X-Git-Url: https://git.lizzy.rs/?a=blobdiff_plain;f=clippy_lints%2Fsrc%2Fmisc.rs;h=0680f6b41a16257185a23ace4b2dac94e7cc5c17;hb=778ce4dfd359eb8071ad76aa6447b23ce7a0c752;hp=3d9795b0c38f142cd225de796d9843546e93deba;hpb=d85b8062e3e5fea82e8eeed06432d2a0157d589b;p=rust.git

diff --git a/clippy_lints/src/misc.rs b/clippy_lints/src/misc.rs
index 3d9795b0c38..0680f6b41a1 100644
--- a/clippy_lints/src/misc.rs
+++ b/clippy_lints/src/misc.rs
@@ -8,45 +8,172 @@
 use rustc_const_eval::eval_const_expr_partial;
 use rustc_const_math::ConstFloat;
 use syntax::codemap::{Span, Spanned, ExpnFormat};
-use syntax::ptr::P;
 use utils::{
-    get_item_name, get_parent_expr, implements_trait, is_integer_literal, match_path, snippet,
-    span_lint, span_lint_and_then, walk_ptrs_ty
+    get_item_name, get_parent_expr, implements_trait, in_macro, is_integer_literal, match_path,
+    snippet, span_lint, span_lint_and_then, walk_ptrs_ty, last_path_segment
 };
+use utils::sugg::Sugg;
 
-/// **What it does:** This lint checks for function arguments and let bindings denoted as `ref`.
+/// **What it does:** Checks for function arguments and let bindings denoted as `ref`.
 ///
-/// **Why is this bad?** The `ref` declaration makes the function take an owned value, but turns the argument into a reference (which means that the value is destroyed when exiting the function). This adds not much value: either take a reference type, or take an owned value and create references in the body.
+/// **Why is this bad?** The `ref` declaration makes the function take an owned
+/// value, but turns the argument into a reference (which means that the value
+/// is destroyed when exiting the function). This adds not much value: either
+/// take a reference type, or take an owned value and create references in the
+/// body.
 ///
-/// For let bindings, `let x = &foo;` is preferred over `let ref x = foo`. The type of `x` is more obvious with the former.
+/// For let bindings, `let x = &foo;` is preferred over `let ref x = foo`. The
+/// type of `x` is more obvious with the former.
 ///
-/// **Known problems:** If the argument is dereferenced within the function, removing the `ref` will lead to errors. This can be fixed by removing the dereferences, e.g. changing `*x` to `x` within the function.
+/// **Known problems:** If the argument is dereferenced within the function,
+/// removing the `ref` will lead to errors. This can be fixed by removing the
+/// dereferences, e.g. changing `*x` to `x` within the function.
 ///
-/// **Example:** `fn foo(ref x: u8) -> bool { .. }`
+/// **Example:**
+/// ```rust
+/// fn foo(ref x: u8) -> bool { .. }
+/// ```
+declare_lint! {
+    pub TOPLEVEL_REF_ARG,
+    Warn,
+    "an entire binding declared as `ref`, in a function argument or a `let` statement"
+}
+
+/// **What it does:** Checks for comparisons to NaN.
+///
+/// **Why is this bad?** NaN does not compare meaningfully to anything – not
+/// even itself – so those comparisons are simply wrong.
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+/// ```rust
+/// x == NAN
+/// ```
 declare_lint! {
-    pub TOPLEVEL_REF_ARG, Warn,
-    "An entire binding was declared as `ref`, in a function argument (`fn foo(ref x: Bar)`), \
-     or a `let` statement (`let ref x = foo()`). In such cases, it is preferred to take \
-     references with `&`."
+    pub CMP_NAN,
+    Deny,
+    "comparisons to NAN, which will always return false, probably not intended"
 }
 
-#[allow(missing_copy_implementations)]
-pub struct TopLevelRefPass;
+/// **What it does:** Checks for (in-)equality comparisons on floating-point
+/// values (apart from zero), except in functions called `*eq*` (which probably
+/// implement equality for a type involving floats).
+///
+/// **Why is this bad?** Floating point calculations are usually imprecise, so
+/// asking if two values are *exactly* equal is asking for trouble. For a good
+/// guide on what to do, see [the floating point
+/// guide](http://www.floating-point-gui.de/errors/comparison).
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+/// ```rust
+/// y == 1.23f64
+/// y != x  // where both are floats
+/// ```
+declare_lint! {
+    pub FLOAT_CMP,
+    Warn,
+    "using `==` or `!=` on float values instead of comparing difference with an epsilon"
+}
 
-impl LintPass for TopLevelRefPass {
+/// **What it does:** Checks for conversions to owned values just for the sake
+/// of a comparison.
+///
+/// **Why is this bad?** The comparison can operate on a reference, so creating
+/// an owned value effectively throws it away directly afterwards, which is
+/// needlessly consuming code and heap space.
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+/// ```rust
+/// x.to_owned() == y
+/// ```
+declare_lint! {
+    pub CMP_OWNED,
+    Warn,
+    "creating owned instances for comparing with others, e.g. `x == \"foo\".to_string()`"
+}
+
+/// **What it does:** Checks for getting the remainder of a division by one.
+///
+/// **Why is this bad?** The result can only ever be zero. No one will write
+/// such code deliberately, unless trying to win an Underhanded Rust
+/// Contest. Even for that contest, it's probably a bad idea. Use something more
+/// underhanded.
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+/// ```rust
+/// x % 1
+/// ```
+declare_lint! {
+    pub MODULO_ONE,
+    Warn,
+    "taking a number modulo 1, which always returns 0"
+}
+
+/// **What it does:** Checks for patterns in the form `name @ _`.
+///
+/// **Why is this bad?** It's almost always more readable to just use direct bindings.
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+/// ```rust
+/// match v {
+///     Some(x) => (),
+///     y @ _   => (), // easier written as `y`,
+/// }
+/// ```
+declare_lint! {
+    pub REDUNDANT_PATTERN,
+    Warn,
+    "using `name @ _` in a pattern"
+}
+
+/// **What it does:** Checks for the use of bindings with a single leading underscore.
+///
+/// **Why is this bad?** A single leading underscore is usually used to indicate
+/// that a binding will not be used. Using such a binding breaks this
+/// expectation.
+///
+/// **Known problems:** The lint does not work properly with desugaring and
+/// macro, it has been allowed in the mean time.
+///
+/// **Example:**
+/// ```rust
+/// let _x = 0;
+/// let y = _x + 1; // Here we are using `_x`, even though it has a leading underscore.
+///                 // We should rename `_x` to `x`
+/// ```
+declare_lint! {
+    pub USED_UNDERSCORE_BINDING,
+    Allow,
+    "using a binding which is prefixed with an underscore"
+}
+
+#[derive(Copy, Clone)]
+pub struct Pass;
+
+impl LintPass for Pass {
     fn get_lints(&self) -> LintArray {
-        lint_array!(TOPLEVEL_REF_ARG)
+        lint_array!(TOPLEVEL_REF_ARG, CMP_NAN, FLOAT_CMP, CMP_OWNED, MODULO_ONE, REDUNDANT_PATTERN,
+                    USED_UNDERSCORE_BINDING)
     }
 }
 
-impl LateLintPass for TopLevelRefPass {
-    fn check_fn(&mut self, cx: &LateContext, k: FnKind, decl: &FnDecl, _: &Block, _: Span, _: NodeId) {
+impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
+    fn check_fn(&mut self, cx: &LateContext<'a, 'tcx>, k: FnKind<'tcx>, decl: &'tcx FnDecl, _: &'tcx Expr, _: Span, _: NodeId) {
         if let FnKind::Closure(_) = k {
             // Does not apply to closures
             return;
         }
-        for ref arg in &decl.inputs {
-            if let PatKind::Binding(BindByRef(_), _, _) = arg.pat.node {
+        for arg in &decl.inputs {
+            if let PatKind::Binding(BindByRef(_), _, _, _) = arg.pat.node {
                 span_lint(cx,
                           TOPLEVEL_REF_ARG,
                           arg.pat.span,
@@ -54,130 +181,144 @@ fn check_fn(&mut self, cx: &LateContext, k: FnKind, decl: &FnDecl, _: &Block, _:
             }
         }
     }
-    fn check_stmt(&mut self, cx: &LateContext, s: &Stmt) {
+
+    fn check_stmt(&mut self, cx: &LateContext<'a, 'tcx>, s: &'tcx Stmt) {
         if_let_chain! {[
             let StmtDecl(ref d, _) = s.node,
             let DeclLocal(ref l) = d.node,
-            let PatKind::Binding(BindByRef(_), i, None) = l.pat.node,
+            let PatKind::Binding(BindByRef(mt), _, i, None) = l.pat.node,
             let Some(ref init) = l.init
         ], {
+            let init = Sugg::hir(cx, init, "..");
+            let (mutopt,initref) = if mt == Mutability::MutMutable {
+                ("mut ", init.mut_addr())
+            } else {
+                ("", init.addr())
+            };
             let tyopt = if let Some(ref ty) = l.ty {
-                format!(": {}", snippet(cx, ty.span, "_"))
+                format!(": &{mutopt}{ty}", mutopt=mutopt, ty=snippet(cx, ty.span, "_"))
             } else {
                 "".to_owned()
             };
             span_lint_and_then(cx,
                 TOPLEVEL_REF_ARG,
                 l.pat.span,
-                "`ref` on an entire `let` pattern is discouraged, take a reference with & instead",
+                "`ref` on an entire `let` pattern is discouraged, take a reference with `&` instead",
                 |db| {
                     db.span_suggestion(s.span,
                                        "try",
-                                       format!("let {}{} = &{};",
-                                               snippet(cx, i.span, "_"),
-                                               tyopt,
-                                               snippet(cx, init.span, "_")));
+                                       format!("let {name}{tyopt} = {initref};",
+                                               name=snippet(cx, i.span, "_"),
+                                               tyopt=tyopt,
+                                               initref=initref));
                 }
             );
         }}
     }
-}
-
-/// **What it does:** This lint checks for comparisons to NAN.
-///
-/// **Why is this bad?** NAN does not compare meaningfully to anything – not even itself – so those comparisons are simply wrong.
-///
-/// **Known problems:** None
-///
-/// **Example:** `x == NAN`
-declare_lint!(pub CMP_NAN, Deny,
-              "comparisons to NAN (which will always return false, which is probably not intended)");
-
-#[derive(Copy,Clone)]
-pub struct CmpNan;
-
-impl LintPass for CmpNan {
-    fn get_lints(&self) -> LintArray {
-        lint_array!(CMP_NAN)
-    }
-}
 
-impl LateLintPass for CmpNan {
-    fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
+    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
         if let ExprBinary(ref cmp, ref left, ref right) = expr.node {
-            if cmp.node.is_comparison() {
-                if let ExprPath(_, ref path) = left.node {
+            let op = cmp.node;
+            if op.is_comparison() {
+                if let ExprPath(QPath::Resolved(_, ref path)) = left.node {
                     check_nan(cx, path, expr.span);
                 }
-                if let ExprPath(_, ref path) = right.node {
+                if let ExprPath(QPath::Resolved(_, ref path)) = right.node {
                     check_nan(cx, path, expr.span);
                 }
+                check_to_owned(cx, left, right, true, cmp.span);
+                check_to_owned(cx, right, left, false, cmp.span)
             }
-        }
-    }
-}
-
-fn check_nan(cx: &LateContext, path: &Path, span: Span) {
-    path.segments.last().map(|seg| {
-        if seg.name.as_str() == "NAN" {
-            span_lint(cx,
-                      CMP_NAN,
-                      span,
-                      "doomed comparison with NAN, use `std::{f32,f64}::is_nan()` instead");
-        }
-    });
-}
-
-/// **What it does:** This lint checks for (in-)equality comparisons on floating-point values (apart from zero), except in functions called `*eq*` (which probably implement equality for a type involving floats).
-///
-/// **Why is this bad?** Floating point calculations are usually imprecise, so asking if two values are *exactly* equal is asking for trouble. For a good guide on what to do, see [the floating point guide](http://www.floating-point-gui.de/errors/comparison).
-///
-/// **Known problems:** None
-///
-/// **Example:** `y == 1.23f64`
-declare_lint!(pub FLOAT_CMP, Warn,
-              "using `==` or `!=` on float values (as floating-point operations \
-               usually involve rounding errors, it is always better to check for approximate \
-               equality within small bounds)");
-
-#[derive(Copy,Clone)]
-pub struct FloatCmp;
-
-impl LintPass for FloatCmp {
-    fn get_lints(&self) -> LintArray {
-        lint_array!(FLOAT_CMP)
-    }
-}
-
-impl LateLintPass for FloatCmp {
-    fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
-        if let ExprBinary(ref cmp, ref left, ref right) = expr.node {
-            let op = cmp.node;
             if (op == BiEq || op == BiNe) && (is_float(cx, left) || is_float(cx, right)) {
                 if is_allowed(cx, left) || is_allowed(cx, right) {
                     return;
                 }
                 if let Some(name) = get_item_name(cx, expr) {
-                    let name = name.as_str();
+                    let name = &*name.as_str();
                     if name == "eq" || name == "ne" || name == "is_nan" || name.starts_with("eq_") ||
-                       name.ends_with("_eq") {
+                        name.ends_with("_eq") {
                         return;
                     }
                 }
+                span_lint_and_then(cx,
+                                   FLOAT_CMP,
+                                   expr.span,
+                                   "strict comparison of f32 or f64",
+                                   |db| {
+                                       let lhs = Sugg::hir(cx, left, "..");
+                                       let rhs = Sugg::hir(cx, right, "..");
+
+                                       db.span_suggestion(expr.span,
+                                                          "consider comparing them within some error",
+                                                          format!("({}).abs() < error", lhs - rhs));
+                                       db.span_note(expr.span, "std::f32::EPSILON and std::f64::EPSILON are available.");
+                                   });
+            } else if op == BiRem && is_integer_literal(right, 1) {
+                span_lint(cx, MODULO_ONE, expr.span, "any number modulo 1 will be 0");
+            }
+        }
+        if in_attributes_expansion(cx, expr) {
+            // Don't lint things expanded by #[derive(...)], etc
+            return;
+        }
+        let binding = match expr.node {
+            ExprPath(ref qpath) => {
+                let binding = last_path_segment(qpath).name.as_str();
+                if binding.starts_with('_') &&
+                    !binding.starts_with("__") &&
+                    &*binding != "_result" && // FIXME: #944
+                    is_used(cx, expr) &&
+                    // don't lint if the declaration is in a macro
+                    non_macro_local(cx, &cx.tcx.tables().qpath_def(qpath, expr.id)) {
+                    Some(binding)
+                } else {
+                    None
+                }
+            }
+            ExprField(_, spanned) => {
+                let name = spanned.node.as_str();
+                if name.starts_with('_') && !name.starts_with("__") {
+                    Some(name)
+                } else {
+                    None
+                }
+            }
+            _ => None,
+        };
+        if let Some(binding) = binding {
+            span_lint(cx,
+                      USED_UNDERSCORE_BINDING,
+                      expr.span,
+                      &format!("used binding `{}` which is prefixed with an underscore. A leading \
+                                underscore signals that a binding will not be used.", binding));
+        }
+    }
+
+    fn check_pat(&mut self, cx: &LateContext<'a, 'tcx>, pat: &'tcx Pat) {
+        if let PatKind::Binding(_, _, ref ident, Some(ref right)) = pat.node {
+            if right.node == PatKind::Wild {
                 span_lint(cx,
-                          FLOAT_CMP,
-                          expr.span,
-                          &format!("{}-comparison of f32 or f64 detected. Consider changing this to `({} - {}).abs() < \
-                                    epsilon` for some suitable value of epsilon. \
-                                    std::f32::EPSILON and std::f64::EPSILON are available.",
-                                   op.as_str(),
-                                   snippet(cx, left.span, ".."),
-                                   snippet(cx, right.span, "..")));
+                          REDUNDANT_PATTERN,
+                          pat.span,
+                          &format!("the `{} @ _` pattern can be written as just `{}`",
+                                   ident.node,
+                                   ident.node));
             }
         }
     }
 }
 
+fn check_nan(cx: &LateContext, path: &Path, span: Span) {
+    path.segments.last().map(|seg| {
+        if &*seg.name.as_str() == "NAN" {
+            span_lint(cx,
+                      CMP_NAN,
+                      span,
+                      "doomed comparison with NAN, use `std::{f32,f64}::is_nan()` instead");
+        }
+    });
+}
+
 fn is_allowed(cx: &LateContext, expr: &Expr) -> bool {
     let res = eval_const_expr_partial(cx.tcx, expr, ExprTypeChecked, None);
     if let Ok(ConstVal::Float(val)) = res {
@@ -207,52 +348,23 @@ fn is_allowed(cx: &LateContext, expr: &Expr) -> bool {
 }
 
 fn is_float(cx: &LateContext, expr: &Expr) -> bool {
-    matches!(walk_ptrs_ty(cx.tcx.expr_ty(expr)).sty, ty::TyFloat(_))
-}
-
-/// **What it does:** This lint checks for conversions to owned values just for the sake of a comparison.
-///
-/// **Why is this bad?** The comparison can operate on a reference, so creating an owned value effectively throws it away directly afterwards, which is needlessly consuming code and heap space.
-///
-/// **Known problems:** None
-///
-/// **Example:** `x.to_owned() == y`
-declare_lint!(pub CMP_OWNED, Warn,
-              "creating owned instances for comparing with others, e.g. `x == \"foo\".to_string()`");
-
-#[derive(Copy,Clone)]
-pub struct CmpOwned;
-
-impl LintPass for CmpOwned {
-    fn get_lints(&self) -> LintArray {
-        lint_array!(CMP_OWNED)
-    }
-}
-
-impl LateLintPass for CmpOwned {
-    fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
-        if let ExprBinary(ref cmp, ref left, ref right) = expr.node {
-            if cmp.node.is_comparison() {
-                check_to_owned(cx, left, right, true, cmp.span);
-                check_to_owned(cx, right, left, false, cmp.span)
-            }
-        }
-    }
+    matches!(walk_ptrs_ty(cx.tcx.tables().expr_ty(expr)).sty, ty::TyFloat(_))
 }
 
 fn check_to_owned(cx: &LateContext, expr: &Expr, other: &Expr, left: bool, op: Span) {
     let (arg_ty, snip) = match expr.node {
         ExprMethodCall(Spanned { node: ref name, .. }, _, ref args) if args.len() == 1 => {
-            if name.as_str() == "to_string" || name.as_str() == "to_owned" && is_str_arg(cx, args) {
-                (cx.tcx.expr_ty(&args[0]), snippet(cx, args[0].span, ".."))
+            let name = &*name.as_str();
+            if name == "to_string" || name == "to_owned" && is_str_arg(cx, args) {
+                (cx.tcx.tables().expr_ty(&args[0]), snippet(cx, args[0].span, ".."))
             } else {
                 return;
             }
         }
         ExprCall(ref path, ref v) if v.len() == 1 => {
-            if let ExprPath(None, ref path) = path.node {
+            if let ExprPath(ref path) = path.node {
                 if match_path(path, &["String", "from_str"]) || match_path(path, &["String", "from"]) {
-                    (cx.tcx.expr_ty(&v[0]), snippet(cx, v[0].span, ".."))
+                    (cx.tcx.tables().expr_ty(&v[0]), snippet(cx, v[0].span, ".."))
                 } else {
                     return;
                 }
@@ -263,7 +375,7 @@ fn check_to_owned(cx: &LateContext, expr: &Expr, other: &Expr, left: bool, op: S
         _ => return,
     };
 
-    let other_ty = cx.tcx.expr_ty(other);
+    let other_ty = cx.tcx.tables().expr_ty(other);
     let partial_eq_trait_id = match cx.tcx.lang_items.eq_trait() {
         Some(id) => id,
         None => return,
@@ -295,155 +407,15 @@ fn check_to_owned(cx: &LateContext, expr: &Expr, other: &Expr, left: bool, op: S
 
 }
 
-fn is_str_arg(cx: &LateContext, args: &[P<Expr>]) -> bool {
+fn is_str_arg(cx: &LateContext, args: &[Expr]) -> bool {
     args.len() == 1 &&
-        matches!(walk_ptrs_ty(cx.tcx.expr_ty(&args[0])).sty, ty::TyStr)
-}
-
-/// **What it does:** This lint checks for getting the remainder of a division by one.
-///
-/// **Why is this bad?** The result can only ever be zero. No one will write such code deliberately, unless trying to win an Underhanded Rust Contest. Even for that contest, it's probably a bad idea. Use something more underhanded.
-///
-/// **Known problems:** None
-///
-/// **Example:** `x % 1`
-declare_lint!(pub MODULO_ONE, Warn, "taking a number modulo 1, which always returns 0");
-
-#[derive(Copy,Clone)]
-pub struct ModuloOne;
-
-impl LintPass for ModuloOne {
-    fn get_lints(&self) -> LintArray {
-        lint_array!(MODULO_ONE)
-    }
-}
-
-impl LateLintPass for ModuloOne {
-    fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
-        if let ExprBinary(ref cmp, _, ref right) = expr.node {
-            if let Spanned { node: BinOp_::BiRem, .. } = *cmp {
-                if is_integer_literal(right, 1) {
-                    span_lint(cx, MODULO_ONE, expr.span, "any number modulo 1 will be 0");
-                }
-            }
-        }
-    }
-}
-
-/// **What it does:** This lint checks for patterns in the form `name @ _`.
-///
-/// **Why is this bad?** It's almost always more readable to just use direct bindings.
-///
-/// **Known problems:** None
-///
-/// **Example**:
-/// ```
-/// match v {
-///     Some(x) => (),
-///     y @ _   => (), // easier written as `y`,
-/// }
-/// ```
-declare_lint!(pub REDUNDANT_PATTERN, Warn, "using `name @ _` in a pattern");
-
-#[derive(Copy,Clone)]
-pub struct PatternPass;
-
-impl LintPass for PatternPass {
-    fn get_lints(&self) -> LintArray {
-        lint_array!(REDUNDANT_PATTERN)
-    }
-}
-
-impl LateLintPass for PatternPass {
-    fn check_pat(&mut self, cx: &LateContext, pat: &Pat) {
-        if let PatKind::Binding(_, ref ident, Some(ref right)) = pat.node {
-            if right.node == PatKind::Wild {
-                span_lint(cx,
-                          REDUNDANT_PATTERN,
-                          pat.span,
-                          &format!("the `{} @ _` pattern can be written as just `{}`",
-                                   ident.node,
-                                   ident.node));
-            }
-        }
-    }
-}
-
-
-/// **What it does:** This lint checks for the use of bindings with a single leading underscore
-///
-/// **Why is this bad?** A single leading underscore is usually used to indicate that a binding
-/// will not be used. Using such a binding breaks this expectation.
-///
-/// **Known problems:** The lint does not work properly with desugaring and macro, it has been
-/// allowed in the mean time.
-///
-/// **Example**:
-/// ```
-/// let _x = 0;
-/// let y = _x + 1; // Here we are using `_x`, even though it has a leading underscore.
-///                 // We should rename `_x` to `x`
-/// ```
-declare_lint!(pub USED_UNDERSCORE_BINDING, Allow,
-              "using a binding which is prefixed with an underscore");
-
-#[derive(Copy, Clone)]
-pub struct UsedUnderscoreBinding;
-
-impl LintPass for UsedUnderscoreBinding {
-    fn get_lints(&self) -> LintArray {
-        lint_array!(USED_UNDERSCORE_BINDING)
-    }
-}
-
-impl LateLintPass for UsedUnderscoreBinding {
-    #[cfg_attr(rustfmt, rustfmt_skip)]
-    fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
-        if in_attributes_expansion(cx, expr) {
-            // Don't lint things expanded by #[derive(...)], etc
-            return;
-        }
-        let binding = match expr.node {
-            ExprPath(_, ref path) => {
-                let segment = path.segments
-                                .last()
-                                .expect("path should always have at least one segment")
-                                .name;
-                if segment.as_str().starts_with('_') &&
-                   !segment.as_str().starts_with("__") &&
-                   segment != segment.unhygienize() && // not in bang macro
-                   is_used(cx, expr) {
-                    Some(segment.as_str())
-                } else {
-                    None
-                }
-            }
-            ExprField(_, spanned) => {
-                let name = spanned.node.as_str();
-                if name.starts_with('_') && !name.starts_with("__") {
-                    Some(name)
-                } else {
-                    None
-                }
-            }
-            _ => None,
-        };
-        if let Some(binding) = binding {
-            if binding != "_result" { // FIXME: #944
-                span_lint(cx,
-                          USED_UNDERSCORE_BINDING,
-                          expr.span,
-                          &format!("used binding `{}` which is prefixed with an underscore. A leading \
-                                    underscore signals that a binding will not be used.", binding));
-            }
-        }
-    }
+        matches!(walk_ptrs_ty(cx.tcx.tables().expr_ty(&args[0])).sty, ty::TyStr)
 }
 
 /// Heuristic to see if an expression is used. Should be compatible with `unused_variables`'s idea
 /// of what it means for an expression to be "used".
 fn is_used(cx: &LateContext, expr: &Expr) -> bool {
-    if let Some(ref parent) = get_parent_expr(cx, expr) {
+    if let Some(parent) = get_parent_expr(cx, expr) {
         match parent.node {
             ExprAssign(_, ref rhs) |
             ExprAssignOp(_, _, ref rhs) => **rhs == *expr,
@@ -463,3 +435,17 @@ fn in_attributes_expansion(cx: &LateContext, expr: &Expr) -> bool {
         })
     })
 }
+
+/// Test whether `def` is a variable defined outside a macro.
+fn non_macro_local(cx: &LateContext, def: &def::Def) -> bool {
+    match *def {
+        def::Def::Local(id) | def::Def::Upvar(id, _, _) => {
+            if let Some(span) = cx.tcx.map.span_if_local(id) {
+                !in_macro(cx, span)
+            } else {
+                true
+            }
+        }
+        _ => false,
+    }
+}