Display name of crate requiring rustc_private in error messages.

src/librustc/middle/stability.rs

@@ -431,12 +431,15 @@ impl<'tcx> Index<'tcx> {
             // while maintaining the invariant that all sysroot crates are unstable
             // by default and are unable to be used.
             if tcx.sess.opts.debugging_opts.force_unstable_if_unmarked {
-                let reason = "this crate is being loaded from the sysroot, an \
-                              unstable location; did you mean to load this crate \
-                              from crates.io via `Cargo.toml` instead?";
+                let reason = format!(
+                    "crate \"{}\" is being loaded from the sysroot, an \
+                     unstable location; did you mean to load this crate \
+                     from crates.io via `Cargo.toml` instead?",
+                     tcx.crate_name(LOCAL_CRATE)
+                );
                 let stability = tcx.intern_stability(Stability {
                     level: attr::StabilityLevel::Unstable {
-                        reason: Some(Symbol::intern(reason)),
+                        reason: Some(Symbol::intern(&reason)),
                         issue: 27812,
                     },
                     feature: sym::rustc_private,

src/libtest/lib.rs

@@ -22,7 +22,8 @@
 #![unstable(feature = "test", issue = "27812")]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/", test(attr(deny(warnings))))]
 #![feature(asm)]
-#![cfg_attr(any(unix, target_os = "cloudabi"), feature(libc, rustc_private))]
+#![feature(libc)]
+#![feature(rustc_private)]
 #![feature(nll)]
 #![feature(set_stdio)]
 #![feature(panic_unwind)]

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

@@ -12,4 +12,5 @@ use rustc_macros::HashStable;
 
 #[derive(HashStable)]
 //~^ use of unstable library feature 'rustc_private'
+//~| use of unstable library feature 'rustc_private'
 struct Test;

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': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
+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': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
+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': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
+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': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
+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': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
+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)]
@@ -47,7 +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: aborting due to 6 previous errors
+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)]
+   |          ^^^^^^^^^^
+   |
+   = 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 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.rs

@@ -0,0 +1,4 @@
+fn main() {
+    extern crate libc; //~ ERROR use of unstable
+    use libc::*; //~ ERROR unresolved import
+}

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

@@ -0,0 +1,19 @@
+error[E0432]: unresolved import `libc`
+  --> $DIR/feature-gate-rustc_private-libc.rs:3:9
+   |
+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?
+  --> $DIR/feature-gate-rustc_private-libc.rs:2:5
+   |
+LL |     extern crate libc;
+   |     ^^^^^^^^^^^^^^^^^^
+   |
+   = 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 have detailed explanations: E0432, E0658.
+For more information about an error, try `rustc --explain E0432`.

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

@@ -0,0 +1,4 @@
+fn main() {
+    extern crate test; //~ ERROR use of unstable
+    use test::*; //~ ERROR unresolved import
+}

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

@@ -0,0 +1,19 @@
+error[E0432]: unresolved import `test`
+  --> $DIR/feature-gate-test.rs:3:9
+   |
+LL |     use test::*;
+   |         ^^^^ maybe a missing `extern crate test;`?
+
+error[E0658]: use of unstable library feature 'test'
+  --> $DIR/feature-gate-test.rs:2:5
+   |
+LL |     extern crate test;
+   |     ^^^^^^^^^^^^^^^^^^
+   |
+   = 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 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': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
+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;