mark some target features as 'forbidden' so they cannot be (un)set with -Ctarget-feature

compiler/rustc_codegen_gcc/messages.ftl

@@ -8,6 +8,9 @@ codegen_gcc_invalid_minimum_alignment =
 codegen_gcc_lto_not_supported =
     LTO is not supported. You may get a linker error.
 
+codegen_gcc_forbidden_ctarget_feature =
+    target feature `{$feature}` cannot be toggled with `-Ctarget-feature`
+
 codegen_gcc_unwinding_inline_asm =
     GCC backend does not support unwinding from inline asm
 
@@ -24,11 +27,15 @@ codegen_gcc_lto_dylib = lto cannot be used for `dylib` crate type without `-Zdyl
 codegen_gcc_lto_bitcode_from_rlib = failed to get bitcode from object file for LTO ({$gcc_err})
 
 codegen_gcc_unknown_ctarget_feature =
-    unknown feature specified for `-Ctarget-feature`: `{$feature}`
-    .note = it is still passed through to the codegen backend
+    unknown and unstable feature specified for `-Ctarget-feature`: `{$feature}`
+    .note = it is still passed through to the codegen backend, but use of this feature might be unsound and the behavior of this feature can change in the future
     .possible_feature = you might have meant: `{$rust_feature}`
     .consider_filing_feature_request = consider filing a feature request
 
+codegen_gcc_unstable_ctarget_feature =
+    unstable feature specified for `-Ctarget-feature`: `{$feature}`
+    .note = this feature is not stably supported; its behavior can change in the future
+
 codegen_gcc_missing_features =
     add the missing features in a `target_feature` attribute
 

compiler/rustc_codegen_gcc/src/errors.rs

@@ -17,6 +17,19 @@ pub(crate) struct UnknownCTargetFeature<'a> {
     pub rust_feature: PossibleFeature<'a>,
 }
 
+#[derive(Diagnostic)]
+#[diag(codegen_gcc_unstable_ctarget_feature)]
+#[note]
+pub(crate) struct UnstableCTargetFeature<'a> {
+    pub feature: &'a str,
+}
+
+#[derive(Diagnostic)]
+#[diag(codegen_gcc_forbidden_ctarget_feature)]
+pub(crate) struct ForbiddenCTargetFeature<'a> {
+    pub feature: &'a str,
+}
+
 #[derive(Subdiagnostic)]
 pub(crate) enum PossibleFeature<'a> {
     #[help(codegen_gcc_possible_feature)]

compiler/rustc_codegen_gcc/src/gcc_util.rs

@@ -5,10 +5,13 @@ use rustc_codegen_ssa::errors::TargetFeatureDisableOrEnable;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_middle::bug;
 use rustc_session::Session;
-use rustc_target::target_features::RUSTC_SPECIFIC_FEATURES;
+use rustc_target::target_features::{RUSTC_SPECIFIC_FEATURES, Stability};
 use smallvec::{SmallVec, smallvec};
 
-use crate::errors::{PossibleFeature, UnknownCTargetFeature, UnknownCTargetFeaturePrefix};
+use crate::errors::{
+    ForbiddenCTargetFeature, PossibleFeature, UnknownCTargetFeature, UnknownCTargetFeaturePrefix,
+    UnstableCTargetFeature,
+};
 
 /// The list of GCC features computed from CLI flags (`-Ctarget-cpu`, `-Ctarget-feature`,
 /// `--target` and similar).
@@ -43,7 +46,7 @@ pub(crate) fn global_gcc_features(sess: &Session, diagnostics: bool) -> Vec<Stri
     );
 
     // -Ctarget-features
-    let supported_features = sess.target.supported_target_features();
+    let known_features = sess.target.rust_target_features();
     let mut featsmap = FxHashMap::default();
     let feats = sess
         .opts
@@ -62,37 +65,49 @@ pub(crate) fn global_gcc_features(sess: &Session, diagnostics: bool) -> Vec<Stri
                 }
             };
 
+            // Get the backend feature name, if any.
+            // This excludes rustc-specific features, that do not get passed down to GCC.
             let feature = backend_feature_name(s)?;
             // Warn against use of GCC specific feature names on the CLI.
-            if diagnostics && !supported_features.iter().any(|&(v, _, _)| v == feature) {
-                let rust_feature = supported_features.iter().find_map(|&(rust_feature, _, _)| {
-                    let gcc_features = to_gcc_features(sess, rust_feature);
-                    if gcc_features.contains(&feature) && !gcc_features.contains(&rust_feature) {
-                        Some(rust_feature)
-                    } else {
-                        None
+            if diagnostics {
+                let feature_state = known_features.iter().find(|&&(v, _, _)| v == feature);
+                match feature_state {
+                    None => {
+                        let rust_feature =
+                            known_features.iter().find_map(|&(rust_feature, _, _)| {
+                                let gcc_features = to_gcc_features(sess, rust_feature);
+                                if gcc_features.contains(&feature)
+                                    && !gcc_features.contains(&rust_feature)
+                                {
+                                    Some(rust_feature)
+                                } else {
+                                    None
+                                }
+                            });
+                        let unknown_feature = if let Some(rust_feature) = rust_feature {
+                            UnknownCTargetFeature {
+                                feature,
+                                rust_feature: PossibleFeature::Some { rust_feature },
+                            }
+                        } else {
+                            UnknownCTargetFeature { feature, rust_feature: PossibleFeature::None }
+                        };
+                        sess.dcx().emit_warn(unknown_feature);
                     }
-                });
-                let unknown_feature = if let Some(rust_feature) = rust_feature {
-                    UnknownCTargetFeature {
-                        feature,
-                        rust_feature: PossibleFeature::Some { rust_feature },
+                    Some((_, Stability::Stable, _)) => {}
+                    Some((_, Stability::Unstable(_), _)) => {
+                        // An unstable feature. Warn about using it.
+                        sess.dcx().emit_warn(UnstableCTargetFeature { feature });
                     }
-                } else {
-                    UnknownCTargetFeature { feature, rust_feature: PossibleFeature::None }
-                };
-                sess.dcx().emit_warn(unknown_feature);
-            }
+                    Some((_, Stability::Forbidden { .. }, _)) => {
+                        sess.dcx().emit_err(ForbiddenCTargetFeature { feature });
+                    }
+                }
 
-            if diagnostics {
                 // FIXME(nagisa): figure out how to not allocate a full hashset here.
                 featsmap.insert(feature, enable_disable == '+');
             }
 
-            // rustc-specific features do not get passed down to GCC…
-            if RUSTC_SPECIFIC_FEATURES.contains(&feature) {
-                return None;
-            }
             // ... otherwise though we run through `to_gcc_features` when
             // passing requests down to GCC. This means that all in-language
             // features also work on the command line instead of having two

compiler/rustc_codegen_gcc/src/lib.rs

@@ -491,8 +491,9 @@ pub fn target_features(
 ) -> Vec<Symbol> {
     // TODO(antoyo): use global_gcc_features.
     sess.target
-        .supported_target_features()
+        .rust_target_features()
         .iter()
+        .filter(|(_, gate, _)| gate.is_supported())
         .filter_map(|&(feature, gate, _)| {
             if sess.is_nightly_build() || allow_unstable || gate.is_stable() {
                 Some(feature)

compiler/rustc_codegen_llvm/messages.ftl

@@ -7,6 +7,11 @@ codegen_llvm_dynamic_linking_with_lto =
 
 codegen_llvm_fixed_x18_invalid_arch = the `-Zfixed-x18` flag is not supported on the `{$arch}` architecture
 
+codegen_llvm_forbidden_ctarget_feature =
+    target feature `{$feature}` cannot be toggled with `-Ctarget-feature`: {$reason}
+    .note = this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+codegen_llvm_forbidden_ctarget_feature_issue = for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>
+
 codegen_llvm_from_llvm_diag = {$message}
 
 codegen_llvm_from_llvm_optimization_diag = {$filename}:{$line}:{$column} {$pass_name} ({$kind}): {$message}

compiler/rustc_codegen_llvm/src/errors.rs

@@ -31,6 +31,15 @@ pub(crate) struct UnstableCTargetFeature<'a> {
     pub feature: &'a str,
 }
 
+#[derive(Diagnostic)]
+#[diag(codegen_llvm_forbidden_ctarget_feature)]
+#[note]
+#[note(codegen_llvm_forbidden_ctarget_feature_issue)]
+pub(crate) struct ForbiddenCTargetFeature<'a> {
+    pub feature: &'a str,
+    pub reason: &'a str,
+}
+
 #[derive(Subdiagnostic)]
 pub(crate) enum PossibleFeature<'a> {
     #[help(codegen_llvm_possible_feature)]

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -16,12 +16,12 @@ use rustc_session::Session;
 use rustc_session::config::{PrintKind, PrintRequest};
 use rustc_span::symbol::Symbol;
 use rustc_target::spec::{MergeFunctions, PanicStrategy, SmallDataThresholdSupport};
-use rustc_target::target_features::{RUSTC_SPECIAL_FEATURES, RUSTC_SPECIFIC_FEATURES};
+use rustc_target::target_features::{RUSTC_SPECIAL_FEATURES, RUSTC_SPECIFIC_FEATURES, Stability};
 
 use crate::back::write::create_informational_target_machine;
 use crate::errors::{
-    FixedX18InvalidArch, InvalidTargetFeaturePrefix, PossibleFeature, UnknownCTargetFeature,
-    UnknownCTargetFeaturePrefix, UnstableCTargetFeature,
+    FixedX18InvalidArch, ForbiddenCTargetFeature, InvalidTargetFeaturePrefix, PossibleFeature,
+    UnknownCTargetFeature, UnknownCTargetFeaturePrefix, UnstableCTargetFeature,
 };
 use crate::llvm;
 
@@ -280,19 +280,29 @@ pub(crate) fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> Option<LLVMFea
     }
 }
 
-/// Used to generate cfg variables and apply features
-/// Must express features in the way Rust understands them
+/// Used to generate cfg variables and apply features.
+/// Must express features in the way Rust understands them.
+///
+/// We do not have to worry about RUSTC_SPECIFIC_FEATURES here, those are handled outside codegen.
 pub fn target_features(sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
-    let mut features = vec![];
-
-    // Add base features for the target
+    let mut features: FxHashSet<Symbol> = Default::default();
+
+    // Add base features for the target.
+    // We do *not* add the -Ctarget-features there, and instead duplicate the logic for that below.
+    // The reason is that if LLVM considers a feature implied but we do not, we don't want that to
+    // show up in `cfg`. That way, `cfg` is entirely under our control -- except for the handling of
+    // the target CPU, that is still expanded to target features (with all their implied features) by
+    // LLVM.
     let target_machine = create_informational_target_machine(sess, true);
+    // Compute which of the known target features are enabled in the 'base' target machine.
+    // We only consider "supported" features; "forbidden" features are not reflected in `cfg` as of now.
     features.extend(
         sess.target
-            .supported_target_features()
+            .rust_target_features()
             .iter()
+            .filter(|(_, gate, _)| gate.is_supported())
             .filter(|(feature, _, _)| {
-                // skip checking special features, as LLVM may not understands them
+                // skip checking special features, as LLVM may not understand them
                 if RUSTC_SPECIAL_FEATURES.contains(feature) {
                     return true;
                 }
@@ -323,16 +333,22 @@ pub fn target_features(sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
         if enabled {
             features.extend(sess.target.implied_target_features(std::iter::once(feature)));
         } else {
+            // We don't care about the order in `features` since the only thing we use it for is the
+            // `features.contains` below.
+            #[allow(rustc::potential_query_instability)]
             features.retain(|f| {
+                // Keep a feature if it does not imply `feature`. Or, equivalently,
+                // remove the reverse-dependencies of `feature`.
                 !sess.target.implied_target_features(std::iter::once(*f)).contains(&feature)
             });
         }
     }
 
     // Filter enabled features based on feature gates
     sess.target
-        .supported_target_features()
+        .rust_target_features()
         .iter()
+        .filter(|(_, gate, _)| gate.is_supported())
         .filter_map(|&(feature, gate, _)| {
             if sess.is_nightly_build() || allow_unstable || gate.is_stable() {
                 Some(feature)
@@ -392,9 +408,13 @@ fn print_target_features(out: &mut String, sess: &Session, tm: &llvm::TargetMach
     let mut known_llvm_target_features = FxHashSet::<&'static str>::default();
     let mut rustc_target_features = sess
         .target
-        .supported_target_features()
+        .rust_target_features()
         .iter()
-        .filter_map(|(feature, _gate, _implied)| {
+        .filter_map(|(feature, gate, _implied)| {
+            if !gate.is_supported() {
+                // Only list (experimentally) supported features.
+                return None;
+            }
             // LLVM asserts that these are sorted. LLVM and Rust both use byte comparison for these
             // strings.
             let llvm_feature = to_llvm_features(sess, *feature)?.llvm_feature_name;
@@ -567,7 +587,7 @@ pub(crate) fn global_llvm_features(
 
     // -Ctarget-features
     if !only_base_features {
-        let supported_features = sess.target.supported_target_features();
+        let known_features = sess.target.rust_target_features();
         let mut featsmap = FxHashMap::default();
 
         // insert implied features
@@ -601,50 +621,53 @@ pub(crate) fn global_llvm_features(
                     }
                 };
 
+                // Get the backend feature name, if any.
+                // This excludes rustc-specific features, which do not get passed to LLVM.
                 let feature = backend_feature_name(sess, s)?;
                 // Warn against use of LLVM specific feature names and unstable features on the CLI.
                 if diagnostics {
-                    let feature_state = supported_features.iter().find(|&&(v, _, _)| v == feature);
-                    if feature_state.is_none() {
-                        let rust_feature =
-                            supported_features.iter().find_map(|&(rust_feature, _, _)| {
-                                let llvm_features = to_llvm_features(sess, rust_feature)?;
-                                if llvm_features.contains(feature)
-                                    && !llvm_features.contains(rust_feature)
-                                {
-                                    Some(rust_feature)
-                                } else {
-                                    None
+                    let feature_state = known_features.iter().find(|&&(v, _, _)| v == feature);
+                    match feature_state {
+                        None => {
+                            let rust_feature =
+                                known_features.iter().find_map(|&(rust_feature, _, _)| {
+                                    let llvm_features = to_llvm_features(sess, rust_feature)?;
+                                    if llvm_features.contains(feature)
+                                        && !llvm_features.contains(rust_feature)
+                                    {
+                                        Some(rust_feature)
+                                    } else {
+                                        None
+                                    }
+                                });
+                            let unknown_feature = if let Some(rust_feature) = rust_feature {
+                                UnknownCTargetFeature {
+                                    feature,
+                                    rust_feature: PossibleFeature::Some { rust_feature },
                                 }
-                            });
-                        let unknown_feature = if let Some(rust_feature) = rust_feature {
-                            UnknownCTargetFeature {
-                                feature,
-                                rust_feature: PossibleFeature::Some { rust_feature },
-                            }
-                        } else {
-                            UnknownCTargetFeature { feature, rust_feature: PossibleFeature::None }
-                        };
-                        sess.dcx().emit_warn(unknown_feature);
-                    } else if feature_state
-                        .is_some_and(|(_name, feature_gate, _implied)| !feature_gate.is_stable())
-                    {
-                        // An unstable feature. Warn about using it.
-                        sess.dcx().emit_warn(UnstableCTargetFeature { feature });
+                            } else {
+                                UnknownCTargetFeature {
+                                    feature,
+                                    rust_feature: PossibleFeature::None,
+                                }
+                            };
+                            sess.dcx().emit_warn(unknown_feature);
+                        }
+                        Some((_, Stability::Stable, _)) => {}
+                        Some((_, Stability::Unstable(_), _)) => {
+                            // An unstable feature. Warn about using it.
+                            sess.dcx().emit_warn(UnstableCTargetFeature { feature });
+                        }
+                        Some((_, Stability::Forbidden { reason }, _)) => {
+                            sess.dcx().emit_warn(ForbiddenCTargetFeature { feature, reason });
+                        }
                     }
-                }
 
-                if diagnostics {
                     // FIXME(nagisa): figure out how to not allocate a full hashset here.
                     featsmap.insert(feature, enable_disable == '+');
                 }
 
-                // rustc-specific features do not get passed down to LLVM…
-                if RUSTC_SPECIFIC_FEATURES.contains(&feature) {
-                    return None;
-                }
-
-                // ... otherwise though we run through `to_llvm_features` when
+                // We run through `to_llvm_features` when
                 // passing requests down to LLVM. This means that all in-language
                 // features also work on the command line instead of having two
                 // different names when the LLVM name and the Rust name differ.

compiler/rustc_codegen_ssa/messages.ftl

@@ -58,6 +58,9 @@ codegen_ssa_failed_to_write = failed to write {$path}: {$error}
 
 codegen_ssa_field_associated_value_expected = associated value expected for `{$name}`
 
+codegen_ssa_forbidden_target_feature_attr =
+    target feature `{$feature}` cannot be toggled with `#[target_feature]`: {$reason}
+
 codegen_ssa_ignoring_emit_path = ignoring emit path because multiple .{$extension} files were produced
 
 codegen_ssa_ignoring_output = ignoring -o because multiple .{$extension} files were produced