go/types: add Info.PkgName method

[adonovan]

Any time a client of go/types needs to find the types.PkgName (or just the name) defined by an import, they must write logic equivalent to this:

// PkgName returns the local package name defined by the specified import.
func (info *types.Info) PkgName(imp *ast.ImportSpec) (*types.PkgName, bool) {
	var obj types.Object
	if imp.Name != nil {
		obj = info.Defs[imp.Name]
	} else {
		obj = info.Implicits[imp]
	}
	pkgname, ok := obj.(*types.PkgName)
	return pkgname, ok
}

I propose that we add this method to types.Info.

[findleyr]

Makes sense to me, in general, though I think we should discuss the name/signature.

The proposed API is fine, but perhaps inconsistent with the existing methods. Currently, types.Info has two methods: ObjectOf and TypeOf, both of which accept an argument from the go/ast domain and return a possibly nil go/types result. Therefore, would a more consistent API be something like this?

// ImportOf returns the local package name defined by the specified import, or nil if not found.
func (info *types.Info) ImportOf(imp *ast.ImportSpec) *types.PkgName

[adonovan]

Fair point about nil (vs bool). How about a name of PkgNameOf?

[findleyr]

PkgNameOf SGTM. Indeed, it is the most natural name since ObjectOf returns an Object, and TypeOf returns a Type.

[adonovan]

I found at least 5 copies of this function in x/tools:

cmd/guru/describe.go
623-		if n.Name != nil {
624-			obj = qpos.info.Defs[n.Name]
625-		} else {
626:			obj = qpos.info.Implicits[n]
627-		}
628-		pkgname, _ := obj.(*types.PkgName)
629-		if pkgname == nil {

go/analysis/passes/pkgfact/pkgfact.go
120-}
121-
122-func imported(info *types.Info, spec *ast.ImportSpec) *types.Package {
123:	obj, ok := info.Implicits[spec]
124-	if !ok {
125-		obj = info.Defs[spec.Name] // renaming import
126-	}

go/analysis/passes/cgocall/cgocall.go
371-
372-// TODO(adonovan): make this a library function or method of Info.
373-func imported(info *types.Info, spec *ast.ImportSpec) *types.Package {
374:	obj, ok := info.Implicits[spec]
375-	if !ok {
376-		obj = info.Defs[spec.Name] // renaming import
377-	}

gopls/internal/lsp/source/highlight.go
473-	if imp.Name != nil {
474-		obj = info.Defs[imp.Name]
475-	} else {
476:		obj = info.Implicits[imp]
477-	}
478-	pkgname, ok := obj.(*types.PkgName)
479-	return pkgname, ok

internal/refactor/inline/inline.go
2088-	if imp.Name != nil {
2089-		obj = info.Defs[imp.Name]
2090-	} else {
2091:		obj = info.Implicits[imp]
2092-	}
2093-	pkgname, ok := obj.(*types.PkgName)
2094-	return pkgname, ok

[rsc]

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

[rsc]

Have all remaining concerns about this proposal been addressed?

In go/types, add a new method:

// PkgName returns the local package name defined by the import.
func (info *types.Info) PkgName(imp *ast.ImportSpec)

[griesemer]

For consistency in naming with the existing functions, and per the comment above, this should probably be:

// PkgNameOf returns the local package name defined by the import,
// or nil if it is not found.
//
// Precondition: the Defs and Implicts maps are populated.
func (info *types.Info) PkgNameOf(imp *ast.ImportSpec) *PkgName