Auto merge of #33130 - eddyb:mir-const, r=nikomatsakis

Implement constant support in MIR.

All of the intended features in `trans::consts` are now supported by `mir::constant`.
The implementation is considered a temporary measure until `miri` replaces it.

A `-Z orbit` bootstrap build will only translate LLVM IR from AST for `#[rustc_no_mir]` functions.

Furthermore, almost all checks of constant expressions have been moved to MIR.
In non-`const` functions, trees of temporaries are promoted, as per RFC 1414 (rvalue promotion).
Promotion before MIR borrowck would allow reasoning about promoted values' lifetimes.

The improved checking comes at the cost of four `[breaking-change]`s:
* repeat counts must contain a constant expression, e.g.:
`let arr = [0; { println!("foo"); 5 }];` used to be allowed (it behaved like `let arr = [0; 5];`)
* dereference of a reference to a `static` cannot be used in another `static`, e.g.:
`static X: [u8; 1] = [1]; static Y: u8 = (&X)[0];` was unintentionally allowed before
* the type of a `static` *must* be `Sync`, irrespective of the initializer, e.g.
`static FOO: *const T = &BAR;` worked as `&T` is `Sync`, but it shouldn't because `*const T` isn't
* a `static` cannot wrap `UnsafeCell` around a type that *may* need drop, e.g.
`static X: MakeSync<UnsafeCell<Option<String>>> = MakeSync(UnsafeCell::new(None));`
was previously allowed based on the fact `None` alone doesn't need drop, but in `UnsafeCell`
it can be later changed to `Some(String)` which *does* need dropping

The drop restrictions are relaxed by RFC 1440 (#33156), which is implemented, but feature-gated.
However, creating `UnsafeCell` from constants is unstable, so users can just enable the feature gate.

Author: bors

mk/crates.mk

@@ -111,7 +111,7 @@ DEPS_rustc_lint := rustc log syntax rustc_const_eval
 DEPS_rustc_llvm := native:rustllvm libc std rustc_bitflags
 DEPS_rustc_metadata := rustc syntax rbml rustc_const_math
 DEPS_rustc_passes := syntax rustc core rustc_const_eval
-DEPS_rustc_mir := rustc syntax rustc_const_math rustc_const_eval
+DEPS_rustc_mir := rustc syntax rustc_const_math rustc_const_eval rustc_bitflags
 DEPS_rustc_resolve := arena rustc log syntax
 DEPS_rustc_platform_intrinsics := std
 DEPS_rustc_plugin := rustc rustc_metadata syntax rustc_mir

src/librustc/dep_graph/dep_node.rs

@@ -64,6 +64,7 @@ pub enum DepNode<D: Clone + Debug> {
     IntrinsicCheck(D),
     MatchCheck(D),
     MirMapConstruction(D),
+    MirPass(D),
     MirTypeck(D),
     BorrowCheck(D),
     RvalueCheck(D),
@@ -186,6 +187,7 @@ impl<D: Clone + Debug> DepNode<D> {
             IntrinsicCheck(ref d) => op(d).map(IntrinsicCheck),
             MatchCheck(ref d) => op(d).map(MatchCheck),
             MirMapConstruction(ref d) => op(d).map(MirMapConstruction),
+            MirPass(ref d) => op(d).map(MirPass),
             MirTypeck(ref d) => op(d).map(MirTypeck),
             BorrowCheck(ref d) => op(d).map(BorrowCheck),
             RvalueCheck(ref d) => op(d).map(RvalueCheck),

src/librustc/hir/map/def_collector.rs

@@ -97,6 +97,31 @@ impl<'ast> DefCollector<'ast> {
         f(self);
         self.parent_def = parent;
     }
+
+    fn visit_ast_const_integer(&mut self, expr: &'ast Expr) {
+        // Find the node which will be used after lowering.
+        if let ExprKind::Paren(ref inner) = expr.node {
+            return self.visit_ast_const_integer(inner);
+        }
+
+        // FIXME(eddyb) Closures should have separate
+        // function definition IDs and expression IDs.
+        if let ExprKind::Closure(..) = expr.node {
+            return;
+        }
+
+        self.create_def(expr.id, DefPathData::Initializer);
+    }
+
+    fn visit_hir_const_integer(&mut self, expr: &'ast hir::Expr) {
+        // FIXME(eddyb) Closures should have separate
+        // function definition IDs and expression IDs.
+        if let hir::ExprClosure(..) = expr.node {
+            return;
+        }
+
+        self.create_def(expr.id, DefPathData::Initializer);
+    }
 }
 
 impl<'ast> visit::Visitor<'ast> for DefCollector<'ast> {
@@ -126,14 +151,17 @@ impl<'ast> visit::Visitor<'ast> for DefCollector<'ast> {
                         let variant_def_index =
                             this.create_def(v.node.data.id(),
                                             DefPathData::EnumVariant(v.node.name.name));
-
-                        for (index, field) in v.node.data.fields().iter().enumerate() {
-                            let name = field.ident.map(|ident| ident.name)
-                                .unwrap_or(token::intern(&index.to_string()));
-                            this.create_def_with_parent(Some(variant_def_index),
-                                                        field.id,
-                                                        DefPathData::Field(name));
-                        }
+                        this.with_parent(variant_def_index, |this| {
+                            for (index, field) in v.node.data.fields().iter().enumerate() {
+                                let name = field.ident.map(|ident| ident.name)
+                                    .unwrap_or_else(|| token::intern(&index.to_string()));
+                                this.create_def(field.id, DefPathData::Field(name));
+                            }
+
+                            if let Some(ref expr) = v.node.disr_expr {
+                                this.visit_ast_const_integer(expr);
+                            }
+                        });
                     }
                 }
                 ItemKind::Struct(ref struct_def, _) => {
@@ -221,6 +249,10 @@ impl<'ast> visit::Visitor<'ast> for DefCollector<'ast> {
     fn visit_expr(&mut self, expr: &'ast Expr) {
         let parent_def = self.parent_def;
 
+        if let ExprKind::Repeat(_, ref count) = expr.node {
+            self.visit_ast_const_integer(count);
+        }
+
         if let ExprKind::Closure(..) = expr.node {
             let def = self.create_def(expr.id, DefPathData::ClosureExpr);
             self.parent_def = Some(def);
@@ -230,6 +262,13 @@ impl<'ast> visit::Visitor<'ast> for DefCollector<'ast> {
         self.parent_def = parent_def;
     }
 
+    fn visit_ty(&mut self, ty: &'ast Ty) {
+        if let TyKind::FixedLengthVec(_, ref length) = ty.node {
+            self.visit_ast_const_integer(length);
+        }
+        visit::walk_ty(self, ty);
+    }
+
     fn visit_lifetime_def(&mut self, def: &'ast LifetimeDef) {
         self.create_def(def.lifetime.id, DefPathData::LifetimeDef(def.lifetime.name));
     }
@@ -276,11 +315,15 @@ impl<'ast> intravisit::Visitor<'ast> for DefCollector<'ast> {
                             this.create_def(v.node.data.id(),
                                             DefPathData::EnumVariant(v.node.name));
 
-                        for field in v.node.data.fields() {
-                            this.create_def_with_parent(Some(variant_def_index),
-                                                        field.id,
-                                                        DefPathData::Field(field.name));
-                        }
+                        this.with_parent(variant_def_index, |this| {
+                            for field in v.node.data.fields() {
+                                this.create_def(field.id,
+                                                DefPathData::Field(field.name));
+                            }
+                            if let Some(ref expr) = v.node.disr_expr {
+                                this.visit_hir_const_integer(expr);
+                            }
+                        });
                     }
                 }
                 hir::ItemStruct(ref struct_def, _) => {
@@ -365,6 +408,10 @@ impl<'ast> intravisit::Visitor<'ast> for DefCollector<'ast> {
     fn visit_expr(&mut self, expr: &'ast hir::Expr) {
         let parent_def = self.parent_def;
 
+        if let hir::ExprRepeat(_, ref count) = expr.node {
+            self.visit_hir_const_integer(count);
+        }
+
         if let hir::ExprClosure(..) = expr.node {
             let def = self.create_def(expr.id, DefPathData::ClosureExpr);
             self.parent_def = Some(def);
@@ -374,11 +421,18 @@ impl<'ast> intravisit::Visitor<'ast> for DefCollector<'ast> {
         self.parent_def = parent_def;
     }
 
+    fn visit_ty(&mut self, ty: &'ast hir::Ty) {
+        if let hir::TyFixedLengthVec(_, ref length) = ty.node {
+            self.visit_hir_const_integer(length);
+        }
+        intravisit::walk_ty(self, ty);
+    }
+
     fn visit_lifetime_def(&mut self, def: &'ast hir::LifetimeDef) {
         self.create_def(def.lifetime.id, DefPathData::LifetimeDef(def.lifetime.name));
     }
 
     fn visit_macro_def(&mut self, macro_def: &'ast hir::MacroDef) {
         self.create_def(macro_def.id, DefPathData::MacroDef(macro_def.name));
     }
-}
+}

src/librustc/mir/repr.rs

@@ -36,6 +36,11 @@ pub struct Mir<'tcx> {
     /// used (eventually) for debuginfo. Indexed by a `ScopeId`.
     pub scopes: Vec<ScopeData>,
 
+    /// Rvalues promoted from this function, such as borrows of constants.
+    /// Each of them is the Mir of a constant with the fn's type parameters
+    /// in scope, but no vars or args and a separate set of temps.
+    pub promoted: Vec<Mir<'tcx>>,
+
     /// Return type of the function.
     pub return_ty: FnOutput<'tcx>,
 
@@ -987,6 +992,10 @@ pub enum Literal<'tcx> {
     Value {
         value: ConstVal,
     },
+    Promoted {
+        // Index into the `promoted` vector of `Mir`.
+        index: usize
+    },
 }
 
 impl<'tcx> Debug for Constant<'tcx> {
@@ -1007,6 +1016,9 @@ impl<'tcx> Debug for Literal<'tcx> {
                 write!(fmt, "const ")?;
                 fmt_const_val(fmt, value)
             }
+            Promoted { index } => {
+                write!(fmt, "promoted{}", index)
+            }
         }
     }
 }

src/librustc/mir/transform.rs

@@ -8,31 +8,102 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use dep_graph::DepNode;
+use hir;
+use hir::map::DefPathData;
+use hir::def_id::DefId;
 use mir::mir_map::MirMap;
 use mir::repr::Mir;
 use ty::TyCtxt;
 use syntax::ast::NodeId;
 
+/// Where a specific Mir comes from.
+#[derive(Copy, Clone)]
+pub enum MirSource {
+    /// Functions and methods.
+    Fn(NodeId),
+
+    /// Constants and associated constants.
+    Const(NodeId),
+
+    /// Initializer of a `static` item.
+    Static(NodeId, hir::Mutability),
+
+    /// Promoted rvalues within a function.
+    Promoted(NodeId, usize)
+}
+
+impl MirSource {
+    pub fn from_node(tcx: &TyCtxt, id: NodeId) -> MirSource {
+        use hir::*;
+
+        // Handle constants in enum discriminants, types, and repeat expressions.
+        let def_id = tcx.map.local_def_id(id);
+        let def_key = tcx.def_key(def_id);
+        if def_key.disambiguated_data.data == DefPathData::Initializer {
+            return MirSource::Const(id);
+        }
+
+        match tcx.map.get(id) {
+            map::NodeItem(&Item { node: ItemConst(..), .. }) |
+            map::NodeTraitItem(&TraitItem { node: ConstTraitItem(..), .. }) |
+            map::NodeImplItem(&ImplItem { node: ImplItemKind::Const(..), .. }) => {
+                MirSource::Const(id)
+            }
+            map::NodeItem(&Item { node: ItemStatic(_, m, _), .. }) => {
+                MirSource::Static(id, m)
+            }
+            // Default to function if it's not a constant or static.
+            _ => MirSource::Fn(id)
+        }
+    }
+
+    pub fn item_id(&self) -> NodeId {
+        match *self {
+            MirSource::Fn(id) |
+            MirSource::Const(id) |
+            MirSource::Static(id, _) |
+            MirSource::Promoted(id, _) => id
+        }
+    }
+}
+
 /// Various information about pass.
 pub trait Pass {
     // fn name() for printouts of various sorts?
     // fn should_run(Session) to check if pass should run?
+    fn dep_node(&self, def_id: DefId) -> DepNode<DefId> {
+        DepNode::MirPass(def_id)
+    }
 }
 
 /// A pass which inspects the whole MirMap.
 pub trait MirMapPass<'tcx>: Pass {
-    fn run_pass(&mut self, cx: &TyCtxt<'tcx>, map: &mut MirMap<'tcx>);
+    fn run_pass(&mut self, tcx: &TyCtxt<'tcx>, map: &mut MirMap<'tcx>);
 }
 
 /// A pass which inspects Mir of functions in isolation.
 pub trait MirPass<'tcx>: Pass {
-    fn run_pass(&mut self, cx: &TyCtxt<'tcx>, id: NodeId, mir: &mut Mir<'tcx>);
+    fn run_pass_on_promoted(&mut self, tcx: &TyCtxt<'tcx>,
+                            item_id: NodeId, index: usize,
+                            mir: &mut Mir<'tcx>) {
+        self.run_pass(tcx, MirSource::Promoted(item_id, index), mir);
+    }
+    fn run_pass(&mut self, tcx: &TyCtxt<'tcx>, src: MirSource, mir: &mut Mir<'tcx>);
 }
 
 impl<'tcx, T: MirPass<'tcx>> MirMapPass<'tcx> for T {
     fn run_pass(&mut self, tcx: &TyCtxt<'tcx>, map: &mut MirMap<'tcx>) {
         for (&id, mir) in &mut map.map {
-            MirPass::run_pass(self, tcx, id, mir);
+            let def_id = tcx.map.local_def_id(id);
+            let _task = tcx.dep_graph.in_task(self.dep_node(def_id));
+
+            let src = MirSource::from_node(tcx, id);
+            MirPass::run_pass(self, tcx, src, mir);
+
+            for (i, mir) in mir.promoted.iter_mut().enumerate() {
+                self.run_pass_on_promoted(tcx, id, i, mir);
+            }
         }
     }
 }

src/librustc/mir/visit.rs

@@ -244,6 +244,7 @@ macro_rules! make_mir_visitor {
                 let Mir {
                     ref $($mutability)* basic_blocks,
                     ref $($mutability)* scopes,
+                    promoted: _, // Visited by passes separately.
                     ref $($mutability)* return_ty,
                     ref $($mutability)* var_decls,
                     ref $($mutability)* arg_decls,
@@ -649,10 +650,11 @@ macro_rules! make_mir_visitor {
                                     ref $($mutability)* substs } => {
                         self.visit_def_id(def_id);
                         self.visit_substs(substs);
-                    },
+                    }
                     Literal::Value { ref $($mutability)* value } => {
                         self.visit_const_val(value);
                     }
+                    Literal::Promoted { index: _ } => {}
                 }
             }
 

src/librustc/ty/mod.rs

@@ -25,7 +25,7 @@ use middle::cstore::{self, LOCAL_CRATE};
 use hir::def::{self, Def, ExportMap};
 use hir::def_id::DefId;
 use middle::lang_items::{FnTraitLangItem, FnMutTraitLangItem, FnOnceTraitLangItem};
-use middle::region::{CodeExtent};
+use middle::region::{CodeExtent, ROOT_CODE_EXTENT};
 use traits;
 use ty;
 use ty::subst::{Subst, Substs, VecPerParamSpace};
@@ -1376,6 +1376,7 @@ impl<'a, 'tcx> ParameterEnvironment<'a, 'tcx> {
                     }
                     hir::ItemEnum(..) |
                     hir::ItemStruct(..) |
+                    hir::ItemTy(..) |
                     hir::ItemImpl(..) |
                     hir::ItemConst(..) |
                     hir::ItemStatic(..) => {
@@ -1408,6 +1409,15 @@ impl<'a, 'tcx> ParameterEnvironment<'a, 'tcx> {
                 // This is a convenience to allow closures to work.
                 ParameterEnvironment::for_item(cx, cx.map.get_parent(id))
             }
+            Some(ast_map::NodeForeignItem(item)) => {
+                let def_id = cx.map.local_def_id(id);
+                let scheme = cx.lookup_item_type(def_id);
+                let predicates = cx.lookup_predicates(def_id);
+                cx.construct_parameter_environment(item.span,
+                                                   &scheme.generics,
+                                                   &predicates,
+                                                   ROOT_CODE_EXTENT)
+            }
             _ => {
                 bug!("ParameterEnvironment::from_item(): \
                       `{}` is not an item",

src/librustc_driver/driver.rs

@@ -37,7 +37,7 @@ use rustc_privacy;
 use rustc_plugin::registry::Registry;
 use rustc_plugin as plugin;
 use rustc::hir::lowering::{lower_crate, LoweringContext};
-use rustc_passes::{no_asm, loops, consts, const_fn, rvalues, static_recursion};
+use rustc_passes::{no_asm, loops, consts, rvalues, static_recursion};
 use rustc_const_eval::check_match;
 use super::Compilation;
 
@@ -726,10 +726,6 @@ pub fn phase_2_configure_and_expand(sess: &Session,
         })
     })?;
 
-    time(time_passes,
-         "const fn bodies and arguments",
-         || const_fn::check_crate(sess, &krate))?;
-
     if sess.opts.debugging_opts.input_stats {
         println!("Post-expansion node count: {}", count_nodes(&krate));
     }
@@ -903,6 +899,7 @@ pub fn phase_3_run_analysis_passes<'tcx, F, R>(sess: &'tcx Session,
             let mut passes = sess.mir_passes.borrow_mut();
             // Push all the built-in passes.
             passes.push_pass(box mir::transform::remove_dead_blocks::RemoveDeadBlocks);
+            passes.push_pass(box mir::transform::qualify_consts::QualifyAndPromoteConstants);
             passes.push_pass(box mir::transform::type_check::TypeckMir);
             passes.push_pass(box mir::transform::simplify_cfg::SimplifyCfg);
             passes.push_pass(box mir::transform::remove_dead_blocks::RemoveDeadBlocks);

src/librustc_metadata/decoder.rs

@@ -855,6 +855,9 @@ pub fn maybe_get_item_mir<'tcx>(cdata: Cmd,
         };
 
         def_id_and_span_translator.visit_mir(&mut mir);
+        for promoted in &mut mir.promoted {
+            def_id_and_span_translator.visit_mir(promoted);
+        }
 
         mir
     });

src/librustc_mir/Cargo.toml

@@ -16,4 +16,5 @@ rustc_back = { path = "../librustc_back" }
 rustc_const_eval = { path = "../librustc_const_eval" }
 rustc_const_math = { path = "../librustc_const_math" }
 rustc_data_structures = { path = "../librustc_data_structures" }
+rustc_bitflags = { path = "../librustc_bitflags" }
 syntax = { path = "../libsyntax" }