Use a codeunit type-assert that is easier for inference to elide

[timholy]

Circumvents #37135

[omus]

I was working on a performance improvement for ==(::AbstractString, ::AbstractString) but I discovered this PR addresses the performance issue I was noticing (mostly allocations):

Before this PR (6aee988):

julia> str = "A" ^ 1000;

julia> @btime invoke(==, Tuple{AbstractString, AbstractString}, $"ABC", $"ABC")
  4.357 ns (0 allocations: 0 bytes)
true

julia> @btime $(SubString("ABC")) == $"ABCD"
  29.574 ns (2 allocations: 64 bytes)
false

julia> @btime $(SubString("ABC")) == $"ABC"
  32.492 ns (2 allocations: 64 bytes)
true

julia> @btime $(SubString("ABC")) == $"DEF"
  31.985 ns (2 allocations: 64 bytes)
false

julia> @btime $(SubString(str)) == $str
  65.086 ns (2 allocations: 64 bytes)
true

With this PR (dbf5e00):

julia> str = "A" ^ 1000;

julia> @btime invoke(==, Tuple{AbstractString, AbstractString}, $"ABC", $"ABC")
  3.907 ns (0 allocations: 0 bytes)
true

julia> @btime $(SubString("ABC")) == $"ABCD"
  0.033 ns (0 allocations: 0 bytes)
false

julia> @btime $(SubString("ABC")) == $"ABC"
  4.003 ns (0 allocations: 0 bytes)
true

julia> @btime $(SubString("ABC")) == $"DEF"
  3.282 ns (0 allocations: 0 bytes)
false

julia> @btime $(SubString(str)) == $str
  38.978 ns (0 allocations: 0 bytes)
true

I'll be adding some benchmarks for this.

[KristofferC]

Is this v1.5? Because AFAIU this just fixed an earlier regression on nightly. But good to have benchmarks anyway.

[omus]

I noticed similar slow performance on Julia 1.3.1, 1.4.2, and 1.5.1. All of these versions were substantially worse with the final test:

julia> @btime $(SubString(str)) == $str
  4.043 μs (2 allocations: 112 bytes)
true

[KristofferC]

Seems more likely that the improvement you see was because of #35973

[omus]

You are correct that #35973 is the main driver behind the performance improvement. I'll update my benchmarks to reflect this.

[omus]

I've also opened #37467 which improves performance of the fallback ==(::AbstractString, ::AbstractString) call.