Auto merge of #6528 - Jarcho:redundant_slicing, r=flip1995

New lint: redundant_slicing

changelog: Added lint: `redundant_slicing`
fixes #6519

This will trigger on any type which implements `Index<RangeFull>` that returns the input type. This would be a false positive if the implementation does something other than return itself, but I'm not sure why you would ever want to do that.

6 files changed, 101 insertions, 1 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index b0e9ad55b4f..85f6929f924 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2162,6 +2162,7 @@ Released 2018-09-13
 [`redundant_pattern`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_pattern
 [`redundant_pattern_matching`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_pattern_matching
 [`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_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
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 4f9ebb4af3d..70fdfd22caa 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -301,6 +301,7 @@ mod redundant_closure_call;
 mod redundant_else;
 mod redundant_field_names;
 mod redundant_pub_crate;
+mod redundant_slicing;
 mod redundant_static_lifetimes;
 mod ref_option_ref;
 mod reference;
@@ -849,6 +850,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &redundant_else::REDUNDANT_ELSE,
         &redundant_field_names::REDUNDANT_FIELD_NAMES,
         &redundant_pub_crate::REDUNDANT_PUB_CRATE,
+        &redundant_slicing::REDUNDANT_SLICING,
         &redundant_static_lifetimes::REDUNDANT_STATIC_LIFETIMES,
         &ref_option_ref::REF_OPTION_REF,
         &reference::DEREF_ADDROF,
@@ -1229,6 +1231,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| box vec_init_then_push::VecInitThenPush::default());
     store.register_late_pass(move || box types::PtrAsPtr::new(msrv));
     store.register_late_pass(|| box case_sensitive_file_extension_comparisons::CaseSensitiveFileExtensionComparisons);
+    store.register_late_pass(|| box redundant_slicing::RedundantSlicing);
 
     store.register_group(true, "clippy::restriction", Some("clippy_restriction"), vec![
         LintId::of(&arithmetic::FLOAT_ARITHMETIC),
@@ -1591,6 +1594,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&redundant_clone::REDUNDANT_CLONE),
         LintId::of(&redundant_closure_call::REDUNDANT_CLOSURE_CALL),
         LintId::of(&redundant_field_names::REDUNDANT_FIELD_NAMES),
+        LintId::of(&redundant_slicing::REDUNDANT_SLICING),
         LintId::of(&redundant_static_lifetimes::REDUNDANT_STATIC_LIFETIMES),
         LintId::of(&reference::DEREF_ADDROF),
         LintId::of(&reference::REF_IN_DEREF),
@@ -1835,6 +1839,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&ptr_offset_with_cast::PTR_OFFSET_WITH_CAST),
         LintId::of(&ranges::RANGE_ZIP_WITH_LEN),
         LintId::of(&redundant_closure_call::REDUNDANT_CLOSURE_CALL),
+        LintId::of(&redundant_slicing::REDUNDANT_SLICING),
         LintId::of(&reference::DEREF_ADDROF),
         LintId::of(&reference::REF_IN_DEREF),
         LintId::of(&repeat_once::REPEAT_ONCE),
diff --git a/clippy_lints/src/map_unit_fn.rs b/clippy_lints/src/map_unit_fn.rs
index e50d11a4d71..01126e86199 100644
--- a/clippy_lints/src/map_unit_fn.rs
+++ b/clippy_lints/src/map_unit_fn.rs
@@ -131,7 +131,7 @@ fn reduce_unit_expression<'a>(cx: &LateContext<'_>, expr: &'a hir::Expr<'_>) ->
             Some(expr.span)
         },
         hir::ExprKind::Block(ref block, _) => {
-            match (&block.stmts[..], block.expr.as_ref()) {
+            match (block.stmts, block.expr.as_ref()) {
                 (&[], Some(inner_expr)) => {
                     // If block only contains an expression,
                     // reduce `{ X }` to `X`
diff --git a/clippy_lints/src/redundant_slicing.rs b/clippy_lints/src/redundant_slicing.rs
new file mode 100644
index 00000000000..e5ced13514f
--- /dev/null
+++ b/clippy_lints/src/redundant_slicing.rs
@@ -0,0 +1,67 @@
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind, LangItem};
+use rustc_lint::{LateContext, LateLintPass, LintContext};
+use rustc_middle::{lint::in_external_macro, ty::TyS};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+use crate::utils::{is_type_lang_item, snippet_with_applicability, span_lint_and_sugg};
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for redundant slicing expressions which use the full range, and
+    /// do not change the type.
+    ///
+    /// **Why is this bad?** It unnecessarily adds complexity to the expression.
+    ///
+    /// **Known problems:** If the type being sliced has an implementation of `Index<RangeFull>`
+    /// that actually changes anything then it can't be removed. However, this would be surprising
+    /// to people reading the code and should have a note with it.
+    ///
+    /// **Example:**
+    ///
+    /// ```ignore
+    /// fn get_slice(x: &[u32]) -> &[u32] {
+    ///     &x[..]
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```ignore
+    /// fn get_slice(x: &[u32]) -> &[u32] {
+    ///     x
+    /// }
+    /// ```
+    pub REDUNDANT_SLICING,
+    complexity,
+    "redundant slicing of the whole range of a type"
+}
+
+declare_lint_pass!(RedundantSlicing => [REDUNDANT_SLICING]);
+
+impl LateLintPass<'_> for RedundantSlicing {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        if in_external_macro(cx.sess(), expr.span) {
+            return;
+        }
+
+        if_chain! {
+            if let ExprKind::AddrOf(_, _, addressee) = expr.kind;
+            if let ExprKind::Index(indexed, range) = addressee.kind;
+            if is_type_lang_item(cx, cx.typeck_results().expr_ty_adjusted(range), LangItem::RangeFull);
+            if TyS::same_type(cx.typeck_results().expr_ty(expr), cx.typeck_results().expr_ty(indexed));
+            then {
+                let mut app = Applicability::MachineApplicable;
+                let hint = snippet_with_applicability(cx, indexed.span, "..", &mut app).into_owned();
+
+                span_lint_and_sugg(
+                    cx,
+                    REDUNDANT_SLICING,
+                    expr.span,
+                    "redundant slicing of the whole range",
+                    "use the original slice instead",
+                    hint,
+                    app,
+                );
+            }
+        }
+    }
+}
diff --git a/tests/ui/redundant_slicing.rs b/tests/ui/redundant_slicing.rs
new file mode 100644
index 00000000000..922b8b4ce57
--- /dev/null
+++ b/tests/ui/redundant_slicing.rs
@@ -0,0 +1,11 @@
+#![allow(unused)]
+#![warn(clippy::redundant_slicing)]
+
+fn main() {
+    let x: &[u32] = &[0];
+    let err = &x[..];
+
+    let v = vec![0];
+    let ok = &v[..];
+    let err = &(&v[..])[..];
+}
diff --git a/tests/ui/redundant_slicing.stderr b/tests/ui/redundant_slicing.stderr
new file mode 100644
index 00000000000..9efd6484ad0
--- /dev/null
+++ b/tests/ui/redundant_slicing.stderr
@@ -0,0 +1,16 @@
+error: redundant slicing of the whole range
+  --> $DIR/redundant_slicing.rs:6:15
+   |
+LL |     let err = &x[..];
+   |               ^^^^^^ help: use the original slice instead: `x`
+   |
+   = note: `-D clippy::redundant-slicing` implied by `-D warnings`
+
+error: redundant slicing of the whole range
+  --> $DIR/redundant_slicing.rs:10:15
+   |
+LL |     let err = &(&v[..])[..];
+   |               ^^^^^^^^^^^^^ help: use the original slice instead: `(&v[..])`
+
+error: aborting due to 2 previous errors
+