feat: add build.warnings config option to control rustc warnings

src/cargo/core/compiler/compilation.rs

@@ -122,6 +122,9 @@ pub struct Compilation<'gctx> {
     target_runners: HashMap<CompileKind, Option<(PathBuf, Vec<String>)>>,
     /// The linker to use for each host or target.
     target_linkers: HashMap<CompileKind, Option<PathBuf>>,
+
+    /// The total number of warnings emitted by the compilation.
+    pub warning_count: usize,
 }
 
 impl<'gctx> Compilation<'gctx> {
@@ -169,6 +172,7 @@ impl<'gctx> Compilation<'gctx> {
                 .chain(Some(&CompileKind::Host))
                 .map(|kind| Ok((*kind, target_linker(bcx, *kind)?)))
                 .collect::<CargoResult<HashMap<_, _>>>()?,
+            warning_count: 0,
         })
     }
 

src/cargo/core/compiler/job_queue/mod.rs

@@ -140,6 +140,7 @@ use crate::core::compiler::future_incompat::{
 };
 use crate::core::resolver::ResolveBehavior;
 use crate::core::{PackageId, Shell, TargetKind};
+use crate::util::context::WarningHandling;
 use crate::util::diagnostic_server::{self, DiagnosticPrinter};
 use crate::util::errors::AlreadyPrintedError;
 use crate::util::machine_message::{self, Message as _};
@@ -601,6 +602,7 @@ impl<'gctx> DrainState<'gctx> {
         plan: &mut BuildPlan,
         event: Message,
     ) -> Result<(), ErrorToHandle> {
+        let warning_handling = build_runner.bcx.gctx.warning_handling()?;
         match event {
             Message::Run(id, cmd) => {
                 build_runner
@@ -638,7 +640,9 @@ impl<'gctx> DrainState<'gctx> {
                 }
             }
             Message::Warning { id, warning } => {
-                build_runner.bcx.gctx.shell().warn(warning)?;
+                if warning_handling != WarningHandling::Allow {
+                    build_runner.bcx.gctx.shell().warn(warning)?;
+                }
                 self.bump_warning_count(id, true, false);
             }
             Message::WarningCount {
@@ -826,6 +830,9 @@ impl<'gctx> DrainState<'gctx> {
             // `display_error` inside `handle_error`.
             Some(anyhow::Error::new(AlreadyPrintedError::new(error)))
         } else if self.queue.is_empty() && self.pending_queue.is_empty() {
+            build_runner.compilation.warning_count +=
+                self.warning_count.values().map(|c| c.total).sum::<usize>();
+
             let profile_link = build_runner.bcx.gctx.shell().err_hyperlink(
                 "https://doc.rust-lang.org/cargo/reference/profiles.html#default-profiles",
             );
@@ -1023,7 +1030,7 @@ impl<'gctx> DrainState<'gctx> {
         id: JobId,
         rustc_workspace_wrapper: &Option<PathBuf>,
     ) {
-        let count = match self.warning_count.remove(&id) {
+        let count = match self.warning_count.get(&id) {
             // An error could add an entry for a `Unit`
             // with 0 warnings but having fixable
             // warnings be disallowed

src/cargo/core/compiler/mod.rs

@@ -90,6 +90,7 @@ pub use crate::core::compiler::unit::{Unit, UnitInterner};
 use crate::core::manifest::TargetSourcePath;
 use crate::core::profiles::{PanicStrategy, Profile, StripInner};
 use crate::core::{Feature, PackageId, Target, Verbosity};
+use crate::util::context::WarningHandling;
 use crate::util::errors::{CargoResult, VerboseError};
 use crate::util::interning::InternedString;
 use crate::util::machine_message::{self, Message};
@@ -201,13 +202,15 @@ fn compile<'gctx>(
         } else {
             // We always replay the output cache,
             // since it might contain future-incompat-report messages
+            let show_diagnostics = unit.show_warnings(bcx.gctx)
+                && build_runner.bcx.gctx.warning_handling()? != WarningHandling::Allow;
             let work = replay_output_cache(
                 unit.pkg.package_id(),
                 PathBuf::from(unit.pkg.manifest_path()),
                 &unit.target,
                 build_runner.files().message_cache_path(unit),
                 build_runner.bcx.build_config.message_format,
-                unit.show_warnings(bcx.gctx),
+                show_diagnostics,
             );
             // Need to link targets on both the dirty and fresh.
             work.then(link_targets(build_runner, unit, true)?)
@@ -1665,10 +1668,12 @@ impl OutputOptions {
         // Remove old cache, ignore ENOENT, which is the common case.
         drop(fs::remove_file(&path));
         let cache_cell = Some((path, LazyCell::new()));
+        let show_diagnostics =
+            build_runner.bcx.gctx.warning_handling().unwrap_or_default() != WarningHandling::Allow;
         OutputOptions {
             format: build_runner.bcx.build_config.message_format,
             cache_cell,
-            show_diagnostics: true,
+            show_diagnostics,
             warnings_seen: 0,
             errors_seen: 0,
         }

src/cargo/core/features.rs

@@ -790,6 +790,7 @@ unstable_cli_options!(
     target_applies_to_host: bool = ("Enable the `target-applies-to-host` key in the .cargo/config.toml file"),
     trim_paths: bool = ("Enable the `trim-paths` option in profiles"),
     unstable_options: bool = ("Allow the usage of unstable options"),
+    warnings: bool = ("Allow use of the build.warnings config key"),
 );
 
 const STABILIZED_COMPILE_PROGRESS: &str = "The progress bar is now always \
@@ -1293,6 +1294,7 @@ impl CliUnstable {
             "script" => self.script = parse_empty(k, v)?,
             "target-applies-to-host" => self.target_applies_to_host = parse_empty(k, v)?,
             "unstable-options" => self.unstable_options = parse_empty(k, v)?,
+            "warnings" => self.warnings = parse_empty(k, v)?,
             _ => bail!("\
             unknown `-Z` flag specified: {k}\n\n\
             For available unstable features, see https://doc.rust-lang.org/nightly/cargo/reference/unstable.html\n\

src/cargo/ops/cargo_compile/mod.rs

@@ -52,7 +52,7 @@ use crate::core::{PackageId, PackageSet, SourceId, TargetKind, Workspace};
 use crate::drop_println;
 use crate::ops;
 use crate::ops::resolve::WorkspaceResolve;
-use crate::util::context::GlobalContext;
+use crate::util::context::{GlobalContext, WarningHandling};
 use crate::util::interning::InternedString;
 use crate::util::{CargoResult, StableHasher};
 
@@ -138,7 +138,11 @@ pub fn compile_with_exec<'a>(
     exec: &Arc<dyn Executor>,
 ) -> CargoResult<Compilation<'a>> {
     ws.emit_warnings()?;
-    compile_ws(ws, options, exec)
+    let compilation = compile_ws(ws, options, exec)?;
+    if ws.gctx().warning_handling()? == WarningHandling::Deny && compilation.warning_count > 0 {
+        anyhow::bail!("warnings are denied by `build.warnings` configuration")
+    }
+    Ok(compilation)
 }
 
 /// Like [`compile_with_exec`] but without warnings from manifest parsing.

src/cargo/util/context/mod.rs

@@ -2004,6 +2004,15 @@ impl GlobalContext {
         })?;
         Ok(deferred.borrow_mut())
     }
+
+    /// Get the global [`WarningHandling`] configuration.
+    pub fn warning_handling(&self) -> CargoResult<WarningHandling> {
+        if self.unstable_flags.warnings {
+            Ok(self.build_config()?.warnings.unwrap_or_default())
+        } else {
+            Ok(Default::default())
+        }
+    }
 }
 
 /// Internal error for serde errors.
@@ -2615,6 +2624,20 @@ pub struct CargoBuildConfig {
     // deprecated alias for artifact-dir
     pub out_dir: Option<ConfigRelativePath>,
     pub artifact_dir: Option<ConfigRelativePath>,
+    pub warnings: Option<WarningHandling>,
+}
+
+/// Whether warnings should warn, be allowed, or cause an error.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Deserialize, Default)]
+#[serde(rename_all = "kebab-case")]
+pub enum WarningHandling {
+    #[default]
+    /// Output warnings.
+    Warn,
+    /// Allow warnings (do not output them).
+    Allow,
+    /// Error if  warnings are emitted.
+    Deny,
 }
 
 /// Configuration for `build.target`.

src/doc/src/reference/unstable.md

@@ -118,6 +118,7 @@ Each new feature described below should explain how to use it.
     * [lockfile-path](#lockfile-path) --- Allows to specify a path to lockfile other than the default path `<workspace_root>/Cargo.lock`.
     * [package-workspace](#package-workspace) --- Allows for packaging and publishing multiple crates in a workspace.
     * [native-completions](#native-completions) --- Move cargo shell completions to native completions.
+    * [warnings](#warnings) --- controls warning behavior; options for allowing or denying warnings.
 
 ## allow-features
 
@@ -1957,3 +1958,22 @@ default behavior.
 
 See the [build script documentation](build-scripts.md#rustc-check-cfg) for information
 about specifying custom cfgs.
+
+## warnings
+
+The `-Z warnings` feature enables the `build.warnings` configuration option to control how
+Cargo handles warnings. If the `-Z warnings` unstable flag is not enabled, then
+the `build.warnings` config will be ignored.
+
+### `build.warnings`
+* Type: string
+* Default: `warn`
+* Environment: `CARGO_BUILD_WARNINGS`
+
+Controls how Cargo handles warnings. Allowed values are:
+* `warn`: warnings are emitted as warnings (default).
+* `allow`: warnings are hidden.
+* `deny`: if warnings are emitted, an error will be raised at the end of the operation and the process will exit with a failure exit code. 
+
+This setting currently only applies to rustc warnings. It may apply to additional warnings (such as Cargo lints or Cargo warnings)
+in the future.

tests/testsuite/warning_override.rs

@@ -8,19 +8,13 @@ use snapbox::data::Inline;
 const ALLOW_CLEAN: LazyLock<Inline> = LazyLock::new(|| {
     str![[r#"
 [CHECKING] foo v0.0.1 ([ROOT]/foo)
-[WARNING] unused variable: `x`
-...
-[WARNING] `foo` (bin "foo") generated 1 warning
 [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
 
 "#]]
 });
 
 const ALLOW_CACHED: LazyLock<Inline> = LazyLock::new(|| {
     str![[r#"
-[WARNING] unused variable: `x`
-...
-[WARNING] `foo` (bin "foo") generated 1 warning
 [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
 
 "#]]
@@ -44,6 +38,7 @@ const DENY: LazyLock<Inline> = LazyLock::new(|| {
 ...
 [WARNING] `foo` (bin "foo") generated 1 warning
 [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+[ERROR] warnings are denied by `build.warnings` configuration
 
 "#]]
 });
@@ -68,33 +63,74 @@ fn make_project(main_src: &str) -> Project {
 #[cargo_test]
 fn rustc_caching_allow_first() {
     let p = make_project("let x = 3;");
-    p.cargo("check").with_stderr_data(ALLOW_CLEAN.clone()).run();
+    p.cargo("check")
+        .masquerade_as_nightly_cargo(&["warnings"])
+        .arg("-Zwarnings")
+        .arg("--config")
+        .arg("build.warnings='allow'")
+        .with_stderr_data(ALLOW_CLEAN.clone())
+        .run();
 
-    p.cargo("check").with_stderr_data(DENY.clone()).run();
+    p.cargo("check")
+        .masquerade_as_nightly_cargo(&["warnings"])
+        .arg("-Zwarnings")
+        .arg("--config")
+        .arg("build.warnings='deny'")
+        .with_stderr_data(DENY.clone())
+        .with_status(101)
+        .run();
 }
 
 #[cargo_test]
 fn rustc_caching_deny_first() {
     let p = make_project("let x = 3;");
-    p.cargo("check").with_stderr_data(DENY.clone()).run();
+    p.cargo("check")
+        .masquerade_as_nightly_cargo(&["warnings"])
+        .arg("-Zwarnings")
+        .arg("--config")
+        .arg("build.warnings='deny'")
+        .with_stderr_data(DENY.clone())
+        .with_status(101)
+        .run();
 
     p.cargo("check")
+        .masquerade_as_nightly_cargo(&["warnings"])
+        .arg("-Zwarnings")
+        .arg("--config")
+        .arg("build.warnings='allow'")
         .with_stderr_data(ALLOW_CACHED.clone())
         .run();
 }
 
 #[cargo_test]
 fn config() {
     let p = make_project("let x = 3;");
-    p.cargo("check").with_stderr_data(DENY.clone()).run();
+    p.cargo("check")
+        .masquerade_as_nightly_cargo(&["warnings"])
+        .arg("-Zwarnings")
+        .env("CARGO_BUILD_WARNINGS", "deny")
+        .with_stderr_data(DENY.clone())
+        .with_status(101)
+        .run();
 
-    // CLI has precedence over env.
-    p.cargo("check").with_stderr_data(WARN.clone()).run();
+    // CLI has precedence over env
+    p.cargo("check")
+        .masquerade_as_nightly_cargo(&["warnings"])
+        .arg("-Zwarnings")
+        .arg("--config")
+        .arg("build.warnings='warn'")
+        .env("CARGO_BUILD_WARNINGS", "deny")
+        .with_stderr_data(WARN.clone())
+        .run();
 }
 
 #[cargo_test]
 fn requires_nightly() {
     // build.warnings has no effect without -Zwarnings.
     let p = make_project("let x = 3;");
-    p.cargo("check").with_stderr_data(WARN.clone()).run();
+    p.cargo("check")
+        .arg("--config")
+        .arg("build.warnings='deny'")
+        .with_stderr_data(WARN.clone())
+        .run();
 }