new lint to check for doomed comparisons to NAN

src/lib.rs

@@ -28,6 +28,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_lint_pass(box misc::MiscPass as LintPassObject);
     reg.register_lint_pass(box misc::StrToStringPass as LintPassObject);
     reg.register_lint_pass(box misc::TopLevelRefPass as LintPassObject);
+    reg.register_lint_pass(box misc::CmpNan as LintPassObject);
     reg.register_lint_pass(box eq_op::EqOp as LintPassObject);
     reg.register_lint_pass(box bit_mask::BitMask as LintPassObject);
     reg.register_lint_pass(box ptr_arg::PtrArg as LintPassObject);
@@ -39,6 +40,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
                                            misc::TOPLEVEL_REF_ARG, eq_op::EQ_OP,
                                            bit_mask::BAD_BIT_MASK, ptr_arg::PTR_ARG,
                                            needless_bool::NEEDLESS_BOOL,
-                                           approx_const::APPROX_CONSTANT
+                                           approx_const::APPROX_CONSTANT,
+                                           misc::CMP_NAN
                                            ]);
 }

src/misc.rs

@@ -1,6 +1,7 @@
 use syntax::ptr::P;
 use syntax::ast;
 use syntax::ast::*;
+use syntax::ast_util::is_comparison_binop;
 use syntax::visit::{FnKind};
 use rustc::lint::{Context, LintPass, LintArray, Lint, Level};
 use rustc::middle::ty::{self, expr_ty, ty_str, ty_ptr, ty_rptr};
@@ -108,3 +109,33 @@ impl LintPass for TopLevelRefPass {
         }
     }
 }
+
+declare_lint!(pub CMP_NAN, Allow, "Deny comparisons to std::f32::NAN or std::f64::NAN");
+
+#[derive(Copy,Clone)]
+pub struct CmpNan;
+
+impl LintPass for CmpNan {
+	fn get_lints(&self) -> LintArray {
+        lint_array!(CMP_NAN)
+	}
+	
+	fn check_expr(&mut self, cx: &Context, expr: &Expr) {
+		if let ExprBinary(ref cmp, ref left, ref right) = expr.node {
+			if is_comparison_binop(cmp.node) {
+				if let &ExprPath(_, ref path) = &left.node {
+					check_nan(cx, path, expr.span);
+				}
+				if let &ExprPath(_, ref path) = &right.node {
+					check_nan(cx, path, expr.span);
+				}
+			}
+		}
+	}
+}
+
+fn check_nan(cx: &Context, path: &Path, span: Span) {
+	path.segments.last().map(|seg| if seg.identifier.as_str() == "NAN" {
+		cx.span_lint(CMP_NAN, span, "Doomed comparison with NAN, use std::{f32,f64}::is_nan instead");
+	});
+}

tests/compile-fail/cmp_nan.rs

@@ -0,0 +1,21 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+#[deny(cmp_nan)]
+fn main() {
+	let x = 5f32;
+	x == std::f32::NAN; //~ERROR
+	x != std::f32::NAN; //~ERROR
+	x < std::f32::NAN; //~ERROR	
+	x > std::f32::NAN; //~ERROR
+	x <= std::f32::NAN; //~ERROR
+	x >= std::f32::NAN; //~ERROR
+
+	let y = 0f64;
+	y == std::f64::NAN; //~ERROR
+	y != std::f64::NAN; //~ERROR
+	y < std::f64::NAN; //~ERROR
+	y > std::f64::NAN; //~ERROR
+	y <= std::f64::NAN; //~ERROR
+	y >= std::f64::NAN; //~ERROR
+}