Change the default edition for cargo new to 2018

As it says on the tin! Some tests were updated to explicitly pass 2015 so they
can continue to work on stable, and otherwise `cargo new` should now by default
generate a 2018 project.

src/cargo/ops/cargo_new.rs

@@ -535,18 +535,16 @@ path = {}
             r#"[package]
 name = "{}"
 version = "0.1.0"
-authors = [{}]{}
+authors = [{}]
+edition = {}
 
 [dependencies]
 {}"#,
             name,
             toml::Value::String(author),
             match opts.edition {
-                Some(edition) => {
-                    let edition = toml::Value::String(edition.to_string());
-                    format!("\nedition = {}", edition)
-                }
-                None => String::new(),
+                Some(edition) => toml::Value::String(edition.to_string()),
+                None => toml::Value::String("2018".to_string()),
             },
             cargotoml_path_specifier
         ).as_bytes(),

tests/testsuite/init.rs

@@ -13,7 +13,7 @@ fn cargo_process(s: &str) -> Execs {
 
 #[test]
 fn simple_lib() {
-    cargo_process("init --lib --vcs none")
+    cargo_process("init --lib --vcs none --edition 2015")
         .env("USER", "foo")
         .with_stderr("[CREATED] library project")
         .run();
@@ -29,7 +29,7 @@ fn simple_lib() {
 fn simple_bin() {
     let path = paths::root().join("foo");
     fs::create_dir(&path).unwrap();
-    cargo_process("init --bin --vcs none")
+    cargo_process("init --bin --vcs none --edition 2015")
         .env("USER", "foo")
         .cwd(&path)
         .with_stderr("[CREATED] binary (application) project")

tests/testsuite/new.rs

@@ -14,7 +14,7 @@ fn create_empty_gitconfig() {
 
 #[test]
 fn simple_lib() {
-    cargo_process("new --lib foo --vcs none")
+    cargo_process("new --lib foo --vcs none --edition 2015")
         .env("USER", "foo")
         .with_stderr("[CREATED] library `foo` project")
         .run();
@@ -47,7 +47,7 @@ mod tests {
 
 #[test]
 fn simple_bin() {
-    cargo_process("new --bin foo")
+    cargo_process("new --bin foo --edition 2015")
         .env("USER", "foo")
         .with_stderr("[CREATED] binary (application) `foo` project")
         .run();
@@ -75,7 +75,7 @@ fn both_lib_and_bin() {
 
 #[test]
 fn simple_git() {
-    cargo_process("new --lib foo").env("USER", "foo").run();
+    cargo_process("new --lib foo --edition 2015").env("USER", "foo").run();
 
     assert!(paths::root().is_dir());
     assert!(paths::root().join("foo/Cargo.toml").is_file());
@@ -474,6 +474,15 @@ fn new_with_edition_2018() {
     assert!(manifest.contains("edition = \"2018\""));
 }
 
+#[test]
+fn new_default_edition() {
+    cargo_process("new foo")
+        .env("USER", "foo")
+        .run();
+    let manifest = fs::read_to_string(paths::root().join("foo/Cargo.toml")).unwrap();
+    assert!(manifest.contains("edition = \"2018\""));
+}
+
 #[test]
 fn new_with_bad_edition() {
     cargo_process("new --edition something_else foo")