The Uses, Defs and Scope information provided by go/types is
inadequate for answering "what if?" queries about the
structure of the lexical environment.
In this code, for example,
var x int
func f() {
print(x)
x := ""
print(x)
}
the two referring Idents x appear at the same lexical depth,
inside the function f's Scope object, yet they resolve to
different objects.
This package associates a lexical.Environment instance with
every reference to capture these differences. Each
environment is a linked list of enclosing Blocks, and for each
block, a number indicating what prefix of its bindings are
visible. (Zero for the first 'x' reference above, 1 for the
second.)
+ Smoke test over stdlib.
This functionality could be integrated with the type checker
in lieu of the not-so-useful types.Info.Scopes data, at little
extra cost in code or in running time/space. We should talk
about that.
LGTM=sameer
R=gri, sameer
CC=golang-codereviews
https://golang.org/cl/143790043
