Add a migrating message for cargo fix --edition.

This helps indicate which edition you are moving from and to.

crates/cargo-test-support/src/lib.rs

@@ -1535,6 +1535,7 @@ fn substitute_macros(input: &str) -> String {
         ("[LOGOUT]", "      Logout"),
         ("[YANK]", "        Yank"),
         ("[OWNER]", "       Owner"),
+        ("[MIGRATING]", "   Migrating"),
     ];
     let mut result = input.to_owned();
     for &(pat, subst) in &macros {

src/cargo/ops/fix.rs

@@ -301,7 +301,7 @@ fn rustfix_crate(
     filename: &Path,
     args: &FixArgs,
 ) -> Result<FixedCrate, Error> {
-    args.verify_not_preparing_for_enabled_edition()?;
+    args.check_edition_and_send_status()?;
 
     // 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
@@ -685,15 +685,10 @@ impl FixArgs {
         }
     }
 
-    /// Verifies that we're not both preparing for an enabled edition and enabling
-    /// the edition.
-    ///
-    /// This indicates that `cargo fix --prepare-for` is being executed out of
-    /// order with enabling the edition itself, meaning that we wouldn't
-    /// actually be able to fix anything! If it looks like this is happening
-    /// then yield an error to the user, indicating that this is happening.
-    fn verify_not_preparing_for_enabled_edition(&self) -> CargoResult<()> {
-        let edition = match self.prepare_for_edition {
+    /// Validates the edition, and sends a message indicating what is being
+    /// done.
+    fn check_edition_and_send_status(&self) -> CargoResult<()> {
+        let to_edition = match self.prepare_for_edition {
             Some(s) => s,
             None => {
                 return Message::Fixing {
@@ -702,20 +697,21 @@ impl FixArgs {
                 .post();
             }
         };
-        let enabled = match self.enabled_edition {
-            Some(s) => s,
-            None => return Ok(()),
-        };
-        if edition != enabled {
-            return Ok(());
-        }
-
-        Message::EditionAlreadyEnabled {
-            file: self.file.display().to_string(),
-            edition,
+        let from_edition = self.enabled_edition.unwrap_or(Edition::Edition2015);
+        if from_edition == to_edition {
+            Message::EditionAlreadyEnabled {
+                file: self.file.display().to_string(),
+                edition: to_edition,
+            }
+            .post()?;
+            process::exit(1);
+        } else {
+            Message::Migrating {
+                file: self.file.display().to_string(),
+                from_edition,
+                to_edition,
+            }
+            .post()
         }
-        .post()?;
-
-        process::exit(1);
     }
 }

src/cargo/util/diagnostic_server.rs

@@ -31,6 +31,11 @@ const PLEASE_REPORT_THIS_BUG: &str =
 
 #[derive(Deserialize, Serialize, Hash, Eq, PartialEq, Clone)]
 pub enum Message {
+    Migrating {
+        file: String,
+        from_edition: Edition,
+        to_edition: Edition,
+    },
     Fixing {
         file: String,
     },
@@ -92,6 +97,19 @@ impl<'a> DiagnosticPrinter<'a> {
 
     pub fn print(&mut self, msg: &Message) -> CargoResult<()> {
         match msg {
+            Message::Migrating {
+                file,
+                from_edition,
+                to_edition,
+            } => {
+                if !self.dedupe.insert(msg.clone()) {
+                    return Ok(());
+                }
+                self.config.shell().status(
+                    "Migrating",
+                    &format!("{} from {} edition to {}", file, from_edition, to_edition),
+                )
+            }
             Message::Fixing { file } => self
                 .config
                 .shell()

tests/testsuite/fix.rs

@@ -286,6 +286,7 @@ fn prepare_for_2018() {
 
     let stderr = "\
 [CHECKING] foo v0.0.1 ([..])
+[MIGRATING] src/lib.rs from 2015 edition to 2018
 [FIXED] src/lib.rs (2 fixes)
 [FINISHED] [..]
 ";
@@ -324,6 +325,7 @@ fn local_paths() {
         .with_stderr(
             "\
 [CHECKING] foo v0.0.1 ([..])
+[MIGRATING] src/lib.rs from 2015 edition to 2018
 [FIXED] src/lib.rs (1 fix)
 [FINISHED] [..]
 ",
@@ -409,6 +411,7 @@ fn specify_rustflags() {
         .with_stderr(
             "\
 [CHECKING] foo v0.0.1 ([..])
+[MIGRATING] src/lib.rs from 2015 edition to 2018
 [FIXED] src/lib.rs (1 fix)
 [FINISHED] [..]
 ",
@@ -842,6 +845,7 @@ fn fix_overlapping() {
         .with_stderr(
             "\
 [CHECKING] foo [..]
+[MIGRATING] src/lib.rs from 2015 edition to 2018
 [FIXED] src/lib.rs (2 fixes)
 [FINISHED] dev [..]
 ",
@@ -1164,6 +1168,7 @@ fn only_warn_for_relevant_crates() {
             "\
 [CHECKING] a v0.1.0 ([..])
 [CHECKING] foo v0.1.0 ([..])
+[MIGRATING] src/lib.rs from 2015 edition to 2018
 [FINISHED] dev [unoptimized + debuginfo] target(s) in [..]
 ",
         )