rustdoc: when merging target features, keep the highest stability

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -10,7 +10,7 @@ use rustc_middle::query::Providers;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::parse::feature_err;
 use rustc_span::{Span, Symbol, sym};
-use rustc_target::target_features;
+use rustc_target::target_features::{self, Stability};
 
 use crate::errors;
 
@@ -87,12 +87,17 @@ pub(crate) fn from_target_feature_attr(
                     // But ensure the ABI does not forbid enabling this.
                     // Here we do assume that LLVM doesn't add even more implied features
                     // we don't know about, at least no features that would have ABI effects!
-                    if abi_feature_constraints.incompatible.contains(&name.as_str()) {
-                        tcx.dcx().emit_err(errors::ForbiddenTargetFeatureAttr {
-                            span: item.span(),
-                            feature: name.as_str(),
-                            reason: "this feature is incompatible with the target ABI",
-                        });
+                    // We skip this logic in rustdoc, where we want to allow all target features of
+                    // all targets, so we can't check their ABI compatibility and anyway we are not
+                    // generating code so "it's fine".
+                    if !tcx.sess.opts.actually_rustdoc {
+                        if abi_feature_constraints.incompatible.contains(&name.as_str()) {
+                            tcx.dcx().emit_err(errors::ForbiddenTargetFeatureAttr {
+                                span: item.span(),
+                                feature: name.as_str(),
+                                reason: "this feature is incompatible with the target ABI",
+                            });
+                        }
                     }
                     target_features.push(TargetFeature { name, implied: name != feature_sym })
                 }
@@ -142,11 +147,38 @@ pub(crate) fn provide(providers: &mut Providers) {
         rust_target_features: |tcx, cnum| {
             assert_eq!(cnum, LOCAL_CRATE);
             if tcx.sess.opts.actually_rustdoc {
-                // rustdoc needs to be able to document functions that use all the features, so
-                // whitelist them all
-                rustc_target::target_features::all_rust_features()
-                    .map(|(a, b)| (a.to_string(), b))
-                    .collect()
+                // HACK: rustdoc would like to pretend that we have all the target features, so we
+                // have to merge all the lists into one. To ensure an unstable target never prevents
+                // a stable one from working, we merge the stability info of all instances of the
+                // same target feature name, with the "most stable" taking precedence. And then we
+                // hope that this doesn't cause issues anywhere else in the compiler...
+                let mut result: UnordMap<String, Stability> = Default::default();
+                for (name, stability) in rustc_target::target_features::all_rust_features() {
+                    use std::collections::hash_map::Entry;
+                    match result.entry(name.to_owned()) {
+                        Entry::Vacant(vacant_entry) => {
+                            vacant_entry.insert(stability);
+                        }
+                        Entry::Occupied(mut occupied_entry) => {
+                            // Merge the two stabilities, "more stable" taking precedence.
+                            match (occupied_entry.get(), stability) {
+                                (Stability::Stable, _)
+                                | (
+                                    Stability::Unstable { .. },
+                                    Stability::Unstable { .. } | Stability::Forbidden { .. },
+                                )
+                                | (Stability::Forbidden { .. }, Stability::Forbidden { .. }) => {
+                                    // The stability in the entry is at least as good as the new one, just keep it.
+                                }
+                                _ => {
+                                    // Overwrite stabilite.
+                                    occupied_entry.insert(stability);
+                                }
+                            }
+                        }
+                    }
+                }
+                result
             } else {
                 tcx.sess
                     .target

tests/rustdoc-ui/target-feature-stability.rs

@@ -0,0 +1,28 @@
+//! This is a regression test for <https://github.com/rust-lang/rust/issues/137366>, ensuring
+//! that we can use the `neon` target feature on ARM32 targets in rustdoc despite there
+//! being a "forbidden" feature of the same name for aarch64, and rustdoc merging the
+//! target features of all targets.
+//@ check-pass
+//@ revisions: arm aarch64
+//@[arm] compile-flags: --target armv7-unknown-linux-gnueabihf
+//@[arm] needs-llvm-components: arm
+//@[aarch64] compile-flags: --target aarch64-unknown-none-softfloat
+//@[aarch64] needs-llvm-components: aarch64
+
+#![crate_type = "lib"]
+#![feature(no_core, lang_items)]
+#![feature(arm_target_feature)]
+#![no_core]
+
+#[lang = "sized"]
+pub trait Sized {}
+
+// `fp-armv8` is "forbidden" on aarch64 as we tie it to `neon`.
+#[target_feature(enable = "fp-armv8")]
+pub fn fun1() {}
+
+// This would usually be rejected as it changes the ABI.
+// But we disable that check in rustdoc since we are building "for all targets" and the
+// check can't really handle that.
+#[target_feature(enable = "soft-float")]
+pub fn fun2() {}