Rollup merge of #51073 - dtolnay:empty, r=alexcrichton

Rename TokenStream::empty to TokenStream::new

There is no precedent for the `empty` name -- we do not have `Vec::empty` or `HashMap::empty` etc.

I would propose landing this but reflecting it in a non-breaking release of proc-macro2 that provides both `new` and a deprecated `empty` constructor.

Tracking issue: #38356

r? @alexcrichton

src/test/run-pass-fulldeps/proc-macro/auxiliary/modify-ast.rs

@@ -26,7 +26,7 @@ pub fn assert1(_a: TokenStream, b: TokenStream) -> TokenStream {
 #[proc_macro_derive(Foo, attributes(foo))]
 pub fn assert2(a: TokenStream) -> TokenStream {
     assert_eq(a, "pub struct MyStructc { _a: i32, }".parse().unwrap());
-    TokenStream::empty()
+    TokenStream::new()
 }
 
 fn assert_eq(a: TokenStream, b: TokenStream) {

src/test/run-pass-fulldeps/proc-macro/auxiliary/not-joint.rs

@@ -20,13 +20,13 @@ use proc_macro::*;
 #[proc_macro]
 pub fn tokens(input: TokenStream) -> TokenStream {
     assert_nothing_joint(input);
-    TokenStream::empty()
+    TokenStream::new()
 }
 
 #[proc_macro_attribute]
 pub fn nothing(_: TokenStream, input: TokenStream) -> TokenStream {
     assert_nothing_joint(input);
-    TokenStream::empty()
+    TokenStream::new()
 }
 
 fn assert_nothing_joint(s: TokenStream) {

src/test/ui-fulldeps/proc-macro/auxiliary/three-equals.rs

@@ -50,7 +50,7 @@ fn parse(input: TokenStream) -> Result<(), Diagnostic> {
 pub fn three_equals(input: TokenStream) -> TokenStream {
     if let Err(diag) = parse(input) {
         diag.emit();
-        return TokenStream::empty();
+        return TokenStream::new();
     }
 
     "3".parse().unwrap()