No longer warn about #![feature(rust_2018_preview)]

This is no longer needed, and so the warning is outdated.

Fixes rust-lang#6050

src/cargo/ops/fix.rs

@@ -233,7 +233,6 @@ fn rustfix_crate(lock_addr: &str, rustc: &Path, filename: &Path, args: &FixArgs)
     -> Result<FixedCrate, Error>
 {
     args.verify_not_preparing_for_enabled_edition()?;
-    args.warn_if_preparing_probably_inert()?;
 
     // First up we want to make sure that each crate is only checked by one
     // process at a time. If two invocations concurrently check a crate then
@@ -596,38 +595,6 @@ impl FixArgs {
         process::exit(1);
     }
 
-    /// If we're preparing for an edition and we *don't* find the
-    /// `rust_2018_preview` feature, for example, in the entry point file then
-    /// it probably means that the edition isn't actually enabled, so we can't
-    /// actually fix anything.
-    ///
-    /// If this is the case, issue a warning.
-    fn warn_if_preparing_probably_inert(&self) -> CargoResult<()> {
-        let edition = match self.prepare_for_edition_resolve() {
-            Some(s) => s,
-            None => return Ok(()),
-        };
-        let path = match &self.file {
-            Some(s) => s,
-            None => return Ok(()),
-        };
-        let contents = match fs::read_to_string(path) {
-            Ok(s) => s,
-            Err(_) => return Ok(())
-        };
-
-        let feature_name = format!("rust_{}_preview", edition);
-        if contents.contains(&feature_name) {
-            return Ok(())
-        }
-        Message::PreviewNotFound {
-            file: path.display().to_string(),
-            edition: edition.to_string(),
-        }.post()?;
-
-        Ok(())
-    }
-
     fn prepare_for_edition_resolve(&self) -> Option<&str> {
         match &self.prepare_for_edition {
             PrepareFor::Edition(s) => Some(s),

tests/testsuite/fix.rs

@@ -335,9 +335,6 @@ fn local_paths_no_fix() {
 
     let stderr = "\
 [CHECKING] foo v0.0.1 ([..])
-warning: failed to find `#![feature(rust_2018_preview)]` in `src/lib.rs`
-this may cause `cargo fix` to not be able to fix all
-issues in preparation for the 2018 edition
 [FINISHED] [..]
 ";
     p.cargo("fix --edition --allow-no-vcs")
@@ -853,26 +850,6 @@ information about transitioning to the 2018 edition see:
         .run();
 }
 
-#[test]
-fn prepare_for_without_feature_issues_warning() {
-    if !is_nightly() {
-        return;
-    }
-    let p = project().file("src/lib.rs", "").build();
-
-    let stderr = "\
-[CHECKING] foo v0.0.1 ([..])
-warning: failed to find `#![feature(rust_2018_preview)]` in `src/lib.rs`
-this may cause `cargo fix` to not be able to fix all
-issues in preparation for the 2018 edition
-[FINISHED] [..]
-";
-    p.cargo("fix --edition --allow-no-vcs")
-        .masquerade_as_nightly_cargo()
-        .with_stderr(stderr)
-        .run();
-}
-
 #[test]
 fn fix_overlapping() {
     if !is_nightly() {