Add lint to suggest into()

Author: rudyardrichter

CHANGELOG.md

@@ -842,6 +842,7 @@ All notable changes to this project will be documented in this file.
 [`unused_label`]: https://rust-lang-nursery.github.io/rust-clippy/master/index.html#unused_label
 [`use_debug`]: https://rust-lang-nursery.github.io/rust-clippy/master/index.html#use_debug
 [`use_self`]: https://rust-lang-nursery.github.io/rust-clippy/master/index.html#use_self
+[`use_shared_from_slice`]: https://rust-lang-nursery.github.io/rust-clippy/master/index.html#use_shared_from_slice
 [`used_underscore_binding`]: https://rust-lang-nursery.github.io/rust-clippy/master/index.html#used_underscore_binding
 [`useless_asref`]: https://rust-lang-nursery.github.io/rust-clippy/master/index.html#useless_asref
 [`useless_attribute`]: https://rust-lang-nursery.github.io/rust-clippy/master/index.html#useless_attribute

clippy_lints/src/lib.rs

@@ -200,6 +200,7 @@ pub mod unsafe_removed_from_name;
 pub mod unused_io_amount;
 pub mod unused_label;
 pub mod use_self;
+pub mod use_shared_from_slice;
 pub mod vec;
 pub mod write;
 pub mod zero_div_zero;
@@ -914,6 +915,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
         mutex_atomic::MUTEX_INTEGER,
         needless_borrow::NEEDLESS_BORROW,
         ranges::RANGE_PLUS_ONE,
+        use_shared_from_slice::USE_SHARED_FROM_SLICE,
     ]);
 }
 

clippy_lints/src/use_shared_from_slice.rs

@@ -0,0 +1,69 @@
+use rustc::hir::Expr;
+use rustc::lint::{LateContext, LateLintPass, LintArray, LintContext, LintPass};
+use rustc::ty;
+use utils::{match_type, span_lint_and_sugg, walk_ptrs_ty};
+use utils::paths;
+
+/// **What it does:**
+/// Checks for usage of `Rc<String>` or `Rc<Vec<T>>`.
+///
+/// **Why is this bad?**
+/// Using a `Rc<str>` or `Rc<[T]>` is more efficient and easy to construct with
+/// `into()`.
+///
+/// **Known problems:**
+/// None.
+///
+/// **Example:**
+///
+/// ```rust
+/// use std::rc::Rc;
+///
+/// // Bad
+/// let bad_ref: Rc<Vec<usize>> = Rc::new(vec!(1, 2, 3));
+///
+/// // Good
+/// let good_ref: Rc<[usize]> = Rc::new(vec!(1, 2, 3).into());
+/// ```
+declare_clippy_lint! {
+    pub USE_SHARED_FROM_SLICE,
+    nursery,
+    "use `into()` to construct `Rc` from slice"
+}
+
+#[derive(Copy, Clone, Debug)]
+pub struct Pass;
+
+impl LintPass for Pass {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(USE_SHARED_FROM_SLICE)
+    }
+}
+
+impl <'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
+    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
+        let expr_ty = walk_ptrs_ty(cx.tables.expr_ty(expr));
+
+        // Check for expressions with the type `Rc<Vec<T>>`.
+        if_chain! {
+            if let ty::TyAdt(_, subst) = expr_ty.sty;
+            if match_type(cx, expr_ty, &paths::RC);
+            if match_type(cx, subst.type_at(1), &paths::VEC);
+            then {
+                span_lint_and_sugg(
+                    cx,
+                    USE_SHARED_FROM_SLICE,
+                    expr.span,
+                    "constructing reference-counted type from vec",
+                    "consider using `into()`",
+                    format!("TODO"),
+                );
+            }
+        }
+
+        // TODO
+        // Check for expressions with the type `Rc<String>`.
+        // Check for expressions with the type `Arc<String>`.
+        // Check for expressions with the type `Arc<Vec<T>>`.
+    }
+}

tests/ui/use_shared_from_slice.rs

@@ -0,0 +1,7 @@
+use std::rc::Rc;
+
+fn main() {
+    let bad_ref: Rc<Vec<usize>> = Rc::new(vec!(1, 2, 3));
+
+    let good_ref: Rc<[usize]> = vec!(1, 2, 3).into();
+}