rollup merge of #23282: nikomatsakis/fn-trait-inheritance

The primary motivation here is to sidestep #19032 -- for a time, I thought that we should improve coherence or otherwise extend the language, but I now think that any such changes will require more time to bake. In the meantime, inheritance amongst the fn traits is both logically correct *and* a simple solution to that obstacle. This change introduces inheritance and modifies the compiler so that it can properly generate impls for closures and fns.

Things enabled by this PR (but not included in this PR):

1. An impl of `FnMut` for `&mut F` where `F : FnMut` (#23015).
2. A better version of `Thunk` I've been calling `FnBox`.

I did not include either of these in the PR because:

1. Adding the impls in 1 currently induces a coherence conflict with the pattern trait. This is interesting and merits some discussion.
2. `FnBox` deserves to be a PR of its own.

The main downside to this design is (a) the need to write impls by hand; (b) the possibility of implementing `FnMut` with different semantics from `Fn`, etc. Point (a) is minor -- in particular, it does not affect normal closure usage -- and could be addressed in the future in many ways (better defaults; convenient macros; specialization; etc). Point (b) is unfortunate but "just a bug" from my POV, and certainly not unique to these traits (c.f. Copy/Clone, PartialEq/Eq, etc). (Until we lift the feature-gate on implementing the Fn traits, in any case, there is room to correct both of these if we find a nice way.)

Note that I believe this change is reversible in the future if we decide on another course of action, due to the feature gate on implementing the `Fn` traits, though I do not (currently) think we should reverse it.

Fixes #18835.

r? @nrc

Author: alexcrichton

src/libcollectionstest/btree/set.rs

@@ -43,15 +43,21 @@ struct Counter<'a, 'b> {
 }
 
 impl<'a, 'b, 'c> FnMut<(&'c i32,)> for Counter<'a, 'b> {
-    type Output = bool;
-
     extern "rust-call" fn call_mut(&mut self, (&x,): (&'c i32,)) -> bool {
         assert_eq!(x, self.expected[*self.i]);
         *self.i += 1;
         true
     }
 }
 
+impl<'a, 'b, 'c> FnOnce<(&'c i32,)> for Counter<'a, 'b> {
+    type Output = bool;
+
+    extern "rust-call" fn call_once(mut self, args: (&'c i32,)) -> bool {
+        self.call_mut(args)
+    }
+}
+
 fn check<F>(a: &[i32], b: &[i32], expected: &[i32], f: F) where
     // FIXME Replace Counter with `Box<FnMut(_) -> _>`
     F: FnOnce(&BTreeSet<i32>, &BTreeSet<i32>, Counter) -> bool,

src/libcore/ops.rs

@@ -1148,6 +1148,7 @@ impl<'a, T: ?Sized> DerefMut for &'a mut T {
 #[lang="fn"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_paren_sugar]
+#[cfg(stage0)]
 pub trait Fn<Args> {
     /// The returned type after the call operator is used.
     type Output;
@@ -1156,10 +1157,21 @@ pub trait Fn<Args> {
     extern "rust-call" fn call(&self, args: Args) -> Self::Output;
 }
 
+/// A version of the call operator that takes an immutable receiver.
+#[lang="fn"]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_paren_sugar]
+#[cfg(not(stage0))]
+pub trait Fn<Args> : FnMut<Args> {
+    /// This is called when the call operator is used.
+    extern "rust-call" fn call(&self, args: Args) -> Self::Output;
+}
+
 /// A version of the call operator that takes a mutable receiver.
 #[lang="fn_mut"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_paren_sugar]
+#[cfg(stage0)]
 pub trait FnMut<Args> {
     /// The returned type after the call operator is used.
     type Output;
@@ -1168,6 +1180,16 @@ pub trait FnMut<Args> {
     extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output;
 }
 
+/// A version of the call operator that takes a mutable receiver.
+#[lang="fn_mut"]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_paren_sugar]
+#[cfg(not(stage0))]
+pub trait FnMut<Args> : FnOnce<Args> {
+    /// This is called when the call operator is used.
+    extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output;
+}
+
 /// A version of the call operator that takes a by-value receiver.
 #[lang="fn_once"]
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1180,6 +1202,7 @@ pub trait FnOnce<Args> {
     extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
 }
 
+#[cfg(stage0)]
 impl<F: ?Sized, A> FnMut<A> for F
     where F : Fn<A>
 {
@@ -1190,6 +1213,7 @@ impl<F: ?Sized, A> FnMut<A> for F
     }
 }
 
+#[cfg(stage0)]
 impl<F,A> FnOnce<A> for F
     where F : FnMut<A>
 {

src/libcore/str/mod.rs

@@ -29,7 +29,7 @@ use iter::{Map, Iterator, IteratorExt, DoubleEndedIterator};
 use marker::Sized;
 use mem;
 use num::Int;
-use ops::{Fn, FnMut};
+use ops::{Fn, FnMut, FnOnce};
 use option::Option::{self, None, Some};
 use raw::{Repr, Slice};
 use result::Result::{self, Ok, Err};
@@ -541,6 +541,7 @@ delegate_iter!{exact u8 : Bytes<'a>}
 #[derive(Copy, Clone)]
 struct BytesDeref;
 
+#[cfg(stage0)]
 impl<'a> Fn<(&'a u8,)> for BytesDeref {
     type Output = u8;
 
@@ -550,6 +551,32 @@ impl<'a> Fn<(&'a u8,)> for BytesDeref {
     }
 }
 
+#[cfg(not(stage0))]
+impl<'a> Fn<(&'a u8,)> for BytesDeref {
+    #[inline]
+    extern "rust-call" fn call(&self, (ptr,): (&'a u8,)) -> u8 {
+        *ptr
+    }
+}
+
+#[cfg(not(stage0))]
+impl<'a> FnMut<(&'a u8,)> for BytesDeref {
+    #[inline]
+    extern "rust-call" fn call_mut(&mut self, (ptr,): (&'a u8,)) -> u8 {
+        Fn::call(&*self, (ptr,))
+    }
+}
+
+#[cfg(not(stage0))]
+impl<'a> FnOnce<(&'a u8,)> for BytesDeref {
+    type Output = u8;
+
+    #[inline]
+    extern "rust-call" fn call_once(self, (ptr,): (&'a u8,)) -> u8 {
+        Fn::call(&self, (ptr,))
+    }
+}
+
 /// An iterator over the substrings of a string, separated by `sep`.
 struct CharSplits<'a, P: Pattern<'a>> {
     /// The slice remaining to be iterated

src/librustc/middle/traits/project.rs

@@ -789,10 +789,13 @@ fn confirm_callable_candidate<'cx,'tcx>(
            obligation.repr(tcx),
            fn_sig.repr(tcx));
 
+    // the `Output` associated type is declared on `FnOnce`
+    let fn_once_def_id = tcx.lang_items.fn_once_trait().unwrap();
+
     // Note: we unwrap the binder here but re-create it below (1)
     let ty::Binder((trait_ref, ret_type)) =
         util::closure_trait_ref_and_return_type(tcx,
-                                                obligation.predicate.trait_ref.def_id,
+                                                fn_once_def_id,
                                                 obligation.predicate.trait_ref.self_ty(),
                                                 fn_sig,
                                                 flag);

src/librustc/middle/traits/select.rs

@@ -1069,7 +1069,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         match self.closure_typer.closure_kind(closure_def_id) {
             Some(closure_kind) => {
                 debug!("assemble_unboxed_candidates: closure_kind = {:?}", closure_kind);
-                if closure_kind == kind {
+                if closure_kind.extends(kind) {
                     candidates.vec.push(ClosureCandidate(closure_def_id, substs.clone()));
                 }
             }
@@ -1088,10 +1088,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                                       candidates: &mut SelectionCandidateSet<'tcx>)
                                       -> Result<(),SelectionError<'tcx>>
     {
-        // We provide a `Fn` impl for fn pointers. There is no need to provide
-        // the other traits (e.g. `FnMut`) since those are provided by blanket
-        // impls.
-        if Some(obligation.predicate.def_id()) != self.tcx().lang_items.fn_trait() {
+        // We provide impl of all fn traits for fn pointers.
+        if self.tcx().lang_items.fn_trait_kind(obligation.predicate.def_id()).is_none() {
             return Ok(());
         }
 

src/librustc/middle/ty.rs

@@ -2462,8 +2462,11 @@ pub struct ItemSubsts<'tcx> {
     pub substs: Substs<'tcx>,
 }
 
-#[derive(Clone, Copy, PartialEq, Eq, Debug, RustcEncodable, RustcDecodable)]
+#[derive(Clone, Copy, PartialOrd, Ord, PartialEq, Eq, Debug, RustcEncodable, RustcDecodable)]
 pub enum ClosureKind {
+    // Warning: Ordering is significant here! The ordering is chosen
+    // because the trait Fn is a subtrait of FnMut and so in turn, and
+    // hence we order it so that Fn < FnMut < FnOnce.
     FnClosureKind,
     FnMutClosureKind,
     FnOnceClosureKind,
@@ -2485,6 +2488,20 @@ impl ClosureKind {
             Err(err) => cx.sess.fatal(&err[..]),
         }
     }
+
+    /// True if this a type that impls this closure kind
+    /// must also implement `other`.
+    pub fn extends(self, other: ty::ClosureKind) -> bool {
+        match (self, other) {
+            (FnClosureKind, FnClosureKind) => true,
+            (FnClosureKind, FnMutClosureKind) => true,
+            (FnClosureKind, FnOnceClosureKind) => true,
+            (FnMutClosureKind, FnMutClosureKind) => true,
+            (FnMutClosureKind, FnOnceClosureKind) => true,
+            (FnOnceClosureKind, FnOnceClosureKind) => true,
+            _ => false,
+        }
+    }
 }
 
 pub trait ClosureTyper<'tcx> {

src/librustc_trans/trans/callee.rs

@@ -264,24 +264,36 @@ fn trans_fn_ref_with_substs_to_callee<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
 /// but for the bare function type given.
 pub fn trans_fn_pointer_shim<'a, 'tcx>(
     ccx: &'a CrateContext<'a, 'tcx>,
+    closure_kind: ty::ClosureKind,
     bare_fn_ty: Ty<'tcx>)
     -> ValueRef
 {
     let _icx = push_ctxt("trans_fn_pointer_shim");
     let tcx = ccx.tcx();
 
+    // Normalize the type for better caching.
     let bare_fn_ty = common::erase_regions(tcx, &bare_fn_ty);
-    match ccx.fn_pointer_shims().borrow().get(&bare_fn_ty) {
+
+    // If this is an impl of `Fn` or `FnMut` trait, the receiver is `&self`.
+    let is_by_ref = match closure_kind {
+        ty::FnClosureKind | ty::FnMutClosureKind => true,
+        ty::FnOnceClosureKind => false,
+    };
+    let bare_fn_ty_maybe_ref = if is_by_ref {
+        ty::mk_imm_rptr(tcx, tcx.mk_region(ty::ReStatic), bare_fn_ty)
+    } else {
+        bare_fn_ty
+    };
+
+    // Check if we already trans'd this shim.
+    match ccx.fn_pointer_shims().borrow().get(&bare_fn_ty_maybe_ref) {
         Some(&llval) => { return llval; }
         None => { }
     }
 
     debug!("trans_fn_pointer_shim(bare_fn_ty={})",
            bare_fn_ty.repr(tcx));
 
-    // This is an impl of `Fn` trait, so receiver is `&self`.
-    let bare_fn_ty_ref = ty::mk_imm_rptr(tcx, tcx.mk_region(ty::ReStatic), bare_fn_ty);
-
     // Construct the "tuply" version of `bare_fn_ty`. It takes two arguments: `self`,
     // which is the fn pointer, and `args`, which is the arguments tuple.
     let (opt_def_id, sig) =
@@ -306,7 +318,7 @@ pub fn trans_fn_pointer_shim<'a, 'tcx>(
                                          unsafety: ast::Unsafety::Normal,
                                          abi: synabi::RustCall,
                                          sig: ty::Binder(ty::FnSig {
-                                             inputs: vec![bare_fn_ty_ref,
+                                             inputs: vec![bare_fn_ty_maybe_ref,
                                                           tuple_input_ty],
                                              output: sig.output,
                                              variadic: false
@@ -337,8 +349,11 @@ pub fn trans_fn_pointer_shim<'a, 'tcx>(
     let mut bcx = init_function(&fcx, false, sig.output);
 
     // the first argument (`self`) will be ptr to the the fn pointer
-    let llfnpointer =
-        Load(bcx, get_param(fcx.llfn, fcx.arg_pos(0) as u32));
+    let llfnpointer = if is_by_ref {
+        Load(bcx, get_param(fcx.llfn, fcx.arg_pos(0) as u32))
+    } else {
+        get_param(fcx.llfn, fcx.arg_pos(0) as u32)
+    };
 
     // the remaining arguments will be the untupled values
     let llargs: Vec<_> =
@@ -361,7 +376,7 @@ pub fn trans_fn_pointer_shim<'a, 'tcx>(
 
     finish_fn(&fcx, bcx, sig.output, DebugLoc::None);
 
-    ccx.fn_pointer_shims().borrow_mut().insert(bare_fn_ty, llfn);
+    ccx.fn_pointer_shims().borrow_mut().insert(bare_fn_ty_maybe_ref, llfn);
 
     llfn
 }