Some cleanups around AllocId management

src/librustc/ich/impls_ty.rs

@@ -338,7 +338,7 @@ impl_stable_hash_for!(
 );
 
 impl_stable_hash_for!(
-    impl<'tcx, M> for enum mir::interpret::AllocType<'tcx, M> [ mir::interpret::AllocType ] {
+    impl<'tcx> for enum mir::interpret::AllocType<'tcx> [ mir::interpret::AllocType ] {
         Function(instance),
         Static(def_id),
         Memory(mem),

src/librustc/mir/interpret/mod.rs

@@ -41,7 +41,6 @@ use ty::{self, TyCtxt, Instance};
 use ty::layout::{self, Size};
 use middle::region;
 use std::io;
-use std::hash::Hash;
 use rustc_serialize::{Encoder, Decodable, Encodable};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sync::{Lock as Mutex, HashMapExt};
@@ -104,7 +103,7 @@ pub fn specialized_encode_alloc_id<
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
     alloc_id: AllocId,
 ) -> Result<(), E::Error> {
-    let alloc_type: AllocType<'tcx, &'tcx Allocation> =
+    let alloc_type: AllocType<'tcx> =
         tcx.alloc_map.lock().get(alloc_id).expect("no value for AllocId");
     match alloc_type {
         AllocType::Memory(alloc) => {
@@ -292,29 +291,29 @@ impl fmt::Display for AllocId {
 }
 
 #[derive(Debug, Clone, Eq, PartialEq, Hash, RustcDecodable, RustcEncodable)]
-pub enum AllocType<'tcx, M> {
+pub enum AllocType<'tcx> {
     /// The alloc id is used as a function pointer
     Function(Instance<'tcx>),
     /// The alloc id points to a "lazy" static variable that did not get computed (yet).
     /// This is also used to break the cycle in recursive statics.
     Static(DefId),
     /// The alloc id points to memory
-    Memory(M)
+    Memory(&'tcx Allocation),
 }
 
-pub struct AllocMap<'tcx, M> {
+pub struct AllocMap<'tcx> {
     /// Lets you know what an AllocId refers to
-    id_to_type: FxHashMap<AllocId, AllocType<'tcx, M>>,
+    id_to_type: FxHashMap<AllocId, AllocType<'tcx>>,
 
-    /// Used to ensure that functions and statics only get one associated AllocId
-    type_interner: FxHashMap<AllocType<'tcx, M>, AllocId>,
+    /// Used to ensure that statics only get one associated AllocId
+    type_interner: FxHashMap<AllocType<'tcx>, AllocId>,
 
     /// The AllocId to assign to the next requested id.
     /// Always incremented, never gets smaller.
     next_id: AllocId,
 }
 
-impl<'tcx, M: fmt::Debug + Eq + Hash + Clone> AllocMap<'tcx, M> {
+impl<'tcx> AllocMap<'tcx> {
     pub fn new() -> Self {
         AllocMap {
             id_to_type: Default::default(),
@@ -323,8 +322,11 @@ impl<'tcx, M: fmt::Debug + Eq + Hash + Clone> AllocMap<'tcx, M> {
         }
     }
 
-    /// obtains a new allocation ID that can be referenced but does not
+    /// Obtains a new allocation ID that can be referenced but does not
     /// yet have an allocation backing it.
+    ///
+    /// Make sure to call `set_id_memory` or `set_id_same_memory` before returning such an
+    /// `AllocId` from a query.
     pub fn reserve(
         &mut self,
     ) -> AllocId {
@@ -337,7 +339,7 @@ impl<'tcx, M: fmt::Debug + Eq + Hash + Clone> AllocMap<'tcx, M> {
         next
     }
 
-    fn intern(&mut self, alloc_type: AllocType<'tcx, M>) -> AllocId {
+    fn intern(&mut self, alloc_type: AllocType<'tcx>) -> AllocId {
         if let Some(&alloc_id) = self.type_interner.get(&alloc_type) {
             return alloc_id;
         }
@@ -348,42 +350,60 @@ impl<'tcx, M: fmt::Debug + Eq + Hash + Clone> AllocMap<'tcx, M> {
         id
     }
 
-    // FIXME: Check if functions have identity. If not, we should not intern these,
-    // but instead create a new id per use.
-    // Alternatively we could just make comparing function pointers an error.
+    /// Functions cannot be identified by pointers, as asm-equal functions can get deduplicated
+    /// by the linker and functions can be duplicated across crates.
+    /// We thus generate a new `AllocId` for every mention of a function. This means that
+    /// `main as fn() == main as fn()` is false, while `let x = main as fn(); x == x` is true.
     pub fn create_fn_alloc(&mut self, instance: Instance<'tcx>) -> AllocId {
-        self.intern(AllocType::Function(instance))
+        let id = self.reserve();
+        self.id_to_type.insert(id, AllocType::Function(instance));
+        id
     }
 
-    pub fn get(&self, id: AllocId) -> Option<AllocType<'tcx, M>> {
+    /// Returns `None` in case the `AllocId` is dangling.
+    /// This function exists to allow const eval to detect the difference between evaluation-
+    /// local dangling pointers and allocations in constants/statics.
+    pub fn get(&self, id: AllocId) -> Option<AllocType<'tcx>> {
         self.id_to_type.get(&id).cloned()
     }
 
-    pub fn unwrap_memory(&self, id: AllocId) -> M {
+    /// Panics if the `AllocId` does not refer to an `Allocation`
+    pub fn unwrap_memory(&self, id: AllocId) -> &'tcx Allocation {
         match self.get(id) {
             Some(AllocType::Memory(mem)) => mem,
             _ => bug!("expected allocation id {} to point to memory", id),
         }
     }
 
+    /// Generate an `AllocId` for a static or return a cached one in case this function has been
+    /// called on the same static before.
     pub fn intern_static(&mut self, static_id: DefId) -> AllocId {
         self.intern(AllocType::Static(static_id))
     }
 
-    pub fn allocate(&mut self, mem: M) -> AllocId {
+    /// Intern the `Allocation` and return a new `AllocId`, even if there's already an identical
+    /// `Allocation` with a different `AllocId`.
+    // FIXME: is this really necessary? Can we ensure `FOO` and `BAR` being different after codegen
+    // in `static FOO: u32 = 42; static BAR: u32 = 42;` even if they reuse the same allocation
+    // inside rustc?
+    pub fn allocate(&mut self, mem: &'tcx Allocation) -> AllocId {
         let id = self.reserve();
         self.set_id_memory(id, mem);
         id
     }
 
-    pub fn set_id_memory(&mut self, id: AllocId, mem: M) {
+    /// Freeze an `AllocId` created with `reserve` by pointing it at an `Allocation`. Trying to
+    /// call this function twice, even with the same `Allocation` will ICE the compiler.
+    pub fn set_id_memory(&mut self, id: AllocId, mem: &'tcx Allocation) {
         if let Some(old) = self.id_to_type.insert(id, AllocType::Memory(mem)) {
             bug!("tried to set allocation id {}, but it was already existing as {:#?}", id, old);
         }
     }
 
-    pub fn set_id_same_memory(&mut self, id: AllocId, mem: M) {
-       self.id_to_type.insert_same(id, AllocType::Memory(mem));
+    /// Freeze an `AllocId` created with `reserve` by pointing it at an `Allocation`. May be called
+    /// twice for the same `(AllocId, Allocation)` pair.
+    pub fn set_id_same_memory(&mut self, id: AllocId, mem: &'tcx Allocation) {
+        self.id_to_type.insert_same(id, AllocType::Memory(mem));
     }
 }
 

src/librustc/ty/context.rs

@@ -946,7 +946,7 @@ pub struct GlobalCtxt<'tcx> {
     /// Stores the value of constants (and deduplicates the actual memory)
     allocation_interner: Lock<FxHashMap<&'tcx Allocation, ()>>,
 
-    pub alloc_map: Lock<interpret::AllocMap<'tcx, &'tcx Allocation>>,
+    pub alloc_map: Lock<interpret::AllocMap<'tcx>>,
 
     layout_interner: Lock<FxHashMap<&'tcx LayoutDetails, ()>>,