Auto merge of #141465 - RalfJung:target-feature-in-trait, r=<try>

forbid #[target_feature] in traits

`target_feature` in trait *implementations* makes basically no sense -- it's almost always unsound since a generic caller has no way of knowing the required target features! In particular, now that we can have *safe* target feature functions, that's *definitely* unsound in combination with `dyn`, see #139368.

As sound version of this would have to check that the same target features are already set in the trait declaration. But only functions with bodies can even have target features in the trait declaration!

So, this was never actually designed in a way that makes any sense, it just kinda did something. Interestingly there were tests for `#[target_feature]` in traits and trait impls... not sure how that didn't set off alarm bells. Oh well.

I propose we just entirely forbid target features in trait impls and trait declarations. This will need an FCW, but before we do that let's get an idea for how big the fallout is by doing a crater run. The diagnostics that result from PR are terrible but it's good enough for crater.

Author: bors

compiler/rustc_passes/src/check_attr.rs

@@ -861,8 +861,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         attrs: &[Attribute],
     ) {
         match target {
-            Target::Method(MethodKind::Trait { body: true } | MethodKind::Inherent)
-            | Target::Fn => {
+            Target::Method(MethodKind::Inherent) | Target::Fn => {
                 // `#[target_feature]` is not allowed in lang items.
                 if let Some((lang_item, _)) = hir::lang_items::extract(attrs)
                     // Calling functions with `#[target_feature]` is

tests/ui/rfcs/rfc-2396-target_feature-11/trait-impl.rs

@@ -90,18 +90,6 @@ impl Foo {}
 
 trait Quux {
     fn foo(); //~ NOTE `foo` from trait
-    //~^ NOTE: type in trait
-}
-
-impl Quux for Foo {
-    #[target_feature(enable = "sse2")]
-    //~^ ERROR `#[target_feature(..)]` cannot be applied to safe trait method
-    //~| NOTE cannot be applied to safe trait method
-    fn foo() {}
-    //~^ NOTE not an `unsafe` function
-    //~| ERROR: incompatible type for trait
-    //~| NOTE: expected safe fn, found unsafe fn
-    //~| NOTE: expected signature `fn()`
 }
 
 fn main() {

tests/ui/rfcs/rfc-2396-target_feature-11/trait-impl.stderr

@@ -132,7 +132,7 @@ LL | impl Foo {}
    | ----------- not a function definition
 
 error: attribute should be applied to a function definition
-  --> $DIR/invalid-attribute.rs:108:5
+  --> $DIR/invalid-attribute.rs:96:5
    |
 LL |       #[target_feature(enable = "sse2")]
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -143,7 +143,7 @@ LL | |     }
    | |_____- not a function definition
 
 error: attribute should be applied to a function definition
-  --> $DIR/invalid-attribute.rs:115:5
+  --> $DIR/invalid-attribute.rs:103:5
    |
 LL |     #[target_feature(enable = "sse2")]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -178,30 +178,6 @@ LL | impl Quux for u8 {}
 LL |     fn foo();
    |     --------- `foo` from trait
 
-error: `#[target_feature(..)]` cannot be applied to safe trait method
-  --> $DIR/invalid-attribute.rs:97:5
-   |
-LL |     #[target_feature(enable = "sse2")]
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot be applied to safe trait method
-...
-LL |     fn foo() {}
-   |     -------- not an `unsafe` function
-
-error[E0053]: method `foo` has an incompatible type for trait
-  --> $DIR/invalid-attribute.rs:100:5
-   |
-LL |     fn foo() {}
-   |     ^^^^^^^^ expected safe fn, found unsafe fn
-   |
-note: type in trait
-  --> $DIR/invalid-attribute.rs:92:5
-   |
-LL |     fn foo();
-   |     ^^^^^^^^^
-   = note: expected signature `fn()`
-              found signature `#[target_features] fn()`
-
-error: aborting due to 23 previous errors
+error: aborting due to 21 previous errors
 
-Some errors have detailed explanations: E0046, E0053.
-For more information about an error, try `rustc --explain E0046`.
+For more information about this error, try `rustc --explain E0046`.

tests/ui/target-feature/invalid-attribute.rs

@@ -0,0 +1,18 @@
+//@ compile-flags: --crate-type=lib
+//@ compile-flags: --target=aarch64-unknown-linux-gnu
+//@ needs-llvm-components: aarch64
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang = "sized"]
+pub trait Sized {}
+
+pub trait T {
+    #[target_feature(enable = "aes")]
+    unsafe fn foo() {} //~^ERROR: should be applied to a function definition
+}
+
+impl T for i32 {
+    #[target_feature(enable = "aes")]
+    unsafe fn foo() {} //~^ERROR: should be applied to a function definition
+}

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

@@ -0,0 +1,18 @@
+error: attribute should be applied to a function definition
+  --> $DIR/not-in-trait.rs:11:5
+   |
+LL |     #[target_feature(enable = "aes")]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     unsafe fn foo() {}
+   |     ------------------ not a function definition
+
+error: attribute should be applied to a function definition
+  --> $DIR/not-in-trait.rs:16:5
+   |
+LL |     #[target_feature(enable = "aes")]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     unsafe fn foo() {}
+   |     ------------------ not a function definition
+
+error: aborting due to 2 previous errors
+