Merge pull request #1861 from CBenoit/master

Add example for needless borrowed ref lint and register it

Author: oli-obk

clippy_lints/src/lib.rs

@@ -264,6 +264,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
     reg.register_late_lint_pass(box mutex_atomic::MutexAtomic);
     reg.register_late_lint_pass(box needless_update::Pass);
     reg.register_late_lint_pass(box needless_borrow::NeedlessBorrow);
+    reg.register_late_lint_pass(box needless_borrowed_ref::NeedlessBorrowedRef);
     reg.register_late_lint_pass(box no_effect::Pass);
     reg.register_late_lint_pass(box map_clone::Pass);
     reg.register_late_lint_pass(box temporary_assignment::Pass);

clippy_lints/src/needless_borrowed_ref.rs

@@ -4,16 +4,31 @@
 
 use rustc::lint::*;
 use rustc::hir::{MutImmutable, Pat, PatKind, BindingAnnotation};
-use rustc::ty;
-use utils::{span_lint, in_macro};
+use utils::{span_lint_and_then, in_macro, snippet};
 
 /// **What it does:** Checks for useless borrowed references.
 ///
-/// **Why is this bad?** It is completely useless and make the code look more
-/// complex than it
+/// **Why is this bad?** It is mostly useless and make the code look more complex than it
 /// actually is.
 ///
-/// **Known problems:** None.
+/// **Known problems:** It seems that the `&ref` pattern is sometimes useful.
+/// For instance in the following snippet:
+/// ```rust
+/// enum Animal {
+///     Cat(u64),
+///     Dog(u64),
+/// }
+///
+/// fn foo(a: &Animal, b: &Animal) {
+///     match (a, b) {
+///         (&Animal::Cat(v), k) | (k, &Animal::Cat(v)) => (), // lifetime mismatch error
+///         (&Animal::Dog(ref c), &Animal::Dog(_)) => ()
+///     }
+/// }
+/// ```
+/// There is a lifetime mismatch error for `k` (indeed a and b have distinct lifetime).
+/// This can be fixed by using the `&ref` pattern.
+/// However, the code can also be fixed by much cleaner ways
 ///
 /// **Example:**
 /// ```rust
@@ -47,15 +62,19 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessBorrowedRef {
         }
 
         if_let_chain! {[
-            // Pat is a pattern whose node
-            // is a binding which "involves" a immutable reference...
-            let PatKind::Binding(BindingAnnotation::Ref, ..) = pat.node,
-            // Pattern's type is a reference. Get the type and mutability of referenced value (tam: TypeAndMut).
-            let ty::TyRef(_, ref tam) = cx.tables.pat_ty(pat).sty,
-            // This is an immutable reference.
-            tam.mutbl == MutImmutable,
+            // Only lint immutable refs, because `&mut ref T` may be useful.
+            let PatKind::Ref(ref sub_pat, MutImmutable) = pat.node,
+
+            // Check sub_pat got a `ref` keyword (excluding `ref mut`).
+            let PatKind::Binding(BindingAnnotation::Ref, _, spanned_name, ..) = sub_pat.node,
         ], {
-            span_lint(cx, NEEDLESS_BORROWED_REFERENCE, pat.span, "this pattern takes a reference on something that is being de-referenced")
+            span_lint_and_then(cx, NEEDLESS_BORROWED_REFERENCE, pat.span,
+                               "this pattern takes a reference on something that is being de-referenced",
+                               |db| {
+                                   let hint = snippet(cx, spanned_name.span, "..").into_owned();
+                                   db.span_suggestion(pat.span, "try removing the `&ref` part and just keep", hint);
+                               });
         }}
     }
 }
+

clippy_tests/examples/needless_borrowed_ref.rs

@@ -0,0 +1,48 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+#[warn(needless_borrowed_reference)]
+#[allow(unused_variables)]
+fn main() {
+    let mut v = Vec::<String>::new();
+    let _ = v.iter_mut().filter(|&ref a| a.is_empty());
+    //                            ^ should be linted
+
+    let var = 3;
+    let thingy = Some(&var);
+    if let Some(&ref v) = thingy {
+        //          ^ should be linted
+    }
+
+    let mut var2 = 5;
+    let thingy2 = Some(&mut var2);
+    if let Some(&mut ref mut v) = thingy2 {
+        //          ^ should *not* be linted
+        // v is borrowed as mutable.
+        *v = 10;
+    }
+    if let Some(&mut ref v) = thingy2 {
+        //          ^ should *not* be linted
+        // here, v is borrowed as immutable.
+        // can't do that:
+        //*v = 15;
+    }
+}
+
+#[allow(dead_code)]
+enum Animal {
+    Cat(u64),
+    Dog(u64),
+}
+
+#[allow(unused_variables)]
+#[allow(dead_code)]
+fn foo(a: &Animal, b: &Animal) {
+    match (a, b) {
+        (&Animal::Cat(v), &ref k) | (&ref k, &Animal::Cat(v)) => (), // lifetime mismatch error if there is no '&ref'
+        //                  ^    and   ^ should *not* be linted
+        (&Animal::Dog(ref a), &Animal::Dog(_)) => ()
+        //              ^ should *not* be linted
+    }
+}
+

clippy_tests/examples/needless_borrowed_ref.stderr

@@ -0,0 +1,38 @@
+error: this pattern takes a reference on something that is being de-referenced
+ --> needless_borrowed_ref.rs:8:34
+  |
+8 |     let _ = v.iter_mut().filter(|&ref a| a.is_empty());
+  |                                  ^^^^^^ help: try removing the `&ref` part and just keep `a`
+  |
+  = note: `-D needless-borrowed-reference` implied by `-D warnings`
+  = help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#needless_borrowed_reference
+
+error: this pattern takes a reference on something that is being de-referenced
+  --> needless_borrowed_ref.rs:13:17
+   |
+13 |     if let Some(&ref v) = thingy {
+   |                 ^^^^^^ help: try removing the `&ref` part and just keep `v`
+   |
+   = help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#needless_borrowed_reference
+
+error: this pattern takes a reference on something that is being de-referenced
+  --> needless_borrowed_ref.rs:42:27
+   |
+42 |         (&Animal::Cat(v), &ref k) | (&ref k, &Animal::Cat(v)) => (), // lifetime mismatch error if there is no '&ref'
+   |                           ^^^^^^ help: try removing the `&ref` part and just keep `k`
+   |
+   = help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#needless_borrowed_reference
+
+error: this pattern takes a reference on something that is being de-referenced
+  --> needless_borrowed_ref.rs:42:38
+   |
+42 |         (&Animal::Cat(v), &ref k) | (&ref k, &Animal::Cat(v)) => (), // lifetime mismatch error if there is no '&ref'
+   |                                      ^^^^^^ help: try removing the `&ref` part and just keep `k`
+   |
+   = help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#needless_borrowed_reference
+
+error: aborting due to previous error(s)
+
+error: Could not compile `clippy_tests`.
+
+To learn more, run the command again with --verbose.

tests/ui/needless_borrow.stderr

@@ -18,11 +18,25 @@ error: this expression borrows a reference that is immediately dereferenced by t
 27 |         46 => &&a,
    |               ^^^
 
+error: this pattern takes a reference on something that is being de-referenced
+  --> $DIR/needless_borrow.rs:49:34
+   |
+49 |     let _ = v.iter_mut().filter(|&ref a| a.is_empty());
+   |                                  ^^^^^^ help: try removing the `&ref` part and just keep: `a`
+   |
+   = note: `-D needless-borrowed-reference` implied by `-D warnings`
+
+error: this pattern takes a reference on something that is being de-referenced
+  --> $DIR/needless_borrow.rs:50:30
+   |
+50 |     let _ = v.iter().filter(|&ref a| a.is_empty());
+   |                              ^^^^^^ help: try removing the `&ref` part and just keep: `a`
+
 error: this pattern creates a reference to a reference
   --> $DIR/needless_borrow.rs:50:31
    |
 50 |     let _ = v.iter().filter(|&ref a| a.is_empty());
    |                               ^^^^^
 
-error: aborting due to 4 previous errors
+error: aborting due to 6 previous errors