[experiment] inject compiler builtins

compiler/rustc_builtin_macros/src/standard_library_imports.rs

@@ -47,6 +47,7 @@ pub fn inject(
     for &name in names.iter().rev() {
         let ident_span = if edition >= Edition2018 { span } else { call_site };
         let item = if name == sym::compiler_builtins {
+            eprintln!("INJECTING BUILTINS");
             // compiler_builtins is a private implementation detail. We only
             // need to insert it into the crate graph for linking and should not
             // expose any of its public API.

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

@@ -685,7 +685,7 @@ impl<'a, 'tcx> CrateLoader<'a, 'tcx> {
 
         match result {
             (LoadResult::Previous(cnum), None) => {
-                info!("library for `{}` was loaded previously", name);
+                info!("library for `{}` was loaded previously, cnum {cnum}", name);
                 // When `private_dep` is none, it indicates the directly dependent crate. If it is
                 // not specified by `--extern` on command line parameters, it may be
                 // `private-dependency` when `register_crate` is called for the first time. Then it must be updated to
@@ -1031,6 +1031,39 @@ impl<'a, 'tcx> CrateLoader<'a, 'tcx> {
         }
     }
 
+    fn inject_compiler_builtins(&mut self, krate: &ast::Crate) {
+        if attr::contains_name(&krate.attrs, sym::compiler_builtins)
+            || attr::contains_name(&krate.attrs, sym::no_core)
+        {
+            // `compiler_builtins` does not get extern builtins, nor do `#![no_core]` crates
+            info!("`compiler_builtins` unneeded");
+            return;
+        }
+
+        for (cnum, cmeta) in self.cstore.iter_crate_data() {
+            if cmeta.is_compiler_builtins() {
+                info!("`compiler_builtins` already exists (cnum = {cnum}); skipping injection");
+                return;
+            }
+        }
+
+        let Ok(cnum) = self.maybe_resolve_crate(
+            sym::compiler_builtins,
+            CrateDepKind::Implicit,
+            CrateOrigin::Injected,
+        ) else {
+            info!("`compiler_builtins` not resolved");
+            return;
+        };
+
+        let cmeta = self.cstore.get_crate_data(cnum);
+
+        // Sanity check the loaded crate to ensure it is indeed compiler_builtins
+        if !cmeta.is_compiler_builtins() {
+            self.dcx().emit_err(errors::CrateNotCompilerBuiltins { crate_name: cmeta.name() });
+        }
+    }
+
     fn inject_dependency_if(
         &mut self,
         krate: CrateNum,
@@ -1140,6 +1173,8 @@ impl<'a, 'tcx> CrateLoader<'a, 'tcx> {
     }
 
     pub fn postprocess(&mut self, krate: &ast::Crate) {
+        info!("POSTPROCESS");
+        self.inject_compiler_builtins(krate);
         self.inject_forced_externs();
         self.inject_profiler_runtime();
         self.inject_allocator_crate(krate);
@@ -1171,12 +1206,18 @@ impl<'a, 'tcx> CrateLoader<'a, 'tcx> {
                     }
                     None => item.ident.name,
                 };
+
                 let dep_kind = if attr::contains_name(&item.attrs, sym::no_link) {
                     CrateDepKind::MacrosOnly
                 } else {
                     CrateDepKind::Explicit
                 };
 
+                if name == sym::compiler_builtins {
+                    info!("BUILTINS DETECTED dep_kind {dep_kind:?}");
+                    return None;
+                }
+
                 let cnum = self.resolve_crate(name, item.span, dep_kind, CrateOrigin::AstExtern)?;
 
                 let path_len = definitions.def_path(def_id).data.len();

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/locator.rs

@@ -260,7 +260,7 @@ pub(crate) struct CrateLocator<'a> {
     crate_rejections: CrateRejections,
 }
 
-#[derive(Clone)]
+#[derive(Clone, Debug)]
 pub(crate) struct CratePaths {
     pub(crate) name: Symbol,
     source: CrateSource,
@@ -272,7 +272,7 @@ impl CratePaths {
     }
 }
 
-#[derive(Copy, Clone, PartialEq)]
+#[derive(Copy, Clone, Debug, PartialEq)]
 pub(crate) enum CrateFlavor {
     Rlib,
     Rmeta,
@@ -893,13 +893,13 @@ fn get_flavor_from_path(path: &Path) -> CrateFlavor {
 
 // ------------------------------------------ Error reporting -------------------------------------
 
-#[derive(Clone)]
+#[derive(Clone, Debug)]
 struct CrateMismatch {
     path: PathBuf,
     got: String,
 }
 
-#[derive(Clone, Default)]
+#[derive(Clone, Debug, Default)]
 struct CrateRejections {
     via_hash: Vec<CrateMismatch>,
     via_triple: Vec<CrateMismatch>,
@@ -912,6 +912,7 @@ struct CrateRejections {
 /// Candidate rejection reasons collected during crate search.
 /// If no candidate is accepted, then these reasons are presented to the user,
 /// otherwise they are ignored.
+#[derive(Debug)]
 pub(crate) struct CombinedLocatorError {
     crate_name: Symbol,
     dep_root: Option<CratePaths>,
@@ -921,6 +922,7 @@ pub(crate) struct CombinedLocatorError {
     crate_rejections: CrateRejections,
 }
 
+#[derive(Debug)]
 pub(crate) enum CrateError {
     NonAsciiName(Symbol),
     ExternLocationNotExist(Symbol, PathBuf),

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