From fff9a8ea9c56d5075c23d9d5b2d8b0838eee0da3 Mon Sep 17 00:00:00 2001
From: Tim Bodeit <tim@bodeit.com>
Date: Sat, 23 Nov 2019 22:35:21 +0100
Subject: [PATCH] [comparison_chain] #4827 Check `core::cmp::Ord` is
 implemented

Only emit lint, if `cmp` is actually available on the type being
compared. Don't emit lint in cases where only `PartialOrd` is
implemented.
---
 clippy_lints/src/comparison_chain.rs | 12 +++++-
 tests/ui/comparison_chain.rs         | 61 ++++++++++++++++++++++++++++
 tests/ui/comparison_chain.stderr     | 42 ++++++++++++++++++-
 3 files changed, 113 insertions(+), 2 deletions(-)

diff --git a/clippy_lints/src/comparison_chain.rs b/clippy_lints/src/comparison_chain.rs
index 6968d8f6559..087bceaffd9 100644
--- a/clippy_lints/src/comparison_chain.rs
+++ b/clippy_lints/src/comparison_chain.rs
@@ -1,4 +1,6 @@
-use crate::utils::{if_sequence, parent_node_is_if_expr, span_help_and_lint, SpanlessEq};
+use crate::utils::{
+    get_trait_def_id, if_sequence, implements_trait, parent_node_is_if_expr, paths, span_help_and_lint, SpanlessEq,
+};
 use rustc::hir::*;
 use rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
 use rustc::{declare_lint_pass, declare_tool_lint};
@@ -84,6 +86,14 @@ fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
                 {
                     return;
                 }
+
+                // Check that the type being compared implements `core::cmp::Ord`
+                let ty = cx.tables.expr_ty(lhs1);
+                let is_ord = get_trait_def_id(cx, &paths::ORD).map_or(false, |id| implements_trait(cx, ty, id, &[]));
+
+                if !is_ord {
+                    return;
+                }
             } else {
                 // We only care about comparison chains
                 return;
diff --git a/tests/ui/comparison_chain.rs b/tests/ui/comparison_chain.rs
index b697413b6e0..9c2128469de 100644
--- a/tests/ui/comparison_chain.rs
+++ b/tests/ui/comparison_chain.rs
@@ -76,4 +76,65 @@ fn f(x: u8, y: u8, z: u8) {
     }
 }
 
+#[allow(clippy::float_cmp)]
+fn g(x: f64, y: f64, z: f64) {
+    // Ignored: f64 doesn't implement Ord
+    if x > y {
+        a()
+    } else if x < y {
+        b()
+    }
+
+    // Ignored: f64 doesn't implement Ord
+    if x > y {
+        a()
+    } else if x < y {
+        b()
+    } else {
+        c()
+    }
+
+    // Ignored: f64 doesn't implement Ord
+    if x > y {
+        a()
+    } else if y > x {
+        b()
+    } else {
+        c()
+    }
+
+    // Ignored: f64 doesn't implement Ord
+    if x > 1.0 {
+        a()
+    } else if x < 1.0 {
+        b()
+    } else if x == 1.0 {
+        c()
+    }
+}
+
+fn h<T: Ord>(x: T, y: T, z: T) {
+    if x > y {
+        a()
+    } else if x < y {
+        b()
+    }
+
+    if x > y {
+        a()
+    } else if x < y {
+        b()
+    } else {
+        c()
+    }
+
+    if x > y {
+        a()
+    } else if y > x {
+        b()
+    } else {
+        c()
+    }
+}
+
 fn main() {}
diff --git a/tests/ui/comparison_chain.stderr b/tests/ui/comparison_chain.stderr
index 575181dd719..69db88b03b5 100644
--- a/tests/ui/comparison_chain.stderr
+++ b/tests/ui/comparison_chain.stderr
@@ -53,5 +53,45 @@ LL | |     }
    |
    = help: Consider rewriting the `if` chain to use `cmp` and `match`.
 
-error: aborting due to 4 previous errors
+error: `if` chain can be rewritten with `match`
+  --> $DIR/comparison_chain.rs:117:5
+   |
+LL | /     if x > y {
+LL | |         a()
+LL | |     } else if x < y {
+LL | |         b()
+LL | |     }
+   | |_____^
+   |
+   = help: Consider rewriting the `if` chain to use `cmp` and `match`.
+
+error: `if` chain can be rewritten with `match`
+  --> $DIR/comparison_chain.rs:123:5
+   |
+LL | /     if x > y {
+LL | |         a()
+LL | |     } else if x < y {
+LL | |         b()
+LL | |     } else {
+LL | |         c()
+LL | |     }
+   | |_____^
+   |
+   = help: Consider rewriting the `if` chain to use `cmp` and `match`.
+
+error: `if` chain can be rewritten with `match`
+  --> $DIR/comparison_chain.rs:131:5
+   |
+LL | /     if x > y {
+LL | |         a()
+LL | |     } else if y > x {
+LL | |         b()
+LL | |     } else {
+LL | |         c()
+LL | |     }
+   | |_____^
+   |
+   = help: Consider rewriting the `if` chain to use `cmp` and `match`.
+
+error: aborting due to 7 previous errors
 
-- 
2.44.0