collectRefs: record idents only for tracked objects

从 internal/collect.go:447 开始，并对比 collect.go:431-438 中的 collectUnkeyedFields；检查 internal/analyzer.go:59-60 中 collectTargets 的顺序，以及 rename.go:70 和 131 中的 readers。将 identifier 和相关 field records 过滤为 tracked objects，同时保留现有的 refs 行为。当 c.idents 仅包含 tracked targets 且 rename readers 仍然正确时，即完成。

Summary

collectRefs (internal/collect.go:447) appends to c.idents for every Def and every Use in the package. Locals, parameters and objects from the standard library are all included. Only tracked targets are ever read, so the map can hold only those.

Current shape

ast.Inspect(fi.file, func(n ast.Node) bool {
	...
	if obj := pass.TypesInfo.Defs[ident]; obj != nil {
		c.idents[obj] = append(c.idents[obj], ident) // every definition
	}
	obj := origin(pass.TypesInfo.Uses[ident])
	...
	c.idents[obj] = append(c.idents[obj], ident) // every use: err, ctx, fmt.Sprintf...
	if tn := embeddedTypeName(obj); tn != nil {
		c.idents[tn] = append(c.idents[tn], ident)
	}
	if _, tracked := c.byObj[obj]; tracked { // refs already filters
		c.refs[obj] = append(c.refs[obj], ref{node: ident, file: fi})
	}
	return true
})

The only readers are rename.go:70 and rename.go:131. Both read c.idents[t.obj], where t is a tracked target.

Why the change helps

c.idents grows with every identifier in the package. Each new key allocates a map bucket and a slice. For a package of 50,000 identifiers, that is tens of thousands of allocations whose results nobody reads. This walk runs on every pass, so every user pays for it.

// tracked reports whether obj is a key of c.byObj.
if obj := pass.TypesInfo.Defs[ident]; obj != nil && c.tracked(obj) {
	c.idents[obj] = append(c.idents[obj], ident)
}
...
if _, tracked := c.byObj[obj]; tracked {
	c.idents[obj] = append(c.idents[obj], ident)
	c.refs[obj] = append(c.refs[obj], ref{node: ident, file: fi})
}
if tn := embeddedTypeName(obj); tn != nil && c.tracked(tn) {
	c.idents[tn] = append(c.idents[tn], ident)
}

With the filter, the map holds entries only for the declarations declscope checks.

Related

collectUnkeyedFields (collect.go:431-438) also records fields that are not targets. Its comment says so: "refs is only ever read for a target, so the entry is never asked for." The same filter applies there.

Risk

Very low. collectTargets runs before collectRefs (internal/analyzer.go:59-60), so c.byObj is complete when the walk starts. refs already uses the same filter.

Not measured yet.