change the strategy for diverging types

The new strategy is as follows. First, the `!` type is assigned
in two cases:

- a block with a diverging statement and no tail expression (e.g.,
  `{return;}`);
- any expression with the type `!` is considered diverging.

Second, we track when we are in a diverging state, and we permit a value
of any type to be coerced **into** `!` if the expression that produced
it is diverging. This means that `fn foo() -> ! { panic!(); 22 }`
type-checks, even though the block has a type of `usize`.

Finally, coercions **from** the `!` type to any other are always
permitted.

Fixes rust-lang#39808.

src/librustc_typeck/check/demand.rs

@@ -67,8 +67,30 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
     }
 
     // Checks that the type of `expr` can be coerced to `expected`.
-    pub fn demand_coerce(&self, expr: &hir::Expr, checked_ty: Ty<'tcx>, expected: Ty<'tcx>) {
+    //
+    // NB: This code relies on `self.diverges` to be accurate.  In
+    // particular, assignments to `!` will be permitted if the
+    // diverges flag is currently "always".
+    pub fn demand_coerce(&self,
+                         expr: &hir::Expr,
+                         checked_ty: Ty<'tcx>,
+                         expected: Ty<'tcx>) {
         let expected = self.resolve_type_vars_with_obligations(expected);
+
+        // If we are "assigning" to a `!` location, then we can permit
+        // any type to be assigned there, so long as we are in
+        // dead-code. This applies to e.g. `fn foo() -> ! { return; 22
+        // }` but also `fn foo() { let x: ! = { return; 22 }; }`.
+        //
+        // You might imagine that we could just *always* bail if we
+        // are in dead-code, but we don't want to do that, because
+        // that leaves a lot of type variables unconstrained. See
+        // e.g. #39808 and #39984.
+        let in_dead_code = self.diverges.get().always();
+        if expected.is_never() && in_dead_code {
+            return;
+        }
+
         if let Err(e) = self.try_coerce(expr, checked_ty, expected) {
             let cause = self.misc(expr.span);
             let expr_ty = self.resolve_type_vars_with_obligations(checked_ty);

src/librustc_typeck/check/mod.rs

@@ -1531,18 +1531,13 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
     #[inline]
     pub fn write_ty(&self, node_id: ast::NodeId, ty: Ty<'tcx>) {
         debug!("write_ty({}, {:?}) in fcx {}",
-               node_id, ty, self.tag());
+               node_id, self.resolve_type_vars_if_possible(&ty), self.tag());
         self.tables.borrow_mut().node_types.insert(node_id, ty);
 
         if ty.references_error() {
             self.has_errors.set(true);
             self.set_tainted_by_errors();
         }
-
-        // FIXME(canndrew): This is_never should probably be an is_uninhabited
-        if ty.is_never() || self.type_var_diverges(ty) {
-            self.diverges.set(self.diverges.get() | Diverges::Always);
-        }
     }
 
     pub fn write_substs(&self, node_id: ast::NodeId, substs: ty::ItemSubsts<'tcx>) {
@@ -3280,6 +3275,11 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             _ => self.warn_if_unreachable(expr.id, expr.span, "expression")
         }
 
+        // Any expression that produces a value of type `!` must have diverged
+        if ty.is_never() {
+            self.diverges.set(self.diverges.get() | Diverges::Always);
+        }
+
         // Record the type, which applies it effects.
         // We need to do this after the warning above, so that
         // we don't warn for the diverging expression itself.
@@ -3965,7 +3965,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         self.diverges.set(Diverges::Maybe);
         self.has_errors.set(false);
 
-        let (node_id, span) = match stmt.node {
+        let (node_id, _span) = match stmt.node {
             hir::StmtDecl(ref decl, id) => {
                 let span = match decl.node {
                     hir::DeclLocal(ref l) => {
@@ -3991,9 +3991,6 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 
         if self.has_errors.get() {
             self.write_error(node_id);
-        } else if self.diverges.get().always() {
-            self.write_ty(node_id, self.next_diverging_ty_var(
-                TypeVariableOrigin::DivergingStmt(span)));
         } else {
             self.write_nil(node_id);
         }
@@ -4044,14 +4041,19 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                         cause = self.misc(e.span);
                     },
                     None => {
-                        e_ty = self.tcx.mk_nil();
+                        e_ty = if self.diverges.get().always() {
+                            self.next_diverging_ty_var(TypeVariableOrigin::DivergingBlockExpr(blk.span))
+                        } else {
+                            self.tcx.mk_nil()
+                        };
                         cause = self.misc(blk.span);
                     }
                 };
 
                 (e_ty, cause)
             });
 
+        if let ExpectHasType(ety) = expected {
             if let Some(ref e) = blk.expr {
                 let result = if !ctxt.may_break {
                     self.try_coerce(e, e_ty, ctxt.coerce_to)

src/test/compile-fail/diverging-tuple-parts-39485.rs

@@ -0,0 +1,25 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// After #39485, this test used to pass, but that change was reverted
+// due to numerous inference failures like #39808, so it now fails
+// again. #39485 made it so that diverging types never propagate
+// upward; but we now do propagate such types upward in many more
+// cases.
+
+fn g() {
+    &panic!() //~ ERROR mismatched types
+}
+
+fn f() -> isize {
+    (return 1, return 2) //~ ERROR mismatched types
+}
+
+fn main() {}

src/test/compile-fail/never-assign-wrong-type.rs

@@ -11,6 +11,7 @@
 // Test that we can't use another type in place of !
 
 #![feature(never_type)]
+#![deny(warnings)]
 
 fn main() {
     let x: ! = "hello"; //~ ERROR mismatched types

src/test/run-pass/issue-39808.rs

@@ -0,0 +1,21 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Regression test: even though `Ok` is dead-code, its type needs to
+// be influenced by the result of `Err` or else we get a "type
+// variable unconstrained" error.
+
+fn main() {
+    let _ = if false {
+        Ok(return)
+    } else {
+        Err("")
+    };
+}