Ask the user to use feature(rustc_private) when linking to rustc_driver

compiler/rustc_metadata/messages.ftl

@@ -41,6 +41,9 @@ metadata_crate_dep_multiple =
 metadata_crate_dep_not_static =
     `{$crate_name}` was unavailable as a static crate, preventing fully static linking
 
+metadata_crate_dep_rustc_driver =
+    `feature(rustc_private)` is needed to link to the compiler's `rustc_driver` library
+
 metadata_crate_location_unknown_type =
     extern location for {$crate_name} is of an unknown type: {$path}
 

compiler/rustc_metadata/src/dependency_format.rs

@@ -59,12 +59,14 @@ use rustc_middle::ty::TyCtxt;
 use rustc_session::config::CrateType;
 use rustc_session::cstore::CrateDepKind;
 use rustc_session::cstore::LinkagePreference::{self, RequireDynamic, RequireStatic};
+use rustc_span::sym;
 use tracing::info;
 
 use crate::creader::CStore;
 use crate::errors::{
     BadPanicStrategy, CrateDepMultiple, IncompatiblePanicInDropStrategy, LibRequired,
-    NonStaticCrateDep, RequiredPanicStrategy, RlibRequired, RustcLibRequired, TwoPanicRuntimes,
+    NonStaticCrateDep, RequiredPanicStrategy, RlibRequired, RustcDriverHelp, RustcLibRequired,
+    TwoPanicRuntimes,
 };
 
 pub(crate) fn calculate(tcx: TyCtxt<'_>) -> Dependencies {
@@ -197,14 +199,14 @@ fn calculate_type(tcx: TyCtxt<'_>, ty: CrateType) -> DependencyList {
         }
 
         let name = tcx.crate_name(cnum);
-            info!("adding dylib: {}", name);
-            add_library(tcx, cnum, RequireDynamic, &mut formats, &mut unavailable_as_static);
-            let deps = tcx.dylib_dependency_formats(cnum);
-            for &(depnum, style) in deps.iter() {
-                info!("adding {:?}: {}", style, tcx.crate_name(depnum));
-                add_library(tcx, depnum, style, &mut formats, &mut unavailable_as_static);
-            }
+        info!("adding dylib: {}", name);
+        add_library(tcx, cnum, RequireDynamic, &mut formats, &mut unavailable_as_static);
+        let deps = tcx.dylib_dependency_formats(cnum);
+        for &(depnum, style) in deps.iter() {
+            info!("adding {:?}: {}", style, tcx.crate_name(depnum));
+            add_library(tcx, depnum, style, &mut formats, &mut unavailable_as_static);
         }
+    }
 
     // Collect what we've got so far in the return vector.
     let last_crate = tcx.crates(()).len();
@@ -292,12 +294,15 @@ fn add_library(
             // This error is probably a little obscure, but I imagine that it
             // can be refined over time.
             if link2 != link || link == RequireStatic {
+                let linking_to_rustc_driver = tcx.sess.psess.unstable_features.is_nightly_build()
+                    && tcx.crates(()).iter().any(|&cnum| tcx.crate_name(cnum) == sym::rustc_driver);
                 tcx.dcx().emit_err(CrateDepMultiple {
                     crate_name: tcx.crate_name(cnum),
                     non_static_deps: unavailable_as_static
                         .drain(..)
                         .map(|cnum| NonStaticCrateDep { crate_name: tcx.crate_name(cnum) })
                         .collect(),
+                    rustc_driver_help: linking_to_rustc_driver.then_some(RustcDriverHelp),
                 });
             }
         }

compiler/rustc_metadata/src/errors.rs

@@ -38,6 +38,8 @@ pub struct CrateDepMultiple {
     pub crate_name: Symbol,
     #[subdiagnostic]
     pub non_static_deps: Vec<NonStaticCrateDep>,
+    #[subdiagnostic]
+    pub rustc_driver_help: Option<RustcDriverHelp>,
 }
 
 #[derive(Subdiagnostic)]
@@ -46,6 +48,10 @@ pub struct NonStaticCrateDep {
     pub crate_name: Symbol,
 }
 
+#[derive(Subdiagnostic)]
+#[help(metadata_crate_dep_rustc_driver)]
+pub struct RustcDriverHelp;
+
 #[derive(Diagnostic)]
 #[diag(metadata_two_panic_runtimes)]
 pub struct TwoPanicRuntimes {

compiler/rustc_span/src/symbol.rs

@@ -1619,6 +1619,7 @@ symbols! {
         rustc_dirty,
         rustc_do_not_const_check,
         rustc_doc_primitive,
+        rustc_driver,
         rustc_dummy,
         rustc_dump_def_parents,
         rustc_dump_item_bounds,