reflect: avoid TypeOf in init

Calling TypeOf to initialize variables forces any import of "reflect"
to link in the declared types of "reflect" even if they are unused.
TypeOf operates on Type and which will pull in
all transitive dependencies of Type, which includes Value as well.
Avoid this problem by declaring a rtypeOf function that
directly extracts the *rtype from an interface value
without going through Type as an intermediate type.

For a program that blank imports "reflect",
this reduces the binary size by ~34 KiB.

Updates #54097

Change-Id: I8dc7d8da8fedc48cc0dd842b69f510d17144827e
Reviewed-on: https://go-review.googlesource.com/c/go/+/419757
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

Author: dsnet

src/reflect/type.go

@@ -1433,6 +1433,12 @@ func TypeOf(i any) Type {
 	return toType(eface.typ)
 }
 
+// rtypeOf directly extracts the *rtype of the provided value.
+func rtypeOf(i any) *rtype {
+	eface := *(*emptyInterface)(unsafe.Pointer(&i))
+	return eface.typ
+}
+
 // ptrMap is the cache for PointerTo.
 var ptrMap sync.Map // map[*rtype]*ptrType
 

src/reflect/value.go

@@ -290,7 +290,7 @@ func (v Value) panicNotBool() {
 	v.mustBe(Bool)
 }
 
-var bytesType = TypeOf(([]byte)(nil)).(*rtype)
+var bytesType = rtypeOf(([]byte)(nil))
 
 // Bytes returns v's underlying value.
 // It panics if v's underlying value is not a slice of bytes or
@@ -1381,7 +1381,7 @@ func (v Value) Float() float64 {
 	panic(&ValueError{"reflect.Value.Float", v.kind()})
 }
 
-var uint8Type = TypeOf(uint8(0)).(*rtype)
+var uint8Type = rtypeOf(uint8(0))
 
 // Index returns v's i'th element.
 // It panics if v's Kind is not Array, Slice, or String or i is out of range.
@@ -1640,7 +1640,7 @@ func (v Value) lenNonSlice() int {
 	panic(&ValueError{"reflect.Value.Len", v.kind()})
 }
 
-var stringType = TypeOf("").(*rtype)
+var stringType = rtypeOf("")
 
 // MapIndex returns the value associated with key in the map v.
 // It panics if v's Kind is not Map.