Update

Author: tgross35

compiler/rustc_metadata/messages.ftl

@@ -47,6 +47,9 @@ metadata_crate_dep_rustc_driver =
 metadata_crate_location_unknown_type =
     extern location for {$crate_name} is of an unknown type: {$path}
 
+metadata_crate_not_compiler_builtins =
+    the crate `{$crate_name}` resolved as `compiler_builtins` but is not `#![compiler_builtins]`
+
 metadata_crate_not_panic_runtime =
     the crate `{$crate_name}` is not a panic runtime
 

compiler/rustc_metadata/src/creader.rs

@@ -1040,18 +1040,20 @@ impl<'a, 'tcx> CrateLoader<'a, 'tcx> {
             return;
         }
 
-        let Some(cnum) = self.resolve_crate(
+        let Ok(cnum) = self.maybe_resolve_crate(
             sym::compiler_builtins,
             CrateDepKind::Implicit,
             CrateOrigin::Injected,
         ) else {
+            info!("`compiler_builtins` not resolved");
             return;
         };
 
-        if res.is_ok() {
-            info!("`compiler_builtins` found");
-        } else {
-            info!("`compiler_builtins` not found, skipping");
+        let data = self.cstore.get_crate_data(cnum);
+
+        // Sanity check the loaded crate to ensure it is indeed compiler_builtins
+        if !data.is_compiler_builtins() {
+            self.dcx().emit_err(errors::CrateNotCompilerBuiltins { crate_name: data.name() });
         }
     }
 

compiler/rustc_metadata/src/errors.rs

@@ -332,6 +332,12 @@ pub struct CrateNotPanicRuntime {
     pub crate_name: Symbol,
 }
 
+#[derive(Diagnostic)]
+#[diag(metadata_crate_not_compiler_builtins)]
+pub struct CrateNotCompilerBuiltins {
+    pub crate_name: Symbol,
+}
+
 #[derive(Diagnostic)]
 #[diag(metadata_no_panic_strategy)]
 pub struct NoPanicStrategy {

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1928,6 +1928,10 @@ impl CrateMetadata {
         self.root.profiler_runtime
     }
 
+    pub(crate) fn is_compiler_builtins(&self) -> bool {
+        self.root.compiler_builtins
+    }
+
     pub(crate) fn needs_allocator(&self) -> bool {
         self.root.needs_allocator
     }

tests/ui/proc-macro/issue-59191-replace-root-with-fn.stderr

@@ -1,4 +1,11 @@
+error: `#[panic_handler]` function required, but not found
+
+error: unwinding panics are not supported without std
+   |
+   = help: using nightly cargo, use -Zbuild-std with panic="abort" to avoid unwinding
+   = note: since the core library is usually precompiled with panic="unwind", rebuilding your crate with panic="abort" may not be enough to fix the problem
+
 error: requires `sized` lang_item
 
-error: aborting due to 1 previous error
+error: aborting due to 3 previous errors