Auto merge of #129884 - RalfJung:forbidden-target-features, r=workingjubilee

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

The context for this is https://github.com/rust-lang/rust/issues/116344: some target features change the way floats are passed between functions. Changing those target features is unsound as code compiled for the same target may now use different ABIs.

So this introduces a new concept of "forbidden" target features (on top of the existing "stable " and "unstable" categories), and makes it a hard error to (un)set such a target feature. For now, the x86 and ARM feature `soft-float` is on that list. We'll have to make some effort to collect more relevant features, and similar features from other targets, but that can happen after the basic infrastructure for this landed. (These features are being collected in https://github.com/rust-lang/rust/issues/131799.)

I've made this a warning for now to give people some time to speak up if this would break something.

MCP: https://github.com/rust-lang/compiler-team/issues/780

9 files changed, 87 insertions, 0 deletions

diff --git a/tests/ui/target-feature/auxiliary/using-target-feature-unstable.rs b/tests/ui/target-feature/auxiliary/using-target-feature-unstable.rs
new file mode 100644
index 00000000000..2682028936c
--- /dev/null
+++ b/tests/ui/target-feature/auxiliary/using-target-feature-unstable.rs
@@ -0,0 +1,5 @@
+#![feature(avx512_target_feature)]
+
+#[inline]
+#[target_feature(enable = "avx512ifma")]
+pub unsafe fn foo() {}
diff --git a/tests/ui/target-feature/forbidden-target-feature-attribute.rs b/tests/ui/target-feature/forbidden-target-feature-attribute.rs
new file mode 100644
index 00000000000..91c56b43689
--- /dev/null
+++ b/tests/ui/target-feature/forbidden-target-feature-attribute.rs
@@ -0,0 +1,12 @@
+//@ 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"]
+pub trait Sized {}
+
+#[target_feature(enable = "soft-float")]
+//~^ERROR: cannot be toggled with
+pub unsafe fn my_fun() {}
diff --git a/tests/ui/target-feature/forbidden-target-feature-attribute.stderr b/tests/ui/target-feature/forbidden-target-feature-attribute.stderr
new file mode 100644
index 00000000000..fb318531f7e
--- /dev/null
+++ b/tests/ui/target-feature/forbidden-target-feature-attribute.stderr
@@ -0,0 +1,8 @@
+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")]
+   |                  ^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
diff --git a/tests/ui/target-feature/forbidden-target-feature-cfg.rs b/tests/ui/target-feature/forbidden-target-feature-cfg.rs
new file mode 100644
index 00000000000..5df26e26793
--- /dev/null
+++ b/tests/ui/target-feature/forbidden-target-feature-cfg.rs
@@ -0,0 +1,15 @@
+//@ compile-flags: --target=x86_64-unknown-none --crate-type=lib
+//@ needs-llvm-components: x86
+//@ check-pass
+#![feature(no_core, lang_items)]
+#![no_std]
+#![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(target_feature = "soft-float")]
+compile_error!("the soft-float feature should not be exposed in `cfg`");
diff --git a/tests/ui/target-feature/forbidden-target-feature-flag-disable.rs b/tests/ui/target-feature/forbidden-target-feature-flag-disable.rs
new file mode 100644
index 00000000000..b27e8a10afe
--- /dev/null
+++ b/tests/ui/target-feature/forbidden-target-feature-flag-disable.rs
@@ -0,0 +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 {}
diff --git a/tests/ui/target-feature/forbidden-target-feature-flag-disable.stderr b/tests/ui/target-feature/forbidden-target-feature-flag-disable.stderr
new file mode 100644
index 00000000000..508e1fe0cf4
--- /dev/null
+++ b/tests/ui/target-feature/forbidden-target-feature-flag-disable.stderr
@@ -0,0 +1,7 @@
+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>
+
+warning: 1 warning emitted
+
diff --git a/tests/ui/target-feature/forbidden-target-feature-flag.rs b/tests/ui/target-feature/forbidden-target-feature-flag.rs
new file mode 100644
index 00000000000..93cebc6b536
--- /dev/null
+++ b/tests/ui/target-feature/forbidden-target-feature-flag.rs
@@ -0,0 +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 {}
diff --git a/tests/ui/target-feature/forbidden-target-feature-flag.stderr b/tests/ui/target-feature/forbidden-target-feature-flag.stderr
new file mode 100644
index 00000000000..508e1fe0cf4
--- /dev/null
+++ b/tests/ui/target-feature/forbidden-target-feature-flag.stderr
@@ -0,0 +1,7 @@
+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>
+
+warning: 1 warning emitted
+
diff --git a/tests/ui/target-feature/using-target-feature-unstable.rs b/tests/ui/target-feature/using-target-feature-unstable.rs
new file mode 100644
index 00000000000..5ec0bda5eef
--- /dev/null
+++ b/tests/ui/target-feature/using-target-feature-unstable.rs
@@ -0,0 +1,11 @@
+//@ run-pass
+//@ only-x86_64
+//@ aux-build:using-target-feature-unstable.rs
+
+extern crate using_target_feature_unstable;
+
+fn main() {
+    unsafe {
+        using_target_feature_unstable::foo();
+    }
+}