Auto merge of #76260 - xd009642:rfc/2867, r=jonas-schievink

Implementation of RFC2867

https://github.com/rust-lang/rust/issues/74727

So I've started work on this, I think my next steps are to make use of the `instruction_set` value in the llvm codegen but this is the point where I begin to get a bit lost. I'm looking at the code but it would be nice to have some guidance on what I've currently done and what I'm doing next 😄

src/test/ui/error-codes/E0778.rs

@@ -0,0 +1,8 @@
+#![feature(isa_attribute)]
+
+#[instruction_set()] //~ ERROR
+fn no_isa_defined() {
+}
+
+fn main() {
+}

src/test/ui/error-codes/E0778.stderr

@@ -0,0 +1,9 @@
+error[E0778]: `#[instruction_set]` requires an argument
+  --> $DIR/E0778.rs:3:1
+   |
+LL | #[instruction_set()]
+   | ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0778`.

src/test/ui/error-codes/E0779.rs

@@ -0,0 +1,6 @@
+#![feature(isa_attribute)]
+
+#[instruction_set(arm::magic)] //~ ERROR
+fn main() {
+
+}

src/test/ui/error-codes/E0779.stderr

@@ -0,0 +1,9 @@
+error[E0779]: invalid instruction set specified
+  --> $DIR/E0779.rs:3:1
+   |
+LL | #[instruction_set(arm::magic)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0779`.

src/test/ui/feature-gate-isa_attribute.rs

@@ -0,0 +1,6 @@
+#[instruction_set]
+//~^ ERROR the `#[instruction_set]` attribute is an experimental feature [E0658]
+//~| ERROR malformed `instruction_set` attribute input
+//~| ERROR must specify an instruction set [E0778]
+fn main() {
+}

src/test/ui/feature-gate-isa_attribute.stderr

@@ -0,0 +1,25 @@
+error: malformed `instruction_set` attribute input
+  --> $DIR/feature-gate-isa_attribute.rs:1:1
+   |
+LL | #[instruction_set]
+   | ^^^^^^^^^^^^^^^^^^ help: must be of the form: `#[instruction_set(set)]`
+
+error[E0658]: the `#[instruction_set]` attribute is an experimental feature
+  --> $DIR/feature-gate-isa_attribute.rs:1:1
+   |
+LL | #[instruction_set]
+   | ^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #74727 <https://github.com/rust-lang/rust/issues/74727> for more information
+   = help: add `#![feature(isa_attribute)]` to the crate attributes to enable
+
+error[E0778]: must specify an instruction set
+  --> $DIR/feature-gate-isa_attribute.rs:1:1
+   |
+LL | #[instruction_set]
+   | ^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0658, E0778.
+For more information about an error, try `rustc --explain E0658`.