Add help for the error message when missing rustc_driver

compiler/rustc_error_messages/locales/en-US/metadata.ftl

@@ -4,6 +4,11 @@ metadata_rlib_required =
 metadata_lib_required =
     crate `{$crate_name}` required to be available in {$kind} format, but was not found in this form
 
+metadata_rustc_lib_required =
+    crate `{$crate_name}` required to be available in {$kind} format, but was not found in this form
+    .note = only .rmeta files are distributed for `rustc_private` crates other than `rustc_driver`
+    .help = try adding `extern crate rustc_driver;` at the top level of this crate
+
 metadata_crate_dep_multiple =
     cannot satisfy dependencies so `{$crate_name}` only shows up once
     .help = having upstream crates all available in one format will likely make this go away

compiler/rustc_metadata/src/dependency_format.rs

@@ -54,7 +54,7 @@
 use crate::creader::CStore;
 use crate::errors::{
     BadPanicStrategy, CrateDepMultiple, IncompatiblePanicInDropStrategy, LibRequired,
-    RequiredPanicStrategy, RlibRequired, TwoPanicRuntimes,
+    RequiredPanicStrategy, RlibRequired, RustcLibRequired, TwoPanicRuntimes,
 };
 
 use rustc_data_structures::fx::FxHashMap;
@@ -224,7 +224,12 @@ fn calculate_type(tcx: TyCtxt<'_>, ty: CrateType) -> DependencyList {
                     Linkage::Static => "rlib",
                     _ => "dylib",
                 };
-                sess.emit_err(LibRequired { crate_name: tcx.crate_name(cnum), kind: kind });
+                let crate_name = tcx.crate_name(cnum);
+                if crate_name.as_str().starts_with("rustc_") {
+                    sess.emit_err(RustcLibRequired { crate_name, kind });
+                } else {
+                    sess.emit_err(LibRequired { crate_name, kind });
+                }
             }
         }
     }

compiler/rustc_metadata/src/errors.rs

@@ -24,6 +24,14 @@ pub struct LibRequired<'a> {
     pub kind: &'a str,
 }
 
+#[derive(Diagnostic)]
+#[diag(metadata_rustc_lib_required)]
+#[help]
+pub struct RustcLibRequired<'a> {
+    pub crate_name: Symbol,
+    pub kind: &'a str,
+}
+
 #[derive(Diagnostic)]
 #[diag(metadata_crate_dep_multiple)]
 #[help]

src/test/ui-fulldeps/missing-rustc-driver-error.rs

@@ -0,0 +1,9 @@
+// Test that we get the following hint when trying to use a compiler crate without rustc_driver.
+// error-pattern: try adding `extern crate rustc_driver;` at the top level of this crate
+// compile-flags: --emit link
+
+#![feature(rustc_private)]
+
+extern crate rustc_serialize;
+
+fn main() {}

src/test/ui-fulldeps/missing-rustc-driver-error.stderr

@@ -0,0 +1,24 @@
+error: crate `rustc_serialize` required to be available in rlib format, but was not found in this form
+   |
+   = help: try adding `extern crate rustc_driver;` at the top level of this crate
+
+error: crate `smallvec` required to be available in rlib format, but was not found in this form
+
+error: crate `thin_vec` required to be available in rlib format, but was not found in this form
+
+error: crate `indexmap` required to be available in rlib format, but was not found in this form
+
+error: crate `hashbrown` required to be available in rlib format, but was not found in this form
+
+error: crate `ahash` required to be available in rlib format, but was not found in this form
+
+error: crate `once_cell` required to be available in rlib format, but was not found in this form
+
+error: crate `getrandom` required to be available in rlib format, but was not found in this form
+
+error: crate `cfg_if` required to be available in rlib format, but was not found in this form
+
+error: crate `libc` required to be available in rlib format, but was not found in this form
+
+error: aborting due to 10 previous errors
+