Auto merge of #9617 - llogiq:cast-nan-to-int, r=Alexendoo

add `cast-nan-to-int` lint

This fixes #371.

r? `@Alexendoo`

---

changelog: add [`cast-nan-to-int`] lint

CHANGELOG.md

@@ -3773,6 +3773,7 @@ Released 2018-09-13
 [`cast_enum_constructor`]: https://rust-lang.github.io/rust-clippy/master/index.html#cast_enum_constructor
 [`cast_enum_truncation`]: https://rust-lang.github.io/rust-clippy/master/index.html#cast_enum_truncation
 [`cast_lossless`]: https://rust-lang.github.io/rust-clippy/master/index.html#cast_lossless
+[`cast_nan_to_int`]: https://rust-lang.github.io/rust-clippy/master/index.html#cast_nan_to_int
 [`cast_possible_truncation`]: https://rust-lang.github.io/rust-clippy/master/index.html#cast_possible_truncation
 [`cast_possible_wrap`]: https://rust-lang.github.io/rust-clippy/master/index.html#cast_possible_wrap
 [`cast_precision_loss`]: https://rust-lang.github.io/rust-clippy/master/index.html#cast_precision_loss

clippy_lints/src/casts/cast_nan_to_int.rs

@@ -0,0 +1,28 @@
+use super::CAST_NAN_TO_INT;
+
+use clippy_utils::consts::{constant, Constant};
+use clippy_utils::diagnostics::span_lint_and_note;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::Ty;
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, cast_expr: &Expr<'_>, from_ty: Ty<'_>, to_ty: Ty<'_>) {
+    if from_ty.is_floating_point() && to_ty.is_integral() && is_known_nan(cx, cast_expr) {
+        span_lint_and_note(
+            cx,
+            CAST_NAN_TO_INT,
+            expr.span,
+            &format!("casting a known NaN to {to_ty}"),
+            None,
+            "this always evaluates to 0",
+        );
+    }
+}
+
+fn is_known_nan(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
+    match constant(cx, cx.typeck_results(), e) {
+        Some((Constant::F64(n), _)) => n.is_nan(),
+        Some((Constant::F32(n), _)) => n.is_nan(),
+        _ => false,
+    }
+}

clippy_lints/src/casts/mod.rs

@@ -4,6 +4,7 @@ mod borrow_as_ptr;
 mod cast_abs_to_unsigned;
 mod cast_enum_constructor;
 mod cast_lossless;
+mod cast_nan_to_int;
 mod cast_possible_truncation;
 mod cast_possible_wrap;
 mod cast_precision_loss;
@@ -570,6 +571,7 @@ declare_clippy_lint! {
     pedantic,
     "borrowing just to cast to a raw pointer"
 }
+
 declare_clippy_lint! {
     /// ### What it does
     /// Checks for a raw slice being cast to a slice pointer
@@ -623,6 +625,28 @@ declare_clippy_lint! {
     "casting the result of the `&self`-taking `as_ptr` to a mutabe pointer"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for a known NaN float being cast to an integer
+    ///
+    /// ### Why is this bad?
+    /// NaNs are cast into zero, so one could simply use this and make the
+    /// code more readable. The lint could also hint at a programmer error.
+    ///
+    /// ### Example
+    /// ```rust,ignore
+    /// let _: (0.0_f32 / 0.0) as u64;
+    /// ```
+    /// Use instead:
+    /// ```rust,ignore
+    /// let _: = 0_u64;
+    /// ```
+    #[clippy::version = "1.64.0"]
+    pub CAST_NAN_TO_INT,
+    suspicious,
+    "casting a known floating-point NaN into an integer"
+}
+
 pub struct Casts {
     msrv: Option<RustcVersion>,
 }
@@ -656,6 +680,7 @@ impl_lint_pass!(Casts => [
     BORROW_AS_PTR,
     CAST_SLICE_FROM_RAW_PARTS,
     AS_PTR_CAST_MUT,
+    CAST_NAN_TO_INT,
 ]);
 
 impl<'tcx> LateLintPass<'tcx> for Casts {
@@ -693,6 +718,7 @@ impl<'tcx> LateLintPass<'tcx> for Casts {
                     cast_precision_loss::check(cx, expr, cast_from, cast_to);
                     cast_sign_loss::check(cx, expr, cast_expr, cast_from, cast_to);
                     cast_abs_to_unsigned::check(cx, expr, cast_expr, cast_from, cast_to, self.msrv);
+                    cast_nan_to_int::check(cx, expr, cast_expr, cast_from, cast_to);
                 }
                 cast_lossless::check(cx, expr, cast_expr, cast_from, cast_to, self.msrv);
                 cast_enum_constructor::check(cx, expr, cast_expr, cast_from);

clippy_lints/src/lib.register_all.rs

@@ -25,6 +25,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(casts::CAST_ABS_TO_UNSIGNED),
     LintId::of(casts::CAST_ENUM_CONSTRUCTOR),
     LintId::of(casts::CAST_ENUM_TRUNCATION),
+    LintId::of(casts::CAST_NAN_TO_INT),
     LintId::of(casts::CAST_REF_TO_MUT),
     LintId::of(casts::CAST_SLICE_DIFFERENT_SIZES),
     LintId::of(casts::CAST_SLICE_FROM_RAW_PARTS),

clippy_lints/src/lib.register_lints.rs

@@ -73,6 +73,7 @@ store.register_lints(&[
     casts::CAST_ENUM_CONSTRUCTOR,
     casts::CAST_ENUM_TRUNCATION,
     casts::CAST_LOSSLESS,
+    casts::CAST_NAN_TO_INT,
     casts::CAST_POSSIBLE_TRUNCATION,
     casts::CAST_POSSIBLE_WRAP,
     casts::CAST_PRECISION_LOSS,

clippy_lints/src/lib.register_suspicious.rs

@@ -11,6 +11,7 @@ store.register_group(true, "clippy::suspicious", Some("clippy_suspicious"), vec!
     LintId::of(casts::CAST_ABS_TO_UNSIGNED),
     LintId::of(casts::CAST_ENUM_CONSTRUCTOR),
     LintId::of(casts::CAST_ENUM_TRUNCATION),
+    LintId::of(casts::CAST_NAN_TO_INT),
     LintId::of(casts::CAST_SLICE_FROM_RAW_PARTS),
     LintId::of(crate_in_macro_def::CRATE_IN_MACRO_DEF),
     LintId::of(drop_forget_ref::DROP_NON_DROP),

src/docs.rs

@@ -61,6 +61,7 @@ docs! {
     "cast_enum_constructor",
     "cast_enum_truncation",
     "cast_lossless",
+    "cast_nan_to_int",
     "cast_possible_truncation",
     "cast_possible_wrap",
     "cast_precision_loss",

src/docs/cast_nan_to_int.txt

@@ -0,0 +1,15 @@
+### What it does
+Checks for a known NaN float being cast to an integer
+
+### Why is this bad?
+NaNs are cast into zero, so one could simply use this and make the
+code more readable. The lint could also hint at a programmer error.
+
+### Example
+```
+let _: (0.0_f32 / 0.0) as u64;
+```
+Use instead:
+```
+let _: = 0_u64;
+```

tests/ui/cast_nan_to_int.rs

@@ -0,0 +1,18 @@
+#![warn(clippy::cast_nan_to_int)]
+#![allow(clippy::eq_op)]
+
+fn main() {
+    let _ = (0.0_f32 / -0.0) as usize;
+    let _ = (f64::INFINITY * -0.0) as usize;
+    let _ = (0.0 * f32::INFINITY) as usize;
+
+    let _ = (f64::INFINITY + f64::NEG_INFINITY) as usize;
+    let _ = (f32::INFINITY - f32::INFINITY) as usize;
+    let _ = (f32::INFINITY / f32::NEG_INFINITY) as usize;
+
+    // those won't be linted:
+    let _ = (1.0_f32 / 0.0) as usize;
+    let _ = (f32::INFINITY * f32::NEG_INFINITY) as usize;
+    let _ = (f32::INFINITY - f32::NEG_INFINITY) as usize;
+    let _ = (f64::INFINITY - 0.0) as usize;
+}

tests/ui/cast_nan_to_int.stderr

@@ -0,0 +1,51 @@
+error: casting a known NaN to usize
+  --> $DIR/cast_nan_to_int.rs:5:13
+   |
+LL |     let _ = (0.0_f32 / -0.0) as usize;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this always evaluates to 0
+   = note: `-D clippy::cast-nan-to-int` implied by `-D warnings`
+
+error: casting a known NaN to usize
+  --> $DIR/cast_nan_to_int.rs:6:13
+   |
+LL |     let _ = (f64::INFINITY * -0.0) as usize;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this always evaluates to 0
+
+error: casting a known NaN to usize
+  --> $DIR/cast_nan_to_int.rs:7:13
+   |
+LL |     let _ = (0.0 * f32::INFINITY) as usize;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this always evaluates to 0
+
+error: casting a known NaN to usize
+  --> $DIR/cast_nan_to_int.rs:9:13
+   |
+LL |     let _ = (f64::INFINITY + f64::NEG_INFINITY) as usize;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this always evaluates to 0
+
+error: casting a known NaN to usize
+  --> $DIR/cast_nan_to_int.rs:10:13
+   |
+LL |     let _ = (f32::INFINITY - f32::INFINITY) as usize;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this always evaluates to 0
+
+error: casting a known NaN to usize
+  --> $DIR/cast_nan_to_int.rs:11:13
+   |
+LL |     let _ = (f32::INFINITY / f32::NEG_INFINITY) as usize;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this always evaluates to 0
+
+error: aborting due to 6 previous errors
+