Rollup merge of #134329 - taiki-e:m68k-target-feature, r=workingjubilee

Add m68k_target_feature

This adds the following unstable target features (tracking issue: https://github.com/rust-lang/rust/issues/134328):

- isa-68000
- isa-68010
- isa-68020
- isa-68030
- isa-68040
- isa-68060
- isa-68881
- isa-68882

The feature names and implied features are match with [definitions in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/M68k/M68k.td#L21-L57).

isa-68881 and isa-68882 are FPU ISA features.
isa-68881 is needed to support input/output in floating-point regs in inline assembly. isa-68020 is needed to implement https://github.com/taiki-e/atomic-maybe-uninit/pull/28 more robustly.

cc `@glaubitz` `@ricky26` (designated developers  of [m68k-unknown-linux-gnu](https://doc.rust-lang.org/nightly/rustc/platform-support/m68k-unknown-linux-gnu.html#designated-developers))
r? workingjubilee

`@rustbot` label +O-motorola68k +A-target-feature

compiler/rustc_feature/src/unstable.rs

@@ -332,6 +332,7 @@ declare_features! (
     (unstable, hexagon_target_feature, "1.27.0", Some(44839)),
     (unstable, lahfsahf_target_feature, "1.78.0", Some(44839)),
     (unstable, loongarch_target_feature, "1.73.0", Some(44839)),
+    (unstable, m68k_target_feature, "CURRENT_RUSTC_VERSION", Some(134328)),
     (unstable, mips_target_feature, "1.27.0", Some(44839)),
     (unstable, powerpc_target_feature, "1.27.0", Some(44839)),
     (unstable, prfchw_target_feature, "1.78.0", Some(44839)),

compiler/rustc_span/src/symbol.rs

@@ -1190,6 +1190,7 @@ symbols! {
         loongarch_target_feature,
         loop_break_value,
         lt,
+        m68k_target_feature,
         macro_at_most_once_rep,
         macro_attributes_in_derive_output,
         macro_escape,

compiler/rustc_target/src/target_features.rs

@@ -668,6 +668,20 @@ const SPARC_FEATURES: &[(&str, StabilityUncomputed, ImpliedFeatures)] = &[
     // tidy-alphabetical-end
 ];
 
+const M68K_FEATURES: &[(&str, StabilityUncomputed, ImpliedFeatures)] = &[
+    // tidy-alphabetical-start
+    ("isa-68000", unstable(sym::m68k_target_feature), &[]),
+    ("isa-68010", unstable(sym::m68k_target_feature), &["isa-68000"]),
+    ("isa-68020", unstable(sym::m68k_target_feature), &["isa-68010"]),
+    ("isa-68030", unstable(sym::m68k_target_feature), &["isa-68020"]),
+    ("isa-68040", unstable(sym::m68k_target_feature), &["isa-68030", "isa-68882"]),
+    ("isa-68060", unstable(sym::m68k_target_feature), &["isa-68040"]),
+    // FPU
+    ("isa-68881", unstable(sym::m68k_target_feature), &[]),
+    ("isa-68882", unstable(sym::m68k_target_feature), &["isa-68881"]),
+    // tidy-alphabetical-end
+];
+
 /// When rustdoc is running, provide a list of all known features so that all their respective
 /// primitives may be documented.
 ///
@@ -687,6 +701,7 @@ pub fn all_rust_features() -> impl Iterator<Item = (&'static str, StabilityUncom
         .chain(LOONGARCH_FEATURES)
         .chain(IBMZ_FEATURES)
         .chain(SPARC_FEATURES)
+        .chain(M68K_FEATURES)
         .cloned()
         .map(|(f, s, _)| (f, s))
 }
@@ -734,6 +749,7 @@ impl Target {
             "loongarch64" => LOONGARCH_FEATURES,
             "s390x" => IBMZ_FEATURES,
             "sparc" | "sparc64" => SPARC_FEATURES,
+            "m68k" => M68K_FEATURES,
             _ => &[],
         }
     }
@@ -751,7 +767,7 @@ impl Target {
             "sparc" | "sparc64" => SPARC_FEATURES_FOR_CORRECT_VECTOR_ABI,
             "hexagon" => HEXAGON_FEATURES_FOR_CORRECT_VECTOR_ABI,
             "mips" | "mips32r6" | "mips64" | "mips64r6" => MIPS_FEATURES_FOR_CORRECT_VECTOR_ABI,
-            "bpf" => &[], // no vector ABI
+            "bpf" | "m68k" => &[], // no vector ABI
             "csky" => CSKY_FEATURES_FOR_CORRECT_VECTOR_ABI,
             // FIXME: for some tier3 targets, we are overly cautious and always give warnings
             // when passing args in vector registers.

tests/ui/check-cfg/target_feature.stderr

@@ -118,6 +118,14 @@ LL |     cfg!(target_feature = "_UNEXPECTED_VALUE");
 `hvx-length128b`
 `hwdiv`
 `i8mm`
+`isa-68000`
+`isa-68010`
+`isa-68020`
+`isa-68030`
+`isa-68040`
+`isa-68060`
+`isa-68881`
+`isa-68882`
 `jsconv`
 `lahfsahf`
 `lasx`

tests/ui/target-feature/gate.rs

@@ -25,6 +25,7 @@
 // gate-test-s390x_target_feature
 // gate-test-sparc_target_feature
 // gate-test-x87_target_feature
+// gate-test-m68k_target_feature
 
 #[target_feature(enable = "avx512bw")]
 //~^ ERROR: currently unstable

tests/ui/target-feature/gate.stderr

@@ -1,5 +1,5 @@
 error[E0658]: the target feature `avx512bw` is currently unstable
-  --> $DIR/gate.rs:29:18
+  --> $DIR/gate.rs:30:18
    |
 LL | #[target_feature(enable = "avx512bw")]
    |                  ^^^^^^^^^^^^^^^^^^^