Rollup merge of rust-lang#58118 - h-michael:libtest-theme-2018, r=Centril

Transition libtest to 2018 edition

Transitions libtest to Rust 2018; cc rust-lang#58099
r? @Centril

Author: kennytm

src/libtest/Cargo.toml

@@ -2,6 +2,7 @@
 authors = ["The Rust Project Developers"]
 name = "test"
 version = "0.0.0"
+edition = "2018"
 
 [lib]
 name = "test"

src/libtest/formatters/json.rs

@@ -145,7 +145,7 @@ impl<T: Write> OutputFormatter for JsonFormatter<T> {
 struct EscapedString<S: AsRef<str>>(S);
 
 impl<S: AsRef<str>> ::std::fmt::Display for EscapedString<S> {
-    fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+    fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
         let mut start = 0;
 
         for (i, byte) in self.0.as_ref().bytes().enumerate() {