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Rollup merge of #44103 - zackmdavis:cmp_op_must_use, r=arielb1

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add comparison operators to must-use lint (under `fn_must_use` feature)

Although RFC 1940 is about annotating functions with `#[must_use]`, a
key part of the motivation was linting unused equality operators.

(See
https://github.com/rust-lang/rfcs/pull/1812#issuecomment-265695898—it
seems to have not been clear to discussants at the time that marking the
comparison methods as `must_use` would not give us the lints on
comparison operators, at least in (what the present author understood
as) the most straightforward implementation, as landed in #43728
(3645b062).)

To rectify the situation, we here lint unused comparison operators as
part of the unused-must-use lint (feature gated by the `fn_must_use`
feature flag, which now arguably becomes a slight (tolerable in the
opinion of the present author) misnomer).

This is in the matter of #43302.

cc @crumblingstatue
This commit is contained in:
Guillaume Gomez 2017-09-24 14:01:49 +02:00 • committed by GitHub
commit 8a25ec019e
4 changed files with 52 additions and 12 deletions
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10
src/test/ui/rfc_1940-must_use_on_functions/fn_must_use.rs
View file

@ -11,6 +11,7 @@
#![feature(fn_must_use)]
#![warn(unused_must_use)]
#[derive(PartialEq, Eq)]
struct MyStruct {
n: usize,
}
@ -58,13 +59,18 @@ fn main() {
need_to_use_this_value();
let mut m = MyStruct { n: 2 };
let n = MyStruct { n: 3 };
m.need_to_use_this_method_value();
m.is_even(); // trait method!
m.replace(3);
m.replace(3); // won't warn (annotation needs to be in trait definition)
// comparison methods are `must_use`
2.eq(&3);
m.eq(&n);
// FIXME: operators should probably be `must_use` if underlying method is
// lint includes comparison operators
2 == 3;
m == n;
}

34
src/test/ui/rfc_1940-must_use_on_functions/fn_must_use.stderr
View file

@ -1,7 +1,7 @@
warning: unused return value of `need_to_use_this_value` which must be used: it's important
--> $DIR/fn_must_use.rs:58:5
--> $DIR/fn_must_use.rs:59:5
|
58 | need_to_use_this_value();
59 | need_to_use_this_value();
| ^^^^^^^^^^^^^^^^^^^^^^^^^
|
note: lint level defined here
@ -11,20 +11,38 @@ note: lint level defined here
| ^^^^^^^^^^^^^^^
warning: unused return value of `MyStruct::need_to_use_this_method_value` which must be used
--> $DIR/fn_must_use.rs:61:5
--> $DIR/fn_must_use.rs:64:5
|
61 | m.need_to_use_this_method_value();
64 | m.need_to_use_this_method_value();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: unused return value of `EvenNature::is_even` which must be used: no side effects
--> $DIR/fn_must_use.rs:62:5
--> $DIR/fn_must_use.rs:65:5
|
62 | m.is_even(); // trait method!
65 | m.is_even(); // trait method!
| ^^^^^^^^^^^^
warning: unused return value of `std::cmp::PartialEq::eq` which must be used
--> $DIR/fn_must_use.rs:66:5
--> $DIR/fn_must_use.rs:70:5
|
66 | 2.eq(&3);
70 | 2.eq(&3);
| ^^^^^^^^^
warning: unused return value of `std::cmp::PartialEq::eq` which must be used
--> $DIR/fn_must_use.rs:71:5
|
71 | m.eq(&n);
| ^^^^^^^^^
warning: unused comparison which must be used
--> $DIR/fn_must_use.rs:74:5
|
74 | 2 == 3;
| ^^^^^^
warning: unused comparison which must be used
--> $DIR/fn_must_use.rs:75:5
|
75 | m == n;
| ^^^^^^