x/tools/go/ast/inspector: add (*Cursor).Valid() bool method

[adonovan]

Background: When using inspector.Cursor, It is often necessary to check whether a cursor is valid. This can be done by comparing it to the zero value, or by testing whether its Inspector field is nil:

if cur != (Cursor{}) { use(cur) }
if cur.Inspector() != nil { use(cur) }

However, both are rather roundabout, and the second is unclear.

Proposal: we propose to add a Valid method that reports this property directly:

package inspector // golang.org/x/tools/go/ast/inspector

type Cursor ...

// Valid reports whether the cursor is valid.
func (Cursor) Valid() bool 

[gabyhelp]

Related Issues

• x/tools/go/ast/inspector: add Cursor, to enable partial and multi-level traversals #70859 (closed)
• proposal: go/types: add TypeAndValue.IsConstant #73402

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[aclements]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— aclements for the proposal review group

[adonovan]

See also #77195.

[aclements]

Based on the discussion above, this proposal seems like a likely accept.
— aclements for the proposal review group

The proposal is given in #76361 (comment)