Auto merge of #133099 - RalfJung:forbidden-hardfloat-features, r=workingjubilee

forbid toggling x87 and fpregs on hard-float targets

Part of https://github.com/rust-lang/rust/issues/116344, follow-up to https://github.com/rust-lang/rust/pull/129884:

The `x87`  target feature on x86 and the `fpregs` target feature on ARM must not be disabled on a hardfloat target, as that would change the float ABI. However, *enabling* `fpregs` on ARM is [explicitly requested](https://github.com/rust-lang/rust/issues/130988) as it seems to be useful. Therefore, we need to refine the distinction of "forbidden" target features and "allowed" target features: all (un)stable target features can determine on a per-target basis whether they should be allowed to be toggled or not. `fpregs` then checks whether the current target has the `soft-float` feature, and if yes, `fpregs` is permitted -- otherwise, it is not. (Same for `x87` on x86).

Also fixes https://github.com/rust-lang/rust/issues/132351. Since `fpregs` and `x87` can be enabled on some builds and disabled on others, it would make sense that one can query it via `cfg`. Therefore, I made them behave in `cfg` like any other unstable target feature.

The first commit prepares the infrastructure, but does not change behavior. The second commit then wires up `fpregs` and `x87` with that new infrastructure.

r? `@workingjubilee`

16 files changed, 81 insertions, 6 deletions

diff --git a/tests/ui/check-cfg/target_feature.stderr b/tests/ui/check-cfg/target_feature.stderr
index 3df1545cd4a..e2ceb669482 100644
--- a/tests/ui/check-cfg/target_feature.stderr
+++ b/tests/ui/check-cfg/target_feature.stderr
@@ -98,6 +98,7 @@ LL |     cfg!(target_feature = "_UNEXPECTED_VALUE");
 `fp8dot2`
 `fp8dot4`
 `fp8fma`
+`fpregs`
 `fpuv2_df`
 `fpuv2_sf`
 `fpuv3_df`
@@ -261,6 +262,7 @@ LL |     cfg!(target_feature = "_UNEXPECTED_VALUE");
 `vsx`
 `wfxt`
 `wide-arithmetic`
+`x87`
 `xop`
 `xsave`
 `xsavec`
diff --git a/tests/ui/target-feature/allowed-softfloat-target-feature-attribute.rs b/tests/ui/target-feature/allowed-softfloat-target-feature-attribute.rs
new file mode 100644
index 00000000000..8b60820cc9b
--- /dev/null
+++ b/tests/ui/target-feature/allowed-softfloat-target-feature-attribute.rs
@@ -0,0 +1,11 @@
+//@ compile-flags: --target=x86_64-unknown-none --crate-type=lib
+//@ needs-llvm-components: x86
+//@ build-pass
+#![feature(no_core, lang_items, x87_target_feature)]
+#![no_core]
+
+#[lang = "sized"]
+pub trait Sized {}
+
+#[target_feature(enable = "x87")]
+pub unsafe fn my_fun() {}
diff --git a/tests/ui/target-feature/allowed-softfloat-target-feature-flag-disable.rs b/tests/ui/target-feature/allowed-softfloat-target-feature-flag-disable.rs
new file mode 100644
index 00000000000..e34faf5a983
--- /dev/null
+++ b/tests/ui/target-feature/allowed-softfloat-target-feature-flag-disable.rs
@@ -0,0 +1,9 @@
+//@ compile-flags: --target=x86_64-unknown-none --crate-type=lib
+//@ needs-llvm-components: x86
+//@ compile-flags: -Ctarget-feature=-x87
+//@ build-pass
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang = "sized"]
+pub trait Sized {}
diff --git a/tests/ui/target-feature/allowed-softfloat-target-feature-flag-disable.stderr b/tests/ui/target-feature/allowed-softfloat-target-feature-flag-disable.stderr
new file mode 100644
index 00000000000..309b64afd92
--- /dev/null
+++ b/tests/ui/target-feature/allowed-softfloat-target-feature-flag-disable.stderr
@@ -0,0 +1,6 @@
+warning: unstable feature specified for `-Ctarget-feature`: `x87`
+   |
+   = note: this feature is not stably supported; its behavior can change in the future
+
+warning: 1 warning emitted
+
diff --git a/tests/ui/target-feature/forbidden-hardfloat-target-feature-attribute.rs b/tests/ui/target-feature/forbidden-hardfloat-target-feature-attribute.rs
new file mode 100644
index 00000000000..b3171d52c51
--- /dev/null
+++ b/tests/ui/target-feature/forbidden-hardfloat-target-feature-attribute.rs
@@ -0,0 +1,11 @@
+//@ compile-flags: --target=x86_64-unknown-linux-gnu --crate-type=lib
+//@ needs-llvm-components: x86
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang = "sized"]
+pub trait Sized {}
+
+#[target_feature(enable = "x87")]
+//~^ERROR: cannot be toggled with
+pub unsafe fn my_fun() {}
diff --git a/tests/ui/target-feature/forbidden-hardfloat-target-feature-attribute.stderr b/tests/ui/target-feature/forbidden-hardfloat-target-feature-attribute.stderr
new file mode 100644
index 00000000000..3ebbe69d8ae
--- /dev/null
+++ b/tests/ui/target-feature/forbidden-hardfloat-target-feature-attribute.stderr
@@ -0,0 +1,8 @@
+error: target feature `x87` cannot be toggled with `#[target_feature]`: unsound on hard-float targets because it changes float ABI
+  --> $DIR/forbidden-hardfloat-target-feature-attribute.rs:9:18
+   |
+LL | #[target_feature(enable = "x87")]
+   |                  ^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
diff --git a/tests/ui/target-feature/forbidden-hardfloat-target-feature-cfg.rs b/tests/ui/target-feature/forbidden-hardfloat-target-feature-cfg.rs
new file mode 100644
index 00000000000..8755791c1c0
--- /dev/null
+++ b/tests/ui/target-feature/forbidden-hardfloat-target-feature-cfg.rs
@@ -0,0 +1,14 @@
+//@ compile-flags: --target=x86_64-unknown-linux-gnu --crate-type=lib
+//@ needs-llvm-components: x86
+//@ check-pass
+#![feature(no_core, lang_items)]
+#![no_core]
+#![allow(unexpected_cfgs)]
+
+#[lang = "sized"]
+pub trait Sized {}
+
+// The compile_error macro does not exist, so if the `cfg` evaluates to `true` this
+// complains about the missing macro rather than showing the error... but that's good enough.
+#[cfg(not(target_feature = "x87"))]
+compile_error!("the x87 feature *should* be exposed in `cfg`");
diff --git a/tests/ui/target-feature/forbidden-hardfloat-target-feature-flag-disable.rs b/tests/ui/target-feature/forbidden-hardfloat-target-feature-flag-disable.rs
new file mode 100644
index 00000000000..fd8023664da
--- /dev/null
+++ b/tests/ui/target-feature/forbidden-hardfloat-target-feature-flag-disable.rs
@@ -0,0 +1,10 @@
+//@ compile-flags: --target=x86_64-unknown-linux-gnu --crate-type=lib
+//@ needs-llvm-components: x86
+//@ compile-flags: -Ctarget-feature=-x87
+// For now this is just a warning.
+//@ build-pass
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang = "sized"]
+pub trait Sized {}
diff --git a/tests/ui/target-feature/forbidden-hardfloat-target-feature-flag-disable.stderr b/tests/ui/target-feature/forbidden-hardfloat-target-feature-flag-disable.stderr
new file mode 100644
index 00000000000..604ad2f991a
--- /dev/null
+++ b/tests/ui/target-feature/forbidden-hardfloat-target-feature-flag-disable.stderr
@@ -0,0 +1,7 @@
+warning: target feature `x87` cannot be toggled with `-Ctarget-feature`: unsound on hard-float targets 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>
+
+warning: 1 warning emitted
+
diff --git a/tests/ui/target-feature/forbidden-target-feature-attribute.rs b/tests/ui/target-feature/forbidden-target-feature-attribute.rs
index 91c56b43689..f13cdd17da6 100644
--- a/tests/ui/target-feature/forbidden-target-feature-attribute.rs
+++ b/tests/ui/target-feature/forbidden-target-feature-attribute.rs
@@ -1,7 +1,6 @@
 //@ compile-flags: --target=x86_64-unknown-linux-gnu --crate-type=lib
 //@ needs-llvm-components: x86
 #![feature(no_core, lang_items)]
-#![no_std]
 #![no_core]
 
 #[lang = "sized"]
diff --git a/tests/ui/target-feature/forbidden-target-feature-attribute.stderr b/tests/ui/target-feature/forbidden-target-feature-attribute.stderr
index fb318531f7e..27ac4aaf960 100644
--- a/tests/ui/target-feature/forbidden-target-feature-attribute.stderr
+++ b/tests/ui/target-feature/forbidden-target-feature-attribute.stderr
@@ -1,5 +1,5 @@
 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
+  --> $DIR/forbidden-target-feature-attribute.rs:9:18
    |
 LL | #[target_feature(enable = "soft-float")]
    |                  ^^^^^^^^^^^^^^^^^^^^^
diff --git a/tests/ui/target-feature/forbidden-target-feature-cfg.rs b/tests/ui/target-feature/forbidden-target-feature-cfg.rs
index 5df26e26793..1f001e9f8ff 100644
--- a/tests/ui/target-feature/forbidden-target-feature-cfg.rs
+++ b/tests/ui/target-feature/forbidden-target-feature-cfg.rs
@@ -2,7 +2,6 @@
 //@ needs-llvm-components: x86
 //@ check-pass
 #![feature(no_core, lang_items)]
-#![no_std]
 #![no_core]
 #![allow(unexpected_cfgs)]
 
diff --git a/tests/ui/target-feature/forbidden-target-feature-flag-disable.rs b/tests/ui/target-feature/forbidden-target-feature-flag-disable.rs
index b27e8a10afe..b09c53bd46a 100644
--- a/tests/ui/target-feature/forbidden-target-feature-flag-disable.rs
+++ b/tests/ui/target-feature/forbidden-target-feature-flag-disable.rs
@@ -4,7 +4,6 @@
 // For now this is just a warning.
 //@ build-pass
 #![feature(no_core, lang_items)]
-#![no_std]
 #![no_core]
 
 #[lang = "sized"]
diff --git a/tests/ui/target-feature/forbidden-target-feature-flag.rs b/tests/ui/target-feature/forbidden-target-feature-flag.rs
index 93cebc6b536..0f688fde7f4 100644
--- a/tests/ui/target-feature/forbidden-target-feature-flag.rs
+++ b/tests/ui/target-feature/forbidden-target-feature-flag.rs
@@ -4,7 +4,6 @@
 // For now this is just a warning.
 //@ build-pass
 #![feature(no_core, lang_items)]
-#![no_std]
 #![no_core]
 
 #[lang = "sized"]
diff --git a/tests/ui/target-feature/gate.rs b/tests/ui/target-feature/gate.rs
index f35fbd11155..2626685fa0a 100644
--- a/tests/ui/target-feature/gate.rs
+++ b/tests/ui/target-feature/gate.rs
@@ -24,6 +24,7 @@
 // gate-test-prfchw_target_feature
 // gate-test-s390x_target_feature
 // gate-test-sparc_target_feature
+// gate-test-x87_target_feature
 
 #[target_feature(enable = "avx512bw")]
 //~^ ERROR: currently unstable
diff --git a/tests/ui/target-feature/gate.stderr b/tests/ui/target-feature/gate.stderr
index b84bab370be..ba5ae79f942 100644
--- a/tests/ui/target-feature/gate.stderr
+++ b/tests/ui/target-feature/gate.stderr
@@ -1,5 +1,5 @@
 error[E0658]: the target feature `avx512bw` is currently unstable
-  --> $DIR/gate.rs:28:18
+  --> $DIR/gate.rs:29:18
    |
 LL | #[target_feature(enable = "avx512bw")]
    |                  ^^^^^^^^^^^^^^^^^^^