Implement numeric fallback

Doesn't yet converge on a fixed point, but generally works. A better algorithm
will come with the implementation of default type parameter fallback.

If inference fails to determine an exact integral or floating point type, it
will set the type to i32 or f64, respectively.

Closes #16968

Author: emberian

src/librustc/middle/infer/mod.rs

@@ -23,7 +23,7 @@ pub use self::freshen::TypeFreshener;
 
 use middle::subst;
 use middle::subst::Substs;
-use middle::ty::{TyVid, IntVid, FloatVid, RegionVid};
+use middle::ty::{TyVid, IntVid, FloatVid, RegionVid, UnconstrainedNumeric};
 use middle::ty::replace_late_bound_regions;
 use middle::ty::{mod, Ty};
 use middle::ty_fold::{TypeFolder, TypeFoldable};
@@ -525,6 +525,25 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         freshen::TypeFreshener::new(self)
     }
 
+    pub fn type_is_unconstrained_numeric(&'a self, ty: Ty) -> UnconstrainedNumeric {
+        use middle::ty::UnconstrainedNumeric::{Neither, UnconstrainedInt, UnconstrainedFloat};
+        match ty.sty {
+            ty::ty_infer(ty::IntVar(vid)) => {
+                match self.int_unification_table.borrow_mut().get(self.tcx, vid).value {
+                    None => UnconstrainedInt,
+                    _ => Neither,
+                }
+            },
+            ty::ty_infer(ty::FloatVar(vid)) => {
+                match self.float_unification_table.borrow_mut().get(self.tcx, vid).value {
+                    None => return UnconstrainedFloat,
+                    _ => Neither,
+                }
+            },
+            _ => Neither,
+        }
+    }
+
     pub fn combine_fields<'b>(&'b self, a_is_expected: bool, trace: TypeTrace<'tcx>)
                               -> CombineFields<'b, 'tcx> {
         CombineFields {infcx: self,

src/librustc/middle/ty.rs

@@ -1641,6 +1641,14 @@ pub enum InferTy {
     FreshIntTy(u32),
 }
 
+#[deriving(Clone, RustcEncodable, RustcDecodable, PartialEq, Eq, Hash, Show, Copy)]
+pub enum UnconstrainedNumeric {
+    UnconstrainedFloat,
+    UnconstrainedInt,
+    Neither,
+}
+
+
 #[deriving(Clone, RustcEncodable, RustcDecodable, Eq, Hash, Show, Copy)]
 pub enum InferRegion {
     ReVar(RegionVid),

src/librustc_typeck/check/mod.rs

@@ -445,7 +445,6 @@ fn check_bare_fn<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
 
             vtable::select_all_fcx_obligations_or_error(&fcx);
             regionck::regionck_fn(&fcx, id, decl, body);
-            fcx.default_diverging_type_variables_to_nil();
             writeback::resolve_type_vars_in_fn(&fcx, decl, body);
         }
         _ => ccx.tcx.sess.impossible_case(body.span,
@@ -1654,10 +1653,24 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
-    pub fn default_diverging_type_variables_to_nil(&self) {
+    /// Apply "fallbacks" to some types
+    /// ! gets replaced with (), unconstrained ints with i32, and unconstrained floats with f64.
+    pub fn default_type_parameters(&self) {
+        use middle::ty::UnconstrainedNumeric::{UnconstrainedInt, UnconstrainedFloat, Neither};
         for (_, &ref ty) in self.inh.node_types.borrow_mut().iter_mut() {
-            if self.infcx().type_var_diverges(self.infcx().resolve_type_vars_if_possible(ty)) {
+            let resolved = self.infcx().resolve_type_vars_if_possible(ty);
+            if self.infcx().type_var_diverges(resolved) {
                 demand::eqtype(self, codemap::DUMMY_SP, *ty, ty::mk_nil(self.tcx()));
+            } else {
+                match self.infcx().type_is_unconstrained_numeric(resolved) {
+                    UnconstrainedInt => {
+                        demand::eqtype(self, codemap::DUMMY_SP, *ty, self.tcx().types.i32)
+                    },
+                    UnconstrainedFloat => {
+                        demand::eqtype(self, codemap::DUMMY_SP, *ty, self.tcx().types.f64)
+                    }
+                    Neither => { }
+                }
             }
         }
     }

src/librustc_typeck/check/vtable.rs

@@ -403,6 +403,9 @@ pub fn register_object_cast_obligations<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
 pub fn select_all_fcx_obligations_or_error(fcx: &FnCtxt) {
     debug!("select_all_fcx_obligations_or_error");
 
+    select_fcx_obligations_where_possible(fcx);
+    fcx.default_type_parameters();
+
     let mut fulfillment_cx = fcx.inh.fulfillment_cx.borrow_mut();
     let r = fulfillment_cx.select_all_or_error(fcx.infcx(),
                                                &fcx.inh.param_env,

src/test/compile-fail/issue-16966.rs

@@ -8,8 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// error-pattern: unable to infer enough type information
 fn main() {
     panic!(
-        1.2 //~ ERROR cannot determine a type for this expression
+        std::default::Default::default()
     );
 }

src/test/compile-fail/traits-multidispatch-convert-ambig-dest.rs

@@ -33,7 +33,7 @@ where T : Convert<U>
 }
 
 fn a() {
-    test(22_i32, 44); //~ ERROR unable to infer
+    test(22_i32, std::default::Default::default()); //~ ERROR unable to infer
 }
 
 fn main() {}

src/test/compile-fail/issue-6458-1.rs renamed to src/test/run-fail/issue-6458-1.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// error-pattern:explicit panic
+
 fn foo<T>(t: T) {}
 fn main() { foo(panic!()) }
-    //~^ ERROR type annotations required

src/test/compile-fail/integer-literal-suffix-inference-2.rs renamed to src/test/run-pass/integer-literal-suffix-inference-2.rs

@@ -11,7 +11,7 @@
 fn foo(_: *const ()) {}
 
 fn main() {
-    let a = 3;  //~ ERROR cannot determine a type for this local variable
+    let a = 3;
     foo(&a as *const _ as *const ());
 }
 

src/test/compile-fail/integer-literal-suffix-inference-3.rs renamed to src/test/run-pass/integer-literal-suffix-inference-3.rs

@@ -10,6 +10,5 @@
 
 fn main() {
     println!("{}", std::mem::size_of_val(&1));
-    //~^ ERROR cannot determine a type for this expression
 }
 

src/test/compile-fail/issue-11382.rs renamed to src/test/run-pass/issue-11382.rs

@@ -9,8 +9,5 @@
 // except according to those terms.
 
 fn main() {
-panic!(
-    1.2
-//~^ ERROR cannot determine the type of this number; add a suffix to specify the type explicitly
-);
+    println!("{}", 1.2);
 }

src/test/compile-fail/issue-15730.rs renamed to src/test/run-pass/issue-15730.rs

@@ -12,6 +12,5 @@
 
 fn main() {
     let mut array = [1, 2, 3];
-//~^ ERROR cannot determine a type for this local variable: cannot determine the type of this integ
     let pie_slice = array[1..2];
 }

src/test/compile-fail/issue-16783.rs renamed to src/test/run-pass/issue-16783.rs

@@ -10,6 +10,5 @@
 
 pub fn main() {
     let x = [1, 2, 3];
-    //~^ ERROR cannot determine a type for this local variable: cannot determine the type of this
     let y = x.as_slice();
 }

src/test/run-pass/traits-multidispatch-infer-convert-source-and-target.rs

@@ -36,11 +36,10 @@ where T : Convert<U>
 }
 
 fn main() {
+    use std::default::Default;
     // T = i16, U = u32
-    test(22_i16, 44,  2, 4);
-    test(22, 44_u32, 2, 4);
+    test(22_i16, Default::default(),  2, 4);
 
     // T = u32, U = i16
-    test(22_u32, 44, 4, 2);
-    test(22, 44_i16, 4, 2);
+    test(22_u32, Default::default(), 4, 2);
 }