Auto merge of rust-lang#13222 - BorysMinaiev:master, r=flodiebold

Remove redundant 'resolve_obligations_as_possible' call

Hi! I was looking for a "good first issue" and saw this one: rust-lang/rust-analyzer#7542. I like searching for performance improvements, so I wanted to try to find something useful there.

There are two tests in integrated_benchmarks.rs, I looked at 'integrated_highlighting_benchmark' (not the one discussed in the issue above).

Profile from that test looks like this:
```
$ RUN_SLOW_BENCHES=1 cargo test --release --package rust-analyzer --lib -- integrated_benchmarks::integrated_highlighting_benchmark --exact --nocapture
    Finished release [optimized] target(s) in 0.06s
     Running unittests src/lib.rs (target/release/deps/rust_analyzer-a80ca6bb8f877458)

running 1 test
workspace loading: 358.45ms
initial: 9.60s
change: 13.96µs
cpu profiling is disabled, uncomment `default = [ "cpu_profiler" ]` in Cargo.toml to enable.
  273ms - highlight
      143ms - infer:wait @ per_query_memory_usage
          143ms - infer_query
                0   - crate_def_map:wait (3165 calls)
                4ms - deref_by_trait (967 calls)
               96ms - resolve_obligations_as_possible (22106 calls)
                0   - trait_solve::wait (2068 calls)
       21ms - Semantics::analyze_impl (18 calls)
        0   - SourceBinder::to_module_def (20 calls)
       36ms - classify_name (19 calls)
       19ms - classify_name_ref (308 calls)
        0   - crate_def_map:wait (461 calls)
        4ms - descend_into_macros (628 calls)
        0   - generic_params_query (4 calls)
        0   - impl_data_with_diagnostics_query (1 calls)
       45ms - infer:wait (37 calls)
        0   - resolve_obligations_as_possible (2 calls)
        0   - source_file_to_def (1 calls)
        0   - trait_solve::wait (42 calls)
after change: 275.23ms
test integrated_benchmarks::integrated_highlighting_benchmark ... ok
```
22106 calls to `resolve_obligations_as_possible` seem like the main issue there.

One thing I noticed (and fixed in this PR) is that `InferenceContext::resolve_ty_shallow` first calls `resolve_obligations_as_possible`, and then calls `InferenceTable::resolve_ty_shallow`. But `InferenceTable::resolve_ty_shallow` [inside](https://github.com/rust-lang/rust-analyzer/blob/2e9f1204ca01c3e20898d4a67c8b84899d394a88/crates/hir-ty/src/infer/unify.rs#L372) again calls `resolve_obligations_as_possible`.

`resolve_obligations_as_possible` inside has a while loop, which works until it can't find any helpful information. So calling this function for the second time does nothing, so one of the calls could be safely removed.

`InferenceContext::resolve_ty_shallow` is actually quite a hot place, and after fixing it, the total number of `resolve_obligations_as_possible` in this test is reduced to 15516 (from 22106). "After change" time also improves from ~270ms to ~240ms, which is not a very huge win, but still something measurable.

Same profile after PR:
```
$ RUN_SLOW_BENCHES=1 cargo test --release --package rust-analyzer --lib -- integrated_benchmarks::integrated_highlighting_benchmark --exact --nocapture
    Finished release [optimized] target(s) in 0.06s
     Running unittests src/lib.rs (target/release/deps/rust_analyzer-a80ca6bb8f877458)

running 1 test
workspace loading: 339.86ms
initial: 9.28s
change: 10.69µs
cpu profiling is disabled, uncomment `default = [ "cpu_profiler" ]` in Cargo.toml to enable.
  236ms - highlight
      110ms - infer:wait @ per_query_memory_usage
          110ms - infer_query
                0   - crate_def_map:wait (3165 calls)
                4ms - deref_by_trait (967 calls)
               64ms - resolve_obligations_as_possible (15516 calls)
                0   - trait_solve::wait (2068 calls)
       21ms - Semantics::analyze_impl (18 calls)
        0   - SourceBinder::to_module_def (20 calls)
       34ms - classify_name (19 calls)
       18ms - classify_name_ref (308 calls)
        0   - crate_def_map:wait (461 calls)
        3ms - descend_into_macros (628 calls)
        0   - generic_params_query (4 calls)
        0   - impl_data_with_diagnostics_query (1 calls)
       45ms - infer:wait (37 calls)
        0   - resolve_obligations_as_possible (2 calls)
        0   - source_file_to_def (1 calls)
        0   - trait_solve::wait (42 calls)
after change: 238.15ms
test integrated_benchmarks::integrated_highlighting_benchmark ... ok
```

The performance of this test could be further improved but at the cost of making code more complicated, so I wanted to check if such a change is desirable before sending another PR.

`resolve_obligations_as_possible` is actually called a lot of times even when no new information was provided. As I understand, `resolve_obligations_as_possible` could do something useful only if some variables/values were unified since the last check. We can store a boolean variable inside `InferenceTable`, which indicates if `try_unify` was called after last `resolve_obligations_as_possible`. If it wasn't called, we can safely not call `resolve_obligations_as_possible` again.

I tested this change locally, and it reduces the number of `resolve_obligations_as_possible` to several thousand (it is not shown in the profile anymore, so don't know the exact number), and the total time is reduced to ~180ms. Here is a generated profile:
```
$ RUN_SLOW_BENCHES=1 cargo test --release --package rust-analyzer --lib -- integrated_benchmarks::integrated_highlighting_benchmark --exact --nocapture
    Finished release [optimized] target(s) in 0.06s
     Running unittests src/lib.rs (target/release/deps/rust_analyzer-a80ca6bb8f877458)

running 1 test
workspace loading: 349.92ms
initial: 8.56s
change: 11.32µs
cpu profiling is disabled, uncomment `default = [ "cpu_profiler" ]` in Cargo.toml to enable.
  175ms - highlight
       21ms - Semantics::analyze_impl (18 calls)
        0   - SourceBinder::to_module_def (20 calls)
       33ms - classify_name (19 calls)
       17ms - classify_name_ref (308 calls)
        0   - crate_def_map:wait (461 calls)
        3ms - descend_into_macros (628 calls)
        0   - generic_params_query (4 calls)
        0   - impl_data_with_diagnostics_query (1 calls)
       97ms - infer:wait (38 calls)
        0   - resolve_obligations_as_possible (2 calls)
        0   - source_file_to_def (1 calls)
        0   - trait_solve::wait (42 calls)
after change: 177.04ms
test integrated_benchmarks::integrated_highlighting_benchmark ... ok
```
Let me know if adding a new bool field seems like a reasonable tradeoff, so I can send a PR.

crates/hir-ty/src/infer.rs

@@ -673,10 +673,6 @@ impl<'a> InferenceContext<'a> {
         )
     }
 
-    fn resolve_obligations_as_possible(&mut self) {
-        self.table.resolve_obligations_as_possible();
-    }
-
     fn push_obligation(&mut self, o: DomainGoal) {
         self.table.register_obligation(o.cast(Interner));
     }
@@ -696,7 +692,6 @@ impl<'a> InferenceContext<'a> {
     }
 
     fn resolve_ty_shallow(&mut self, ty: &Ty) -> Ty {
-        self.resolve_obligations_as_possible();
         self.table.resolve_ty_shallow(ty)
     }