Reimplement the `fn_to_numeric_cast` lint

CHANGELOG.md

@@ -688,6 +688,7 @@ All notable changes to this project will be documented in this file.
 [`float_arithmetic`]: https://rust-lang-nursery.github.io/rust-clippy/master/index.html#float_arithmetic
 [`float_cmp`]: https://rust-lang-nursery.github.io/rust-clippy/master/index.html#float_cmp
 [`float_cmp_const`]: https://rust-lang-nursery.github.io/rust-clippy/master/index.html#float_cmp_const
+[`fn_to_numeric_cast`]: https://rust-lang-nursery.github.io/rust-clippy/master/index.html#fn_to_numeric_cast
 [`for_kv_map`]: https://rust-lang-nursery.github.io/rust-clippy/master/index.html#for_kv_map
 [`for_loop_over_option`]: https://rust-lang-nursery.github.io/rust-clippy/master/index.html#for_loop_over_option
 [`for_loop_over_result`]: https://rust-lang-nursery.github.io/rust-clippy/master/index.html#for_loop_over_result

README.md

@@ -9,7 +9,7 @@ We are currently in the process of discussing Clippy 1.0 via the RFC process in
 
 A collection of lints to catch common mistakes and improve your [Rust](https://github.com/rust-lang/rust) code.
 
-[There are 277 lints included in this crate!](https://rust-lang-nursery.github.io/rust-clippy/master/index.html)
+[There are 278 lints included in this crate!](https://rust-lang-nursery.github.io/rust-clippy/master/index.html)
 
 We have a bunch of lint categories to allow you to choose how much Clippy is supposed to ~~annoy~~ help you:
 

clippy_lints/src/lib.rs

@@ -697,6 +697,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         types::CAST_LOSSLESS,
         types::CAST_PTR_ALIGNMENT,
         types::CHAR_LIT_AS_U8,
+        types::FN_TO_NUMERIC_CAST,
         types::IMPLICIT_HASHER,
         types::LET_UNIT_VALUE,
         types::OPTION_OPTION,
@@ -789,6 +790,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         returns::LET_AND_RETURN,
         returns::NEEDLESS_RETURN,
         strings::STRING_LIT_AS_BYTES,
+        types::FN_TO_NUMERIC_CAST,
         types::IMPLICIT_HASHER,
         types::LET_UNIT_VALUE,
         unsafe_removed_from_name::UNSAFE_REMOVED_FROM_NAME,

clippy_lints/src/types.rs

@@ -719,6 +719,30 @@ declare_clippy_lint! {
     "cast from a pointer to a more-strictly-aligned pointer"
 }
 
+/// **What it does:** Checks for casts of function pointers to something other than usize
+///
+/// **Why is this bad?**
+/// Depending on the system architechture, casting a function pointer to something other than
+/// `usize` will result in incorrect pointer addresses.
+/// `usize` will always be able to store the function pointer on the given architechture.
+///
+/// **Example**
+///
+/// ```rust
+/// // Bad
+/// fn fun() -> i32 {}
+/// let a = fun as i64;
+///
+/// // Good
+/// fn fun2() -> i32 {}
+/// let a = fun2 as usize;
+/// ```
+declare_clippy_lint! {
+    pub FN_TO_NUMERIC_CAST,
+    style,
+    "casting a function pointer to a numeric type other than usize"
+}
+
 /// 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<'_>, tcx: TyCtxt<'_, '_, '_>) -> u64 {
@@ -913,6 +937,7 @@ impl LintPass for CastPass {
             CAST_LOSSLESS,
             UNNECESSARY_CAST,
             CAST_PTR_ALIGNMENT,
+            FN_TO_NUMERIC_CAST
         )
     }
 }
@@ -921,6 +946,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for CastPass {
     fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
         if let ExprKind::Cast(ref ex, _) = expr.node {
             let (cast_from, cast_to) = (cx.tables.expr_ty(ex), cx.tables.expr_ty(expr));
+            lint_fn_to_numeric_cast(cx, expr, ex, cast_from, cast_to);
             if let ExprKind::Lit(ref lit) = ex.node {
                 use crate::syntax::ast::{LitIntType, LitKind};
                 match lit.node {
@@ -1021,6 +1047,25 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for CastPass {
     }
 }
 
+fn lint_fn_to_numeric_cast(cx: &LateContext<'_, '_>, expr: &Expr, cast_expr: &Expr, cast_from: Ty, cast_to: Ty) {
+    match cast_from.sty {
+        ty::FnDef(..) | ty::FnPtr(_) => {
+            let from_snippet = snippet(cx, cast_expr.span, "x");
+            if cast_to.sty != ty::Uint(UintTy::Usize) {
+                span_lint_and_sugg(
+                    cx,
+                    FN_TO_NUMERIC_CAST,
+                    expr.span,
+                    &format!("casting function pointer `{}` to `{}`", from_snippet, cast_to),
+                    "try",
+                    format!("{} as usize", from_snippet)
+                );
+            }
+        },
+        _ => {}
+    }
+}
+
 /// **What it does:** Checks for types used in structs, parameters and `let`
 /// declarations above a certain complexity threshold.
 ///

tests/ui/fn_to_numeric_cast.rs

@@ -0,0 +1,49 @@
+#![feature(tool_lints)]
+
+#[warn(clippy::fn_to_numeric_cast)]
+
+fn foo() -> String { String::new() }
+
+fn test_function_to_numeric_cast() {
+    let _ = foo as i8;
+    let _ = foo as i16;
+    let _ = foo as i32;
+    let _ = foo as i64;
+    let _ = foo as i128;
+    let _ = foo as isize;
+
+    let _ = foo as u8;
+    let _ = foo as u16;
+    let _ = foo as u32;
+    let _ = foo as u64;
+    let _ = foo as u128;
+
+    // Casting to usize is OK and should not warn
+    let _ = foo as usize;
+}
+
+fn test_function_var_to_numeric_cast() {
+    let abc: fn() -> String = foo;
+
+    let _ = abc as i8;
+    let _ = abc as i16;
+    let _ = abc as i32;
+    let _ = abc as i64;
+    let _ = abc as i128;
+    let _ = abc as isize;
+
+    let _ = abc as u8;
+    let _ = abc as u16;
+    let _ = abc as u32;
+    let _ = abc as u64;
+    let _ = abc as u128;
+
+    // Casting to usize is OK and should not warn
+    let _ = abc as usize;
+}
+
+fn fn_with_fn_args(f: fn(i32) -> i32) -> i32 {
+    f as i32
+}
+
+fn main() {}