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Allow #[must_use] on traits #55663

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Nov 20, 2018
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59 changes: 43 additions & 16 deletions src/librustc_lint/unused.rs
Original file line number Diff line number Diff line change
Expand Up @@ -60,18 +60,39 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedResults {
}

let t = cx.tables.expr_ty(&expr);
// FIXME(varkor): replace with `t.is_unit() || t.conservative_is_uninhabited()`.
let type_permits_no_use = match t.sty {
ty::Tuple(ref tys) if tys.is_empty() => true,
ty::Never => true,
ty::Adt(def, _) => {
if def.variants.is_empty() {
true
} else {
check_must_use(cx, def.did, s.span, "")
let type_permits_lack_of_use = if t.is_unit()
|| cx.tcx.is_ty_uninhabited_from(cx.tcx.hir.get_module_parent(expr.id), t) {
true
} else {
match t.sty {
ty::Adt(def, _) => check_must_use(cx, def.did, s.span, "", ""),
ty::Opaque(def, _) => {
let mut must_use = false;
for (predicate, _) in &cx.tcx.predicates_of(def).predicates {
if let ty::Predicate::Trait(ref poly_trait_predicate) = predicate {
let trait_ref = poly_trait_predicate.skip_binder().trait_ref;
if check_must_use(cx, trait_ref.def_id, s.span, "implementer of ", "") {
must_use = true;
break;
}
}
}
must_use
}
ty::Dynamic(binder, _) => {
let mut must_use = false;
for predicate in binder.skip_binder().iter() {
if let ty::ExistentialPredicate::Trait(ref trait_ref) = predicate {
if check_must_use(cx, trait_ref.def_id, s.span, "", " trait object") {
must_use = true;
break;
}
}
}
must_use
}
_ => false,
}
_ => false,
};

let mut fn_warned = false;
Expand All @@ -98,8 +119,8 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedResults {
};
if let Some(def) = maybe_def {
let def_id = def.def_id();
fn_warned = check_must_use(cx, def_id, s.span, "return value of ");
} else if type_permits_no_use {
fn_warned = check_must_use(cx, def_id, s.span, "return value of ", "");
} else if type_permits_lack_of_use {
// We don't warn about unused unit or uninhabited types.
// (See https://github.com/rust-lang/rust/issues/43806 for details.)
return;
Expand Down Expand Up @@ -148,15 +169,21 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedResults {
op_warned = true;
}

if !(type_permits_no_use || fn_warned || op_warned) {
if !(type_permits_lack_of_use || fn_warned || op_warned) {
cx.span_lint(UNUSED_RESULTS, s.span, "unused result");
}

fn check_must_use(cx: &LateContext, def_id: DefId, sp: Span, describe_path: &str) -> bool {
fn check_must_use(
cx: &LateContext,
def_id: DefId,
sp: Span,
descr_pre_path: &str,
descr_post_path: &str,
) -> bool {
for attr in cx.tcx.get_attrs(def_id).iter() {
if attr.check_name("must_use") {
let msg = format!("unused {}`{}` that must be used",
describe_path, cx.tcx.item_path_str(def_id));
let msg = format!("unused {}`{}`{} that must be used",
descr_pre_path, cx.tcx.item_path_str(def_id), descr_post_path);
let mut err = cx.struct_span_lint(UNUSED_MUST_USE, sp, &msg);
// check for #[must_use = "..."]
if let Some(note) = attr.value_str() {
Expand Down
22 changes: 22 additions & 0 deletions src/test/ui/lint/must_use-trait.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,22 @@
#![deny(unused_must_use)]

#[must_use]
trait Critical {}

trait NotSoCritical {}

trait DecidedlyUnimportant {}

struct Anon;

impl Critical for Anon {}
impl NotSoCritical for Anon {}
impl DecidedlyUnimportant for Anon {}

fn get_critical() -> impl NotSoCritical + Critical + DecidedlyUnimportant {
Anon {}
}

fn main() {
get_critical(); //~ ERROR unused implementer of `Critical` that must be used
}
14 changes: 14 additions & 0 deletions src/test/ui/lint/must_use-trait.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,14 @@
error: unused implementer of `Critical` that must be used
--> $DIR/must_use-trait.rs:21:5
|
LL | get_critical(); //~ ERROR unused implementer of `Critical` that must be used
| ^^^^^^^^^^^^^^^
|
note: lint level defined here
--> $DIR/must_use-trait.rs:1:9
|
LL | #![deny(unused_must_use)]
| ^^^^^^^^^^^^^^^

error: aborting due to previous error