Enable support for fix --edition for 2021.

src/cargo/core/features.rs

@@ -208,15 +208,15 @@ impl Edition {
 
     /// Whether or not this edition supports the `rust_*_compatibility` lint.
     ///
-    /// Ideally this would not be necessary, but currently 2021 does not have
-    /// any lints, and thus `rustc` doesn't recognize it. Perhaps `rustc`
-    /// could create an empty group instead?
+    /// Ideally this would not be necessary, but editions may not have any
+    /// lints, and thus `rustc` doesn't recognize it. Perhaps `rustc` could
+    /// create an empty group instead?
     pub(crate) fn supports_compat_lint(&self) -> bool {
         use Edition::*;
         match self {
             Edition2015 => false,
             Edition2018 => true,
-            Edition2021 => false,
+            Edition2021 => true,
         }
     }
 

src/cargo/ops/fix.rs

@@ -330,7 +330,7 @@ pub fn fix_maybe_exec_rustc(config: &Config) -> CargoResult<bool> {
     // that we have to back it all out.
     if !fixes.files.is_empty() {
         let mut cmd = rustc.build_command();
-        args.apply(&mut cmd);
+        args.apply(&mut cmd, config);
         cmd.arg("--error-format=json");
         let output = cmd.output().context("failed to spawn rustc")?;
 
@@ -369,7 +369,7 @@ pub fn fix_maybe_exec_rustc(config: &Config) -> CargoResult<bool> {
     // - If `--broken-code`, show the error messages.
     // - If the fix succeeded, show any remaining warnings.
     let mut cmd = rustc.build_command();
-    args.apply(&mut cmd);
+    args.apply(&mut cmd, config);
     for arg in args.format_args {
         // Add any json/error format arguments that Cargo wants. This allows
         // things like colored output to work correctly.
@@ -457,7 +457,7 @@ fn rustfix_crate(
             // We'll generate new errors below.
             file.errors_applying_fixes.clear();
         }
-        rustfix_and_fix(&mut fixes, rustc, filename, args)?;
+        rustfix_and_fix(&mut fixes, rustc, filename, args, config)?;
         let mut progress_yet_to_be_made = false;
         for (path, file) in fixes.files.iter_mut() {
             if file.errors_applying_fixes.is_empty() {
@@ -499,14 +499,15 @@ fn rustfix_and_fix(
     rustc: &ProcessBuilder,
     filename: &Path,
     args: &FixArgs,
+    config: &Config,
 ) -> Result<(), Error> {
     // If not empty, filter by these lints.
     // TODO: implement a way to specify this.
     let only = HashSet::new();
 
     let mut cmd = rustc.build_command();
     cmd.arg("--error-format=json");
-    args.apply(&mut cmd);
+    args.apply(&mut cmd, config);
     let output = cmd.output().with_context(|| {
         format!(
             "failed to execute `{}`",
@@ -763,7 +764,7 @@ impl FixArgs {
         })
     }
 
-    fn apply(&self, cmd: &mut Command) {
+    fn apply(&self, cmd: &mut Command, config: &Config) {
         cmd.arg(&self.file);
         cmd.args(&self.other).arg("--cap-lints=warn");
         if let Some(edition) = self.enabled_edition {
@@ -775,7 +776,13 @@ impl FixArgs {
 
         if let Some(edition) = self.prepare_for_edition {
             if edition.supports_compat_lint() {
-                cmd.arg("-W").arg(format!("rust-{}-compatibility", edition));
+                if config.nightly_features_allowed {
+                    cmd.arg("--force-warns")
+                        .arg(format!("rust-{}-compatibility", edition))
+                        .arg("-Zunstable-options");
+                } else {
+                    cmd.arg("-W").arg(format!("rust-{}-compatibility", edition));
+                }
             }
         }
     }

tests/testsuite/fix.rs

@@ -1501,3 +1501,49 @@ fn rustfix_handles_multi_spans() {
     p.cargo("fix --allow-no-vcs").run();
     assert!(p.read_file("src/lib.rs").contains(r#"panic!("hey");"#));
 }
+
+#[cargo_test]
+fn fix_edition_2021() {
+    // Can migrate 2021, even when lints are allowed.
+    if !is_nightly() {
+        // 2021 is unstable
+        return;
+    }
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2018"
+            "#,
+        )
+        .file(
+            "src/lib.rs",
+            r#"
+                #![allow(ellipsis_inclusive_range_patterns)]
+
+                pub fn f() -> bool {
+                    let x = 123;
+                    match x {
+                        0...100 => true,
+                        _ => false,
+                    }
+                }
+            "#,
+        )
+        .build();
+    p.cargo("fix --edition --allow-no-vcs")
+        .masquerade_as_nightly_cargo()
+        .with_stderr(
+            "\
+[CHECKING] foo v0.1.0 [..]
+[MIGRATING] src/lib.rs from 2018 edition to 2021
+[FIXED] src/lib.rs (1 fix)
+[FINISHED] [..]
+",
+        )
+        .run();
+    assert!(p.read_file("src/lib.rs").contains(r#"0..=100 => true,"#));
+}