Add a help to use in_band_lifetimes in nightly

src/librustc_resolve/late/diagnostics.rs

@@ -1044,6 +1044,7 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
             lifetime_ref
         );
         err.span_label(lifetime_ref.span, "undeclared lifetime");
+        let mut suggests_in_band = false;
         for missing in &self.missing_named_lifetime_spots {
             match missing {
                 MissingLifetimeSpot::Generics(generics) => {
@@ -1057,6 +1058,7 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
                         }) {
                         (param.span.shrink_to_lo(), format!("{}, ", lifetime_ref))
                     } else {
+                        suggests_in_band = true;
                         (generics.span, format!("<{}>", lifetime_ref))
                     };
                     err.span_suggestion(
@@ -1084,6 +1086,15 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
                 }
             }
         }
+        if nightly_options::is_nightly_build()
+            && !self.tcx.features().in_band_lifetimes
+            && suggests_in_band
+        {
+            err.help(
+                "if you want to experiment with in-band lifetime bindings, \
+                    add `#![feature(in_band_lifetimes)]` to the crate attributes",
+            );
+        }
         err.emit();
     }
 

src/test/ui/error-codes/E0261.stderr

@@ -5,6 +5,8 @@ LL | fn foo(x: &'a str) { }
    |       -    ^^ undeclared lifetime
    |       |
    |       help: consider introducing lifetime `'a` here: `<'a>`
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'a`
   --> $DIR/E0261.rs:5:9
@@ -13,6 +15,8 @@ LL | struct Foo {
    |           - help: consider introducing lifetime `'a` here: `<'a>`
 LL |     x: &'a str,
    |         ^^ undeclared lifetime
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error: aborting due to 2 previous errors
 

src/test/ui/feature-gates/feature-gate-in_band_lifetimes.stderr

@@ -5,6 +5,8 @@ LL | fn foo(x: &'x u8) -> &'x u8 { x }
    |       -    ^^ undeclared lifetime
    |       |
    |       help: consider introducing lifetime `'x` here: `<'x>`
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'x`
   --> $DIR/feature-gate-in_band_lifetimes.rs:3:23
@@ -13,6 +15,8 @@ LL | fn foo(x: &'x u8) -> &'x u8 { x }
    |       -               ^^ undeclared lifetime
    |       |
    |       help: consider introducing lifetime `'x` here: `<'x>`
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'b`
   --> $DIR/feature-gate-in_band_lifetimes.rs:15:12
@@ -28,6 +32,7 @@ error[E0261]: use of undeclared lifetime name `'b`
 LL |     fn inner_2(&self) -> &'b u8 {
    |                           ^^ undeclared lifetime
    |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 help: consider introducing lifetime `'b` here
    |
 LL | impl<'b, 'a> X<'b> {
@@ -44,13 +49,16 @@ LL | impl X<'b> {
    |     -  ^^ undeclared lifetime
    |     |
    |     help: consider introducing lifetime `'b` here: `<'b>`
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'b`
   --> $DIR/feature-gate-in_band_lifetimes.rs:25:27
    |
 LL |     fn inner_3(&self) -> &'b u8 {
    |                           ^^ undeclared lifetime
    |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 help: consider introducing lifetime `'b` here
    |
 LL | impl<'b> X<'b> {
@@ -67,13 +75,16 @@ LL | impl Y<&'a u8> {
    |     -   ^^ undeclared lifetime
    |     |
    |     help: consider introducing lifetime `'a` here: `<'a>`
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'a`
   --> $DIR/feature-gate-in_band_lifetimes.rs:35:25
    |
 LL |     fn inner(&self) -> &'a u8 {
    |                         ^^ undeclared lifetime
    |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 help: consider introducing lifetime `'a` here
    |
 LL | impl<'a> Y<&'a u8> {
@@ -89,6 +100,7 @@ error[E0261]: use of undeclared lifetime name `'b`
 LL |     fn any_lifetime() -> &'b u8;
    |                           ^^ undeclared lifetime
    |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 help: consider introducing lifetime `'b` here
    |
 LL | trait MyTrait<'b, 'a> {
@@ -104,6 +116,7 @@ error[E0261]: use of undeclared lifetime name `'b`
 LL |     fn borrowed_lifetime(&'b self) -> &'b u8;
    |                           ^^ undeclared lifetime
    |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 help: consider introducing lifetime `'b` here
    |
 LL | trait MyTrait<'b, 'a> {
@@ -119,6 +132,7 @@ error[E0261]: use of undeclared lifetime name `'b`
 LL |     fn borrowed_lifetime(&'b self) -> &'b u8;
    |                                        ^^ undeclared lifetime
    |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 help: consider introducing lifetime `'b` here
    |
 LL | trait MyTrait<'b, 'a> {
@@ -135,6 +149,8 @@ LL | impl MyTrait<'a> for Y<&'a u8> {
    |     -        ^^ undeclared lifetime
    |     |
    |     help: consider introducing lifetime `'a` here: `<'a>`
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'a`
   --> $DIR/feature-gate-in_band_lifetimes.rs:50:25
@@ -143,13 +159,16 @@ LL | impl MyTrait<'a> for Y<&'a u8> {
    |     -                   ^^ undeclared lifetime
    |     |
    |     help: consider introducing lifetime `'a` here: `<'a>`
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'a`
   --> $DIR/feature-gate-in_band_lifetimes.rs:53:31
    |
 LL |     fn my_lifetime(&self) -> &'a u8 { self.0 }
    |                               ^^ undeclared lifetime
    |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 help: consider introducing lifetime `'a` here
    |
 LL | impl<'a> MyTrait<'a> for Y<&'a u8> {
@@ -165,6 +184,7 @@ error[E0261]: use of undeclared lifetime name `'b`
 LL |     fn any_lifetime() -> &'b u8 { &0 }
    |                           ^^ undeclared lifetime
    |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 help: consider introducing lifetime `'b` here
    |
 LL | impl<'b> MyTrait<'a> for Y<&'a u8> {
@@ -180,6 +200,7 @@ error[E0261]: use of undeclared lifetime name `'b`
 LL |     fn borrowed_lifetime(&'b self) -> &'b u8 { &*self.0 }
    |                           ^^ undeclared lifetime
    |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 help: consider introducing lifetime `'b` here
    |
 LL | impl<'b> MyTrait<'a> for Y<&'a u8> {
@@ -195,6 +216,7 @@ error[E0261]: use of undeclared lifetime name `'b`
 LL |     fn borrowed_lifetime(&'b self) -> &'b u8 { &*self.0 }
    |                                        ^^ undeclared lifetime
    |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 help: consider introducing lifetime `'b` here
    |
 LL | impl<'b> MyTrait<'a> for Y<&'a u8> {

src/test/ui/generic-associated-types/generic_associated_type_undeclared_lifetimes.stderr

@@ -4,6 +4,7 @@ error[E0261]: use of undeclared lifetime name `'b`
 LL |         + Deref<Target = Self::Item<'b>>;
    |                                     ^^ undeclared lifetime
    |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 help: consider introducing lifetime `'b` here
    |
 LL | trait Iterable<'b> {
@@ -19,6 +20,7 @@ error[E0261]: use of undeclared lifetime name `'undeclared`
 LL |     fn iter<'a>(&'a self) -> Self::Iter<'undeclared>;
    |                                         ^^^^^^^^^^^ undeclared lifetime
    |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 help: consider introducing lifetime `'undeclared` here
    |
 LL | trait Iterable<'undeclared> {

src/test/ui/methods/method-call-lifetime-args-unresolved.stderr

@@ -5,6 +5,8 @@ LL | fn main() {
    |        - help: consider introducing lifetime `'a` here: `<'a>`
 LL |     0.clone::<'a>();
    |               ^^ undeclared lifetime
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error: aborting due to previous error
 

src/test/ui/regions/regions-in-enums.stderr

@@ -5,6 +5,8 @@ LL | enum No0 {
    |         - help: consider introducing lifetime `'foo` here: `<'foo>`
 LL |     X5(&'foo usize)
    |         ^^^^ undeclared lifetime
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'a`
   --> $DIR/regions-in-enums.rs:17:9
@@ -13,6 +15,8 @@ LL | enum No1 {
    |         - help: consider introducing lifetime `'a` here: `<'a>`
 LL |     X6(&'a usize)
    |         ^^ undeclared lifetime
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error: aborting due to 2 previous errors
 

src/test/ui/regions/regions-in-structs.stderr

@@ -5,6 +5,8 @@ LL | struct StructDecl {
    |                  - help: consider introducing lifetime `'a` here: `<'a>`
 LL |     a: &'a isize,
    |         ^^ undeclared lifetime
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'a`
   --> $DIR/regions-in-structs.rs:11:9
@@ -14,6 +16,8 @@ LL | struct StructDecl {
 LL |     a: &'a isize,
 LL |     b: &'a isize,
    |         ^^ undeclared lifetime
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error: aborting due to 2 previous errors
 

src/test/ui/regions/regions-name-undeclared.stderr

@@ -4,6 +4,7 @@ error[E0261]: use of undeclared lifetime name `'b`
 LL |     fn m4(&self, arg: &'b isize) { }
    |                        ^^ undeclared lifetime
    |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 help: consider introducing lifetime `'b` here
    |
 LL | impl<'b, 'a> Foo<'a> {
@@ -19,6 +20,7 @@ error[E0261]: use of undeclared lifetime name `'b`
 LL |     fn m5(&'b self) { }
    |            ^^ undeclared lifetime
    |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 help: consider introducing lifetime `'b` here
    |
 LL | impl<'b, 'a> Foo<'a> {
@@ -34,6 +36,7 @@ error[E0261]: use of undeclared lifetime name `'b`
 LL |     fn m6(&self, arg: Foo<'b>) { }
    |                           ^^ undeclared lifetime
    |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 help: consider introducing lifetime `'b` here
    |
 LL | impl<'b, 'a> Foo<'a> {
@@ -50,6 +53,8 @@ LL |     type X = Option<&'a isize>;
    |           -          ^^ undeclared lifetime
    |           |
    |           help: consider introducing lifetime `'a` here: `<'a>`
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'a`
   --> $DIR/regions-name-undeclared.rs:27:13
@@ -58,6 +63,8 @@ LL |     enum E {
    |           - help: consider introducing lifetime `'a` here: `<'a>`
 LL |         E1(&'a isize)
    |             ^^ undeclared lifetime
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'a`
   --> $DIR/regions-name-undeclared.rs:30:13
@@ -66,6 +73,8 @@ LL |     struct S {
    |             - help: consider introducing lifetime `'a` here: `<'a>`
 LL |         f: &'a isize
    |             ^^ undeclared lifetime
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'a`
   --> $DIR/regions-name-undeclared.rs:32:14
@@ -74,6 +83,8 @@ LL |     fn f(a: &'a isize) { }
    |         -    ^^ undeclared lifetime
    |         |
    |         help: consider introducing lifetime `'a` here: `<'a>`
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'a`
   --> $DIR/regions-name-undeclared.rs:40:17
@@ -82,6 +93,8 @@ LL | fn fn_types(a: &'a isize,
    |            -    ^^ undeclared lifetime
    |            |
    |            help: consider introducing lifetime `'a` here: `<'a>`
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'b`
   --> $DIR/regions-name-undeclared.rs:42:36
@@ -90,6 +103,7 @@ LL | ...                   &'b isize,
    |                        ^^ undeclared lifetime
    |
    = note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 help: consider introducing lifetime `'b` here
    |
 LL | fn fn_types<'b>(a: &'a isize,
@@ -106,6 +120,7 @@ LL | ...                   &'b isize)>,
    |                        ^^ undeclared lifetime
    |
    = note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 help: consider introducing lifetime `'b` here
    |
 LL | fn fn_types<'b>(a: &'a isize,
@@ -123,6 +138,8 @@ LL | fn fn_types(a: &'a isize,
 ...
 LL |             c: &'a isize)
    |                 ^^ undeclared lifetime
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error: aborting due to 11 previous errors
 

src/test/ui/regions/regions-undeclared.stderr

@@ -11,6 +11,8 @@ LL | enum EnumDecl {
    |              - help: consider introducing lifetime `'a` here: `<'a>`
 LL |     Foo(&'a isize),
    |          ^^ undeclared lifetime
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'a`
   --> $DIR/regions-undeclared.rs:5:10
@@ -20,6 +22,8 @@ LL | enum EnumDecl {
 LL |     Foo(&'a isize),
 LL |     Bar(&'a isize),
    |          ^^ undeclared lifetime
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'a`
   --> $DIR/regions-undeclared.rs:8:15
@@ -28,6 +32,8 @@ LL | fn fnDecl(x: &'a isize,
    |          -    ^^ undeclared lifetime
    |          |
    |          help: consider introducing lifetime `'a` here: `<'a>`
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'a`
   --> $DIR/regions-undeclared.rs:9:15
@@ -36,6 +42,8 @@ LL | fn fnDecl(x: &'a isize,
    |          - help: consider introducing lifetime `'a` here: `<'a>`
 LL |           y: &'a isize)
    |               ^^ undeclared lifetime
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error: aborting due to 5 previous errors
 

src/test/ui/where-clauses/where-lifetime-resolution.stderr

@@ -6,6 +6,8 @@ LL | fn f() where
 LL |     for<'a> dyn Trait1<'a>: Trait1<'a>, // OK
 LL |     (dyn for<'a> Trait1<'a>): Trait1<'a>,
    |                                      ^^ undeclared lifetime
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error[E0261]: use of undeclared lifetime name `'b`
   --> $DIR/where-lifetime-resolution.rs:8:52
@@ -15,6 +17,8 @@ LL | fn f() where
 ...
 LL |     for<'a> dyn for<'b> Trait2<'a, 'b>: Trait2<'a, 'b>,
    |                                                    ^^ undeclared lifetime
+   |
+   = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
 
 error: aborting due to 2 previous errors