Add --prepare-for flag

cargo-fix/src/cli.rs

@@ -33,6 +33,13 @@ pub fn run() -> Result<(), Error> {
                     Arg::with_name("broken-code")
                         .long("broken-code")
                         .help("Fix code even if it already has compiler errors"),
+                )
+                .arg(
+                    Arg::with_name("edition")
+                        .long("prepare-for")
+                        .help("Fix warnings in preparation of an edition upgrade")
+                        .takes_value(true)
+                        .possible_values(&["2018"]),
                 ),
         )
         .get_matches();
@@ -77,6 +84,15 @@ pub fn run() -> Result<(), Error> {
         cmd.env("RUSTC_ORIGINAL", rustc);
     }
 
+    // Trigger edition-upgrade mode. Currently only supports the 2018 edition.
+    info!("edition upgrade? {:?}", matches.value_of("edition"));
+    if let Some("2018") = matches.value_of("edition") {
+        info!("edition upgrade!");
+        let mut rustc_flags = env::var_os("RUSTFLAGS").unwrap_or_else(|| "".into());
+        rustc_flags.push("-W rust-2018-breakage");
+        cmd.env("RUSTFLAGS", &rustc_flags);
+    }
+
     // An now execute all of Cargo! This'll fix everything along the way.
     //
     // TODO: we probably want to do something fancy here like collect results

cargo-fix/tests/all/edition_upgrade.rs

@@ -0,0 +1,46 @@
+//! Test that we can use cargo-fix to upgrade our code to work with the 2018
+//! edition.
+//!
+//! We'll trigger the `absolute_path_starting_with_module` lint which should
+//! transform a `use ::foo;` where `foo` is local to `use crate::foo;`.
+
+use super::project;
+
+#[test]
+fn prepare_for_2018() {
+    let p = project()
+        .file(
+            "src/lib.rs",
+            r#"
+                #![allow(unused)]
+                #![feature(rust_2018_preview)]
+
+                mod foo {
+                    pub const FOO: &str = "fooo";
+                }
+
+                mod bar {
+                    use ::foo::FOO;
+                }
+
+                fn main() {
+                    let x = ::foo::FOO;
+                }
+            "#,
+        )
+        .build();
+
+    let stderr = "\
+[CHECKING] foo v0.1.0 (CWD)
+[FIXING] src/lib.rs (2 fixes)
+[FINISHED] dev [unoptimized + debuginfo]
+";
+    p.expect_cmd("cargo-fix fix --prepare-for 2018")
+        .stdout("")
+        .stderr(stderr)
+        .run();
+
+    println!("{}", p.read("src/lib.rs"));
+    assert!(p.read("src/lib.rs").contains("use crate::foo::FOO;"));
+    assert!(p.read("src/lib.rs").contains("let x = crate::foo::FOO;"));
+}

cargo-fix/tests/all/main.rs

@@ -318,6 +318,7 @@ fn diff(expected: &str, actual: &str) {
 mod broken_build;
 mod broken_lints;
 mod dependencies;
+mod edition_upgrade;
 mod smoke;
 mod subtargets;
 mod warnings;