Auto merge of #76850 - ecstatic-morse:const-checking-refactor, r=oli-obk

Remove `qualify_min_const_fn`

~~Blocked on #76807 (the first six commits).~~

With this PR, all checks in `qualify_min_const_fn` are replicated in `check_consts`, and the former is no longer invoked. My goal was to have as few changes to test output as possible, since making sweeping changes to the code *while* doing big batches of diagnostics updates turned out to be a headache. To this end, there's a few `HACK`s in `check_consts` to achieve parity with `qualify_min_const_fn`.

The new system that replaces `is_min_const_fn` is referred to as "const-stability"  My end goal for the const-stability rules is this:
* Const-stability is only applicable to functions defined in `staged_api` crates.
* All functions not marked `rustc_const_unstable` are considered "const-stable".
    - NB. This is currently not implemented. `#[unstable]` functions are also const-unstable. This causes problems when searching for feature gates.
    - All "const-unstable" functions have an associated feature gate
* const-stable functions can only call other const-stable functions
     - `allow_internal_unstable` can be used to circumvent this.
* All const-stable functions are subject to some additional checks (the ones that were unique to `qualify_min_const_fn`)

The plan is to remove each `HACK` individually in subsequent PRs. That way, changes to error message output can be reviewed in isolation.

Author: bors

compiler/rustc_ast/src/lib.rs

@@ -7,6 +7,7 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/", test(attr(deny(warnings))))]
 #![feature(box_syntax)]
 #![feature(const_fn)] // For the `transmute` in `P::new`
+#![feature(const_fn_transmute)]
 #![feature(const_panic)]
 #![feature(crate_visibility_modifier)]
 #![feature(label_break_value)]

compiler/rustc_mir/src/transform/check_consts/mod.rs

@@ -51,6 +51,12 @@ impl ConstCx<'mir, 'tcx> {
     pub fn const_kind(&self) -> hir::ConstContext {
         self.const_kind.expect("`const_kind` must not be called on a non-const fn")
     }
+
+    pub fn is_const_stable_const_fn(&self) -> bool {
+        self.const_kind == Some(hir::ConstContext::ConstFn)
+            && self.tcx.features().staged_api
+            && is_const_stable_const_fn(self.tcx, self.def_id.to_def_id())
+    }
 }
 
 /// Returns `true` if this `DefId` points to one of the official `panic` lang items.
@@ -63,3 +69,37 @@ pub fn allow_internal_unstable(tcx: TyCtxt<'tcx>, def_id: DefId, feature_gate: S
     attr::allow_internal_unstable(&tcx.sess, attrs)
         .map_or(false, |mut features| features.any(|name| name == feature_gate))
 }
+
+// Returns `true` if the given `const fn` is "const-stable".
+//
+// Panics if the given `DefId` does not refer to a `const fn`.
+//
+// Const-stability is only relevant for `const fn` within a `staged_api` crate. Only "const-stable"
+// functions can be called in a const-context by users of the stable compiler. "const-stable"
+// functions are subject to more stringent restrictions than "const-unstable" functions: They
+// cannot use unstable features and can only call other "const-stable" functions.
+pub fn is_const_stable_const_fn(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
+    use attr::{ConstStability, Stability, StabilityLevel};
+
+    // Const-stability is only relevant for `const fn`.
+    assert!(tcx.is_const_fn_raw(def_id));
+
+    // Functions with `#[rustc_const_unstable]` are const-unstable.
+    match tcx.lookup_const_stability(def_id) {
+        Some(ConstStability { level: StabilityLevel::Unstable { .. }, .. }) => return false,
+        Some(ConstStability { level: StabilityLevel::Stable { .. }, .. }) => return true,
+        None => {}
+    }
+
+    // Functions with `#[unstable]` are const-unstable.
+    //
+    // FIXME(ecstaticmorse): We should keep const-stability attributes wholly separate from normal stability
+    // attributes. `#[unstable]` should be irrelevant.
+    if let Some(Stability { level: StabilityLevel::Unstable { .. }, .. }) =
+        tcx.lookup_stability(def_id)
+    {
+        return false;
+    }
+
+    true
+}

compiler/rustc_mir/src/transform/check_consts/ops.rs

@@ -10,33 +10,34 @@ use rustc_span::{Span, Symbol};
 
 use super::ConstCx;
 
-/// Emits an error if `op` is not allowed in the given const context.
-pub fn non_const<O: NonConstOp>(ccx: &ConstCx<'_, '_>, op: O, span: Span) {
+/// Emits an error and returns `true` if `op` is not allowed in the given const context.
+pub fn non_const<O: NonConstOp>(ccx: &ConstCx<'_, '_>, op: O, span: Span) -> bool {
     debug!("illegal_op: op={:?}", op);
 
     let gate = match op.status_in_item(ccx) {
-        Status::Allowed => return,
+        Status::Allowed => return false,
 
         Status::Unstable(gate) if ccx.tcx.features().enabled(gate) => {
-            let unstable_in_stable = ccx.const_kind() == hir::ConstContext::ConstFn
-                && ccx.tcx.features().enabled(sym::staged_api)
-                && !ccx.tcx.has_attr(ccx.def_id.to_def_id(), sym::rustc_const_unstable)
+            let unstable_in_stable = ccx.is_const_stable_const_fn()
                 && !super::allow_internal_unstable(ccx.tcx, ccx.def_id.to_def_id(), gate);
 
             if unstable_in_stable {
                 ccx.tcx.sess
-                    .struct_span_err(span, &format!("`#[feature({})]` cannot be depended on in a const-stable function", gate.as_str()))
+                    .struct_span_err(
+                        span,
+                        &format!("const-stable function cannot use `#[feature({})]`", gate.as_str()),
+                    )
                     .span_suggestion(
                         ccx.body.span,
                         "if it is not part of the public API, make this function unstably const",
                         concat!(r#"#[rustc_const_unstable(feature = "...", issue = "...")]"#, '\n').to_owned(),
                         Applicability::HasPlaceholders,
                     )
-                    .help("otherwise `#[allow_internal_unstable]` can be used to bypass stability checks")
+                    .note("otherwise `#[allow_internal_unstable]` can be used to bypass stability checks")
                     .emit();
             }
 
-            return;
+            return unstable_in_stable;
         }
 
         Status::Unstable(gate) => Some(gate),
@@ -45,12 +46,14 @@ pub fn non_const<O: NonConstOp>(ccx: &ConstCx<'_, '_>, op: O, span: Span) {
 
     if ccx.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
         ccx.tcx.sess.miri_unleashed_feature(span, gate);
-        return;
+        return false;
     }
 
     op.emit_error(ccx, span);
+    true
 }
 
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum Status {
     Allowed,
     Unstable(Symbol),
@@ -59,6 +62,8 @@ pub enum Status {
 
 /// An operation that is not *always* allowed in a const context.
 pub trait NonConstOp: std::fmt::Debug {
+    const STOPS_CONST_CHECKING: bool = false;
+
     /// Returns an enum indicating whether this operation is allowed within the given item.
     fn status_in_item(&self, _ccx: &ConstCx<'_, '_>) -> Status {
         Status::Forbidden
@@ -93,6 +98,34 @@ pub trait NonConstOp: std::fmt::Debug {
     }
 }
 
+#[derive(Debug)]
+pub struct Abort;
+impl NonConstOp for Abort {
+    const STOPS_CONST_CHECKING: bool = true;
+
+    fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+        mcf_status_in_item(ccx)
+    }
+
+    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+        mcf_emit_error(ccx, span, "abort is not stable in const fn")
+    }
+}
+
+#[derive(Debug)]
+pub struct NonPrimitiveOp;
+impl NonConstOp for NonPrimitiveOp {
+    const STOPS_CONST_CHECKING: bool = true;
+
+    fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+        mcf_status_in_item(ccx)
+    }
+
+    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+        mcf_emit_error(ccx, span, "only int, `bool` and `char` operations are stable in const fn")
+    }
+}
+
 /// A function call where the callee is a pointer.
 #[derive(Debug)]
 pub struct FnCallIndirect;
@@ -125,7 +158,8 @@ impl NonConstOp for FnCallNonConst {
 ///
 /// Contains the name of the feature that would allow the use of this function.
 #[derive(Debug)]
-pub struct FnCallUnstable(pub DefId, pub Symbol);
+pub struct FnCallUnstable(pub DefId, pub Option<Symbol>);
+
 impl NonConstOp for FnCallUnstable {
     fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
         let FnCallUnstable(def_id, feature) = *self;
@@ -134,13 +168,51 @@ impl NonConstOp for FnCallUnstable {
             span,
             &format!("`{}` is not yet stable as a const fn", ccx.tcx.def_path_str(def_id)),
         );
-        if nightly_options::is_nightly_build() {
-            err.help(&format!("add `#![feature({})]` to the crate attributes to enable", feature));
+
+        if ccx.is_const_stable_const_fn() {
+            err.help("Const-stable functions can only call other const-stable functions");
+        } else if nightly_options::is_nightly_build() {
+            if let Some(feature) = feature {
+                err.help(&format!(
+                    "add `#![feature({})]` to the crate attributes to enable",
+                    feature
+                ));
+            }
         }
         err.emit();
     }
 }
 
+#[derive(Debug)]
+pub struct FnPtrCast;
+impl NonConstOp for FnPtrCast {
+    const STOPS_CONST_CHECKING: bool = true;
+
+    fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+        mcf_status_in_item(ccx)
+    }
+
+    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+        mcf_emit_error(ccx, span, "function pointer casts are not allowed in const fn");
+    }
+}
+
+#[derive(Debug)]
+pub struct Generator;
+impl NonConstOp for Generator {
+    const STOPS_CONST_CHECKING: bool = true;
+
+    fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+        // FIXME: This means generator-only MIR is only forbidden in const fn. This is for
+        // compatibility with the old code. Such MIR should be forbidden everywhere.
+        mcf_status_in_item(ccx)
+    }
+
+    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+        mcf_emit_error(ccx, span, "const fn generators are unstable");
+    }
+}
+
 #[derive(Debug)]
 pub struct HeapAllocation;
 impl NonConstOp for HeapAllocation {
@@ -403,6 +475,24 @@ impl NonConstOp for ThreadLocalAccess {
     }
 }
 
+#[derive(Debug)]
+pub struct Transmute;
+impl NonConstOp for Transmute {
+    const STOPS_CONST_CHECKING: bool = true;
+
+    fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+        if ccx.const_kind() != hir::ConstContext::ConstFn {
+            Status::Allowed
+        } else {
+            Status::Unstable(sym::const_fn_transmute)
+        }
+    }
+
+    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+        mcf_emit_error(ccx, span, "can only call `transmute` from const items, not `const fn`");
+    }
+}
+
 #[derive(Debug)]
 pub struct UnionAccess;
 impl NonConstOp for UnionAccess {
@@ -425,3 +515,131 @@ impl NonConstOp for UnionAccess {
         .emit();
     }
 }
+
+/// See [#64992].
+///
+/// [#64992]: https://github.com/rust-lang/rust/issues/64992
+#[derive(Debug)]
+pub struct UnsizingCast;
+impl NonConstOp for UnsizingCast {
+    fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+        mcf_status_in_item(ccx)
+    }
+
+    fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+        mcf_emit_error(
+            ccx,
+            span,
+            "unsizing casts to types besides slices are not allowed in const fn",
+        );
+    }
+}
+
+pub mod ty {
+    use super::*;
+
+    #[derive(Debug)]
+    pub struct MutRef;
+    impl NonConstOp for MutRef {
+        const STOPS_CONST_CHECKING: bool = true;
+
+        fn status_in_item(&self, _ccx: &ConstCx<'_, '_>) -> Status {
+            Status::Unstable(sym::const_mut_refs)
+        }
+
+        fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+            mcf_emit_error(ccx, span, "mutable references in const fn are unstable");
+        }
+    }
+
+    #[derive(Debug)]
+    pub struct FnPtr;
+    impl NonConstOp for FnPtr {
+        const STOPS_CONST_CHECKING: bool = true;
+
+        fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+            // FIXME: This attribute a hack to allow the specialization of the `futures` API. See
+            // #59739. We should have a proper feature gate for this.
+            if ccx.tcx.has_attr(ccx.def_id.to_def_id(), sym::rustc_allow_const_fn_ptr) {
+                Status::Allowed
+            } else {
+                mcf_status_in_item(ccx)
+            }
+        }
+
+        fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+            mcf_emit_error(ccx, span, "function pointers in const fn are unstable");
+        }
+    }
+
+    #[derive(Debug)]
+    pub struct ImplTrait;
+    impl NonConstOp for ImplTrait {
+        const STOPS_CONST_CHECKING: bool = true;
+
+        fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+            mcf_status_in_item(ccx)
+        }
+
+        fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+            mcf_emit_error(ccx, span, "`impl Trait` in const fn is unstable");
+        }
+    }
+
+    #[derive(Debug)]
+    pub struct TraitBound;
+    impl NonConstOp for TraitBound {
+        const STOPS_CONST_CHECKING: bool = true;
+
+        fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
+            mcf_status_in_item(ccx)
+        }
+
+        fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+            mcf_emit_error(
+                ccx,
+                span,
+                "trait bounds other than `Sized` on const fn parameters are unstable",
+            );
+        }
+    }
+
+    /// A trait bound with the `?const Trait` opt-out
+    #[derive(Debug)]
+    pub struct TraitBoundNotConst;
+    impl NonConstOp for TraitBoundNotConst {
+        const STOPS_CONST_CHECKING: bool = true;
+
+        fn status_in_item(&self, _: &ConstCx<'_, '_>) -> Status {
+            Status::Unstable(sym::const_trait_bound_opt_out)
+        }
+
+        fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
+            feature_err(
+                &ccx.tcx.sess.parse_sess,
+                sym::const_trait_bound_opt_out,
+                span,
+                "`?const Trait` syntax is unstable",
+            )
+            .emit()
+        }
+    }
+}
+
+fn mcf_status_in_item(ccx: &ConstCx<'_, '_>) -> Status {
+    if ccx.const_kind() != hir::ConstContext::ConstFn {
+        Status::Allowed
+    } else {
+        Status::Unstable(sym::const_fn)
+    }
+}
+
+fn mcf_emit_error(ccx: &ConstCx<'_, '_>, span: Span, msg: &str) {
+    struct_span_err!(ccx.tcx.sess, span, E0723, "{}", msg)
+        .note(
+            "see issue #57563 <https://github.com/rust-lang/rust/issues/57563> \
+             for more information",
+        )
+        .help("add `#![feature(const_fn)]` to the crate attributes to enable")
+        .emit();
+}