ab91a6b4df
In the comments of (closed, defunct) pull request #54884, Mazdak "Centril" Farrokhzad noted that must-use annotations didn't work on an associated function (what other communities might call a "static method"). Subsequent logging revealed that in this case we have a `Def::Method`, whereas the lint pass was only matching on `Def::Fn`. (One could argue that those def-names are thereby misleading—must-use for self-ful methods have always worked—but documenting or reworking that can be left to another day.)
86 lines
2.2 KiB
Rust
86 lines
2.2 KiB
Rust
// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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// compile-pass
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#![warn(unused_must_use)]
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#[derive(PartialEq, Eq)]
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struct MyStruct {
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n: usize,
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}
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impl MyStruct {
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#[must_use]
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fn need_to_use_this_method_value(&self) -> usize {
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self.n
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}
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#[must_use]
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fn need_to_use_this_associated_function_value() -> isize {
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-1
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}
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}
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trait EvenNature {
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#[must_use = "no side effects"]
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fn is_even(&self) -> bool;
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}
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impl EvenNature for MyStruct {
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fn is_even(&self) -> bool {
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self.n % 2 == 0
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}
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}
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trait Replaceable {
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fn replace(&mut self, substitute: usize) -> usize;
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}
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impl Replaceable for MyStruct {
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// ↓ N.b.: `#[must_use]` attribute on a particular trait implementation
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// method won't work; the attribute should be on the method signature in
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// the trait's definition.
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#[must_use]
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fn replace(&mut self, substitute: usize) -> usize {
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let previously = self.n;
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self.n = substitute;
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previously
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}
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}
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#[must_use = "it's important"]
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fn need_to_use_this_value() -> bool {
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false
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}
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fn main() {
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need_to_use_this_value(); //~ WARN unused return value
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let mut m = MyStruct { n: 2 };
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let n = MyStruct { n: 3 };
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m.need_to_use_this_method_value(); //~ WARN unused return value
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m.is_even(); // trait method!
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//~^ WARN unused return value
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MyStruct::need_to_use_this_associated_function_value();
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//~^ WARN unused return value
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m.replace(3); // won't warn (annotation needs to be in trait definition)
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// comparison methods are `must_use`
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2.eq(&3); //~ WARN unused return value
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m.eq(&n); //~ WARN unused return value
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// lint includes comparison operators
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2 == 3; //~ WARN unused comparison
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m == n; //~ WARN unused comparison
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}
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