fix(fix): Migrate cargo script manifests across editions

At this point it is unlikely for a script to be written for pre-2024
edition and migrated to 2024+ but this code is independent of Editions
so this means we have one less bug and are better prepared for the next
edition.

When we add `cargo fix` support for regular manifest warnings, we'll
need to take into account cargo scripts.

This is a part of rust-lang#12207.

src/cargo/ops/fix.rs

@@ -61,6 +61,7 @@ use crate::ops::resolve::WorkspaceResolve;
 use crate::ops::{self, CompileOptions};
 use crate::util::diagnostic_server::{Message, RustfixDiagnosticServer};
 use crate::util::errors::CargoResult;
+use crate::util::toml_mut::manifest::LocalManifest;
 use crate::util::GlobalContext;
 use crate::util::{existing_vcs_repo, LockServer, LockServerClient};
 use crate::{drop_eprint, drop_eprintln};
@@ -272,7 +273,8 @@ fn migrate_manifests(ws: &Workspace<'_>, pkgs: &[&Package]) -> CargoResult<()> {
             MaybePackage::Virtual(manifest) => manifest.original_toml(),
         };
         if Edition::Edition2024 <= prepare_for_edition {
-            let mut document = pkg.manifest().document().clone().into_mut();
+            let mut manifest_mut = LocalManifest::try_new(pkg.manifest_path())?;
+            let document = &mut manifest_mut.data;
             let mut fixes = 0;
 
             let root = document.as_table_mut();
@@ -335,9 +337,7 @@ fn migrate_manifests(ws: &Workspace<'_>, pkgs: &[&Package]) -> CargoResult<()> {
                 let msg = format!("{file} ({fixes} {verb})");
                 ws.gctx().shell().status("Fixed", msg)?;
 
-                let s = document.to_string();
-                let new_contents_bytes = s.as_bytes();
-                cargo_util::paths::write_atomic(pkg.manifest_path(), new_contents_bytes)?;
+                manifest_mut.write()?;
             }
         }
     }

tests/testsuite/fix.rs

@@ -2479,57 +2479,27 @@ fn main() {
         .with_stderr_data(str![[r#"
 [MIGRATING] foo.rs from 2021 edition to 2024
 [FIXED] foo.rs (1 fix)
-[WARNING] `package.edition` is unspecified, defaulting to `2024`
 [CHECKING] foo v0.0.0 ([ROOT]/foo)
-[WARNING] `foo.rs` is already on the latest edition (2024), unable to migrate further
-
-If you are trying to migrate from the previous edition (2021), the
-process requires following these steps:
-
-1. Start with `edition = "2021"` in `Cargo.toml`
-2. Run `cargo fix --edition`
-3. Modify `Cargo.toml` to set `edition = "2024"`
-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
-
-[ERROR] expected item, found `[`
- --> foo.rs:1:1
-  |
-1 | [[bin]]
-  | ^ expected item
-  |
-  = [NOTE] for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>
-
-[ERROR] could not compile `foo` (bin "foo" test) due to 1 previous error
-[WARNING] build failed, waiting for other jobs to finish...
-[ERROR] could not compile `foo` (bin "foo") due to 1 previous error
+[MIGRATING] [ROOT]/home/.cargo/target/[HASH]/foo.rs from 2021 edition to 2024
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
 
 "#]])
-        .with_status(101)
         .run();
     assert_e2e().eq(
         p.read_file("foo.rs"),
         str![[r#"
-[[bin]]
-name = "foo"
-path = "[ROOT]/home/.cargo/target/[HASH]/foo.rs"
 
-[package]
-autobenches = false
-autobins = false
-autoexamples = false
-autolib = false
-autotests = false
-build = false
-edition = "2021"
+---
+# Before package
+[ package ] # After package header
+# After package header line
 name = "foo"
+edition = "2021"
+# After project table
+---
 
-[profile.release]
-strip = true
-
-[workspace]
+fn main() {
+}
 
 "#]],
     );