Inject compiler_builtins during postprocessing rather than via AST

`compiler_builtins` is currently injected as `extern crate
compiler_builtins as _`. This has made gating via diagnostics difficult
because it appears in the crate graph as a non-private dependency, and
there isn't an easy way to differentiate between the injected AST and
user-specified `extern crate compiler_builtins`.

Resolve this by injecting `compiler_builtins` during postprocessing
rather than early in the AST. Most of the time this isn't even needed
because it shows up in `std` or `core`'s crate graph, but injection is
still needed to ensure `#![no_core]` works correctly.

A similar change was attempted at [1] but this encountered errors
building `proc_macro` and `rustc-std-workspace-std`. Similar failures
showed up while working on this patch, which were traced back to
`compiler_builtins` showing up in the graph twice (once via dependency
and once via injection). This is resolved by not injecting if a
`#![compiler_builtins]` crate already exists.

[1]: rust-lang#113634

Author: tgross35

compiler/rustc_builtin_macros/src/standard_library_imports.rs

@@ -19,16 +19,12 @@ pub fn inject(
     let edition = sess.psess.edition;
 
     // the first name in this list is the crate name of the crate with the prelude
-    let names: &[Symbol] = if attr::contains_name(pre_configured_attrs, sym::no_core) {
+    let name: Symbol = if attr::contains_name(pre_configured_attrs, sym::no_core) {
         return 0;
     } else if attr::contains_name(pre_configured_attrs, sym::no_std) {
-        if attr::contains_name(pre_configured_attrs, sym::compiler_builtins) {
-            &[sym::core]
-        } else {
-            &[sym::core, sym::compiler_builtins]
-        }
+        sym::core
     } else {
-        &[sym::std]
+        sym::std
     };
 
     let expn_id = resolver.expansion_for_ast_pass(
@@ -43,36 +39,16 @@ pub fn inject(
     let ecfg = ExpansionConfig::default("std_lib_injection".to_string(), features);
     let cx = ExtCtxt::new(sess, ecfg, resolver, None);
 
-    // .rev() to preserve ordering above in combination with insert(0, ...)
-    for &name in names.iter().rev() {
-        let ident_span = if edition >= Edition2018 { span } else { call_site };
-        let item = if name == sym::compiler_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.
-            //
-            // FIXME(#113634) We should inject this during post-processing like
-            // we do for the panic runtime, profiler runtime, etc.
-            cx.item(
-                span,
-                Ident::new(kw::Underscore, ident_span),
-                thin_vec![],
-                ast::ItemKind::ExternCrate(Some(name)),
-            )
-        } else {
-            cx.item(
-                span,
-                Ident::new(name, ident_span),
-                thin_vec![cx.attr_word(sym::macro_use, span)],
-                ast::ItemKind::ExternCrate(None),
-            )
-        };
-        krate.items.insert(0, item);
-    }
+    let ident_span = if edition >= Edition2018 { span } else { call_site };
 
-    // The crates have been injected, the assumption is that the first one is
-    // the one with the prelude.
-    let name = names[0];
+    let item = cx.item(
+        span,
+        Ident::new(name, ident_span),
+        thin_vec![cx.attr_word(sym::macro_use, span)],
+        ast::ItemKind::ExternCrate(None),
+    );
+
+    krate.items.insert(0, item);
 
     let root = (edition == Edition2015).then_some(kw::PathRoot);
 
@@ -88,6 +64,7 @@ pub fn inject(
         .map(|&symbol| Ident::new(symbol, span))
         .collect();
 
+    // Inject the relevant crate's prelude.
     let use_item = cx.item(
         span,
         Ident::empty(),

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

@@ -1032,6 +1032,43 @@ impl<'a, 'tcx> CrateLoader<'a, 'tcx> {
         }
     }
 
+    /// Inject the `compiler_builtins` crate if it is not already in the graph.
+    fn inject_compiler_builtins(&mut self, krate: &ast::Crate) {
+        // `compiler_builtins` does not get extern builtins, nor do `#![no_core]` crates
+        if attr::contains_name(&krate.attrs, sym::compiler_builtins)
+            || attr::contains_name(&krate.attrs, sym::no_core)
+        {
+            info!("`compiler_builtins` unneeded");
+            return;
+        }
+
+        // If a `#![compiler_builtins]` crate already exists, avoid injecting it twice. This is
+        // the common case since usually it appears as a dependency of `std` or `core`.
+        for (cnum, cmeta) in self.cstore.iter_crate_data() {
+            if cmeta.is_compiler_builtins() {
+                info!("`compiler_builtins` already exists (cnum = {cnum}); skipping injection");
+                return;
+            }
+        }
+
+        // Allow builtins to remain unresolved, which will just mean linker errors if the
+        // relevant symbols aren't otherwise provided.
+        let Ok(cnum) = self.maybe_resolve_crate(
+            sym::compiler_builtins,
+            CrateDepKind::Implicit,
+            CrateOrigin::Injected,
+        ) else {
+            info!("`compiler_builtins` not resolved");
+            return;
+        };
+
+        // Sanity check that the loaded crate is `#![compiler_builtins]`
+        let cmeta = self.cstore.get_crate_data(cnum);
+        if !cmeta.is_compiler_builtins() {
+            self.dcx().emit_err(errors::CrateNotCompilerBuiltins { crate_name: cmeta.name() });
+        }
+    }
+
     fn inject_dependency_if(
         &mut self,
         krate: CrateNum,
@@ -1141,6 +1178,7 @@ impl<'a, 'tcx> CrateLoader<'a, 'tcx> {
     }
 
     pub fn postprocess(&mut self, krate: &ast::Crate) {
+        self.inject_compiler_builtins(krate);
         self.inject_forced_externs();
         self.inject_profiler_runtime();
         self.inject_allocator_crate(krate);

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

@@ -1932,6 +1932,10 @@ 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.rs

@@ -3,6 +3,7 @@
 
 //@ edition:2018
 //@ proc-macro: issue-59191.rs
+//@ needs-unwind (affects error output)
 //@ error-pattern: requires `sized` lang_item
 
 #![feature(custom_inner_attributes)]

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
 

tests/ui/proc-macro/meta-macro-hygiene.stdout

@@ -20,7 +20,6 @@ Respanned: TokenStream [Ident { ident: "$crate", span: $DIR/auxiliary/make-macro
 use core /* 0#1 */::prelude /* 0#1 */::rust_2018 /* 0#1 */::*;
 #[macro_use /* 0#1 */]
 extern crate core /* 0#1 */;
-extern crate compiler_builtins /* NNN */ as _ /* 0#1 */;
 // Don't load unnecessary hygiene information from std
 extern crate std /* 0#0 */;
 

tests/ui/proc-macro/nonterminal-token-hygiene.stdout

@@ -39,7 +39,6 @@ PRINT-BANG INPUT (DEBUG): TokenStream [
 use ::core /* 0#1 */::prelude /* 0#1 */::rust_2015 /* 0#1 */::*;
 #[macro_use /* 0#1 */]
 extern crate core /* 0#2 */;
-extern crate compiler_builtins /* NNN */ as _ /* 0#2 */;
 // Don't load unnecessary hygiene information from std
 extern crate std /* 0#0 */;