Move metadata generation.

It's currently done within `start_codegen`, which is called from
`codegen_and_build_linker`, and it gets included in the codegen timing
section. All this is surprising, because it's not part of codegen.

This commit moves it into `run_compiler`, just before the call to
`codegen_and_build_linker`. This is a more sensible place for it.
The commit also adds some extra non-obvious information about the
`has_errors_or_delayed_bugs` check, which I learned when I tried
moving metadata generation earlier.

Likewise, the nearby `rustc_delayed_bug_from_inside_query` code is also
moved, because (a) it's not part of codegen, and (b) it needs to be
nearby.

Author: nnethercote

compiler/rustc_driver_impl/src/lib.rs

@@ -60,8 +60,8 @@ use rustc_session::getopts::{self, Matches};
 use rustc_session::lint::{Lint, LintId};
 use rustc_session::output::{CRATE_TYPES, collect_crate_types, invalid_output_for_target};
 use rustc_session::{EarlyDiagCtxt, Session, config};
-use rustc_span::FileName;
 use rustc_span::def_id::LOCAL_CRATE;
+use rustc_span::{FileName, sym};
 use rustc_target::json::ToJson;
 use rustc_target::spec::{Target, TargetTuple};
 use tracing::trace;
@@ -383,7 +383,31 @@ pub fn run_compiler(at_args: &[String], callbacks: &mut (dyn Callbacks + Send))
                 }
             }
 
-            Some(Linker::codegen_and_build_linker(tcx, &*compiler.codegen_backend))
+            // Hook for tests. This must be shortly before the
+            // `has_errors_or_delayed_bugs` check below for
+            // `tests/ui/treat-err-as-bug/span_delayed_bug.rs` to work.
+            if let Some((def_id, _)) = tcx.entry_fn(())
+                && tcx.has_attr(def_id, sym::rustc_delayed_bug_from_inside_query)
+            {
+                tcx.ensure_ok().trigger_delayed_bug(def_id);
+            }
+
+            // Don't emit metadata or do codegen if there were any errors.
+            //
+            // Note: checking for delayed bugs here is aggressive. If we have
+            // no errors but one or more delayed bugs, the `raise_fatal` call
+            // will instantly ICE when the DiagCtxt is dropped. I.e. this check
+            // serves as a kind of barrier, giving no opportunity for a
+            // subsequent error (e.g. during codegen) to nullify a delayed bug.
+            // The rationale is that delayed bugs are likely to cause more ICEs
+            // during codegen, obscuring the original problem.
+            if let Some(guar) = tcx.sess.dcx().has_errors_or_delayed_bugs() {
+                guar.raise_fatal();
+            }
+
+            let metadata = rustc_metadata::fs::encode_and_write_metadata(tcx);
+
+            Some(Linker::codegen_and_build_linker(tcx, &*compiler.codegen_backend, metadata))
         });
 
         // Linking is done outside the `compiler.enter()` so that the

compiler/rustc_interface/src/passes.rs

@@ -21,7 +21,6 @@ use rustc_hir::def_id::{LOCAL_CRATE, StableCrateId, StableCrateIdMap};
 use rustc_hir::definitions::Definitions;
 use rustc_incremental::setup_dep_graph;
 use rustc_lint::{BufferedEarlyLint, EarlyCheckNode, LintStore, unerased_lint_store};
-use rustc_metadata::EncodedMetadata;
 use rustc_metadata::creader::CStore;
 use rustc_middle::arena::Arena;
 use rustc_middle::dep_graph::DepsType;
@@ -1196,33 +1195,17 @@ fn analysis(tcx: TyCtxt<'_>, (): ()) {
 pub(crate) fn start_codegen<'tcx>(
     codegen_backend: &dyn CodegenBackend,
     tcx: TyCtxt<'tcx>,
-) -> (Box<dyn Any>, EncodedMetadata) {
+) -> Box<dyn Any> {
     tcx.sess.timings.start_section(tcx.sess.dcx(), TimingSection::Codegen);
 
-    // Hook for tests.
-    if let Some((def_id, _)) = tcx.entry_fn(())
-        && tcx.has_attr(def_id, sym::rustc_delayed_bug_from_inside_query)
-    {
-        tcx.ensure_ok().trigger_delayed_bug(def_id);
-    }
-
     // Don't run this test assertions when not doing codegen. Compiletest tries to build
     // build-fail tests in check mode first and expects it to not give an error in that case.
     if tcx.sess.opts.output_types.should_codegen() {
         rustc_symbol_mangling::test::report_symbol_names(tcx);
     }
 
-    // Don't do code generation if there were any errors. Likewise if
-    // there were any delayed bugs, because codegen will likely cause
-    // more ICEs, obscuring the original problem.
-    if let Some(guar) = tcx.sess.dcx().has_errors_or_delayed_bugs() {
-        guar.raise_fatal();
-    }
-
     info!("Pre-codegen\n{:?}", tcx.debug_stats());
 
-    let metadata = rustc_metadata::fs::encode_and_write_metadata(tcx);
-
     let codegen = tcx.sess.time("codegen_crate", move || codegen_backend.codegen_crate(tcx));
 
     info!("Post-codegen\n{:?}", tcx.debug_stats());
@@ -1233,7 +1216,7 @@ pub(crate) fn start_codegen<'tcx>(
         tcx.sess.code_stats.print_type_sizes();
     }
 
-    (codegen, metadata)
+    codegen
 }
 
 /// Compute and validate the crate name.

compiler/rustc_interface/src/queries.rs

@@ -28,8 +28,9 @@ impl Linker {
     pub fn codegen_and_build_linker(
         tcx: TyCtxt<'_>,
         codegen_backend: &dyn CodegenBackend,
+        metadata: EncodedMetadata,
     ) -> Linker {
-        let (ongoing_codegen, metadata) = passes::start_codegen(codegen_backend, tcx);
+        let ongoing_codegen = passes::start_codegen(codegen_backend, tcx);
 
         Linker {
             dep_graph: tcx.dep_graph.clone(),

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -2274,6 +2274,8 @@ fn prefetch_mir(tcx: TyCtxt<'_>) {
 // will allow us to slice the metadata to the precise length that we just
 // generated regardless of trailing bytes that end up in it.
 
+// The `Default` impl is for tests.
+#[derive(Default)]
 pub struct EncodedMetadata {
     // The declaration order matters because `full_metadata` should be dropped
     // before `_temp_dir`.

tests/ui-fulldeps/run-compiler-twice.rs

@@ -12,6 +12,7 @@
 
 extern crate rustc_driver;
 extern crate rustc_interface;
+extern crate rustc_metadata;
 extern crate rustc_session;
 extern crate rustc_span;
 
@@ -81,7 +82,8 @@ fn compile(code: String, output: PathBuf, sysroot: Sysroot, linker: Option<&Path
         let krate = rustc_interface::passes::parse(&compiler.sess);
         let linker = rustc_interface::create_and_enter_global_ctxt(&compiler, krate, |tcx| {
             let _ = tcx.analysis(());
-            Linker::codegen_and_build_linker(tcx, &*compiler.codegen_backend)
+            let metadata = Default::default();
+            Linker::codegen_and_build_linker(tcx, &*compiler.codegen_backend, metadata)
         });
         linker.link(&compiler.sess, &*compiler.codegen_backend);
     });