add needless_borrow lint

Author: oli-obk

CHANGELOG.md

@@ -149,6 +149,7 @@ All notable changes to this project will be documented in this file.
 [`mutex_atomic`]: https://github.com/Manishearth/rust-clippy/wiki#mutex_atomic
 [`mutex_integer`]: https://github.com/Manishearth/rust-clippy/wiki#mutex_integer
 [`needless_bool`]: https://github.com/Manishearth/rust-clippy/wiki#needless_bool
+[`needless_borrow`]: https://github.com/Manishearth/rust-clippy/wiki#needless_borrow
 [`needless_lifetimes`]: https://github.com/Manishearth/rust-clippy/wiki#needless_lifetimes
 [`needless_range_loop`]: https://github.com/Manishearth/rust-clippy/wiki#needless_range_loop
 [`needless_return`]: https://github.com/Manishearth/rust-clippy/wiki#needless_return

README.md

@@ -17,7 +17,7 @@ Table of contents:
 
 ## Lints
 
-There are 146 lints included in this crate:
+There are 147 lints included in this crate:
 
 name                                                                                                                 | default | meaning
 ---------------------------------------------------------------------------------------------------------------------|---------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
@@ -97,6 +97,7 @@ name
 [mutex_atomic](https://github.com/Manishearth/rust-clippy/wiki#mutex_atomic)                                         | warn    | using a Mutex where an atomic value could be used instead
 [mutex_integer](https://github.com/Manishearth/rust-clippy/wiki#mutex_integer)                                       | allow   | using a Mutex for an integer type
 [needless_bool](https://github.com/Manishearth/rust-clippy/wiki#needless_bool)                                       | warn    | if-statements with plain booleans in the then- and else-clause, e.g. `if p { true } else { false }`
+[needless_borrow](https://github.com/Manishearth/rust-clippy/wiki#needless_borrow)                                   | warn    | taking a reference that is going to be automatically dereferenced
 [needless_lifetimes](https://github.com/Manishearth/rust-clippy/wiki#needless_lifetimes)                             | warn    | using explicit lifetimes for references in function arguments when elision rules would allow omitting them
 [needless_range_loop](https://github.com/Manishearth/rust-clippy/wiki#needless_range_loop)                           | warn    | for-looping over a range of indices where an iterator over items would do
 [needless_return](https://github.com/Manishearth/rust-clippy/wiki#needless_return)                                   | warn    | using a return statement like `return expr;` where an expression would suffice

src/lib.rs

@@ -96,6 +96,7 @@ pub mod mut_mut;
 pub mod mut_reference;
 pub mod mutex_atomic;
 pub mod needless_bool;
+pub mod needless_borrow;
 pub mod needless_update;
 pub mod neg_multiply;
 pub mod new_without_default;
@@ -210,6 +211,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_late_lint_pass(box zero_div_zero::ZeroDivZeroPass);
     reg.register_late_lint_pass(box mutex_atomic::MutexAtomic);
     reg.register_late_lint_pass(box needless_update::NeedlessUpdatePass);
+    reg.register_late_lint_pass(box needless_borrow::NeedlessBorrow);
     reg.register_late_lint_pass(box no_effect::NoEffectPass);
     reg.register_late_lint_pass(box map_clone::MapClonePass);
     reg.register_late_lint_pass(box temporary_assignment::TemporaryAssignmentPass);
@@ -364,6 +366,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
         mutex_atomic::MUTEX_ATOMIC,
         needless_bool::BOOL_COMPARISON,
         needless_bool::NEEDLESS_BOOL,
+        needless_borrow::NEEDLESS_BORROW,
         needless_update::NEEDLESS_UPDATE,
         neg_multiply::NEG_MULTIPLY,
         new_without_default::NEW_WITHOUT_DEFAULT,

src/needless_borrow.rs

@@ -0,0 +1,50 @@
+//! Checks for needless address of operations (`&`)
+//!
+//! This lint is **warn** by default
+
+use rustc::lint::*;
+use rustc::hir::*;
+use rustc::ty::TyRef;
+use utils::{span_lint, in_macro};
+
+/// **What it does:** This lint checks for address of operations (`&`) that are going to be dereferenced immediately by the compiler
+///
+/// **Why is this bad?** Suggests that the receiver of the expression borrows the expression
+///
+/// **Known problems:**
+///
+/// **Example:** `let x: &i32 = &&&&&&5;`
+declare_lint! {
+    pub NEEDLESS_BORROW,
+    Warn,
+    "taking a reference that is going to be automatically dereferenced"
+}
+
+#[derive(Copy,Clone)]
+pub struct NeedlessBorrow;
+
+impl LintPass for NeedlessBorrow {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(NEEDLESS_BORROW)
+    }
+}
+
+impl LateLintPass for NeedlessBorrow {
+    fn check_expr(&mut self, cx: &LateContext, e: &Expr) {
+        if in_macro(cx, e.span) {
+            return;
+        }
+        if let ExprAddrOf(MutImmutable, ref inner) = e.node {
+            if let TyRef(..) = cx.tcx.expr_ty(inner).sty {
+                let ty = cx.tcx.expr_ty(e);
+                let adj_ty = cx.tcx.expr_ty_adjusted(e);
+                if ty != adj_ty {
+                    span_lint(cx,
+                              NEEDLESS_BORROW,
+                              e.span,
+                              "this expression borrows a reference that is immediately dereferenced by the compiler");
+                }
+            }
+        }
+    }
+}

tests/compile-fail/eta.rs

@@ -18,6 +18,7 @@ fn main() {
     //~| SUGGESTION let c = Some(1u8).map({1+2; foo});
     let d = Some(1u8).map(|a| foo((|b| foo2(b))(a))); //is adjusted?
     all(&[1, 2, 3], &&2, |x, y| below(x, y)); //is adjusted
+    //~^ WARN needless_borrow
     unsafe {
         Some(1u8).map(|a| unsafe_fn(a)); // unsafe fn
     }

tests/compile-fail/needless_borrow.rs

@@ -0,0 +1,27 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+fn x(y: &i32) -> i32 {
+    *y
+}
+
+#[deny(clippy)]
+#[allow(unused_variables)]
+fn main() {
+    let a = 5;
+    let b = x(&a);
+    let c = x(&&a); //~ ERROR: needless_borrow
+    let s = &String::from("hi");
+    let s_ident = f(&s); // should not error, because `&String` implements Copy, but `String` does not
+    let g_val = g(&Vec::new()); // should not error, because `&Vec<T>` derefs to `&[T]`
+    let vec = Vec::new();
+    let vec_val = g(&vec); // should not error, because `&Vec<T>` derefs to `&[T]`
+}
+
+fn f<T:Copy>(y: &T) -> T {
+    *y
+}
+
+fn g(y: &[u8]) -> u8 {
+    y[0]
+}