Only compute the crate hash when necessary.

The crate hash is needed:
- if debug assertions are enabled, or
- if incr. comp. is enabled, or
- if metadata is being generated, or
- if `-C instrumentation-coverage` is enabled.

This commit avoids computing the crate hash when these conditions are
all false, such as when doing a release build of a binary crate.

It uses `Option` to store the hashes when needed, rather than
computing them on demand, because some of them are needed in multiple
places and computing them on demand would make compilation slower.

The commit also removes `Owner::hash_without_bodies`. There is no
benefit to pre-computing that one, it can just be done in the normal
fashion.

compiler/rustc_session/src/session.rs

@@ -224,6 +224,13 @@ pub struct PerfStats {
     pub normalize_projection_ty: AtomicUsize,
 }
 
+#[derive(PartialEq, Eq, PartialOrd, Ord)]
+pub enum MetadataKind {
+    None,
+    Uncompressed,
+    Compressed,
+}
+
 impl Session {
     pub fn miri_unleashed_feature(&self, span: Span, feature_gate: Option<Symbol>) {
         self.miri_unleashed_features.lock().push((span, feature_gate));
@@ -287,6 +294,38 @@ impl Session {
         self.crate_types.get().unwrap().as_slice()
     }
 
+    pub fn needs_crate_hash(&self) -> bool {
+        // Why is the crate hash needed for these configurations?
+        // - debug_assertions: for the "fingerprint the result" check in
+        //   `rustc_query_system::query::plumbing::execute_job`.
+        // - incremental: for query lookups.
+        // - needs_metadata: for putting into crate metadata.
+        // - instrument_coverage: for putting into coverage data (see
+        //   `hash_mir_source`).
+        cfg!(debug_assertions)
+            || self.opts.incremental.is_some()
+            || self.needs_metadata()
+            || self.instrument_coverage()
+    }
+
+    pub fn metadata_kind(&self) -> MetadataKind {
+        self.crate_types()
+            .iter()
+            .map(|ty| match *ty {
+                CrateType::Executable | CrateType::Staticlib | CrateType::Cdylib => {
+                    MetadataKind::None
+                }
+                CrateType::Rlib => MetadataKind::Uncompressed,
+                CrateType::Dylib | CrateType::ProcMacro => MetadataKind::Compressed,
+            })
+            .max()
+            .unwrap_or(MetadataKind::None)
+    }
+
+    pub fn needs_metadata(&self) -> bool {
+        self.metadata_kind() != MetadataKind::None
+    }
+
     pub fn init_crate_types(&self, crate_types: Vec<CrateType>) {
         self.crate_types.set(crate_types).expect("`crate_types` was initialized twice")
     }