Emit warning for migrating to unstable edition in stable channel

src/cargo/ops/fix.rs

@@ -414,7 +414,10 @@ fn rustfix_crate(
     args: &FixArgs,
     config: &Config,
 ) -> Result<FixedCrate, Error> {
-    args.check_edition_and_send_status(config)?;
+    if !args.can_run_rustfix(config)? {
+        // This fix should not be run. Skipping...
+        return Ok(FixedCrate::default());
+    }
 
     // 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
@@ -804,15 +807,16 @@ impl FixArgs {
     }
 
     /// Validates the edition, and sends a message indicating what is being
-    /// done.
-    fn check_edition_and_send_status(&self, config: &Config) -> CargoResult<()> {
+    /// done. Returns a flag indicating whether this fix should be run.
+    fn can_run_rustfix(&self, config: &Config) -> CargoResult<bool> {
         let to_edition = match self.prepare_for_edition {
             Some(s) => s,
             None => {
                 return Message::Fixing {
                     file: self.file.display().to_string(),
                 }
-                .post();
+                .post()
+                .and(Ok(true));
             }
         };
         // Unfortunately determining which cargo targets are being built
@@ -826,18 +830,31 @@ impl FixArgs {
         // multiple jobs run in parallel (the error appears multiple
         // times). Hopefully this doesn't happen often in practice.
         if !to_edition.is_stable() && !config.nightly_features_allowed {
-            bail!(
-                "cannot migrate {} to edition {to_edition}\n\
-                 Edition {to_edition} is unstable and not allowed in this release, \
-                 consider trying the nightly release channel.",
-                self.file.display(),
+            let message = format!(
+                "`{file}` is on the latest edition, but trying to \
+                 migrate to edition {to_edition}.\n\
+                 Edition {to_edition} is unstable and not allowed in \
+                 this release, consider trying the nightly release channel.",
+                file = self.file.display(),
                 to_edition = to_edition
             );
+            return Message::EditionAlreadyEnabled {
+                message,
+                edition: to_edition.previous().unwrap(),
+            }
+            .post()
+            .and(Ok(false)); // Do not run rustfix for this the edition.
         }
         let from_edition = self.enabled_edition.unwrap_or(Edition::Edition2015);
         if from_edition == to_edition {
+            let message = format!(
+                "`{}` is already on the latest edition ({}), \
+                 unable to migrate further",
+                self.file.display(),
+                to_edition
+            );
             Message::EditionAlreadyEnabled {
-                file: self.file.display().to_string(),
+                message,
                 edition: to_edition,
             }
             .post()
@@ -849,5 +866,6 @@ impl FixArgs {
             }
             .post()
         }
+        .and(Ok(true))
     }
 }

src/cargo/util/diagnostic_server.rs

@@ -54,7 +54,7 @@ pub enum Message {
         message: String,
     },
     EditionAlreadyEnabled {
-        file: String,
+        message: String,
         edition: Edition,
     },
 }
@@ -177,18 +177,13 @@ impl<'a> DiagnosticPrinter<'a> {
                 )?;
                 Ok(())
             }
-            Message::EditionAlreadyEnabled { file, edition } => {
+            Message::EditionAlreadyEnabled { message, edition } => {
                 if !self.dedupe.insert(msg.clone()) {
                     return Ok(());
                 }
-                let warning = format!(
-                    "`{}` is already on the latest edition ({}), \
-                     unable to migrate further",
-                    file, edition
-                );
                 // Don't give a really verbose warning if it has already been issued.
                 if self.dedupe.insert(Message::EditionAlreadyEnabled {
-                    file: "".to_string(), // Dummy, so that this only long-warns once.
+                    message: "".to_string(), // Dummy, so that this only long-warns once.
                     edition: *edition,
                 }) {
                     self.config.shell().warn(&format!("\
@@ -205,10 +200,10 @@ process requires following these steps:
 More details may be found at
 https://doc.rust-lang.org/edition-guide/editions/transitioning-an-existing-project-to-a-new-edition.html
 ",
-                        warning, this_edition=edition, prev_edition=edition.previous().unwrap()
+                        message, this_edition=edition, prev_edition=edition.previous().unwrap()
                     ))
                 } else {
-                    self.config.shell().warn(warning)
+                    self.config.shell().warn(message)
                 }
             }
         }

tests/testsuite/fix.rs

@@ -809,6 +809,7 @@ fn prepare_for_unstable() {
         }
     };
     let latest_stable = Edition::LATEST_STABLE;
+    let prev = latest_stable.previous().unwrap();
     let p = project()
         .file(
             "Cargo.toml",
@@ -828,13 +829,24 @@ fn prepare_for_unstable() {
     // -j1 to make the error more deterministic (otherwise there can be
     // multiple errors since they run in parallel).
     p.cargo("fix --edition --allow-no-vcs -j1")
-        .with_status(101)
-        .with_stderr(&format!("\
+        .with_stderr(&format_args!("\
 [CHECKING] foo [..]
-[ERROR] cannot migrate src/lib.rs to edition {next}
+[WARNING] `src/lib.rs` is on the latest edition, but trying to migrate to edition {next}.
 Edition {next} is unstable and not allowed in this release, consider trying the nightly release channel.
-error: could not compile `foo`
-", next=next))
+
+If you are trying to migrate from the previous edition ({prev}), the
+process requires following these steps:
+
+1. Start with `edition = \"{prev}\"` in `Cargo.toml`
+2. Run `cargo fix --edition`
+3. Modify `Cargo.toml` to set `edition = \"{latest_stable}\"`
+4. Run `cargo build` or `cargo test` to verify the fixes worked
+
+More details may be found at
+https://doc.rust-lang.org/edition-guide/editions/transitioning-an-existing-project-to-a-new-edition.html
+
+[FINISHED] [..]
+", next=next, latest_stable=latest_stable, prev=prev))
         .run();
 
     if !is_nightly() {