Auto merge of #43728 - zackmdavis:fnused, r=eddyb
#[must_use] for functions This implements [RFC 1940](https://github.com/rust-lang/rfcs/pull/1940). The RFC and discussion thereof seem to suggest that tagging `PartialEq::eq` and friends as `#[must_use]` would automatically lint for unused comparisons, but it doesn't work out that way (at least the way I've implemented it): unused `.eq` method calls get linted, but not `==` expressions. (The lint operates on the HIR, which sees binary operations as their own thing, even if they ultimately just call `.eq` _&c._.) What do _you_ think?? Resolves #43302.
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3f977baf34
@ -110,11 +110,13 @@ use self::Ordering::*;
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pub trait PartialEq<Rhs: ?Sized = Self> {
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/// This method tests for `self` and `other` values to be equal, and is used
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/// by `==`.
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#[must_use]
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#[stable(feature = "rust1", since = "1.0.0")]
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fn eq(&self, other: &Rhs) -> bool;
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/// This method tests for `!=`.
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#[inline]
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#[must_use]
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#[stable(feature = "rust1", since = "1.0.0")]
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fn ne(&self, other: &Rhs) -> bool { !self.eq(other) }
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}
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@ -625,6 +627,7 @@ pub trait PartialOrd<Rhs: ?Sized = Self>: PartialEq<Rhs> {
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/// let result = std::f64::NAN.partial_cmp(&1.0);
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/// assert_eq!(result, None);
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/// ```
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#[must_use]
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#[stable(feature = "rust1", since = "1.0.0")]
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fn partial_cmp(&self, other: &Rhs) -> Option<Ordering>;
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@ -640,6 +643,7 @@ pub trait PartialOrd<Rhs: ?Sized = Self>: PartialEq<Rhs> {
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/// assert_eq!(result, false);
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/// ```
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#[inline]
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#[must_use]
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#[stable(feature = "rust1", since = "1.0.0")]
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fn lt(&self, other: &Rhs) -> bool {
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match self.partial_cmp(other) {
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@ -661,6 +665,7 @@ pub trait PartialOrd<Rhs: ?Sized = Self>: PartialEq<Rhs> {
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/// assert_eq!(result, true);
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/// ```
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#[inline]
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#[must_use]
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#[stable(feature = "rust1", since = "1.0.0")]
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fn le(&self, other: &Rhs) -> bool {
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match self.partial_cmp(other) {
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@ -681,6 +686,7 @@ pub trait PartialOrd<Rhs: ?Sized = Self>: PartialEq<Rhs> {
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/// assert_eq!(result, false);
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/// ```
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#[inline]
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#[must_use]
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#[stable(feature = "rust1", since = "1.0.0")]
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fn gt(&self, other: &Rhs) -> bool {
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match self.partial_cmp(other) {
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@ -702,6 +708,7 @@ pub trait PartialOrd<Rhs: ?Sized = Self>: PartialEq<Rhs> {
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/// assert_eq!(result, true);
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/// ```
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#[inline]
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#[must_use]
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#[stable(feature = "rust1", since = "1.0.0")]
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fn ge(&self, other: &Rhs) -> bool {
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match self.partial_cmp(other) {
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@ -145,22 +145,38 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedResults {
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}
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let t = cx.tables.expr_ty(&expr);
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let warned = match t.sty {
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ty::TyTuple(ref tys, _) if tys.is_empty() => return,
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ty::TyNever => return,
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ty::TyBool => return,
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ty::TyAdt(def, _) => check_must_use(cx, def.did, s.span),
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let ty_warned = match t.sty {
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ty::TyAdt(def, _) => check_must_use(cx, def.did, s.span, ""),
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_ => false,
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};
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if !warned {
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let mut fn_warned = false;
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let maybe_def = match expr.node {
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hir::ExprCall(ref callee, _) => {
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match callee.node {
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hir::ExprPath(ref qpath) => Some(cx.tables.qpath_def(qpath, callee.id)),
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_ => None
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}
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},
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hir::ExprMethodCall(..) => {
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cx.tables.type_dependent_defs.get(&expr.id).cloned()
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},
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_ => { None }
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};
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if let Some(def) = maybe_def {
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let def_id = def.def_id();
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fn_warned = check_must_use(cx, def_id, s.span, "return value of ");
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}
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if !(ty_warned || fn_warned) {
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cx.span_lint(UNUSED_RESULTS, s.span, "unused result");
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}
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fn check_must_use(cx: &LateContext, def_id: DefId, sp: Span) -> bool {
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fn check_must_use(cx: &LateContext, def_id: DefId, sp: Span, describe_path: &str) -> bool {
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for attr in cx.tcx.get_attrs(def_id).iter() {
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if attr.check_name("must_use") {
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let mut msg = format!("unused `{}` which must be used",
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cx.tcx.item_path_str(def_id));
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let mut msg = format!("unused {}`{}` which must be used",
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describe_path, cx.tcx.item_path_str(def_id));
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// check for #[must_use="..."]
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if let Some(s) = attr.value_str() {
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msg.push_str(": ");
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33
src/test/ui/lint/fn_must_use.rs
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33
src/test/ui/lint/fn_must_use.rs
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@ -0,0 +1,33 @@
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// 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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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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}
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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();
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let m = MyStruct { n: 2 };
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m.need_to_use_this_method_value();
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}
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14
src/test/ui/lint/fn_must_use.stderr
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14
src/test/ui/lint/fn_must_use.stderr
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@ -0,0 +1,14 @@
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warning: unused return value of `need_to_use_this_value` which must be used: it's important
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--> $DIR/fn_must_use.rs:29:5
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29 | need_to_use_this_value();
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| ^^^^^^^^^^^^^^^^^^^^^^^^^
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= note: #[warn(unused_must_use)] on by default
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warning: unused return value of `MyStruct::need_to_use_this_method_value` which must be used
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--> $DIR/fn_must_use.rs:32:5
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32 | m.need_to_use_this_method_value();
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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