Make --force-warn support auto-detect.

src/cargo/core/compiler/build_context/target_info.rs

@@ -47,6 +47,8 @@ pub struct TargetInfo {
     pub rustdocflags: Vec<String>,
     /// Whether or not rustc supports the `-Csplit-debuginfo` flag.
     pub supports_split_debuginfo: bool,
+    /// Whether or not rustc supports the `--force-warn` flag. Remove after 1.56 is stable.
+    pub supports_force_warn: bool,
 }
 
 /// Kind of each file generated by a Unit, part of `FileType`.
@@ -178,6 +180,12 @@ impl TargetInfo {
                 extra_fingerprint,
             )
             .is_ok();
+        let supports_force_warn = rustc
+            .cached_output(
+                process.clone().arg("--force-warn=rust-2021-compatibility"),
+                extra_fingerprint,
+            )
+            .is_ok();
 
         process.arg("--print=sysroot");
         process.arg("--print=cfg");
@@ -253,6 +261,7 @@ impl TargetInfo {
             )?,
             cfg,
             supports_split_debuginfo,
+            supports_force_warn,
         })
     }
 

src/cargo/core/features.rs

@@ -139,6 +139,7 @@ pub enum Edition {
 // - Set LATEST_STABLE to the new version.
 // - Update `is_stable` to `true`.
 // - Set the editionNNNN feature to stable in the features macro below.
+// - Update any tests that are affected.
 // - Update the man page for the --edition flag.
 // - Update unstable.md to move the edition section to the bottom.
 // - Update the documentation:

src/cargo/ops/fix.rs

@@ -51,7 +51,7 @@ use log::{debug, trace, warn};
 use rustfix::diagnostics::Diagnostic;
 use rustfix::{self, CodeFix};
 
-use crate::core::compiler::RustcTargetData;
+use crate::core::compiler::{CompileKind, RustcTargetData, TargetInfo};
 use crate::core::resolver::features::{DiffMap, FeatureOpts, FeatureResolver};
 use crate::core::resolver::{HasDevUnits, Resolve, ResolveBehavior};
 use crate::core::{Edition, MaybePackage, Workspace};
@@ -67,6 +67,7 @@ const FIX_ENV: &str = "__CARGO_FIX_PLZ";
 const BROKEN_CODE_ENV: &str = "__CARGO_FIX_BROKEN_CODE";
 const EDITION_ENV: &str = "__CARGO_FIX_EDITION";
 const IDIOMS_ENV: &str = "__CARGO_FIX_IDIOMS";
+const SUPPORTS_FORCE_WARN: &str = "__CARGO_SUPPORTS_FORCE_WARN";
 
 pub struct FixOptions {
     pub edition: bool,
@@ -122,6 +123,17 @@ pub fn fix(ws: &Workspace<'_>, opts: &mut FixOptions) -> CargoResult<()> {
     let rustc = ws.config().load_global_rustc(Some(ws))?;
     wrapper.arg(&rustc.path);
 
+    // Remove this once 1.56 is stabilized.
+    let target_info = TargetInfo::new(
+        ws.config(),
+        &opts.compile_opts.build_config.requested_kinds,
+        &rustc,
+        CompileKind::Host,
+    )?;
+    if target_info.supports_force_warn {
+        wrapper.env(SUPPORTS_FORCE_WARN, "1");
+    }
+
     // primary crates are compiled using a cargo subprocess to do extra work of applying fixes and
     // repeating build until there are no more changes to be applied
     opts.compile_opts.build_config.primary_unit_rustc = Some(wrapper);
@@ -362,7 +374,7 @@ 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, config);
+        args.apply(&mut cmd);
         cmd.arg("--error-format=json");
         debug!("calling rustc for final verification: {:?}", cmd);
         let output = cmd.output().context("failed to spawn rustc")?;
@@ -403,7 +415,7 @@ 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, config);
+    args.apply(&mut cmd);
     for arg in args.format_args {
         // Add any json/error format arguments that Cargo wants. This allows
         // things like colored output to work correctly.
@@ -497,7 +509,7 @@ fn rustfix_crate(
             // We'll generate new errors below.
             file.errors_applying_fixes.clear();
         }
-        rustfix_and_fix(&mut fixes, rustc, filename, args, config)?;
+        rustfix_and_fix(&mut fixes, rustc, filename, args)?;
         let mut progress_yet_to_be_made = false;
         for (path, file) in fixes.files.iter_mut() {
             if file.errors_applying_fixes.is_empty() {
@@ -539,7 +551,6 @@ 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.
@@ -547,7 +558,7 @@ fn rustfix_and_fix(
 
     let mut cmd = rustc.build_command();
     cmd.arg("--error-format=json");
-    args.apply(&mut cmd, config);
+    args.apply(&mut cmd);
     debug!(
         "calling rustc to collect suggestions and validate previous fixes: {:?}",
         cmd
@@ -822,10 +833,10 @@ impl FixArgs {
         })
     }
 
-    fn apply(&self, cmd: &mut Command, config: &Config) {
+    fn apply(&self, cmd: &mut Command) {
         cmd.arg(&self.file);
         cmd.args(&self.other);
-        if self.prepare_for_edition.is_some() && config.nightly_features_allowed {
+        if self.prepare_for_edition.is_some() && env::var_os(SUPPORTS_FORCE_WARN).is_some() {
             // When migrating an edition, we don't want to fix other lints as
             // they can sometimes add suggestions that fail to apply, causing
             // the entire migration to fail. But those lints aren't needed to
@@ -844,7 +855,7 @@ impl FixArgs {
 
         if let Some(edition) = self.prepare_for_edition {
             if edition.supports_compat_lint() {
-                if config.nightly_features_allowed {
+                if env::var_os(SUPPORTS_FORCE_WARN).is_some() {
                     cmd.arg("--force-warn")
                         .arg(format!("rust-{}-compatibility", edition))
                         .arg("-Zunstable-options");

tests/testsuite/fix.rs

@@ -950,6 +950,10 @@ fn prepare_for_already_on_latest_unstable() {
 #[cargo_test]
 fn prepare_for_already_on_latest_stable() {
     // Stable counterpart of prepare_for_already_on_latest_unstable.
+    if !is_nightly() {
+        // Remove once 1.56 is stabilized.
+        return;
+    }
     if Edition::LATEST_UNSTABLE.is_some() {
         eprintln!("This test cannot run while the latest edition is unstable, skipping.");
         return;
@@ -1434,10 +1438,6 @@ fn fix_color_message() {
 #[cargo_test]
 fn edition_v2_resolver_report() {
     // Show a report if the V2 resolver shows differences.
-    if !is_nightly() {
-        // 2021 is unstable
-        return;
-    }
     Package::new("common", "1.0.0")
         .feature("f1", &[])
         .feature("dev-feat", &[])
@@ -1477,7 +1477,6 @@ fn edition_v2_resolver_report() {
         .build();
 
     p.cargo("fix --edition --allow-no-vcs")
-        .masquerade_as_nightly_cargo()
         .with_stderr_unordered("\
 [UPDATING] [..]
 [DOWNLOADING] crates ...
@@ -1530,7 +1529,7 @@ fn rustfix_handles_multi_spans() {
 fn fix_edition_2021() {
     // Can migrate 2021, even when lints are allowed.
     if !is_nightly() {
-        // 2021 is unstable
+        // Remove once 1.56 is stabilized.
         return;
     }
     let p = project()