proposal: x/tools/go/ast/astutil: PathContainingPos

[cszczepaniak]

Proposal Details

I sometimes find myself writing static analysis tools which load packages and iterate something other than the syntax (like pkg.TypesInfo.Defs) to do some analysis. Sometimes in the course of such analysis, it's useful to map some position (perhaps from a types.Object) to its path in the AST.

The best way I've found to do this is to first find the containing *ast.File by iterating the fileset and checking the position in question against the file's Pos() and End(), then subsequently calling astutil.PathEnclosingInterval to find the path within the file.

I propose the addition of a function that combines these two operations to make this more ergonomic:

// PathContainingPos returns the node that encloses the source
// position and all its ancestors up to the AST root.
func PathContainingPos(fset *token.FileSet, pos token.Position) []ast.Node

[mateusz834]

It also needs an *ast.File in params, it probably should take a token.Pos instead and i think that the fset is not needed, if we pass it a token.Pos.

// PathContainingPos returns the node that encloses the source
// position and all its ancestors up to the AST root.
-func PathContainingPos(fset *token.FileSet, pos token.Position) []ast.Node
+func PathContainingPos(root *ast.File, pos token.Pos) []ast.Node

[adonovan]

We recently published a mechanism that makes this kind of navigation easy: inspector.Cursor. Essentially a cursor represents a Node, but in the context of a traversal, so you can move not only down, but also up, left, and right from it to its neighbors. Use FindPos(pos, end) on the root cursor for a given package to find the largest node within that (pos, end) range.

You may also be interested in the typeindex analyzer, which builds an index mapping objects to their definition (a Cursor) and uses (a list of Cursors). This makes it extremely efficient to visit just, say, the three places that call fmt.Printf, instead of having to visit the whole tree for a package. It's internal, but you can easily fork it.