Auto merge of rust-lang#4821 - Areredify:as_conversions, r=flip1995

Add `as_conversions` lint

changelog: closes rust-lang#4771, adding a new pedantic allow-by-default lint that lints against any usage of `as`.

CHANGELOG.md

@@ -929,6 +929,7 @@ Released 2018-09-13
 [`absurd_extreme_comparisons`]: https://rust-lang.github.io/rust-clippy/master/index.html#absurd_extreme_comparisons
 [`almost_swapped`]: https://rust-lang.github.io/rust-clippy/master/index.html#almost_swapped
 [`approx_constant`]: https://rust-lang.github.io/rust-clippy/master/index.html#approx_constant
+[`as_conversions`]: https://rust-lang.github.io/rust-clippy/master/index.html#as_conversions
 [`assertions_on_constants`]: https://rust-lang.github.io/rust-clippy/master/index.html#assertions_on_constants
 [`assign_op_pattern`]: https://rust-lang.github.io/rust-clippy/master/index.html#assign_op_pattern
 [`assign_ops`]: https://rust-lang.github.io/rust-clippy/master/index.html#assign_ops

README.md

@@ -6,7 +6,7 @@
 
 A collection of lints to catch common mistakes and improve your [Rust](https://github.com/rust-lang/rust) code.
 
-[There are 337 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are 338 lints included in this crate!](https://rust-lang.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/as_conversions.rs

@@ -0,0 +1,56 @@
+use rustc::lint::{in_external_macro, EarlyContext, EarlyLintPass, LintArray, LintContext, LintPass};
+use rustc::{declare_lint_pass, declare_tool_lint};
+use syntax::ast::*;
+
+use crate::utils::span_help_and_lint;
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for usage of `as` conversions.
+    ///
+    /// **Why is this bad?** `as` conversions will perform many kinds of
+    /// conversions, including silently lossy conversions and dangerous coercions.
+    /// There are cases when it makes sense to use `as`, so the lint is
+    /// Allow by default.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust,ignore
+    /// let a: u32;
+    /// ...
+    /// f(a as u16);
+    /// ```
+    ///
+    /// Usually better represents the semantics you expect:
+    /// ```rust,ignore
+    /// f(a.try_into()?);
+    /// ```
+    /// or
+    /// ```rust,ignore
+    /// f(a.try_into().expect("Unexpected u16 overflow in f"));
+    /// ```
+    ///
+    pub AS_CONVERSIONS,
+    restriction,
+    "using a potentially dangerous silent `as` conversion"
+}
+
+declare_lint_pass!(AsConversions => [AS_CONVERSIONS]);
+
+impl EarlyLintPass for AsConversions {
+    fn check_expr(&mut self, cx: &EarlyContext<'_>, expr: &Expr) {
+        if in_external_macro(cx.sess(), expr.span) {
+            return;
+        }
+
+        if let ExprKind::Cast(_, _) = expr.kind {
+            span_help_and_lint(
+                cx,
+                AS_CONVERSIONS,
+                expr.span,
+                "using a potentially dangerous silent `as` conversion",
+                "consider using a safe wrapper for this conversion",
+            );
+        }
+    }
+}

clippy_lints/src/lib.rs

@@ -156,6 +156,7 @@ mod utils;
 // begin lints modules, do not remove this comment, it’s used in `update_lints`
 pub mod approx_const;
 pub mod arithmetic;
+pub mod as_conversions;
 pub mod assertions_on_constants;
 pub mod assign_ops;
 pub mod attrs;
@@ -449,6 +450,7 @@ pub fn register_plugins(store: &mut lint::LintStore, sess: &Session, conf: &Conf
         &approx_const::APPROX_CONSTANT,
         &arithmetic::FLOAT_ARITHMETIC,
         &arithmetic::INTEGER_ARITHMETIC,
+        &as_conversions::AS_CONVERSIONS,
         &assertions_on_constants::ASSERTIONS_ON_CONSTANTS,
         &assign_ops::ASSIGN_OP_PATTERN,
         &assign_ops::MISREFACTORED_ASSIGN_OP,
@@ -960,10 +962,12 @@ pub fn register_plugins(store: &mut lint::LintStore, sess: &Session, conf: &Conf
     store.register_late_pass(|| box to_digit_is_some::ToDigitIsSome);
     let array_size_threshold = conf.array_size_threshold;
     store.register_late_pass(move || box large_stack_arrays::LargeStackArrays::new(array_size_threshold));
+    store.register_early_pass(|| box as_conversions::AsConversions);
 
     store.register_group(true, "clippy::restriction", Some("clippy_restriction"), vec![
         LintId::of(&arithmetic::FLOAT_ARITHMETIC),
         LintId::of(&arithmetic::INTEGER_ARITHMETIC),
+        LintId::of(&as_conversions::AS_CONVERSIONS),
         LintId::of(&dbg_macro::DBG_MACRO),
         LintId::of(&else_if_without_else::ELSE_IF_WITHOUT_ELSE),
         LintId::of(&exit::EXIT),

src/lintlist/mod.rs

@@ -6,7 +6,7 @@ pub use lint::Lint;
 pub use lint::LINT_LEVELS;
 
 // begin lint list, do not remove this comment, it’s used in `update_lints`
-pub const ALL_LINTS: [Lint; 337] = [
+pub const ALL_LINTS: [Lint; 338] = [
     Lint {
         name: "absurd_extreme_comparisons",
         group: "correctness",
@@ -28,6 +28,13 @@ pub const ALL_LINTS: [Lint; 337] = [
         deprecation: None,
         module: "approx_const",
     },
+    Lint {
+        name: "as_conversions",
+        group: "restriction",
+        desc: "using a potentially dangerous silent `as` conversion",
+        deprecation: None,
+        module: "as_conversions",
+    },
     Lint {
         name: "assertions_on_constants",
         group: "style",

tests/ui/as_conversions.rs

@@ -0,0 +1,7 @@
+#[warn(clippy::as_conversions)]
+
+fn main() {
+    let i = 0u32 as u64;
+
+    let j = &i as *const u64 as *mut u64;
+}

tests/ui/as_conversions.stderr

@@ -0,0 +1,27 @@
+error: using a potentially dangerous silent `as` conversion
+  --> $DIR/as_conversions.rs:4:13
+   |
+LL |     let i = 0u32 as u64;
+   |             ^^^^^^^^^^^
+   |
+   = note: `-D clippy::as-conversions` implied by `-D warnings`
+   = help: consider using a safe wrapper for this conversion
+
+error: using a potentially dangerous silent `as` conversion
+  --> $DIR/as_conversions.rs:6:13
+   |
+LL |     let j = &i as *const u64 as *mut u64;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: consider using a safe wrapper for this conversion
+
+error: using a potentially dangerous silent `as` conversion
+  --> $DIR/as_conversions.rs:6:13
+   |
+LL |     let j = &i as *const u64 as *mut u64;
+   |             ^^^^^^^^^^^^^^^^
+   |
+   = help: consider using a safe wrapper for this conversion
+
+error: aborting due to 3 previous errors
+

tests/ui/types.fixed

@@ -1,7 +1,7 @@
 // run-rustfix
 
 #![allow(dead_code, unused_variables)]
-#![warn(clippy::all, clippy::pedantic)]
+#![warn(clippy::cast_lossless)]
 
 // should not warn on lossy casting in constant types
 // because not supported yet

tests/ui/types.rs

@@ -1,7 +1,7 @@
 // run-rustfix
 
 #![allow(dead_code, unused_variables)]
-#![warn(clippy::all, clippy::pedantic)]
+#![warn(clippy::cast_lossless)]
 
 // should not warn on lossy casting in constant types
 // because not supported yet