Auto merge of #66385 - ecstatic-morse:check-only-pass2, r=eddyb

Make dataflow-based const qualification the canonical one

For over a month, dataflow-based const qualification has been running in parallel with `qualify_consts` to check the bodies of `const` and `static`s. This PR removes the old qualification pass completely in favor of the dataflow-based one.

**edit:**
This PR also stops checking `QUALIF_ERROR_BIT` during promotion. This check appears to no longer serve a purpose now that the CTFE engine is more robust.

As a side-effect, this resolves #66167.

r? @eddyb

src/librustc/mir/mod.rs

@@ -2763,6 +2763,17 @@ pub struct BorrowCheckResult<'tcx> {
     pub used_mut_upvars: SmallVec<[Field; 8]>,
 }
 
+/// The result of the `mir_const_qualif` query.
+///
+/// Each field corresponds to an implementer of the `Qualif` trait in
+/// `librustc_mir/transform/check_consts/qualifs.rs`. See that file for more information on each
+/// `Qualif`.
+#[derive(Clone, Copy, Debug, Default, RustcEncodable, RustcDecodable, HashStable)]
+pub struct ConstQualifs {
+    pub has_mut_interior: bool,
+    pub needs_drop: bool,
+}
+
 /// After we borrow check a closure, we are left with various
 /// requirements that we have inferred between the free regions that
 /// appear in the closure's signature or on its field types. These

src/librustc/query/mod.rs

@@ -91,9 +91,9 @@ rustc_queries! {
         }
 
         /// Maps DefId's that have an associated `mir::Body` to the result
-        /// of the MIR qualify_consts pass. The actual meaning of
-        /// the value isn't known except to the pass itself.
-        query mir_const_qualif(key: DefId) -> u8 {
+        /// of the MIR const-checking pass. This is the set of qualifs in
+        /// the final value of a `const`.
+        query mir_const_qualif(key: DefId) -> mir::ConstQualifs {
             desc { |tcx| "const checking `{}`", tcx.def_path_str(key) }
             cache_on_disk_if { key.is_local() }
         }

src/librustc/session/config.rs

@@ -1373,8 +1373,6 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "describes how to render the `rendered` field of json diagnostics"),
     unleash_the_miri_inside_of_you: bool = (false, parse_bool, [TRACKED],
         "take the breaks off const evaluation. NOTE: this is unsound"),
-    suppress_const_validation_back_compat_ice: bool = (false, parse_bool, [TRACKED],
-        "silence ICE triggered when the new const validator disagrees with the old"),
     osx_rpath_install_name: bool = (false, parse_bool, [TRACKED],
         "pass `-install_name @rpath/...` to the macOS linker"),
     sanitizer: Option<Sanitizer> = (None, parse_sanitizer, [TRACKED],

src/librustc_metadata/rmeta/decoder.rs

@@ -952,12 +952,12 @@ impl<'a, 'tcx> CrateMetadata {
             .decode((self, tcx))
     }
 
-    fn mir_const_qualif(&self, id: DefIndex) -> u8 {
+    fn mir_const_qualif(&self, id: DefIndex) -> mir::ConstQualifs {
         match self.kind(id) {
             EntryKind::Const(qualif, _) |
             EntryKind::AssocConst(AssocContainer::ImplDefault, qualif, _) |
             EntryKind::AssocConst(AssocContainer::ImplFinal, qualif, _) => {
-                qualif.mir
+                qualif
             }
             _ => bug!(),
         }

src/librustc_metadata/rmeta/encoder.rs

@@ -875,7 +875,11 @@ impl EncodeContext<'tcx> {
                     hir::print::to_string(self.tcx.hir(), |s| s.print_trait_item(ast_item));
                 let rendered_const = self.lazy(RenderedConst(rendered));
 
-                EntryKind::AssocConst(container, ConstQualif { mir: 0 }, rendered_const)
+                EntryKind::AssocConst(
+                    container,
+                    Default::default(),
+                    rendered_const,
+                )
             }
             ty::AssocKind::Method => {
                 let fn_data = if let hir::TraitItemKind::Method(m_sig, m) = &ast_item.kind {
@@ -955,10 +959,11 @@ impl EncodeContext<'tcx> {
         record!(self.per_def.kind[def_id] <- match impl_item.kind {
             ty::AssocKind::Const => {
                 if let hir::ImplItemKind::Const(_, body_id) = ast_item.kind {
-                    let mir = self.tcx.at(ast_item.span).mir_const_qualif(def_id);
+                    let qualifs = self.tcx.at(ast_item.span).mir_const_qualif(def_id);
 
-                    EntryKind::AssocConst(container,
-                        ConstQualif { mir },
+                    EntryKind::AssocConst(
+                        container,
+                        qualifs,
                         self.encode_rendered_const_for_body(body_id))
                 } else {
                     bug!()
@@ -1089,9 +1094,9 @@ impl EncodeContext<'tcx> {
             hir::ItemKind::Static(_, hir::Mutability::Mutable, _) => EntryKind::MutStatic,
             hir::ItemKind::Static(_, hir::Mutability::Immutable, _) => EntryKind::ImmStatic,
             hir::ItemKind::Const(_, body_id) => {
-                let mir = self.tcx.at(item.span).mir_const_qualif(def_id);
+                let qualifs = self.tcx.at(item.span).mir_const_qualif(def_id);
                 EntryKind::Const(
-                    ConstQualif { mir },
+                    qualifs,
                     self.encode_rendered_const_for_body(body_id)
                 )
             }
@@ -1368,9 +1373,9 @@ impl EncodeContext<'tcx> {
         let id = self.tcx.hir().as_local_hir_id(def_id).unwrap();
         let body_id = self.tcx.hir().body_owned_by(id);
         let const_data = self.encode_rendered_const_for_body(body_id);
-        let mir = self.tcx.mir_const_qualif(def_id);
+        let qualifs = self.tcx.mir_const_qualif(def_id);
 
-        record!(self.per_def.kind[def_id] <- EntryKind::Const(ConstQualif { mir }, const_data));
+        record!(self.per_def.kind[def_id] <- EntryKind::Const(qualifs, const_data));
         record!(self.per_def.visibility[def_id] <- ty::Visibility::Public);
         record!(self.per_def.span[def_id] <- self.tcx.def_span(def_id));
         self.encode_item_type(def_id);

src/librustc_metadata/rmeta/mod.rs

@@ -260,7 +260,7 @@ crate struct LazyPerDefTables<'tcx> {
 
 #[derive(Copy, Clone, RustcEncodable, RustcDecodable)]
 enum EntryKind<'tcx> {
-    Const(ConstQualif, Lazy<RenderedConst>),
+    Const(mir::ConstQualifs, Lazy<RenderedConst>),
     ImmStatic,
     MutStatic,
     ForeignImmStatic,
@@ -288,16 +288,10 @@ enum EntryKind<'tcx> {
     Method(Lazy<MethodData>),
     AssocType(AssocContainer),
     AssocOpaqueTy(AssocContainer),
-    AssocConst(AssocContainer, ConstQualif, Lazy<RenderedConst>),
+    AssocConst(AssocContainer, mir::ConstQualifs, Lazy<RenderedConst>),
     TraitAlias,
 }
 
-/// Additional data for EntryKind::Const and EntryKind::AssocConst
-#[derive(Clone, Copy, RustcEncodable, RustcDecodable)]
-struct ConstQualif {
-    mir: u8,
-}
-
 /// Contains a constant which has been rendered to a String.
 /// Used by rustdoc.
 #[derive(RustcEncodable, RustcDecodable)]

src/librustc_mir/lib.rs

@@ -32,7 +32,6 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 
 #[macro_use] extern crate log;
 #[macro_use] extern crate rustc;
-#[macro_use] extern crate rustc_data_structures;
 #[macro_use] extern crate syntax;
 
 mod borrow_check;

src/librustc_mir/transform/check_consts/mod.rs

@@ -95,16 +95,6 @@ impl ConstKind {
             ConstKind::ConstFn | ConstKind::Const => false,
         }
     }
-
-    /// Returns `true` if the value returned by this item must be `Sync`.
-    ///
-    /// This returns false for `StaticMut` since all accesses to one are `unsafe` anyway.
-    pub fn requires_sync(self) -> bool {
-        match self {
-            ConstKind::Static => true,
-            ConstKind::ConstFn | ConstKind::Const |  ConstKind::StaticMut => false,
-        }
-    }
 }
 
 impl fmt::Display for ConstKind {

src/librustc_mir/transform/check_consts/ops.rs

@@ -138,7 +138,15 @@ impl NonConstOp for HeapAllocation {
 
 #[derive(Debug)]
 pub struct IfOrMatch;
-impl NonConstOp for IfOrMatch {}
+impl NonConstOp for IfOrMatch {
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        // This should be caught by the HIR const-checker.
+        item.tcx.sess.delay_span_bug(
+            span,
+            "complex control flow is forbidden in a const context",
+        );
+    }
+}
 
 #[derive(Debug)]
 pub struct LiveDrop;
@@ -154,7 +162,15 @@ impl NonConstOp for LiveDrop {
 
 #[derive(Debug)]
 pub struct Loop;
-impl NonConstOp for Loop {}
+impl NonConstOp for Loop {
+    fn emit_error(&self, item: &Item<'_, '_>, span: Span) {
+        // This should be caught by the HIR const-checker.
+        item.tcx.sess.delay_span_bug(
+            span,
+            "complex control flow is forbidden in a const context",
+        );
+    }
+}
 
 #[derive(Debug)]
 pub struct MutBorrow(pub BorrowKind);

src/librustc_mir/transform/check_consts/qualifs.rs

@@ -6,12 +6,10 @@ use syntax_pos::DUMMY_SP;
 
 use super::{ConstKind, Item as ConstCx};
 
-#[derive(Clone, Copy)]
-pub struct QualifSet(u8);
-
-impl QualifSet {
-    fn contains<Q: ?Sized + Qualif>(self) -> bool {
-        self.0 & (1 << Q::IDX) != 0
+pub fn in_any_value_of_ty(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> ConstQualifs {
+    ConstQualifs {
+        has_mut_interior: HasMutInterior::in_any_value_of_ty(cx, ty),
+        needs_drop: NeedsDrop::in_any_value_of_ty(cx, ty),
     }
 }
 
@@ -22,14 +20,14 @@ impl QualifSet {
 ///
 /// The default implementations proceed structurally.
 pub trait Qualif {
-    const IDX: usize;
-
     /// The name of the file used to debug the dataflow analysis that computes this qualif.
     const ANALYSIS_NAME: &'static str;
 
     /// Whether this `Qualif` is cleared when a local is moved from.
     const IS_CLEARED_ON_MOVE: bool = false;
 
+    fn in_qualifs(qualifs: &ConstQualifs) -> bool;
+
     /// Return the qualification that is (conservatively) correct for any value
     /// of the type.
     fn in_any_value_of_ty(_cx: &ConstCx<'_, 'tcx>, _ty: Ty<'tcx>) -> bool;
@@ -122,9 +120,8 @@ pub trait Qualif {
                     if cx.tcx.trait_of_item(def_id).is_some() {
                         Self::in_any_value_of_ty(cx, constant.literal.ty)
                     } else {
-                        let bits = cx.tcx.at(constant.span).mir_const_qualif(def_id);
-
-                        let qualif = QualifSet(bits).contains::<Self>();
+                        let qualifs = cx.tcx.at(constant.span).mir_const_qualif(def_id);
+                        let qualif = Self::in_qualifs(&qualifs);
 
                         // Just in case the type is more specific than
                         // the definition, e.g., impl associated const
@@ -210,9 +207,12 @@ pub trait Qualif {
 pub struct HasMutInterior;
 
 impl Qualif for HasMutInterior {
-    const IDX: usize = 0;
     const ANALYSIS_NAME: &'static str = "flow_has_mut_interior";
 
+    fn in_qualifs(qualifs: &ConstQualifs) -> bool {
+        qualifs.has_mut_interior
+    }
+
     fn in_any_value_of_ty(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
         !ty.is_freeze(cx.tcx, cx.param_env, DUMMY_SP)
     }
@@ -275,10 +275,13 @@ impl Qualif for HasMutInterior {
 pub struct NeedsDrop;
 
 impl Qualif for NeedsDrop {
-    const IDX: usize = 1;
     const ANALYSIS_NAME: &'static str = "flow_needs_drop";
     const IS_CLEARED_ON_MOVE: bool = true;
 
+    fn in_qualifs(qualifs: &ConstQualifs) -> bool {
+        qualifs.needs_drop
+    }
+
     fn in_any_value_of_ty(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
         ty.needs_drop(cx.tcx, cx.param_env)
     }