Auto merge of #59953 - eddyb:soa-metadata, r=<try>

 rustc_metadata: replace Entry table with one table for each of its fields (AoS -> SoA).

*Based on top of #59887*

In #59789 (comment) I noticed that for many cross-crate queries (e.g. `predicates_of(def_id)`), we were deserializing the `rustc_metadata::schema::Entry` for `def_id` *only* to read one field (i.e. `predicates`).

But there are several such queries, and `Entry` is not particularly small (in terms of number of fields, the encoding itself is quite compact), so there is a large (and unnecessary) constant factor.

This PR replaces the (random-access) array¹ of `Entry` structures ("AoS"), with many separate arrays¹, one for each field that used to be in `Entry` ("SoA"), resulting in the ability to read individual fields separately, with negligible time overhead (in thoery), and some size overhead (as these arrays are not sparse).

In a way, the new approach is closer to incremental on-disk caches, which store each query's cached results separately, but it would take significantly more work to unify the two.

For stage1 `libcore`'s metadata blob, the size overhead is `8.44%`, and I have another commit (not initially included in this PR because I want to do perf runs with both) that brings it down to `5.88%`.

¹(in the source, these arrays are called "tables", but perhaps they could use a better name)

src/librustc_metadata/creader.rs

@@ -162,7 +162,7 @@ impl<'a> CrateLoader<'a> {
 
     fn verify_no_symbol_conflicts(&self,
                                   span: Span,
-                                  root: &CrateRoot) {
+                                  root: &CrateRoot<'_>) {
         // Check for (potential) conflicts with the local crate
         if self.local_crate_name == root.name &&
            self.sess.local_crate_disambiguator() == root.disambiguator {
@@ -467,7 +467,7 @@ impl<'a> CrateLoader<'a> {
     // Go through the crate metadata and load any crates that it references
     fn resolve_crate_deps(&mut self,
                           root: &Option<CratePaths>,
-                          crate_root: &CrateRoot,
+                          crate_root: &CrateRoot<'_>,
                           metadata: &MetadataBlob,
                           krate: CrateNum,
                           span: Span,
@@ -573,7 +573,7 @@ impl<'a> CrateLoader<'a> {
     /// implemented as dynamic libraries, but we have a possible future where
     /// custom derive (and other macro-1.1 style features) are implemented via
     /// executables and custom IPC.
-    fn load_derive_macros(&mut self, root: &CrateRoot, dylib: Option<PathBuf>, span: Span)
+    fn load_derive_macros(&mut self, root: &CrateRoot<'_>, dylib: Option<PathBuf>, span: Span)
                           -> Vec<(ast::Name, Lrc<SyntaxExtension>)> {
         use std::{env, mem};
         use crate::dynamic_lib::DynamicLibrary;

src/librustc_metadata/cstore.rs

@@ -64,7 +64,11 @@ pub struct CrateMetadata {
     /// Used for decoding interpret::AllocIds in a cached & thread-safe manner.
     pub alloc_decoding_state: AllocDecodingState,
 
-    pub root: schema::CrateRoot,
+    // NOTE(eddyb) we pass `'static` to a `'tcx` parameter because this
+    // lifetime is only used behind `Lazy`, and therefore acts like an
+    // universal (`for<'tcx>`), that is paired up with whichever `TyCtxt`
+    // is being used to decode those values.
+    pub root: schema::CrateRoot<'static>,
 
     /// For each public item in this crate, we encode a key. When the
     /// crate is loaded, we read all the keys and put them in this
@@ -73,7 +77,7 @@ pub struct CrateMetadata {
     /// compilation support.
     pub def_path_table: Lrc<DefPathTable>,
 
-    pub trait_impls: FxHashMap<(u32, DefIndex), schema::LazySeq<DefIndex>>,
+    pub trait_impls: FxHashMap<(u32, DefIndex), schema::Lazy<[DefIndex]>>,
 
     pub dep_kind: Lock<DepKind>,
     pub source: CrateSource,

src/librustc_metadata/cstore_impl.rs

@@ -244,12 +244,7 @@ provide! { <'tcx> tcx, def_id, other, cdata,
 
     used_crate_source => { Lrc::new(cdata.source.clone()) }
 
-    exported_symbols => {
-        let cnum = cdata.cnum;
-        assert!(cnum != LOCAL_CRATE);
-
-        Arc::new(cdata.exported_symbols(tcx))
-    }
+    exported_symbols => { Arc::new(cdata.exported_symbols(tcx)) }
 }
 
 pub fn provide<'tcx>(providers: &mut Providers<'tcx>) {