lint_pass!(NeedlessBorrow => [NEEDLESS_BORROW]);
+struct RefPat {
+    /// Whether every usage of the binding is dereferenced.
+    always_deref: bool,
+    /// The spans of all the ref bindings for this local.
+    spans: Vec<Span>,
+    /// The applicability of this suggestion.
+    app: Applicability,
+    /// All the replacements which need to be made.
+    replacements: Vec<(Span, String)>,
+}
 
 impl<'tcx> LateLintPass<'tcx> for NeedlessBorrow {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
-        if e.span.from_expansion() || self.derived_item.is_some() {
+        if let Some(local) = path_to_local(e) {
+            self.check_local_usage(cx, e, local);
+        }
+
+        if e.span.from_expansion() {
             return;
         }
         if let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Not, inner) = e.kind {
@@ -85,50 +137,131 @@ impl<'tcx> LateLintPass<'tcx> for NeedlessBorrow {
             }
         }
     }
+
     fn check_pat(&mut self, cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>) {
-        if pat.span.from_expansion() || self.derived_item.is_some() {
-            return;
+        if let PatKind::Binding(BindingAnnotation::Ref, id, name, _) = pat.kind {
+            if let Some(opt_prev_pat) = self.ref_locals.get_mut(&id) {
+                // This binding id has been seen before. Add this pattern to the list of changes.
+                if let Some(prev_pat) = opt_prev_pat {
+                    if in_macro(pat.span) {
+                        // Doesn't match the context of the previous pattern. Can't lint here.
+                        *opt_prev_pat = None;
+                    } else {
+                        prev_pat.spans.push(pat.span);
+                        prev_pat.replacements.push((
+                            pat.span,
+                            snippet_with_context(cx, name.span, pat.span.ctxt(), "..", &mut prev_pat.app)
+                                .0
+                                .into(),
+                        ));
+                    }
+                }
+                return;
+            }
+
+            if_chain! {
+                if !in_macro(pat.span);
+                if let ty::Ref(_, tam, _) = *cx.typeck_results().pat_ty(pat).kind();
+                // only lint immutable refs, because borrowed `&mut T` cannot be moved out
+                if let ty::Ref(_, _, Mutability::Not) = *tam.kind();
+                then {
+                    let mut app = Applicability::MachineApplicable;
+                    let snip = snippet_with_context(cx, name.span, pat.span.ctxt(), "..", &mut app).0;
+                    self.current_body = self.current_body.or(cx.enclosing_body);
+                    self.ref_locals.insert(
+                        id,
+                        Some(RefPat {
+                            always_deref: true,
+                            spans: vec![pat.span],
+                            app,
+                            replacements: vec![(pat.span, snip.into())],
+                        }),
+                    );
+                }
+            }
         }
-        if_chain! {
-            if let PatKind::Binding(BindingAnnotation::Ref, .., name, _) = pat.kind;
-            if let ty::Ref(_, tam, mutbl) = *cx.typeck_results().pat_ty(pat).kind();
-            if mutbl == Mutability::Not;
-            if let ty::Ref(_, _, mutbl) = *tam.kind();
-            // only lint immutable refs, because borrowed `&mut T` cannot be moved out
-            if mutbl == Mutability::Not;
-            then {
+    }
+
+    fn check_body_post(&mut self, cx: &LateContext<'tcx>, body: &'tcx Body<'_>) {
+        if Some(body.id()) == self.current_body {
+            for pat in self.ref_locals.drain(..).filter_map(|(_, x)| x) {
+                let replacements = pat.replacements;
+                let app = pat.app;
                 span_lint_and_then(
                     cx,
-                    NEEDLESS_BORROW,
-                    pat.span,
+                    if pat.always_deref {
+                        NEEDLESS_BORROW
+                    } else {
+                        REF_BINDING_TO_REFERENCE
+                    },
+                    pat.spans,
                     "this pattern creates a reference to a reference",
                     |diag| {
-                        if let Some(snippet) = snippet_opt(cx, name.span) {
-                            diag.span_suggestion(
-                                pat.span,
-                                "change this to",
-                                snippet,
-                                Applicability::MachineApplicable,
-                            );
-                        }
-                    }
+                        diag.multipart_suggestion("try this", replacements, app);
+                    },
                 )
             }
+            self.current_body = None;
         }
     }
-
-    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) {
-        let attrs = cx.tcx.hir().attrs(item.hir_id());
-        if is_automatically_derived(attrs) {
-            debug_assert!(self.derived_item.is_none());
-            self.derived_item = Some(item.def_id);
-        }
-    }
-
-    fn check_item_post(&mut self, _: &LateContext<'tcx>, item: &'tcx Item<'_>) {
-        if let Some(id) = self.derived_item {
-            if item.def_id == id {
-                self.derived_item = None;
+}
+impl NeedlessBorrow {
+    fn check_local_usage(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>, local: HirId) {
+        if let Some(outer_pat) = self.ref_locals.get_mut(&local) {
+            if let Some(pat) = outer_pat {
+                // Check for auto-deref
+                if !matches!(
+                    cx.typeck_results().expr_adjustments(e),
+                    [
+                        Adjustment {
+                            kind: Adjust::Deref(_),
+                            ..
+                        },
+                        Adjustment {
+                            kind: Adjust::Deref(_),
+                            ..
+                        },
+                        ..
+                    ]
+                ) {
+                    match get_parent_expr(cx, e) {
+                        // Field accesses are the same no matter the number of references.
+                        Some(Expr {
+                            kind: ExprKind::Field(..),
+                            ..
+                        }) => (),
+                        Some(&Expr {
+                            span,
+                            kind: ExprKind::Unary(UnOp::Deref, _),
+                            ..
+                        }) if !in_macro(span) => {
+                            // Remove explicit deref.
+                            let snip = snippet_with_context(cx, e.span, span.ctxt(), "..", &mut pat.app).0;
+                            pat.replacements.push((span, snip.into()));
+                        },
+                        Some(parent) if !in_macro(parent.span) => {
+                            // Double reference might be needed at this point.
+                            if parent.precedence().order() == PREC_POSTFIX {
+                                // Parentheses would be needed here, don't lint.
+                                *outer_pat = None;
+                            } else {
+                                pat.always_deref = false;
+                                let snip = snippet_with_context(cx, e.span, parent.span.ctxt(), "..", &mut pat.app).0;
+                                pat.replacements.push((e.span, format!("&{}", snip)));
+                            }
+                        },
+                        _ if !in_macro(e.span) => {
+                            // Double reference might be needed at this point.
+                            pat.always_deref = false;
+                            let snip = snippet_with_applicability(cx, e.span, "..", &mut pat.app);
+                            pat.replacements.push((e.span, format!("&{}", snip)));
+                        },
+                        // Edge case for macros. The span of the identifier will usually match the context of the
+                        // binding, but not if the identifier was created in a macro. e.g. `concat_idents` and proc
+                        // macros
+                        _ => *outer_pat = None,
+                    }
+                }
             }
         }
     }

clippy_utils/src/higher.rs

@@ -70,7 +70,7 @@ pub fn range<'a>(expr: &'a hir::Expr<'_>) -> Option<Range<'a>> {
                 limits: ast::RangeLimits::Closed,
             })
         },
-        hir::ExprKind::Struct(ref path, ref fields, None) => match path {
+        hir::ExprKind::Struct(path, ref fields, None) => match path {
             hir::QPath::LangItem(hir::LangItem::RangeFull, _) => Some(Range {
                 start: None,
                 end: None,

clippy_utils/src/lib.rs

@@ -1254,12 +1254,12 @@ pub fn match_function_call<'tcx>(
     path: &[&str],
 ) -> Option<&'tcx [Expr<'tcx>]> {
     if_chain! {
-        if let ExprKind::Call(ref fun, ref args) = expr.kind;
+        if let ExprKind::Call(ref fun, args) = expr.kind;
         if let ExprKind::Path(ref qpath) = fun.kind;
         if let Some(fun_def_id) = cx.qpath_res(qpath, fun.hir_id).opt_def_id();
         if match_def_path(cx, fun_def_id, path);
         then {
-            return Some(&args)
+            return Some(args)
         }
     };
     None

clippy_utils/src/visitors.rs

@@ -189,34 +189,21 @@ impl<'v> Visitor<'v> for LocalUsedVisitor<'v> {
     }
 }
 
+/// A type which can be visited.
 pub trait Visitable<'tcx> {
-    fn visit<V: Visitor<'tcx>>(self, v: &mut V);
+    /// Calls the corresponding `visit_*` function on the visitor.
+    fn visit<V: Visitor<'tcx>>(self, visitor: &mut V);
 }
-impl Visitable<'tcx> for &'tcx Expr<'tcx> {
-    fn visit<V: Visitor<'tcx>>(self, v: &mut V) {
-        v.visit_expr(self)
-    }
-}
-impl Visitable<'tcx> for &'tcx Block<'tcx> {
-    fn visit<V: Visitor<'tcx>>(self, v: &mut V) {
-        v.visit_block(self)
-    }
-}
-impl<'tcx> Visitable<'tcx> for &'tcx Stmt<'tcx> {
-    fn visit<V: Visitor<'tcx>>(self, v: &mut V) {
-        v.visit_stmt(self)
-    }
-}
-impl<'tcx> Visitable<'tcx> for &'tcx Body<'tcx> {
-    fn visit<V: Visitor<'tcx>>(self, v: &mut V) {
-        v.visit_body(self)
-    }
-}
-impl<'tcx> Visitable<'tcx> for &'tcx Arm<'tcx> {
-    fn visit<V: Visitor<'tcx>>(self, v: &mut V) {
-        v.visit_arm(self)
-    }
+macro_rules! visitable_ref {
+    ($t:ident, $f:ident) => {
+        impl Visitable<'tcx> for &'tcx $t<'tcx> {
+            fn visit<V: Visitor<'tcx>>(self, visitor: &mut V) {
+                visitor.$f(self);
+            }
+        }
+    };
 }
+visitable_ref!(Block, visit_block);
 
 /// Calls the given function for each break expression.
 pub fn visit_break_exprs<'tcx>(

tests/ui/needless_borrow.fixed

@@ -18,7 +18,6 @@ fn main() {
     let vec = Vec::new();
     let vec_val = g(&vec); // should not error, because `&Vec<T>` derefs to `&[T]`
     h(&"foo"); // should not error, because the `&&str` is required, due to `&Trait`
-    if let Some(cake) = Some(&5) {}
     let garbl = match 42 {
         44 => &a,
         45 => {
@@ -43,19 +42,3 @@ trait Trait {}
 impl<'a> Trait for &'a str {}
 
 fn h(_: &dyn Trait) {}
-#[warn(clippy::needless_borrow)]
-#[allow(dead_code)]
-fn issue_1432() {
-    let mut v = Vec::<String>::new();
-    let _ = v.iter_mut().filter(|&ref a| a.is_empty());
-    let _ = v.iter().filter(|&a| a.is_empty());
-
-    let _ = v.iter().filter(|&a| a.is_empty());
-}
-
-#[allow(dead_code)]
-#[warn(clippy::needless_borrow)]
-#[derive(Debug)]
-enum Foo<'a> {
-    Str(&'a str),
-}