structurally resolve how does it work

[lcnr]

TODO: summarize and refer to the relevant code from rust-lang/rust#110204

	Projection goal old	Projection goal new	structurally resolve new	structurally resolve new
success incl nested obl	normalizes to	normalizes to	normalizes	normalizes
success shallow, ambig nested	normalizes to	normalizes to	normalizes	normalizes
success shallow, err nested	normalizes to -> fulfill err1	stays as projection	stays as projection	stays as projection
ambig shallow	ambig	normalizes to (ambig)	error because infer var	error because infer var
err shallow	unnormalized	unnormalized	stays as projection	stays as projection

Footnotes

1. if this happens we fail anyways as the projection would not be well-formed. ↩