Auto merge of #116505 - saethlin:infer-inline, r=cjgillot

Automatically enable cross-crate inlining for small functions

This is basically reviving #70550

The `#[inline]` attribute can have a significant impact on code generation or runtime performance (because it enables inlining between CGUs where it would normally not happen) and also on compile-time performance (because it enables MIR inlining). But it has to be added manually, which is awkward.

This PR factors whether a DefId is cross-crate inlinable into a query, and replaces all uses of `CodegenFnAttrs::requests_inline` with this new query. The new query incorporates all the other logic that is used to determine whether a Def should be treated as cross-crate-inlinable, and as a last step inspects the function's optimized_mir to determine if it should be treated as cross-crate-inlinable.

The heuristic implemented here is deliberately conservative; we only infer inlinability for functions whose optimized_mir does not contain any calls or asserts. I plan to study adjusting the cost model later, but for now the compile time implications of this change are so significant that I think this very crude heuristic is well worth landing.

Author: bors

compiler/rustc_interface/src/tests.rs

@@ -770,6 +770,7 @@ fn test_unstable_options_tracking_hash() {
     );
     tracked!(codegen_backend, Some("abc".to_string()));
     tracked!(crate_attr, vec!["abc".to_string()]);
+    tracked!(cross_crate_inline_threshold, Some(200));
     tracked!(debug_info_for_profiling, true);
     tracked!(debug_macros, true);
     tracked!(dep_info_omit_d_target, true);

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1273,6 +1273,10 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         self.root.tables.optimized_mir.get(self, id).is_some()
     }
 
+    fn cross_crate_inlinable(self, id: DefIndex) -> bool {
+        self.root.tables.cross_crate_inlinable.get(self, id).unwrap_or(false)
+    }
+
     fn get_fn_has_self_parameter(self, id: DefIndex, sess: &'a Session) -> bool {
         self.root
             .tables