ParseSess which holds state living beyond what one
It also serves as an input to the parser itself.
Collected spans during parsing for places where a certain feature was
used and should be feature gated accordingly in
Info about a parsing session.
Construct a diagnostic for a language feature error due to the given
Construct a diagnostic for a feature gate error.
The set of keys (and, optionally, values) that define the compilation environment of the crate, used to drive conditional compilation.