Reorder MIR encoding checks

Start from least expensive checks when deciding whether to encode MIR or not.

No functional changes intended.

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -66,6 +66,11 @@ pub(super) struct EncodeContext<'a, 'tcx> {
     required_source_files: Option<GrowableBitSet<usize>>,
     is_proc_macro: bool,
     hygiene_ctxt: &'a HygieneEncodeContext,
+
+    // Determines if MIR used for code generation will be included in the crate
+    // metadata. When emitting only metadata (e.g., cargo check), we can avoid
+    // generating optimized MIR altogether.
+    emit_codegen_mir: bool,
 }
 
 /// If the current crate is a proc-macro, returns early with `Lazy:empty()`.
@@ -1032,11 +1037,6 @@ impl EncodeContext<'a, 'tcx> {
         }
     }
 
-    fn metadata_output_only(&self) -> bool {
-        // MIR optimisation can be skipped when we're just interested in the metadata.
-        !self.tcx.sess.opts.output_types.should_codegen()
-    }
-
     fn encode_info_for_impl_item(&mut self, def_id: DefId) {
         debug!("EncodeContext::encode_info_for_impl_item({:?})", def_id);
         let tcx = self.tcx;
@@ -1105,13 +1105,12 @@ impl EncodeContext<'a, 'tcx> {
         let (mir, mir_const) = match ast_item.kind {
             hir::ImplItemKind::Const(..) => (false, true),
             hir::ImplItemKind::Fn(ref sig, _) => {
-                let generics = self.tcx.generics_of(def_id);
-                let needs_inline = (generics.requires_monomorphization(self.tcx)
-                    || tcx.codegen_fn_attrs(def_id).requests_inline())
-                    && !self.metadata_output_only();
+                let opt_mir = tcx.sess.opts.debugging_opts.always_encode_mir
+                    || (self.emit_codegen_mir
+                        && (tcx.generics_of(def_id).requires_monomorphization(tcx)
+                            || tcx.codegen_fn_attrs(def_id).requests_inline()));
                 let is_const_fn = sig.header.constness == hir::Constness::Const;
-                let always_encode_mir = self.tcx.sess.opts.debugging_opts.always_encode_mir;
-                (needs_inline || always_encode_mir, is_const_fn)
+                (opt_mir, is_const_fn)
             }
             hir::ImplItemKind::TyAlias(..) => (false, false),
         };
@@ -1433,16 +1432,13 @@ impl EncodeContext<'a, 'tcx> {
         let (mir, const_mir) = match item.kind {
             hir::ItemKind::Static(..) | hir::ItemKind::Const(..) => (false, true),
             hir::ItemKind::Fn(ref sig, ..) => {
-                let generics = tcx.generics_of(def_id);
-                let needs_inline = (generics.requires_monomorphization(tcx)
-                    || tcx.codegen_fn_attrs(def_id).requests_inline())
-                    && !self.metadata_output_only();
-
+                let opt_mir = self.tcx.sess.opts.debugging_opts.always_encode_mir
+                    || (self.emit_codegen_mir
+                        && (tcx.generics_of(def_id).requires_monomorphization(tcx)
+                            || tcx.codegen_fn_attrs(def_id).requests_inline()));
                 let is_const_fn = sig.header.constness == hir::Constness::Const;
-                let always_encode_mir = self.tcx.sess.opts.debugging_opts.always_encode_mir;
-                let mir = needs_inline || always_encode_mir;
                 // We don't need the optimized MIR for const fns.
-                (mir, is_const_fn)
+                (opt_mir, is_const_fn)
             }
             _ => (false, false),
         };
@@ -2008,10 +2004,9 @@ impl<'tcx, 'v> ParItemLikeVisitor<'v> for PrefetchVisitor<'tcx> {
             }
             hir::ItemKind::Fn(ref sig, ..) => {
                 let def_id = tcx.hir().local_def_id(item.hir_id);
-                let generics = tcx.generics_of(def_id.to_def_id());
-                let needs_inline = generics.requires_monomorphization(tcx)
+                let opt_mir = tcx.generics_of(def_id.to_def_id()).requires_monomorphization(tcx)
                     || tcx.codegen_fn_attrs(def_id.to_def_id()).requests_inline();
-                if needs_inline {
+                if opt_mir {
                     self.prefetch_mir(def_id)
                 }
                 if sig.header.constness == hir::Constness::Const {
@@ -2045,11 +2040,10 @@ impl<'tcx, 'v> ParItemLikeVisitor<'v> for PrefetchVisitor<'tcx> {
             }
             hir::ImplItemKind::Fn(ref sig, _) => {
                 let def_id = tcx.hir().local_def_id(impl_item.hir_id);
-                let generics = tcx.generics_of(def_id.to_def_id());
-                let needs_inline = generics.requires_monomorphization(tcx)
+                let opt_mir = tcx.generics_of(def_id.to_def_id()).requires_monomorphization(tcx)
                     || tcx.codegen_fn_attrs(def_id.to_def_id()).requests_inline();
                 let is_const_fn = sig.header.constness == hir::Constness::Const;
-                if needs_inline {
+                if opt_mir {
                     self.prefetch_mir(def_id)
                 }
                 if is_const_fn {
@@ -2148,6 +2142,7 @@ fn encode_metadata_impl(tcx: TyCtxt<'_>) -> EncodedMetadata {
         required_source_files,
         is_proc_macro: tcx.sess.crate_types().contains(&CrateType::ProcMacro),
         hygiene_ctxt: &hygiene_ctxt,
+        emit_codegen_mir: tcx.sess.opts.output_types.should_codegen(),
     };
 
     // Encode the rustc version string in a predictable location.