Improve needless_borrow lint

Suggest changing usages of ref bindings to match the new type
Split out some cases into new lint `ref_binding_to_reference`

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),
@@ -1890,7 +1892,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| box zero_div_zero::ZeroDiv);
     store.register_late_pass(|| box mutex_atomic::Mutex);
     store.register_late_pass(|| box needless_update::NeedlessUpdate);
-    store.register_late_pass(|| box needless_borrow::NeedlessBorrow);
+    store.register_late_pass(|| box needless_borrow::NeedlessBorrow::default());
     store.register_late_pass(|| box needless_borrowed_ref::NeedlessBorrowedRef);
     store.register_late_pass(|| box no_effect::NoEffect);
     store.register_late_pass(|| box temporary_assignment::TemporaryAssignment);

clippy_lints/src/needless_borrow.rs

@@ -3,14 +3,18 @@
 //! This lint is **warn** by default
 
 use clippy_utils::diagnostics::span_lint_and_then;
-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, 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::Span;
 
 declare_clippy_lint! {
     /// **What it does:** Checks for address of operations (`&`) that are going to
@@ -34,13 +38,65 @@ declare_clippy_lint! {
     "taking a reference that is going to be automatically dereferenced"
 }
 
-#[derive(Default)]
-pub struct NeedlessBorrow;
+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]);
+impl_lint_pass!(NeedlessBorrow => [NEEDLESS_BORROW, REF_BINDING_TO_REFERENCE]);
+#[derive(Default)]
+pub struct NeedlessBorrow {
+    /// 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>>,
+}
+
+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 let Some(local) = path_to_local(e) {
+            self.check_local_usage(cx, e, local);
+        }
+
         if e.span.from_expansion() {
             return;
         }
@@ -81,35 +137,132 @@ impl<'tcx> LateLintPass<'tcx> for NeedlessBorrow {
             }
         }
     }
+
     fn check_pat(&mut self, cx: &LateContext<'tcx>, pat: &'tcx Pat<'_>) {
-        if pat.span.from_expansion() {
-            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;
+        }
+    }
+}
+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),
-}

tests/ui/needless_borrow.rs

@@ -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(ref 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(|&ref 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),
-}

tests/ui/needless_borrow.stderr

@@ -6,23 +6,11 @@ LL |     let c = x(&&a);
    |
    = note: `-D clippy::needless-borrow` implied by `-D warnings`
 
-error: this pattern creates a reference to a reference
-  --> $DIR/needless_borrow.rs:21:17
-   |
-LL |     if let Some(ref cake) = Some(&5) {}
-   |                 ^^^^^^^^ help: change this to: `cake`
-
 error: this expression borrows a reference (`&i32`) that is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:28:15
+  --> $DIR/needless_borrow.rs:27:15
    |
 LL |         46 => &&a,
    |               ^^^ help: change this to: `&a`
 
-error: this pattern creates a reference to a reference
-  --> $DIR/needless_borrow.rs:51:31
-   |
-LL |     let _ = v.iter().filter(|&ref a| a.is_empty());
-   |                               ^^^^^ help: change this to: `a`
-
-error: aborting due to 4 previous errors
+error: aborting due to 2 previous errors
 

tests/ui/needless_borrow_pat.rs

@@ -0,0 +1,151 @@
+// edition:2018
+// FIXME: run-rustfix waiting on multi-span suggestions
+
+#![warn(clippy::needless_borrow)]
+#![allow(clippy::needless_borrowed_reference)]
+
+fn f1(_: &str) {}
+macro_rules! m1 {
+    ($e:expr) => {
+        f1($e);
+    };
+}
+macro_rules! m3 {
+    ($i:ident) => {
+        Some(ref $i)
+    };
+}
+macro_rules! if_chain {
+    (if $e:expr; $($rest:tt)*) => {
+        if $e {
+            if_chain!($($rest)*)
+        }
+    };
+
+    (if let $p:pat = $e:expr; $($rest:tt)*) => {
+        if let $p = $e {
+            if_chain!($($rest)*)
+        }
+    };
+
+    (then $b:block) => {
+        $b
+    };
+}
+
+#[allow(dead_code)]
+fn main() {
+    let x = String::new();
+
+    // Ok, reference to a String.
+    let _: &String = match Some(x.clone()) {
+        Some(ref x) => x,
+        None => return,
+    };
+
+    // Ok, reference to a &mut String
+    let _: &&mut String = match Some(&mut x.clone()) {
+        Some(ref x) => x,
+        None => return,
+    };
+
+    // Ok, the pattern is from a macro
+    let _: &String = match Some(&x) {
+        m3!(x) => x,
+        None => return,
+    };
+
+    // Err, reference to a &String
+    let _: &String = match Some(&x) {
+        Some(ref x) => x,
+        None => return,
+    };
+
+    // Err, reference to a &String.
+    let _: &String = match Some(&x) {
+        Some(ref x) => *x,
+        None => return,
+    };
+
+    // Err, reference to a &String
+    let _: &String = match Some(&x) {
+        Some(ref x) => {
+            f1(x);
+            f1(*x);
+            x
+        },
+        None => return,
+    };
+
+    // Err, reference to a &String
+    match Some(&x) {
+        Some(ref x) => m1!(x),
+        None => return,
+    };
+
+    // Err, reference to a &String
+    let _ = |&ref x: &&String| {
+        let _: &String = x;
+    };
+
+    // Err, reference to a &String
+    let (ref y,) = (&x,);
+    let _: &String = *y;
+
+    let y = &&x;
+    // Ok, different y
+    let _: &String = *y;
+
+    let x = (0, 0);
+    // Err, reference to a &u32. Don't suggest adding a reference to the field access.
+    let _: u32 = match Some(&x) {
+        Some(ref x) => x.0,
+        None => return,
+    };
+
+    enum E {
+        A(&'static u32),
+        B(&'static u32),
+    }
+    // Err, reference to &u32.
+    let _: &u32 = match E::A(&0) {
+        E::A(ref x) | E::B(ref x) => *x,
+    };
+
+    // Err, reference to &String.
+    if_chain! {
+        if true;
+        if let Some(ref x) = Some(&String::new());
+        then {
+            f1(x);
+        }
+    }
+}
+
+// Err, reference to a &String
+fn f2<'a>(&ref x: &&'a String) -> &'a String {
+    let _: &String = x;
+    *x
+}
+
+trait T1 {
+    // Err, reference to a &String
+    fn f(&ref x: &&String) {
+        let _: &String = x;
+    }
+}
+
+struct S;
+impl T1 for S {
+    // Err, reference to a &String
+    fn f(&ref x: &&String) {
+        let _: &String = *x;
+    }
+}
+
+// Ok - used to error due to rustc bug
+#[allow(dead_code)]
+#[derive(Debug)]
+enum Foo<'a> {
+    Str(&'a str),
+}

tests/ui/needless_borrow_pat.stderr

@@ -0,0 +1,112 @@
+error: this pattern creates a reference to a reference
+  --> $DIR/needless_borrow_pat.rs:60:14
+   |
+LL |         Some(ref x) => x,
+   |              ^^^^^ help: try this: `x`
+   |
+   = note: `-D clippy::needless-borrow` implied by `-D warnings`
+
+error: this pattern creates a reference to a reference
+  --> $DIR/needless_borrow_pat.rs:66:14
+   |
+LL |         Some(ref x) => *x,
+   |              ^^^^^
+   |
+help: try this
+   |
+LL |         Some(x) => x,
+   |              ^     ^
+
+error: this pattern creates a reference to a reference
+  --> $DIR/needless_borrow_pat.rs:72:14
+   |
+LL |         Some(ref x) => {
+   |              ^^^^^
+   |
+help: try this
+   |
+LL |         Some(x) => {
+LL |             f1(x);
+LL |             f1(x);
+   |
+
+error: this pattern creates a reference to a reference
+  --> $DIR/needless_borrow_pat.rs:82:14
+   |
+LL |         Some(ref x) => m1!(x),
+   |              ^^^^^ help: try this: `x`
+
+error: this pattern creates a reference to a reference
+  --> $DIR/needless_borrow_pat.rs:87:15
+   |
+LL |     let _ = |&ref x: &&String| {
+   |               ^^^^^ help: try this: `x`
+
+error: this pattern creates a reference to a reference
+  --> $DIR/needless_borrow_pat.rs:92:10
+   |
+LL |     let (ref y,) = (&x,);
+   |          ^^^^^
+   |
+help: try this
+   |
+LL |     let (y,) = (&x,);
+LL |     let _: &String = y;
+   |
+
+error: this pattern creates a reference to a reference
+  --> $DIR/needless_borrow_pat.rs:102:14
+   |
+LL |         Some(ref x) => x.0,
+   |              ^^^^^ help: try this: `x`
+
+error: this pattern creates a reference to a reference
+  --> $DIR/needless_borrow_pat.rs:112:14
+   |
+LL |         E::A(ref x) | E::B(ref x) => *x,
+   |              ^^^^^         ^^^^^
+   |
+help: try this
+   |
+LL |         E::A(x) | E::B(x) => x,
+   |              ^         ^     ^
+
+error: this pattern creates a reference to a reference
+  --> $DIR/needless_borrow_pat.rs:118:21
+   |
+LL |         if let Some(ref x) = Some(&String::new());
+   |                     ^^^^^ help: try this: `x`
+
+error: this pattern creates a reference to a reference
+  --> $DIR/needless_borrow_pat.rs:126:12
+   |
+LL | fn f2<'a>(&ref x: &&'a String) -> &'a String {
+   |            ^^^^^
+   |
+help: try this
+   |
+LL | fn f2<'a>(&x: &&'a String) -> &'a String {
+LL |     let _: &String = x;
+LL |     x
+   |
+
+error: this pattern creates a reference to a reference
+  --> $DIR/needless_borrow_pat.rs:133:11
+   |
+LL |     fn f(&ref x: &&String) {
+   |           ^^^^^ help: try this: `x`
+
+error: this pattern creates a reference to a reference
+  --> $DIR/needless_borrow_pat.rs:141:11
+   |
+LL |     fn f(&ref x: &&String) {
+   |           ^^^^^
+   |
+help: try this
+   |
+LL |     fn f(&x: &&String) {
+LL |         let _: &String = x;
+   |
+
+error: aborting due to 12 previous errors
+

tests/ui/ref_binding_to_reference.rs

@@ -0,0 +1,76 @@
+// edition:2018
+// FIXME: run-rustfix waiting on multi-span suggestions
+
+#![warn(clippy::ref_binding_to_reference)]
+#![allow(clippy::needless_borrowed_reference)]
+
+fn f1(_: &str) {}
+macro_rules! m2 {
+    ($e:expr) => {
+        f1(*$e);
+    };
+}
+macro_rules! m3 {
+    ($i:ident) => {
+        Some(ref $i)
+    };
+}
+
+#[allow(dead_code)]
+fn main() {
+    let x = String::new();
+
+    // Ok, the pattern is from a macro
+    let _: &&String = match Some(&x) {
+        m3!(x) => x,
+        None => return,
+    };
+
+    // Err, reference to a &String
+    let _: &&String = match Some(&x) {
+        Some(ref x) => x,
+        None => return,
+    };
+
+    // Err, reference to a &String
+    let _: &&String = match Some(&x) {
+        Some(ref x) => {
+            f1(x);
+            f1(*x);
+            x
+        },
+        None => return,
+    };
+
+    // Err, reference to a &String
+    match Some(&x) {
+        Some(ref x) => m2!(x),
+        None => return,
+    }
+
+    // Err, reference to a &String
+    let _ = |&ref x: &&String| {
+        let _: &&String = x;
+    };
+}
+
+// Err, reference to a &String
+fn f2<'a>(&ref x: &&'a String) -> &'a String {
+    let _: &&String = x;
+    *x
+}
+
+trait T1 {
+    // Err, reference to a &String
+    fn f(&ref x: &&String) {
+        let _: &&String = x;
+    }
+}
+
+struct S;
+impl T1 for S {
+    // Err, reference to a &String
+    fn f(&ref x: &&String) {
+        let _: &&String = x;
+    }
+}

tests/ui/ref_binding_to_reference.stderr

@@ -0,0 +1,88 @@
+error: this pattern creates a reference to a reference
+  --> $DIR/ref_binding_to_reference.rs:31:14
+   |
+LL |         Some(ref x) => x,
+   |              ^^^^^
+   |
+   = note: `-D clippy::ref-binding-to-reference` implied by `-D warnings`
+help: try this
+   |
+LL |         Some(x) => &x,
+   |              ^     ^^
+
+error: this pattern creates a reference to a reference
+  --> $DIR/ref_binding_to_reference.rs:37:14
+   |
+LL |         Some(ref x) => {
+   |              ^^^^^
+   |
+help: try this
+   |
+LL |         Some(x) => {
+LL |             f1(x);
+LL |             f1(x);
+LL |             &x
+   |
+
+error: this pattern creates a reference to a reference
+  --> $DIR/ref_binding_to_reference.rs:47:14
+   |
+LL |         Some(ref x) => m2!(x),
+   |              ^^^^^
+   |
+help: try this
+   |
+LL |         Some(x) => m2!(&x),
+   |              ^         ^^
+
+error: this pattern creates a reference to a reference
+  --> $DIR/ref_binding_to_reference.rs:52:15
+   |
+LL |     let _ = |&ref x: &&String| {
+   |               ^^^^^
+   |
+help: try this
+   |
+LL |     let _ = |&x: &&String| {
+LL |         let _: &&String = &x;
+   |
+
+error: this pattern creates a reference to a reference
+  --> $DIR/ref_binding_to_reference.rs:58:12
+   |
+LL | fn f2<'a>(&ref x: &&'a String) -> &'a String {
+   |            ^^^^^
+   |
+help: try this
+   |
+LL | fn f2<'a>(&x: &&'a String) -> &'a String {
+LL |     let _: &&String = &x;
+LL |     x
+   |
+
+error: this pattern creates a reference to a reference
+  --> $DIR/ref_binding_to_reference.rs:65:11
+   |
+LL |     fn f(&ref x: &&String) {
+   |           ^^^^^
+   |
+help: try this
+   |
+LL |     fn f(&x: &&String) {
+LL |         let _: &&String = &x;
+   |
+
+error: this pattern creates a reference to a reference
+  --> $DIR/ref_binding_to_reference.rs:73:11
+   |
+LL |     fn f(&ref x: &&String) {
+   |           ^^^^^
+   |
+help: try this
+   |
+LL |     fn f(&x: &&String) {
+LL |         let _: &&String = &x;
+   |
+
+error: aborting due to 7 previous errors
+