Fix unit test errors.

src/test/ui-fulldeps/hash-stable-is-unstable.stderr

@@ -2,7 +2,7 @@ error[E0601]: `main` function not found in crate `hash_stable_is_unstable`
    |
    = note: consider adding a `main` function to `$DIR/hash-stable-is-unstable.rs`
 
-error[E0658]: use of unstable library feature 'rustc_private': crate "rustc_data_structures" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
+error[E0658]: use of unstable library feature 'rustc_private': crate "rustc_data_structures" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
   --> $DIR/hash-stable-is-unstable.rs:3:1
    |
 LL | extern crate rustc_data_structures;
@@ -11,7 +11,7 @@ LL | extern crate rustc_data_structures;
    = note: for more information, see https://github.com/rust-lang/rust/issues/27812
    = help: add `#![feature(rustc_private)]` to the crate attributes to enable
 
-error[E0658]: use of unstable library feature 'rustc_private': crate "rustc" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
+error[E0658]: use of unstable library feature 'rustc_private': crate "rustc" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
   --> $DIR/hash-stable-is-unstable.rs:5:1
    |
 LL | extern crate rustc;
@@ -20,7 +20,7 @@ LL | extern crate rustc;
    = note: for more information, see https://github.com/rust-lang/rust/issues/27812
    = help: add `#![feature(rustc_private)]` to the crate attributes to enable
 
-error[E0658]: use of unstable library feature 'rustc_private': crate "rustc_macros" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
+error[E0658]: use of unstable library feature 'rustc_private': crate "rustc_macros" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
   --> $DIR/hash-stable-is-unstable.rs:7:1
    |
 LL | extern crate rustc_macros;
@@ -29,7 +29,7 @@ LL | extern crate rustc_macros;
    = note: for more information, see https://github.com/rust-lang/rust/issues/27812
    = help: add `#![feature(rustc_private)]` to the crate attributes to enable
 
-error[E0658]: use of unstable library feature 'rustc_private': crate "rustc_macros" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
+error[E0658]: use of unstable library feature 'rustc_private': crate "rustc_macros" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
   --> $DIR/hash-stable-is-unstable.rs:10:5
    |
 LL | use rustc_macros::HashStable;
@@ -38,7 +38,7 @@ LL | use rustc_macros::HashStable;
    = note: for more information, see https://github.com/rust-lang/rust/issues/27812
    = help: add `#![feature(rustc_private)]` to the crate attributes to enable
 
-error[E0658]: use of unstable library feature 'rustc_private': crate "rustc_data_structures" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
+error[E0658]: use of unstable library feature 'rustc_private': crate "rustc_data_structures" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
   --> $DIR/hash-stable-is-unstable.rs:13:10
    |
 LL | #[derive(HashStable)]
@@ -47,15 +47,16 @@ LL | #[derive(HashStable)]
    = note: for more information, see https://github.com/rust-lang/rust/issues/27812
    = help: add `#![feature(rustc_private)]` to the crate attributes to enable
 
-error[E0658]: use of unstable library feature 'rustc_private': crate "rustc" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
+error[E0658]: use of unstable library feature 'rustc_private': crate "rustc" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
   --> $DIR/hash-stable-is-unstable.rs:13:10
    |
 LL | #[derive(HashStable)]
    |          ^^^^^^^^^^
    |
-   = help: add #![feature(rustc_private)] to the crate attributes to enable
+   = note: for more information, see https://github.com/rust-lang/rust/issues/27812
+   = help: add `#![feature(rustc_private)]` to the crate attributes to enable
 
 error: aborting due to 7 previous errors
 
-Some errors occurred: E0601, E0658.
+Some errors have detailed explanations: E0601, E0658.
 For more information about an error, try `rustc --explain E0601`.

src/test/ui/feature-gates/feature-gate-rustc_private-libc.stderr

@@ -4,15 +4,16 @@ error[E0432]: unresolved import `libc`
 LL |     use libc::*;
    |         ^^^^ maybe a missing `extern crate libc;`?
 
-error[E0658]: use of unstable library feature 'rustc_private': crate "libc" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
+error[E0658]: use of unstable library feature 'rustc_private': crate "libc" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
   --> $DIR/feature-gate-rustc_private-libc.rs:2:5
    |
 LL |     extern crate libc;
    |     ^^^^^^^^^^^^^^^^^^
    |
-   = help: add #![feature(rustc_private)] to the crate attributes to enable
+   = note: for more information, see https://github.com/rust-lang/rust/issues/27812
+   = help: add `#![feature(rustc_private)]` to the crate attributes to enable
 
 error: aborting due to 2 previous errors
 
-Some errors occurred: E0432, E0658.
+Some errors have detailed explanations: E0432, E0658.
 For more information about an error, try `rustc --explain E0432`.

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

@@ -4,15 +4,16 @@ error[E0432]: unresolved import `test`
 LL |     use test::*;
    |         ^^^^ maybe a missing `extern crate test;`?
 
-error[E0658]: use of unstable library feature 'test' (see issue #27812)
+error[E0658]: use of unstable library feature 'test'
   --> $DIR/feature-gate-test.rs:2:5
    |
 LL |     extern crate test;
    |     ^^^^^^^^^^^^^^^^^^
    |
-   = help: add #![feature(test)] to the crate attributes to enable
+   = note: for more information, see https://github.com/rust-lang/rust/issues/27812
+   = help: add `#![feature(test)]` to the crate attributes to enable
 
 error: aborting due to 2 previous errors
 
-Some errors occurred: E0432, E0658.
+Some errors have detailed explanations: E0432, E0658.
 For more information about an error, try `rustc --explain E0432`.

src/test/ui/issues/issue-37887.stderr

@@ -4,7 +4,7 @@ error[E0432]: unresolved import `libc`
 LL |     use libc::*;
    |         ^^^^ maybe a missing `extern crate libc;`?
 
-error[E0658]: use of unstable library feature 'rustc_private': crate "libc" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
+error[E0658]: use of unstable library feature 'rustc_private': crate "libc" is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
   --> $DIR/issue-37887.rs:2:5
    |
 LL |     extern crate libc;