make this just a warning (for use with the -C flag) for now; also show a reason why it is forbidden

Author: RalfJung

compiler/rustc_codegen_gcc/src/gcc_util.rs

@@ -97,7 +97,7 @@ pub(crate) fn global_gcc_features(sess: &Session, diagnostics: bool) -> Vec<Stri
                         // An unstable feature. Warn about using it.
                         sess.dcx().emit_warn(UnstableCTargetFeature { feature });
                     }
-                    Some((_, Stability::Forbidden, _)) => {
+                    Some((_, Stability::Forbidden { .. }, _)) => {
                         sess.dcx().emit_err(ForbiddenCTargetFeature { feature });
                     }
                 }

compiler/rustc_codegen_llvm/messages.ftl

@@ -8,7 +8,9 @@ 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 `{$feature}` cannot be toggled with `-Ctarget-feature`
+    target feature `{$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}
 

compiler/rustc_codegen_llvm/src/errors.rs

@@ -33,8 +33,11 @@ pub(crate) struct UnstableCTargetFeature<'a> {
 
 #[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)]

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -679,8 +679,8 @@ pub(crate) fn global_llvm_features(
                             // An unstable feature. Warn about using it.
                             sess.dcx().emit_warn(UnstableCTargetFeature { feature });
                         }
-                        Some((_, Stability::Forbidden, _)) => {
-                            sess.dcx().emit_err(ForbiddenCTargetFeature { feature });
+                        Some((_, Stability::Forbidden { reason }, _)) => {
+                            sess.dcx().emit_warn(ForbiddenCTargetFeature { feature, reason });
                         }
                     }
 

compiler/rustc_codegen_ssa/messages.ftl

@@ -59,7 +59,7 @@ 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 =
-    target feature `{$feature}` cannot be toggled with `#[target_feature]`
+    target feature `{$feature}` cannot be toggled with `#[target_feature]`: {$reason}
 
 codegen_ssa_ignoring_emit_path = ignoring emit path because multiple .{$extension} files were produced
 

compiler/rustc_codegen_ssa/src/errors.rs

@@ -1023,6 +1023,7 @@ pub struct ForbiddenTargetFeature<'a> {
     #[primary_span]
     pub span: Span,
     pub feature: &'a str,
+    pub reason: &'a str,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -66,7 +66,7 @@ pub fn from_target_feature_attr(
 
             // Only allow features whose feature gates have been enabled.
             let allowed = match stability {
-                Stability::Forbidden => false,
+                Stability::Forbidden { .. } => false,
                 Stability::Stable => true,
                 Stability::Unstable(sym::arm_target_feature) => rust_features.arm_target_feature,
                 Stability::Unstable(sym::hexagon_target_feature) => {
@@ -125,10 +125,11 @@ pub fn from_target_feature_attr(
                         )
                         .emit();
                     }
-                    Stability::Forbidden => {
+                    Stability::Forbidden { reason } => {
                         tcx.dcx().emit_err(errors::ForbiddenTargetFeature {
                             span: item.span(),
                             feature,
+                            reason,
                         });
                     }
                 }

compiler/rustc_target/src/target_features.rs

@@ -25,7 +25,7 @@ pub enum Stability {
     Unstable(Symbol),
     /// This feature can not be set via `-Ctarget-feature` or `#[target_feature]`, it can only be set in the basic
     /// target definition. Used in particular for features that change the floating-point ABI.
-    Forbidden,
+    Forbidden { reason: &'static str },
 }
 use Stability::*;
 
@@ -38,7 +38,7 @@ impl<CTX> HashStable<CTX> for Stability {
             Unstable(sym) => {
                 sym.hash_stable(hcx, hasher);
             }
-            Forbidden => {}
+            Forbidden { .. } => {}
         }
     }
 }
@@ -50,7 +50,7 @@ impl Stability {
 
     /// Forbidden features are not supported.
     pub fn is_supported(self) -> bool {
-        !matches!(self, Forbidden)
+        !matches!(self, Forbidden { .. })
     }
 }
 
@@ -101,13 +101,13 @@ const ARM_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("dotprod", Unstable(sym::arm_target_feature), &["neon"]),
     ("dsp", Unstable(sym::arm_target_feature), &[]),
     ("fp-armv8", Unstable(sym::arm_target_feature), &["vfp4"]),
-    ("fpregs", Forbidden, &[]), // changes float ABI
+    ("fpregs", Forbidden { reason: "unsound because it changes float ABI" }, &[]),
     ("i8mm", Unstable(sym::arm_target_feature), &["neon"]),
     ("mclass", Unstable(sym::arm_target_feature), &[]),
     ("neon", Unstable(sym::arm_target_feature), &["vfp3"]),
     ("rclass", Unstable(sym::arm_target_feature), &[]),
     ("sha2", Unstable(sym::arm_target_feature), &["neon"]),
-    ("soft-float", Forbidden, &[]), // changes float ABI
+    ("soft-float", Forbidden { reason: "unsound because it changes float ABI" }, &[]),
     // This is needed for inline assembly, but shouldn't be stabilized as-is
     // since it should be enabled per-function using #[instruction_set], not
     // #[target_feature].
@@ -366,7 +366,7 @@ const X86_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("sha512", Unstable(sym::sha512_sm_x86), &["avx2"]),
     ("sm3", Unstable(sym::sha512_sm_x86), &["avx"]),
     ("sm4", Unstable(sym::sha512_sm_x86), &["avx2"]),
-    ("soft-float", Forbidden, &[]), // changes float ABI
+    ("soft-float", Forbidden { reason: "unsound because it changes float ABI" }, &[]),
     ("sse", Stable, &[]),
     ("sse2", Stable, &["sse"]),
     ("sse3", Stable, &["sse2"]),
@@ -377,7 +377,7 @@ const X86_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("tbm", Unstable(sym::tbm_target_feature), &[]),
     ("vaes", Unstable(sym::avx512_target_feature), &["avx2", "aes"]),
     ("vpclmulqdq", Unstable(sym::avx512_target_feature), &["avx", "pclmulqdq"]),
-    ("x87", Forbidden, &[]), // changes float ABI
+    ("x87", Forbidden { reason: "unsound because it changes float ABI" }, &[]),
     ("xop", Unstable(sym::xop_target_feature), &[/*"fma4", */ "avx", "sse4a"]),
     ("xsave", Stable, &[]),
     ("xsavec", Stable, &["xsave"]),

tests/ui/target-feature/forbidden-target-feature-attribute.stderr

@@ -1,4 +1,4 @@
-error: target feature `soft-float` cannot be toggled with `#[target_feature]`
+error: target feature `soft-float` cannot be toggled with `#[target_feature]`: unsound because it changes float ABI
   --> $DIR/forbidden-target-feature-attribute.rs:10:18
    |
 LL | #[target_feature(enable = "soft-float")]

tests/ui/target-feature/forbidden-target-feature-flag-disable.rs

@@ -1,11 +1,11 @@
 //@ compile-flags: --target=x86_64-unknown-linux-gnu --crate-type=lib
 //@ needs-llvm-components: x86
 //@ compile-flags: -Ctarget-feature=-soft-float
+// For now this is just a warning.
+//@ build-pass
 #![feature(no_core, lang_items)]
 #![no_std]
 #![no_core]
 
 #[lang = "sized"]
 pub trait Sized {}
-
-fn main() {}

tests/ui/target-feature/forbidden-target-feature-flag-disable.stderr

@@ -1,4 +1,7 @@
-error: target feature `soft-float` cannot be toggled with `-Ctarget-feature`
+warning: target feature `soft-float` cannot be toggled with `-Ctarget-feature`: unsound because it changes float ABI
+   |
+   = note: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>
 
-error: aborting due to 1 previous error
+warning: 1 warning emitted
 

tests/ui/target-feature/forbidden-target-feature-flag.rs

@@ -1,11 +1,11 @@
 //@ compile-flags: --target=x86_64-unknown-linux-gnu --crate-type=lib
 //@ needs-llvm-components: x86
 //@ compile-flags: -Ctarget-feature=+soft-float
+// For now this is just a warning.
+//@ build-pass
 #![feature(no_core, lang_items)]
 #![no_std]
 #![no_core]
 
 #[lang = "sized"]
 pub trait Sized {}
-
-fn main() {}

tests/ui/target-feature/forbidden-target-feature-flag.stderr

@@ -1,4 +1,7 @@
-error: target feature `soft-float` cannot be toggled with `-Ctarget-feature`
+warning: target feature `soft-float` cannot be toggled with `-Ctarget-feature`: unsound because it changes float ABI
+   |
+   = note: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>
 
-error: aborting due to 1 previous error
+warning: 1 warning emitted