Auto merge of #55641 - nagisa:optimize-attr, r=pnkfelix

Implement optimize(size) and optimize(speed) attributes

This PR implements both `optimize(size)` and `optimize(speed)` attributes.

While the functionality itself works fine now, this PR is not yet complete: the code might be messy in places and, most importantly, the compiletest must be improved with functionality to run tests with custom optimization levels. Otherwise the new attribute cannot be tested properly. Oh, and not all of the RFC is implemented – attribute propagation is not implemented for example.

# TODO

* [x] Improve compiletest so that tests can be written;
* [x] Assign a proper error number (E9999 currently, no idea how to allocate a number properly);
* [ ] Perhaps reduce the duplication in LLVM attribute assignment code…

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

@@ -0,0 +1,18 @@
+#![crate_type="rlib"]
+#![optimize(speed)] //~ ERROR #54882
+
+#[optimize(size)] //~ ERROR #54882
+mod module {
+
+#[optimize(size)] //~ ERROR #54882
+fn size() {}
+
+#[optimize(speed)] //~ ERROR #54882
+fn speed() {}
+
+#[optimize(banana)]
+//~^ ERROR #54882
+//~| ERROR E0722
+fn not_known() {}
+
+}

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

@@ -0,0 +1,50 @@
+error[E0658]: #[optimize] attribute is an unstable feature (see issue #54882)
+  --> $DIR/feature-gate-optimize_attribute.rs:7:1
+   |
+LL | #[optimize(size)] //~ ERROR #54882
+   | ^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(optimize_attribute)] to the crate attributes to enable
+
+error[E0658]: #[optimize] attribute is an unstable feature (see issue #54882)
+  --> $DIR/feature-gate-optimize_attribute.rs:10:1
+   |
+LL | #[optimize(speed)] //~ ERROR #54882
+   | ^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(optimize_attribute)] to the crate attributes to enable
+
+error[E0658]: #[optimize] attribute is an unstable feature (see issue #54882)
+  --> $DIR/feature-gate-optimize_attribute.rs:13:1
+   |
+LL | #[optimize(banana)]
+   | ^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(optimize_attribute)] to the crate attributes to enable
+
+error[E0658]: #[optimize] attribute is an unstable feature (see issue #54882)
+  --> $DIR/feature-gate-optimize_attribute.rs:4:1
+   |
+LL | #[optimize(size)] //~ ERROR #54882
+   | ^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(optimize_attribute)] to the crate attributes to enable
+
+error[E0658]: #[optimize] attribute is an unstable feature (see issue #54882)
+  --> $DIR/feature-gate-optimize_attribute.rs:2:1
+   |
+LL | #![optimize(speed)] //~ ERROR #54882
+   | ^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(optimize_attribute)] to the crate attributes to enable
+
+error[E0722]: invalid argument
+  --> $DIR/feature-gate-optimize_attribute.rs:13:12
+   |
+LL | #[optimize(banana)]
+   |            ^^^^^^
+
+error: aborting due to 6 previous errors
+
+Some errors occurred: E0658, E0722.
+For more information about an error, try `rustc --explain E0658`.