Prepare const_limit feature gate and attribute

Author: TheSamsa

src/doc/unstable-book/src/language-features/const-limit.md

@@ -0,0 +1,7 @@
+# `const_limit`
+
+The tracking issue for this feature is: [#67217]
+
+[#57563]: https://github.com/rust-lang/rust/issues/67217
+
+The `const_limit` allows someone to limit the evaluation steps the CTFE undertakes to evaluate a `const fn`.

src/librustc/middle/recursion_limit.rs renamed to src/librustc/middle/limits.rs

@@ -1,4 +1,4 @@
-// Recursion limit.
+// Registering limits, recursion_limit, type_length_limit and const_limit
 //
 // There are various parts of the compiler that must impose arbitrary limits
 // on how deeply they recurse to prevent stack overflow. Users can override
@@ -16,6 +16,7 @@ use rustc_data_structures::sync::Once;
 pub fn update_limits(sess: &Session, krate: &ast::Crate) {
     update_limit(sess, krate, &sess.recursion_limit, sym::recursion_limit, 128);
     update_limit(sess, krate, &sess.type_length_limit, sym::type_length_limit, 1048576);
+    update_limit(sess, krate, &sess.const_limit, sym::const_limit, 128);
 }
 
 fn update_limit(

src/librustc_feature/active.rs

@@ -532,6 +532,9 @@ declare_features! (
     /// Allows using `&mut` in constant functions.
     (active, const_mut_refs, "1.41.0", Some(57349), None),
 
+    // Allows limiting the evaluation steps of const expressions
+    (active, const_limit, "1.41.0", Some(67217), None),
+
     /// Allows the use of `loop` and `while` in constants.
     (active, const_loop, "1.41.0", Some(52000), None),
 

src/librustc_feature/builtin_attrs.rs

@@ -239,6 +239,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     // Limits:
     ungated!(recursion_limit, CrateLevel, template!(NameValueStr: "N")),
     ungated!(type_length_limit, CrateLevel, template!(NameValueStr: "N")),
+    gated!(
+        const_limit, CrateLevel, template!(NameValueStr: "N"), const_limit,
+        experimental!(const_limit)
+    ),
 
     // Entry point:
     ungated!(main, Normal, template!(Word)),

src/librustc_session/session.rs

@@ -88,6 +88,9 @@ pub struct Session {
     /// The maximum length of types during monomorphization.
     pub type_length_limit: Once<usize>,
 
+    /// The maximum blocks a const expression can evaluate.
+    pub const_limit: Once<usize>,
+
     /// Map from imported macro spans (which consist of
     /// the localized span for the macro body) to the
     /// macro name and definition span in the source crate.
@@ -1053,6 +1056,7 @@ fn build_session_(
         features: Once::new(),
         recursion_limit: Once::new(),
         type_length_limit: Once::new(),
+        const_limit: Once::new(),
         imported_macro_spans: OneThread::new(RefCell::new(FxHashMap::default())),
         incr_comp_session: OneThread::new(RefCell::new(IncrCompSession::NotInitialized)),
         cgu_reuse_tracker,

src/librustc_span/symbol.rs

@@ -208,6 +208,7 @@ symbols! {
         console,
         const_compare_raw_pointers,
         const_constructor,
+        const_limit,
         const_extern_fn,
         const_fn,
         const_fn_union,

src/test/ui/consts/const_limit/feature-gate-const_limit.rs

@@ -0,0 +1,12 @@
+#![const_limit="1"]
+//~^ ERROR the `#[const_limit]` attribute is an experimental feature [E0658]
+
+const CONSTANT: usize = limit();
+
+fn main() {}
+
+const fn limit() -> usize {
+    let x = 42;
+
+    x * 42
+}

src/test/ui/consts/const_limit/feature-gate-const_limit.stderr

@@ -0,0 +1,12 @@
+error[E0658]: the `#[const_limit]` attribute is an experimental feature
+  --> $DIR/feature-gate-const_limit.rs:1:1
+   |
+LL | #![const_limit="1"]
+   | ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/67217
+   = help: add `#![feature(const_limit)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.