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single life #55432

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Nov 4, 2018
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single life #55432

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fd28753
structured suggestions for single-use lifetime lint on fns and methods
zackmdavis Oct 28, 2018
faea5bb
single-use-lifetime lint: don't false-positive on the anonymous lifetime
zackmdavis Oct 28, 2018
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124 changes: 107 additions & 17 deletions src/librustc/middle/resolve_lifetime.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1443,23 +1443,101 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
/// helper method to determine the span to remove when suggesting the
/// deletion of a lifetime
fn lifetime_deletion_span(&self, name: ast::Ident, generics: &hir::Generics) -> Option<Span> {
if generics.params.len() == 1 {
// if sole lifetime, remove the `<>` brackets
Some(generics.span)
} else {
generics.params.iter().enumerate().find_map(|(i, param)| {
if param.name.ident() == name {
// We also want to delete a leading or trailing comma
// as appropriate
if i >= generics.params.len() - 1 {
Some(generics.params[i - 1].span.shrink_to_hi().to(param.span))
} else {
Some(param.span.to(generics.params[i + 1].span.shrink_to_lo()))
generics.params.iter().enumerate().find_map(|(i, param)| {
if param.name.ident() == name {
let mut in_band = false;
if let hir::GenericParamKind::Lifetime { kind } = param.kind {
if let hir::LifetimeParamKind::InBand = kind {
in_band = true;
}
}
if in_band {
Some(param.span)
} else {
None
if generics.params.len() == 1 {
// if sole lifetime, remove the entire `<>` brackets
Some(generics.span)
} else {
// if removing within `<>` brackets, we also want to
// delete a leading or trailing comma as appropriate
if i >= generics.params.len() - 1 {
Some(generics.params[i - 1].span.shrink_to_hi().to(param.span))
} else {
Some(param.span.to(generics.params[i + 1].span.shrink_to_lo()))
}
}
}
})
} else {
None
}
})
}

// helper method to issue suggestions from `fn rah<'a>(&'a T)` to `fn rah(&T)`
fn suggest_eliding_single_use_lifetime(
&self, err: &mut DiagnosticBuilder<'_>, def_id: DefId, lifetime: &hir::Lifetime
) {
// FIXME: future work: also suggest `impl Foo<'_>` for `impl<'a> Foo<'a>`
let name = lifetime.name.ident();
let mut remove_decl = None;
if let Some(parent_def_id) = self.tcx.parent(def_id) {
if let Some(generics) = self.tcx.hir.get_generics(parent_def_id) {
remove_decl = self.lifetime_deletion_span(name, generics);
}
}

let mut remove_use = None;
let mut find_arg_use_span = |inputs: &hir::HirVec<hir::Ty>| {
for input in inputs {
if let hir::TyKind::Rptr(lt, _) = input.node {
if lt.name.ident() == name {
// include the trailing whitespace between the ampersand and the type name
let lt_through_ty_span = lifetime.span.to(input.span.shrink_to_hi());
remove_use = Some(
self.tcx.sess.source_map()
.span_until_non_whitespace(lt_through_ty_span)
);
break;
}
}
}
};
if let Node::Lifetime(hir_lifetime) = self.tcx.hir.get(lifetime.id) {
if let Some(parent) = self.tcx.hir.find(self.tcx.hir.get_parent(hir_lifetime.id)) {
match parent {
Node::Item(item) => {
if let hir::ItemKind::Fn(decl, _, _, _) = &item.node {
find_arg_use_span(&decl.inputs);
}
},
Node::ImplItem(impl_item) => {
if let hir::ImplItemKind::Method(sig, _) = &impl_item.node {
find_arg_use_span(&sig.decl.inputs);
}
}
_ => {}
}
}
}

if let (Some(decl_span), Some(use_span)) = (remove_decl, remove_use) {
// if both declaration and use deletion spans start at the same
// place ("start at" because the latter includes trailing
// whitespace), then this is an in-band lifetime
if decl_span.shrink_to_lo() == use_span.shrink_to_lo() {
err.span_suggestion_with_applicability(
use_span,
"elide the single-use lifetime",
String::new(),
Applicability::MachineApplicable,
);
} else {
err.multipart_suggestion_with_applicability(
"elide the single-use lifetime",
vec![(decl_span, String::new()), (use_span, String::new())],
Applicability::MachineApplicable,
);
}
}
}

Expand Down Expand Up @@ -1515,14 +1593,26 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
_ => None,
} {
debug!("id = {:?} span = {:?} name = {:?}", node_id, span, name);

if name == keywords::UnderscoreLifetime.ident() {
continue;
}

let mut err = self.tcx.struct_span_lint_node(
lint::builtin::SINGLE_USE_LIFETIMES,
id,
span,
&format!("lifetime parameter `{}` only used once", name),
);
err.span_label(span, "this lifetime...");
err.span_label(lifetime.span, "...is used only here");

if span == lifetime.span {
// spans are the same for in-band lifetime declarations
err.span_label(span, "this lifetime is only used here");
} else {
err.span_label(span, "this lifetime...");
err.span_label(lifetime.span, "...is used only here");
}
self.suggest_eliding_single_use_lifetime(&mut err, def_id, lifetime);
err.emit();
}
}
Expand Down Expand Up @@ -1555,7 +1645,7 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
if let Some(span) = unused_lt_span {
err.span_suggestion_with_applicability(
span,
"remove it",
"elide the unused lifetime",
String::new(),
Applicability::MachineApplicable,
);
Expand Down
28 changes: 28 additions & 0 deletions src/test/ui/single-use-lifetime/one-use-in-fn-argument-in-band.fixed
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@@ -0,0 +1,28 @@
// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

// run-rustfix

#![feature(in_band_lifetimes)]
#![deny(single_use_lifetimes)]
#![allow(dead_code)]
#![allow(unused_variables)]

// Test that we DO warn when lifetime name is used only
// once in a fn argument, even with in band lifetimes.

fn a(x: &u32, y: &u32) {
//~^ ERROR `'a` only used once
//~| ERROR `'b` only used once
//~| HELP elide the single-use lifetime
//~| HELP elide the single-use lifetime
}

fn main() { }
4 changes: 4 additions & 0 deletions src/test/ui/single-use-lifetime/one-use-in-fn-argument-in-band.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,8 @@
// option. This file may not be copied, modified, or distributed
// except according to those terms.

// run-rustfix

#![feature(in_band_lifetimes)]
#![deny(single_use_lifetimes)]
#![allow(dead_code)]
Expand All @@ -19,6 +21,8 @@
fn a(x: &'a u32, y: &'b u32) {
//~^ ERROR `'a` only used once
//~| ERROR `'b` only used once
//~| HELP elide the single-use lifetime
//~| HELP elide the single-use lifetime
}

fn main() { }
18 changes: 9 additions & 9 deletions src/test/ui/single-use-lifetime/one-use-in-fn-argument-in-band.stderr
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@@ -1,26 +1,26 @@
error: lifetime parameter `'a` only used once
--> $DIR/one-use-in-fn-argument-in-band.rs:19:10
--> $DIR/one-use-in-fn-argument-in-band.rs:21:10
|
LL | fn a(x: &'a u32, y: &'b u32) {
| ^^
| ^^-
| |
| this lifetime...
| ...is used only here
| this lifetime is only used here
| help: elide the single-use lifetime
|
note: lint level defined here
--> $DIR/one-use-in-fn-argument-in-band.rs:12:9
--> $DIR/one-use-in-fn-argument-in-band.rs:14:9
|
LL | #![deny(single_use_lifetimes)]
| ^^^^^^^^^^^^^^^^^^^^

error: lifetime parameter `'b` only used once
--> $DIR/one-use-in-fn-argument-in-band.rs:19:22
--> $DIR/one-use-in-fn-argument-in-band.rs:21:22
|
LL | fn a(x: &'a u32, y: &'b u32) {
| ^^
| ^^-
| |
| this lifetime...
| ...is used only here
| this lifetime is only used here
| help: elide the single-use lifetime

error: aborting due to 2 previous errors

1 change: 1 addition & 0 deletions src/test/ui/single-use-lifetime/one-use-in-fn-argument.rs
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Expand Up @@ -16,6 +16,7 @@
// once in a fn argument.

fn a<'a>(x: &'a u32) { //~ ERROR `'a` only used once
//~^ HELP elide the single-use lifetime
}

fn main() { }
4 changes: 4 additions & 0 deletions src/test/ui/single-use-lifetime/one-use-in-fn-argument.stderr
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Expand Up @@ -11,6 +11,10 @@ note: lint level defined here
|
LL | #![deny(single_use_lifetimes)]
| ^^^^^^^^^^^^^^^^^^^^
help: elide the single-use lifetime
|
LL | fn a(x: &u32) { //~ ERROR `'a` only used once
| -- --

error: aborting due to previous error

11 changes: 7 additions & 4 deletions src/test/ui/single-use-lifetime/one-use-in-inherent-impl-header.rs
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Expand Up @@ -12,10 +12,8 @@
#![allow(dead_code)]
#![allow(unused_variables)]

// Test that we DO warn for a lifetime used only once in an impl.
//
// (Actually, until #15872 is fixed, you can't use `'_` here, but
// hopefully that will come soon.)
// Test that we DO warn for a lifetime used only once in an impl, and that we
// don't warn for the anonymous lifetime.

struct Foo<'f> {
data: &'f u32
Expand All @@ -26,4 +24,9 @@ impl<'f> Foo<'f> { //~ ERROR `'f` only used once
}
}

impl Foo<'_> {
fn inherent_b(&self) {}
}


fn main() { }
2 changes: 1 addition & 1 deletion src/test/ui/single-use-lifetime/one-use-in-inherent-impl-header.stderr
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
error: lifetime parameter `'f` only used once
--> $DIR/one-use-in-inherent-impl-header.rs:24:6
--> $DIR/one-use-in-inherent-impl-header.rs:22:6
|
LL | impl<'f> Foo<'f> { //~ ERROR `'f` only used once
| ^^ -- ...is used only here
Expand Down
1 change: 1 addition & 0 deletions src/test/ui/single-use-lifetime/one-use-in-inherent-method-argument.rs
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Expand Up @@ -20,6 +20,7 @@ struct Foo<'f> {

impl<'f> Foo<'f> { //~ ERROR `'f` only used once
fn inherent_a<'a>(&self, data: &'a u32) { //~ ERROR `'a` only used once
//~^ HELP elide the single-use lifetime
}
}

Expand Down
4 changes: 4 additions & 0 deletions src/test/ui/single-use-lifetime/one-use-in-inherent-method-argument.stderr
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Expand Up @@ -11,6 +11,10 @@ note: lint level defined here
|
LL | #![deny(single_use_lifetimes)]
| ^^^^^^^^^^^^^^^^^^^^
help: elide the single-use lifetime
|
LL | fn inherent_a(&self, data: &u32) { //~ ERROR `'a` only used once
| -- --

error: lifetime parameter `'f` only used once
--> $DIR/one-use-in-inherent-method-argument.rs:21:6
Expand Down
1 change: 1 addition & 0 deletions src/test/ui/single-use-lifetime/one-use-in-trait-method-argument.rs
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Expand Up @@ -23,6 +23,7 @@ impl<'f> Iterator for Foo<'f> {
type Item = &'f u32;

fn next<'g>(&'g mut self) -> Option<Self::Item> { //~ ERROR `'g` only used once
//~^ HELP elide the single-use lifetime
None
}
}
Expand Down
4 changes: 4 additions & 0 deletions src/test/ui/single-use-lifetime/one-use-in-trait-method-argument.stderr
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Expand Up @@ -11,6 +11,10 @@ note: lint level defined here
|
LL | #![deny(single_use_lifetimes)]
| ^^^^^^^^^^^^^^^^^^^^
help: elide the single-use lifetime
|
LL | fn next(&mut self) -> Option<Self::Item> { //~ ERROR `'g` only used once
| ----

error: aborting due to previous error

6 changes: 3 additions & 3 deletions src/test/ui/single-use-lifetime/zero-uses-in-fn.fixed
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Expand Up @@ -7,17 +7,17 @@

fn september() {}
//~^ ERROR lifetime parameter `'a` never used
//~| HELP remove it
//~| HELP elide the unused lifetime

fn october<'b, T>(s: &'b T) -> &'b T {
//~^ ERROR lifetime parameter `'a` never used
//~| HELP remove it
//~| HELP elide the unused lifetime
s
}

fn november<'a>(s: &'a str) -> (&'a str) {
//~^ ERROR lifetime parameter `'b` never used
//~| HELP remove it
//~| HELP elide the unused lifetime
s
}

Expand Down
6 changes: 3 additions & 3 deletions src/test/ui/single-use-lifetime/zero-uses-in-fn.rs
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Expand Up @@ -7,17 +7,17 @@

fn september<'a>() {}
//~^ ERROR lifetime parameter `'a` never used
//~| HELP remove it
//~| HELP elide the unused lifetime

fn october<'a, 'b, T>(s: &'b T) -> &'b T {
//~^ ERROR lifetime parameter `'a` never used
//~| HELP remove it
//~| HELP elide the unused lifetime
s
}

fn november<'a, 'b>(s: &'a str) -> (&'a str) {
//~^ ERROR lifetime parameter `'b` never used
//~| HELP remove it
//~| HELP elide the unused lifetime
s
}

Expand Down
6 changes: 3 additions & 3 deletions src/test/ui/single-use-lifetime/zero-uses-in-fn.stderr
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Expand Up @@ -2,7 +2,7 @@ error: lifetime parameter `'a` never used
--> $DIR/zero-uses-in-fn.rs:8:14
|
LL | fn september<'a>() {}
| -^^- help: remove it
| -^^- help: elide the unused lifetime
|
note: lint level defined here
--> $DIR/zero-uses-in-fn.rs:5:9
Expand All @@ -16,15 +16,15 @@ error: lifetime parameter `'a` never used
LL | fn october<'a, 'b, T>(s: &'b T) -> &'b T {
| ^^--
| |
| help: remove it
| help: elide the unused lifetime

error: lifetime parameter `'b` never used
--> $DIR/zero-uses-in-fn.rs:18:17
|
LL | fn november<'a, 'b>(s: &'a str) -> (&'a str) {
| --^^
| |
| help: remove it
| help: elide the unused lifetime
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This...looks funny, no? It seems like we'd be better off making this suggestion "out of line"?


error: aborting due to 3 previous errors

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