PathEnclosingInterval returns the node that encloses the source interval [start, end), and all its ancestors up to the AST root.
The definition of "enclosing" used by this function considers additional whitespace abutting a node to be enclosed by it. In this example:
z := x + y // add them <-A-> <----B----->
the ast.BinaryExpr(+) node is considered to enclose interval B even though its [Pos()..End()) is actually only interval A. This behaviour makes user interfaces more tolerant of imperfect input.
This function treats tokens as nodes, though they are not included in the result. e.g. PathEnclosingInterval("+") returns the enclosing ast.BinaryExpr("x + y").
If start==end, the 1-char interval following start is used instead.
The 'exact' result is true if the interval contains only path and perhaps some adjacent whitespace. It is false if the interval overlaps multiple children of path, or if it contains only interior whitespace of path. In this example:
z := x + y // add them <--C--> <---E--> ^ D
intervals C, D and E are inexact. C is contained by the z-assignment statement, because it spans three of its children (:=, x, +). So too is the 1-char interval D, because it contains only interior whitespace of the assignment. E is considered interior whitespace of the BlockStmt containing the assignment.
Precondition: [start, end) both lie within the same file as root. TODO(adonovan): return (nil, false) in this case and remove precond. Requires FileSet; see loader.tokenFileContainsPos.
Postcondition: path is never nil; it always contains at least 'root'.