go/analysis/passes/modernize: reflecttypefor: simplify .Elem()

The reflecttypefor modernizer already detects patterns involving pointers like:

reflect.TypeOf((*io.Reader)(nil)).Elem()

which are used to extract the type of an interface (since passing the interface directly to TypeOf produces a nil result) and simplifies them to remove the pointer and the call to Elem() when transformed to reflect.TypeFor.

This CL also detects patterns using slices which have the same goal of extracting the interface type:

reflect.TypeOf([]io.Reader(nil)).Elem()

both become:

reflect.TypeFor[io.Reader]()

Fixes golang/go#77724

Change-Id: I1d5b4bdbaa812224a9e6f30687cadde01f99ed75
Reviewed-on: https://go-review.googlesource.com/c/tools/+/749500
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: madelinekalil

go/analysis/passes/modernize/doc.go

@@ -263,11 +263,15 @@ is known at compile time, for example:
 	reflect.TypeOf(uint32(0))        -> reflect.TypeFor[uint32]()
 	reflect.TypeOf((*ast.File)(nil)) -> reflect.TypeFor[*ast.File]()
 
-It also offers a fix to simplify the construction below, which uses
+It also offers a fix to simplify the constructions below, which use
 reflect.TypeOf to return the runtime type for an interface type,
 
 	reflect.TypeOf((*io.Reader)(nil)).Elem()
 
+or:
+
+	reflect.TypeOf([]io.Reader(nil)).Elem()
+
 to:
 
 	reflect.TypeFor[io.Reader]()

go/analysis/passes/modernize/reflect.go

@@ -62,24 +62,30 @@ func reflecttypefor(pass *analysis.Pass) (any, error) {
 		t := info.TypeOf(expr)
 		var edits []analysis.TextEdit
 
-		// Special case for TypeOf((*T)(nil)).Elem(),
-		// needed when T is an interface type.
+		// Special cases for TypeOf((*T)(nil)).Elem(), and
+		// TypeOf([]T(nil)).Elem(), needed when T is an interface type.
 		if curCall.ParentEdgeKind() == edge.SelectorExpr_X {
 			curSel := unparenEnclosing(curCall).Parent()
 			if curSel.ParentEdgeKind() == edge.CallExpr_Fun {
-				call2 := unparenEnclosing(curSel).Parent().Node().(*ast.CallExpr)
+				call2 := unparenEnclosing(curSel).Parent().Node().(*ast.CallExpr) // potentially .Elem()
 				obj := typeutil.Callee(info, call2)
 				if typesinternal.IsMethodNamed(obj, "reflect", "Type", "Elem") {
-					if ptr, ok := t.(*types.Pointer); ok {
+					// reflect.TypeOf(expr).Elem()
+					//                     -------
+					// reflect.TypeOf(expr)
+					removeElem := []analysis.TextEdit{{
+						Pos: call.End(),
+						End: call2.End(),
+					}}
+					switch typ := t.(type) {
+					case *types.Pointer:
 						// Have: TypeOf(expr).Elem() where expr : *T
-						t = ptr.Elem()
-						// reflect.TypeOf(expr).Elem()
-						//                     -------
-						// reflect.TypeOf(expr)
-						edits = []analysis.TextEdit{{
-							Pos: call.End(),
-							End: call2.End(),
-						}}
+						t = typ.Elem()
+						edits = removeElem
+					case *types.Slice:
+						// Have: TypeOf(expr).Elem() where expr : []T
+						t = typ.Elem()
+						edits = removeElem
 					}
 				}
 			}

go/analysis/passes/modernize/testdata/src/reflecttypefor/reflecttypefor.go

@@ -14,19 +14,22 @@ var (
 	x any
 	a A
 	b B[int]
-	_ = reflect.TypeOf(x)                 // nope (dynamic)
-	_ = reflect.TypeOf(0)                 // want "reflect.TypeOf call can be simplified using TypeFor"
-	_ = reflect.TypeOf(nil)               // nope (likely a mistake)
-	_ = reflect.TypeOf(uint(0))           // want "reflect.TypeOf call can be simplified using TypeFor"
-	_ = reflect.TypeOf(error(nil))        // nope (likely a mistake)
-	_ = reflect.TypeOf((*error)(nil))     // want "reflect.TypeOf call can be simplified using TypeFor"
-	_ = reflect.TypeOf(io.Reader(nil))    // nope (likely a mistake)
-	_ = reflect.TypeOf((*io.Reader)(nil)) // want "reflect.TypeOf call can be simplified using TypeFor"
-	_ = reflect.TypeOf(*new(time.Time))   // want "reflect.TypeOf call can be simplified using TypeFor"
-	_ = reflect.TypeOf(time.Time{})       // want "reflect.TypeOf call can be simplified using TypeFor"
-	_ = reflect.TypeOf(time.Duration(0))  // want "reflect.TypeOf call can be simplified using TypeFor"
-	_ = reflect.TypeOf(&a)                // want "reflect.TypeOf call can be simplified using TypeFor"
-	_ = reflect.TypeOf(&b)                // want "reflect.TypeOf call can be simplified using TypeFor"
+	_ = reflect.TypeOf(x)                        // nope (dynamic)
+	_ = reflect.TypeOf(0)                        // want "reflect.TypeOf call can be simplified using TypeFor"
+	_ = reflect.TypeOf(nil)                      // nope (likely a mistake)
+	_ = reflect.TypeOf(uint(0))                  // want "reflect.TypeOf call can be simplified using TypeFor"
+	_ = reflect.TypeOf(error(nil))               // nope (likely a mistake)
+	_ = reflect.TypeOf((*error)(nil))            // want "reflect.TypeOf call can be simplified using TypeFor"
+	_ = reflect.TypeOf(io.Reader(nil))           // nope (likely a mistake)
+	_ = reflect.TypeOf((*io.Reader)(nil))        // want "reflect.TypeOf call can be simplified using TypeFor"
+	_ = reflect.TypeOf(*new(time.Time))          // want "reflect.TypeOf call can be simplified using TypeFor"
+	_ = reflect.TypeOf(time.Time{})              // want "reflect.TypeOf call can be simplified using TypeFor"
+	_ = reflect.TypeOf(time.Duration(0))         // want "reflect.TypeOf call can be simplified using TypeFor"
+	_ = reflect.TypeOf(&a)                       // want "reflect.TypeOf call can be simplified using TypeFor"
+	_ = reflect.TypeOf(&b)                       // want "reflect.TypeOf call can be simplified using TypeFor"
+	_ = reflect.TypeOf([]io.Reader(nil)).Elem()  // want "reflect.TypeOf call can be simplified using TypeFor"
+	_ = reflect.TypeOf([]*io.Reader(nil)).Elem() // want "reflect.TypeOf call can be simplified using TypeFor"
+	_ = reflect.TypeOf((*io.Reader)(nil)).Elem() // want "reflect.TypeOf call can be simplified using TypeFor"
 )
 
 // Eliminate local var if we deleted its last use.

go/analysis/passes/modernize/testdata/src/reflecttypefor/reflecttypefor.go.golden

@@ -25,8 +25,11 @@ var (
 	_ = reflect.TypeFor[time.Time]()     // want "reflect.TypeOf call can be simplified using TypeFor"
 	_ = reflect.TypeFor[time.Time]()     // want "reflect.TypeOf call can be simplified using TypeFor"
 	_ = reflect.TypeFor[time.Duration]() // want "reflect.TypeOf call can be simplified using TypeFor"
-	_ = reflect.TypeFor[*A]() // want "reflect.TypeOf call can be simplified using TypeFor"
-	_ = reflect.TypeFor[*B[int]]() // want "reflect.TypeOf call can be simplified using TypeFor"
+	_ = reflect.TypeFor[*A]()            // want "reflect.TypeOf call can be simplified using TypeFor"
+	_ = reflect.TypeFor[*B[int]]()       // want "reflect.TypeOf call can be simplified using TypeFor"
+	_ = reflect.TypeFor[io.Reader]()     // want "reflect.TypeOf call can be simplified using TypeFor"
+	_ = reflect.TypeFor[*io.Reader]()    // want "reflect.TypeOf call can be simplified using TypeFor"
+	_ = reflect.TypeFor[io.Reader]()     // want "reflect.TypeOf call can be simplified using TypeFor"
 )
 
 // Eliminate local var if we deleted its last use.

gopls/doc/analyzers.md

@@ -3732,10 +3732,14 @@ This analyzer suggests fixes to replace uses of reflect.TypeOf(x) with reflect.T
 	reflect.TypeOf(uint32(0))        -> reflect.TypeFor[uint32]()
 	reflect.TypeOf((*ast.File)(nil)) -> reflect.TypeFor[*ast.File]()
 
-It also offers a fix to simplify the construction below, which uses reflect.TypeOf to return the runtime type for an interface type,
+It also offers a fix to simplify the constructions below, which use reflect.TypeOf to return the runtime type for an interface type,
 
 	reflect.TypeOf((*io.Reader)(nil)).Elem()
 
+or:
+
+	reflect.TypeOf([]io.Reader(nil)).Elem()
+
 to:
 
 	reflect.TypeFor[io.Reader]()

gopls/internal/doc/api.json

@@ -1606,7 +1606,7 @@
 						},
 						{
 							"Name": "\"reflecttypefor\"",
-							"Doc": "replace reflect.TypeOf(x) with TypeFor[T]()\n\nThis analyzer suggests fixes to replace uses of reflect.TypeOf(x) with\nreflect.TypeFor, introduced in go1.22, when the desired runtime type\nis known at compile time, for example:\n\n\treflect.TypeOf(uint32(0))        -\u003e reflect.TypeFor[uint32]()\n\treflect.TypeOf((*ast.File)(nil)) -\u003e reflect.TypeFor[*ast.File]()\n\nIt also offers a fix to simplify the construction below, which uses\nreflect.TypeOf to return the runtime type for an interface type,\n\n\treflect.TypeOf((*io.Reader)(nil)).Elem()\n\nto:\n\n\treflect.TypeFor[io.Reader]()\n\nNo fix is offered in cases when the runtime type is dynamic, such as:\n\n\tvar r io.Reader = ...\n\treflect.TypeOf(r)\n\nor when the operand has potential side effects.",
+							"Doc": "replace reflect.TypeOf(x) with TypeFor[T]()\n\nThis analyzer suggests fixes to replace uses of reflect.TypeOf(x) with\nreflect.TypeFor, introduced in go1.22, when the desired runtime type\nis known at compile time, for example:\n\n\treflect.TypeOf(uint32(0))        -\u003e reflect.TypeFor[uint32]()\n\treflect.TypeOf((*ast.File)(nil)) -\u003e reflect.TypeFor[*ast.File]()\n\nIt also offers a fix to simplify the constructions below, which use\nreflect.TypeOf to return the runtime type for an interface type,\n\n\treflect.TypeOf((*io.Reader)(nil)).Elem()\n\nor:\n\n\treflect.TypeOf([]io.Reader(nil)).Elem()\n\nto:\n\n\treflect.TypeFor[io.Reader]()\n\nNo fix is offered in cases when the runtime type is dynamic, such as:\n\n\tvar r io.Reader = ...\n\treflect.TypeOf(r)\n\nor when the operand has potential side effects.",
 							"Default": "true",
 							"Status": ""
 						},
@@ -3535,7 +3535,7 @@
 		},
 		{
 			"Name": "reflecttypefor",
-			"Doc": "replace reflect.TypeOf(x) with TypeFor[T]()\n\nThis analyzer suggests fixes to replace uses of reflect.TypeOf(x) with\nreflect.TypeFor, introduced in go1.22, when the desired runtime type\nis known at compile time, for example:\n\n\treflect.TypeOf(uint32(0))        -\u003e reflect.TypeFor[uint32]()\n\treflect.TypeOf((*ast.File)(nil)) -\u003e reflect.TypeFor[*ast.File]()\n\nIt also offers a fix to simplify the construction below, which uses\nreflect.TypeOf to return the runtime type for an interface type,\n\n\treflect.TypeOf((*io.Reader)(nil)).Elem()\n\nto:\n\n\treflect.TypeFor[io.Reader]()\n\nNo fix is offered in cases when the runtime type is dynamic, such as:\n\n\tvar r io.Reader = ...\n\treflect.TypeOf(r)\n\nor when the operand has potential side effects.",
+			"Doc": "replace reflect.TypeOf(x) with TypeFor[T]()\n\nThis analyzer suggests fixes to replace uses of reflect.TypeOf(x) with\nreflect.TypeFor, introduced in go1.22, when the desired runtime type\nis known at compile time, for example:\n\n\treflect.TypeOf(uint32(0))        -\u003e reflect.TypeFor[uint32]()\n\treflect.TypeOf((*ast.File)(nil)) -\u003e reflect.TypeFor[*ast.File]()\n\nIt also offers a fix to simplify the constructions below, which use\nreflect.TypeOf to return the runtime type for an interface type,\n\n\treflect.TypeOf((*io.Reader)(nil)).Elem()\n\nor:\n\n\treflect.TypeOf([]io.Reader(nil)).Elem()\n\nto:\n\n\treflect.TypeFor[io.Reader]()\n\nNo fix is offered in cases when the runtime type is dynamic, such as:\n\n\tvar r io.Reader = ...\n\treflect.TypeOf(r)\n\nor when the operand has potential side effects.",
 			"URL": "https://pkg.go.dev/golang.org/x/tools/go/analysis/passes/modernize#reflecttypefor",
 			"Default": true
 		},