ABI-required target features: warn when they are missing in base CPU (rather than silently enabling them)

compiler/rustc_interface/messages.ftl

@@ -1,3 +1,8 @@
+interface_abi_required_feature =
+    target feature `{$feature}` must be {$enabled} to ensure that the ABI of the current target can be implemented correctly
+    .note = this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+interface_abi_required_feature_issue = for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>
+
 interface_cant_emit_mir =
     could not emit MIR: {$error}
 

compiler/rustc_interface/src/errors.rs

@@ -103,3 +103,12 @@ pub struct IgnoringOutDir;
 #[derive(Diagnostic)]
 #[diag(interface_multiple_output_types_to_stdout)]
 pub struct MultipleOutputTypesToStdout;
+
+#[derive(Diagnostic)]
+#[diag(interface_abi_required_feature)]
+#[note]
+#[note(interface_abi_required_feature_issue)]
+pub(crate) struct AbiRequiredTargetFeature<'a> {
+    pub feature: &'a str,
+    pub enabled: &'a str,
+}

compiler/rustc_interface/src/interface.rs

@@ -492,6 +492,8 @@ pub fn run_compiler<R: Send>(config: Config, f: impl FnOnce(&Compiler) -> R + Se
             }
             sess.lint_store = Some(Lrc::new(lint_store));
 
+            util::check_abi_required_features(&sess);
+
             let compiler = Compiler {
                 sess,
                 codegen_backend,

compiler/rustc_interface/src/util.rs

@@ -18,21 +18,25 @@ use rustc_session::{EarlyDiagCtxt, Session, filesearch};
 use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::edition::Edition;
 use rustc_span::source_map::SourceMapInputs;
-use rustc_span::sym;
+use rustc_span::{Symbol, sym};
 use rustc_target::spec::Target;
 use tracing::info;
 
 use crate::errors;
 
 /// Function pointer type that constructs a new CodegenBackend.
-pub type MakeBackendFn = fn() -> Box<dyn CodegenBackend>;
+type MakeBackendFn = fn() -> Box<dyn CodegenBackend>;
 
 /// Adds `target_feature = "..."` cfgs for a variety of platform
 /// specific features (SSE, NEON etc.).
 ///
 /// This is performed by checking whether a set of permitted features
 /// is available on the target machine, by querying the codegen backend.
-pub fn add_configuration(cfg: &mut Cfg, sess: &mut Session, codegen_backend: &dyn CodegenBackend) {
+pub(crate) fn add_configuration(
+    cfg: &mut Cfg,
+    sess: &mut Session,
+    codegen_backend: &dyn CodegenBackend,
+) {
     let tf = sym::target_feature;
 
     let unstable_target_features = codegen_backend.target_features_cfg(sess, true);
@@ -48,6 +52,34 @@ pub fn add_configuration(cfg: &mut Cfg, sess: &mut Session, codegen_backend: &dy
     }
 }
 
+/// Ensures that all target features required by the ABI are present.
+/// Must be called after `unstable_target_features` has been populated!
+pub(crate) fn check_abi_required_features(sess: &Session) {
+    let abi_feature_constraints = sess.target.abi_required_features();
+    // We check this against `unstable_target_features` as that is conveniently already
+    // back-translated to rustc feature names, taking into account `-Ctarget-cpu` and `-Ctarget-feature`.
+    // Just double-check that the features we care about are actually on our list.
+    for feature in
+        abi_feature_constraints.required.iter().chain(abi_feature_constraints.incompatible.iter())
+    {
+        assert!(
+            sess.target.rust_target_features().iter().any(|(name, ..)| feature == name),
+            "target feature {feature} is required/incompatible for the current ABI but not a recognized feature for this target"
+        );
+    }
+
+    for feature in abi_feature_constraints.required {
+        if !sess.unstable_target_features.contains(&Symbol::intern(feature)) {
+            sess.dcx().emit_warn(errors::AbiRequiredTargetFeature { feature, enabled: "enabled" });
+        }
+    }
+    for feature in abi_feature_constraints.incompatible {
+        if sess.unstable_target_features.contains(&Symbol::intern(feature)) {
+            sess.dcx().emit_warn(errors::AbiRequiredTargetFeature { feature, enabled: "disabled" });
+        }
+    }
+}
+
 pub static STACK_SIZE: OnceLock<usize> = OnceLock::new();
 pub const DEFAULT_STACK_SIZE: usize = 8 * 1024 * 1024;