use backticks instead of single quotes when reporting "use of unstable library feature"

This is consistent with all other diagnostics I could find containing
features and enables the use of `DiagSymbolList` for generalizing
diagnostics for unstable library features to multiple features.

Author: dianne

compiler/rustc_hir_analysis/messages.ftl

@@ -311,8 +311,8 @@ hir_analysis_missing_trait_item_suggestion = implement the missing item: `{$snip
 
 hir_analysis_missing_trait_item_unstable = not all trait items implemented, missing: `{$missing_item_name}`
     .note = default implementation of `{$missing_item_name}` is unstable
-    .some_note = use of unstable library feature '{$feature}': {$reason}
-    .none_note = use of unstable library feature '{$feature}'
+    .some_note = use of unstable library feature `{$feature}`: {$reason}
+    .none_note = use of unstable library feature `{$feature}`
 
 hir_analysis_missing_type_params =
     the type {$parameterCount ->

compiler/rustc_middle/src/middle/stability.rs

@@ -112,8 +112,8 @@ pub fn report_unstable(
     soft_handler: impl FnOnce(&'static Lint, Span, String),
 ) {
     let msg = match reason {
-        Some(r) => format!("use of unstable library feature '{feature}': {r}"),
-        None => format!("use of unstable library feature '{feature}'"),
+        Some(r) => format!("use of unstable library feature `{feature}`: {r}"),
+        None => format!("use of unstable library feature `{feature}`"),
     };
 
     if is_soft {

tests/ui-fulldeps/hash-stable-is-unstable.rs

@@ -1,24 +1,24 @@
 //@ compile-flags: -Zdeduplicate-diagnostics=yes
 extern crate rustc_data_structures;
-//~^ use of unstable library feature 'rustc_private'
+//~^ use of unstable library feature `rustc_private`
 //~| NOTE: issue #27812 <https://github.com/rust-lang/rust/issues/27812> for more information
 //~| NOTE: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 extern crate rustc_macros;
-//~^ use of unstable library feature 'rustc_private'
+//~^ use of unstable library feature `rustc_private`
 //~| NOTE: see issue #27812 <https://github.com/rust-lang/rust/issues/27812> for more information
 //~| NOTE: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 extern crate rustc_query_system;
-//~^ use of unstable library feature 'rustc_private'
+//~^ use of unstable library feature `rustc_private`
 //~| NOTE: see issue #27812 <https://github.com/rust-lang/rust/issues/27812> for more information
 //~| NOTE: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 use rustc_macros::HashStable;
-//~^ use of unstable library feature 'rustc_private'
+//~^ use of unstable library feature `rustc_private`
 //~| NOTE: see issue #27812 <https://github.com/rust-lang/rust/issues/27812> for more information
 //~| NOTE: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 #[derive(HashStable)]
-//~^ use of unstable library feature 'rustc_private'
+//~^ use of unstable library feature `rustc_private`
 //~| NOTE: in this expansion of #[derive(HashStable)]
 //~| NOTE: in this expansion of #[derive(HashStable)]
 //~| NOTE: in this expansion of #[derive(HashStable)]

tests/ui-fulldeps/pathless-extern-unstable.rs

@@ -4,6 +4,6 @@
 // Test that `--extern rustc_middle` fails with `rustc_private`.
 
 pub use rustc_middle;
-//~^ ERROR use of unstable library feature 'rustc_private'
+//~^ ERROR use of unstable library feature `rustc_private`
 
 fn main() {}

tests/ui/associated-inherent-types/assoc-inherent-unstable.rs

@@ -4,6 +4,6 @@
 #![feature(inherent_associated_types)]
 #![allow(incomplete_features)]
 
-type Data = aux::Owner::Data; //~ ERROR use of unstable library feature 'data'
+type Data = aux::Owner::Data; //~ ERROR use of unstable library feature `data`
 
 fn main() {}

tests/ui/associated-inherent-types/assoc-inherent-unstable.stderr

@@ -1,4 +1,4 @@
-error[E0658]: use of unstable library feature 'data'
+error[E0658]: use of unstable library feature `data`
   --> $DIR/assoc-inherent-unstable.rs:7:13
    |
 LL | type Data = aux::Owner::Data;

tests/ui/async-await/async-fn/edition-2015.rs

@@ -3,7 +3,7 @@ fn foo(x: impl async Fn()) -> impl async Fn() { x }
 //~| ERROR `async` trait bounds are only allowed in Rust 2018 or later
 //~| ERROR async closures are unstable
 //~| ERROR async closures are unstable
-//~| ERROR use of unstable library feature 'async_closure'
-//~| ERROR use of unstable library feature 'async_closure'
+//~| ERROR use of unstable library feature `async_closure`
+//~| ERROR use of unstable library feature `async_closure`
 
 fn main() {}

tests/ui/async-await/async-fn/edition-2015.stderr

@@ -38,7 +38,7 @@ LL | fn foo(x: impl async Fn()) -> impl async Fn() { x }
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
    = help: to use an async block, remove the `||`: `async {`
 
-error[E0658]: use of unstable library feature 'async_closure'
+error[E0658]: use of unstable library feature `async_closure`
   --> $DIR/edition-2015.rs:1:42
    |
 LL | fn foo(x: impl async Fn()) -> impl async Fn() { x }
@@ -48,7 +48,7 @@ LL | fn foo(x: impl async Fn()) -> impl async Fn() { x }
    = help: add `#![feature(async_closure)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error[E0658]: use of unstable library feature 'async_closure'
+error[E0658]: use of unstable library feature `async_closure`
   --> $DIR/edition-2015.rs:1:22
    |
 LL | fn foo(x: impl async Fn()) -> impl async Fn() { x }

tests/ui/box/alloc-unstable-fail.rs

@@ -2,5 +2,5 @@ use std::boxed::Box;
 
 fn main() {
     let _boxed: Box<u32, _> = Box::new(10);
-    //~^ ERROR use of unstable library feature 'allocator_api'
+    //~^ ERROR use of unstable library feature `allocator_api`
 }

tests/ui/box/alloc-unstable-fail.stderr

@@ -1,4 +1,4 @@
-error[E0658]: use of unstable library feature 'allocator_api'
+error[E0658]: use of unstable library feature `allocator_api`
   --> $DIR/alloc-unstable-fail.rs:4:26
    |
 LL |     let _boxed: Box<u32, _> = Box::new(10);