Auto merge of rust-lang#15436 - Veykril:temp-alloc, r=Veykril

Preserve `resolve_obligations_as_possible` temporary allocation across calls

This saves ~20ms in the highlihting bench on my machine
```
  284ms - highlight
      150ms - infer:wait @ per_query_memory_usage
          150ms - infer_query
                0   - PerNs::filter_visibility (436 calls)
                0   - crate_def_map:wait (336 calls)
                2ms - deref_by_trait (909 calls)
                0   - generic_params_query (1 calls)
                0   - inherent_impls_in_block_query (1 calls)
              107ms - resolve_obligations_as_possible (17013 calls)
                0   - trait_solve::wait (1017 calls)
        0   - PerNs::filter_visibility (13 calls)
       17ms - Semantics::analyze_impl (19 calls)
        0   - SourceBinder::to_module_def (30 calls)
        0   - attrs_query (6 calls)
        0   - classify_lifetime (1 calls)
        0   - classify_lifetime_ref (4 calls)
       35ms - classify_name (28 calls)
       54ms - classify_name_ref (452 calls)
        0   - crate_def_map:wait (375 calls)
        7ms - descend_into_macros (776 calls)
        0   - generic_params_query (5 calls)
        0   - impl_data_with_diagnostics_query (1 calls)
       17ms - infer:wait (32 calls)
        0   - resolve_obligations_as_possible (1 calls)
        0   - source_file_to_def (1 calls)
        0   - trait_solve::wait (1 calls)
```
to
```

  256ms - highlight
      121ms - infer:wait @ per_query_memory_usage
          121ms - infer_query
                0   - PerNs::filter_visibility (436 calls)
                0   - crate_def_map:wait (336 calls)
                2ms - deref_by_trait (909 calls)
                0   - generic_params_query (1 calls)
                0   - inherent_impls_in_block_query (1 calls)
               81ms - resolve_obligations_as_possible (17013 calls)
                0   - trait_solve::wait (1017 calls)
        0   - PerNs::filter_visibility (13 calls)
       17ms - Semantics::analyze_impl (19 calls)
        0   - SourceBinder::to_module_def (30 calls)
        0   - attrs_query (6 calls)
        0   - classify_lifetime (1 calls)
        0   - classify_lifetime_ref (4 calls)
       35ms - classify_name (28 calls)
       56ms - classify_name_ref (452 calls)
        0   - crate_def_map:wait (375 calls)
        7ms - descend_into_macros (776 calls)
        0   - generic_params_query (5 calls)
        0   - impl_data_with_diagnostics_query (1 calls)
       16ms - infer:wait (32 calls)
        0   - resolve_obligations_as_possible (1 calls)
        0   - source_file_to_def (1 calls)
        0   - trait_solve::wait (1 calls)
```

Author: bors

crates/hir-ty/src/infer/unify.rs

@@ -143,6 +143,9 @@ pub(crate) struct InferenceTable<'a> {
     var_unification_table: ChalkInferenceTable,
     type_variable_table: Vec<TypeVariableFlags>,
     pending_obligations: Vec<Canonicalized<InEnvironment<Goal>>>,
+    /// Double buffer used in [`Self::resolve_obligations_as_possible`] to cut down on
+    /// temporary allocations.
+    resolve_obligations_buffer: Vec<Canonicalized<InEnvironment<Goal>>>,
 }
 
 pub(crate) struct InferenceTableSnapshot {
@@ -159,6 +162,7 @@ impl<'a> InferenceTable<'a> {
             var_unification_table: ChalkInferenceTable::new(),
             type_variable_table: Vec::new(),
             pending_obligations: Vec::new(),
+            resolve_obligations_buffer: Vec::new(),
         }
     }
 
@@ -510,10 +514,10 @@ impl<'a> InferenceTable<'a> {
     pub(crate) fn resolve_obligations_as_possible(&mut self) {
         let _span = profile::span("resolve_obligations_as_possible");
         let mut changed = true;
-        let mut obligations = Vec::new();
-        while changed {
-            changed = false;
+        let mut obligations = mem::take(&mut self.resolve_obligations_buffer);
+        while mem::take(&mut changed) {
             mem::swap(&mut self.pending_obligations, &mut obligations);
+
             for canonicalized in obligations.drain(..) {
                 if !self.check_changed(&canonicalized) {
                     self.pending_obligations.push(canonicalized);
@@ -528,6 +532,8 @@ impl<'a> InferenceTable<'a> {
                 self.register_obligation_in_env(uncanonical);
             }
         }
+        self.resolve_obligations_buffer = obligations;
+        self.resolve_obligations_buffer.clear();
     }
 
     pub(crate) fn fudge_inference<T: TypeFoldable<Interner>>(