New zero_ptr lint.

This fixes #1047. I did not bother to add a full suggestion,
because replacing with `std::ptr::null()` may still lead to
inference failures.

CHANGELOG.md

@@ -1,6 +1,7 @@
 # Change Log
 All notable changes to this project will be documented in this file.
 
+* New [`zero_ptr`] lint
 * New [`never_loop`] lint
 * New [`mut_from_ref`] lint
 
@@ -472,5 +473,6 @@ All notable changes to this project will be documented in this file.
 [`wrong_transmute`]: https://github.com/Manishearth/rust-clippy/wiki#wrong_transmute
 [`zero_divided_by_zero`]: https://github.com/Manishearth/rust-clippy/wiki#zero_divided_by_zero
 [`zero_prefixed_literal`]: https://github.com/Manishearth/rust-clippy/wiki#zero_prefixed_literal
+[`zero_ptr`]: https://github.com/Manishearth/rust-clippy/wiki#zero_ptr
 [`zero_width_space`]: https://github.com/Manishearth/rust-clippy/wiki#zero_width_space
 <!-- end autogenerated links to wiki -->

README.md

@@ -180,7 +180,7 @@ transparently:
 
 ## Lints
 
-There are 191 lints included in this crate:
+There are 192 lints included in this crate:
 
 name                                                                                                                   | default | triggers on
 -----------------------------------------------------------------------------------------------------------------------|---------|----------------------------------------------------------------------------------------------------------------------------------
@@ -374,6 +374,7 @@ name
 [wrong_transmute](https://github.com/Manishearth/rust-clippy/wiki#wrong_transmute)                                     | warn    | transmutes that are confusing at best, undefined behaviour at worst and always useless
 [zero_divided_by_zero](https://github.com/Manishearth/rust-clippy/wiki#zero_divided_by_zero)                           | warn    | usage of `0.0 / 0.0` to obtain NaN instead of std::f32::NaN or std::f64::NaN
 [zero_prefixed_literal](https://github.com/Manishearth/rust-clippy/wiki#zero_prefixed_literal)                         | warn    | integer literals starting with `0`
+[zero_ptr](https://github.com/Manishearth/rust-clippy/wiki#zero_ptr)                                                   | warn    | using 0 as *{const, mut} T
 [zero_width_space](https://github.com/Manishearth/rust-clippy/wiki#zero_width_space)                                   | deny    | using a zero-width space in a string literal, which is confusing
 
 More to come, please [file an issue](https://github.com/Manishearth/rust-clippy/issues) if you have ideas!

clippy_lints/src/lib.rs

@@ -450,6 +450,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
         misc_early::REDUNDANT_CLOSURE_CALL,
         misc_early::UNNEEDED_FIELD_PATTERN,
         misc_early::ZERO_PREFIXED_LITERAL,
+        misc_early::ZERO_PTR,
         mut_reference::UNNECESSARY_MUT_PASSED,
         mutex_atomic::MUTEX_ATOMIC,
         needless_bool::BOOL_COMPARISON,

clippy_lints/src/misc_early.rs

@@ -162,6 +162,23 @@ declare_lint! {
     "shadowing a builtin type"
 }
 
+/// **What it does:** Catch casts from `0` to some pointer type
+///
+/// **Why is this bad?** This generally means `null` and is better expressed as
+/// {`std`, `core`}`::ptr::`{`null`, `null_mut`}.
+///
+/// **Known problems:** None.
+///
+/// **Example:**
+///
+/// ```rust
+/// ß as *const u32
+/// ```
+declare_lint! {
+    pub ZERO_PTR,
+    Warn,
+    "using 0 as *{const, mut} T"
+}
 
 #[derive(Copy, Clone)]
 pub struct MiscEarly;
@@ -175,7 +192,8 @@ impl LintPass for MiscEarly {
                     MIXED_CASE_HEX_LITERALS,
                     UNSEPARATED_LITERAL_SUFFIX,
                     ZERO_PREFIXED_LITERAL,
-                    BUILTIN_TYPE_SHADOW)
+                    BUILTIN_TYPE_SHADOW,
+                    ZERO_PTR)
     }
 }
 
@@ -363,6 +381,9 @@ impl EarlyLintPass for MiscEarly {
                     }
                 }}
             },
+            ExprKind::Cast(ref e, ref ty) => {
+                check_cast(cx, expr.span, e, ty);
+            },
             _ => (),
         }
     }
@@ -391,3 +412,18 @@ impl EarlyLintPass for MiscEarly {
         }
     }
 }
+
+fn check_cast(cx: &EarlyContext, span: Span, e: &Expr, ty: &Ty) {
+    if_let_chain! {[
+        let TyKind::Ptr(MutTy { mutbl, .. }) = ty.node,
+        let ExprKind::Lit(ref lit) = e.node,
+        let LitKind::Int(value, ..) = lit.node,
+        value == 0
+    ], {
+        let msg = match mutbl {
+            Mutability::Mutable => "`0 as *mut _` detected. Consider using `ptr::null_mut()`",
+            Mutability::Immutable => "`0 as *const _` detected. Consider using `ptr::null()`",
+        };
+        span_lint(cx, ZERO_PTR, span, msg);
+    }}
+}

tests/ui/transmute.rs

@@ -91,7 +91,7 @@ fn issue1231() {
         bar: &'a T,
     }
 
-    let raw = 0 as *const i32;
+    let raw = 42 as *const i32;
     let _: &Foo<u8> = unsafe { std::mem::transmute::<_, &Foo<_>>(raw) };
 
 
@@ -103,7 +103,7 @@ fn issue1231() {
 
 
     type Bar<'a> = &'a u8;
-    let raw = 0 as *const i32;
+    let raw = 42 as *const i32;
     unsafe { std::mem::transmute::<_, Bar>(raw) };
 
 

tests/ui/zero_ptr.rs

@@ -0,0 +1,11 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+#[allow(unused_variables)]
+fn main() {
+    let x = 0 as *const usize;
+    let y = 0 as *mut f64;
+
+    let z = 0;
+    let z = z as *const usize; // this is currently not caught
+}

tests/ui/zero_ptr.stderr

@@ -0,0 +1,16 @@
+warning: `0 as *const _` detected. Consider using `ptr::null()`
+ --> $DIR/zero_ptr.rs:6:13
+  |
+6 |     let x = 0 as *const usize;
+  |             ^^^^^^^^^^^^^^^^^
+  |
+  = note: #[warn(zero_ptr)] on by default
+
+warning: `0 as *mut _` detected. Consider using `ptr::null_mut()`
+ --> $DIR/zero_ptr.rs:7:13
+  |
+7 |     let y = 0 as *mut f64;
+  |             ^^^^^^^^^^^^^
+  |
+  = note: #[warn(zero_ptr)] on by default
+