Merge pull request #1544 from bood/master

Add lint for unnecessary casts (cast to same type)

CHANGELOG.md

@@ -447,6 +447,7 @@ All notable changes to this project will be documented in this file.
 [`type_complexity`]: https://github.com/Manishearth/rust-clippy/wiki#type_complexity
 [`unicode_not_nfc`]: https://github.com/Manishearth/rust-clippy/wiki#unicode_not_nfc
 [`unit_cmp`]: https://github.com/Manishearth/rust-clippy/wiki#unit_cmp
+[`unnecessary_cast`]: https://github.com/Manishearth/rust-clippy/wiki#unnecessary_cast
 [`unnecessary_mut_passed`]: https://github.com/Manishearth/rust-clippy/wiki#unnecessary_mut_passed
 [`unnecessary_operation`]: https://github.com/Manishearth/rust-clippy/wiki#unnecessary_operation
 [`unneeded_field_pattern`]: https://github.com/Manishearth/rust-clippy/wiki#unneeded_field_pattern

README.md

@@ -351,6 +351,7 @@ name
 [type_complexity](https://github.com/Manishearth/rust-clippy/wiki#type_complexity)                                     | warn    | usage of very complex types that might be better factored into `type` definitions
 [unicode_not_nfc](https://github.com/Manishearth/rust-clippy/wiki#unicode_not_nfc)                                     | allow   | using a unicode literal not in NFC normal form (see [unicode tr15](http://www.unicode.org/reports/tr15/) for further information)
 [unit_cmp](https://github.com/Manishearth/rust-clippy/wiki#unit_cmp)                                                   | warn    | comparing unit values
+[unnecessary_cast](https://github.com/Manishearth/rust-clippy/wiki#unnecessary_cast)                                   | warn    | cast to the same type, e.g `x as i32` where `x: i32`
 [unnecessary_mut_passed](https://github.com/Manishearth/rust-clippy/wiki#unnecessary_mut_passed)                       | warn    | an argument passed as a mutable reference although the callee only demands an immutable reference
 [unnecessary_operation](https://github.com/Manishearth/rust-clippy/wiki#unnecessary_operation)                         | warn    | outer expressions with no effect
 [unneeded_field_pattern](https://github.com/Manishearth/rust-clippy/wiki#unneeded_field_pattern)                       | warn    | struct fields bound to a wildcard instead of using `..`

clippy_lints/src/lib.rs

@@ -497,6 +497,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
         types::LINKEDLIST,
         types::TYPE_COMPLEXITY,
         types::UNIT_CMP,
+        types::UNNECESSARY_CAST,
         unicode::ZERO_WIDTH_SPACE,
         unsafe_removed_from_name::UNSAFE_REMOVED_FROM_NAME,
         unused_io_amount::UNUSED_IO_AMOUNT,

clippy_lints/src/types.rs

@@ -379,6 +379,22 @@ declare_lint! {
      and `x > i32::MAX`"
 }
 
+/// **What it does:** Checks for casts to the same type.
+///
+/// **Why is this bad?** It's just unnecessary.
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+/// ```rust
+/// let _ = 2i32 as i32
+/// ```
+declare_lint! {
+    pub UNNECESSARY_CAST,
+    Warn,
+    "cast to the same type, e.g `x as i32` where `x: i32`"
+}
+
 /// Returns the size in bits of an integral type.
 /// Will return 0 if the type is not an int or uint variant
 fn int_ty_to_nbits(typ: &ty::TyS) -> usize {
@@ -503,7 +519,8 @@ impl LintPass for CastPass {
         lint_array!(CAST_PRECISION_LOSS,
                     CAST_SIGN_LOSS,
                     CAST_POSSIBLE_TRUNCATION,
-                    CAST_POSSIBLE_WRAP)
+                    CAST_POSSIBLE_WRAP,
+                    UNNECESSARY_CAST)
     }
 }
 
@@ -511,6 +528,23 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for CastPass {
     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
         if let ExprCast(ref ex, _) = expr.node {
             let (cast_from, cast_to) = (cx.tables.expr_ty(ex), cx.tables.expr_ty(expr));
+            if let ExprLit(ref lit) = ex.node {
+                use syntax::ast::{LitKind, LitIntType};
+                match lit.node {
+                    LitKind::Int(_, LitIntType::Unsuffixed) |
+                    LitKind::FloatUnsuffixed(_) => {},
+                    _ => {
+                        if cast_from.sty == cast_to.sty && !in_external_macro(cx, expr.span) {
+                            span_lint(cx,
+                                      UNNECESSARY_CAST,
+                                      expr.span,
+                                      &format!("casting to the same type is unnecessary (`{}` -> `{}`)",
+                                               cast_from,
+                                               cast_to));
+                        }
+                    },
+                }
+            }
             if cast_from.is_numeric() && cast_to.is_numeric() && !in_external_macro(cx, expr.span) {
                 match (cast_from.is_integral(), cast_to.is_integral()) {
                     (true, false) => {

tests/ui/cast.rs

@@ -61,4 +61,17 @@ fn main() {
     1u32 as usize; // Should not trigger any lint
     1i32 as isize; // Neither should this
     1i32 as usize;
+
+    // Test cast_unnecessary
+    1i32 as i32;
+    1f32 as f32;
+    false as bool;
+    &1i32 as &i32;
+
+    // Should not trigger
+    1i32 as i64;
+    let v = vec!(1);
+    &v as &[i32];
+    1.0 as f64;
+    1 as u64;
 }

tests/ui/cast.stderr

@@ -274,5 +274,29 @@ error: casting i32 to usize may lose the sign of the value
 63 |     1i32 as usize;
    |     ^^^^^^^^^^^^^
 
+warning: casting to the same type is unnecessary (`i32` -> `i32`)
+  --> $DIR/cast.rs:66:5
+   |
+66 |     1i32 as i32;
+   |     ^^^^^^^^^^^
+   |
+   = note: #[warn(unnecessary_cast)] on by default
+
+warning: casting to the same type is unnecessary (`f32` -> `f32`)
+  --> $DIR/cast.rs:67:5
+   |
+67 |     1f32 as f32;
+   |     ^^^^^^^^^^^
+   |
+   = note: #[warn(unnecessary_cast)] on by default
+
+warning: casting to the same type is unnecessary (`bool` -> `bool`)
+  --> $DIR/cast.rs:68:5
+   |
+68 |     false as bool;
+   |     ^^^^^^^^^^^^^
+   |
+   = note: #[warn(unnecessary_cast)] on by default
+
 error: aborting due to 42 previous errors