Auto merge of #9694 - ehuss:edition-fix-latest, r=alexcrichton

`cargo fix --edition`: extend warning when on latest edition

This extends the warning issued when `cargo fix --edition` is run when the user is already on the latest edition.  Before #9184, there were instructions on what to do, but those probably should not have been completely removed.  It seems likely that some users may get the steps out of order, so this hopefully tries to explain them clearly.

src/cargo/util/diagnostic_server.rs

@@ -181,10 +181,35 @@ impl<'a> DiagnosticPrinter<'a> {
                 if !self.dedupe.insert(msg.clone()) {
                     return Ok(());
                 }
-                self.config.shell().warn(&format!(
-                    "`{}` is already on the latest edition ({}), unable to migrate further",
+                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.
+                    edition: *edition,
+                }) {
+                    self.config.shell().warn(&format!("\
+{}
+
+If you are trying to migrate from the previous edition ({prev_edition}), the
+process requires following these steps:
+
+1. Start with `edition = \"{prev_edition}\"` in `Cargo.toml`
+2. Run `cargo fix --edition`
+3. Modify `Cargo.toml` to set `edition = \"{this_edition}\"`
+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
+",
+                        warning, this_edition=edition, prev_edition=edition.previous().unwrap()
+                    ))
+                } else {
+                    self.config.shell().warn(warning)
+                }
             }
         }
     }

tests/testsuite/fix.rs

@@ -925,11 +925,10 @@ fn prepare_for_already_on_latest_unstable() {
 
     p.cargo("fix --edition --allow-no-vcs")
         .masquerade_as_nightly_cargo()
+        .with_stderr_contains("[CHECKING] foo [..]")
         .with_stderr_contains(&format!(
             "\
-[CHECKING] foo [..]
 [WARNING] `src/lib.rs` is already on the latest edition ({next_edition}), unable to migrate further
-[FINISHED] [..]
 ",
             next_edition = next_edition
         ))
@@ -961,11 +960,10 @@ fn prepare_for_already_on_latest_stable() {
         .build();
 
     p.cargo("fix --edition --allow-no-vcs")
+        .with_stderr_contains("[CHECKING] foo [..]")
         .with_stderr_contains(&format!(
             "\
-[CHECKING] foo [..]
 [WARNING] `src/lib.rs` is already on the latest edition ({latest_stable}), unable to migrate further
-[FINISHED] [..]
 ",
             latest_stable = latest_stable
         ))