GetTypeByMetadataName would benefit greatly from non-alloc optimizations.

[CyrusNajmabadi]

I'm seeing enormous allocations in GetTypeByMetadataName, exacerbated by how many analyzers that run now that need to look up special types. When typing in Roslyn.sln i see the following for example:

This is 600 MB of allocs of strings while just typing a few characters. The core issue here is that we are being fed in large named strings, and continually breaking them into tiny garbage substrings:

Moving to an approach where we used ReadOnlyMemory<char> here to refer to sub spans of the string could be greatly beneficial. I'm suggesting ROM over Span here as lots of places where we need these substrings are when looking up into maps currently keyed by strings. Span won't work there given it's ref-struct nature. But ROM would be a viable option there to keep things working.

[CyrusNajmabadi]

Tagging @jaredpar . Currently the #1 hitter for memory allocs in roslyn. Almost 10% of all allocs during typing is just this :-o

[jcouv]

Assigned to @CyrusNajmabadi since he's already working on a solution. Thanks!