Auto merge of #40224 - nikomatsakis:issue-39808, r=eddyb

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 #39808.
Fixes #39984.

src/librustc/cfg/construct.rs

@@ -74,11 +74,11 @@ pub fn construct<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 
 impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
     fn block(&mut self, blk: &hir::Block, pred: CFGIndex) -> CFGIndex {
-        if let Some(break_to_expr_id) = blk.break_to_expr_id {
+        if blk.targeted_by_break {
             let expr_exit = self.add_ast_node(blk.id, &[]);
 
             self.breakable_block_scopes.push(BlockScope {
-                block_expr_id: break_to_expr_id,
+                block_expr_id: blk.id,
                 break_index: expr_exit,
             });
 
@@ -195,7 +195,7 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
                 //   [..expr..]
                 //
                 let cond_exit = self.expr(&cond, pred);                // 1
-                let then_exit = self.block(&then, cond_exit);          // 2
+                let then_exit = self.expr(&then, cond_exit);          // 2
                 self.add_ast_node(expr.id, &[cond_exit, then_exit])      // 3,4
             }
 
@@ -215,7 +215,7 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
                 //   [..expr..]
                 //
                 let cond_exit = self.expr(&cond, pred);                // 1
-                let then_exit = self.block(&then, cond_exit);          // 2
+                let then_exit = self.expr(&then, cond_exit);          // 2
                 let else_exit = self.expr(&otherwise, cond_exit);      // 3
                 self.add_ast_node(expr.id, &[then_exit, else_exit])      // 4, 5
             }

src/librustc/hir/intravisit.rs

@@ -960,7 +960,7 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr) {
         }
         ExprIf(ref head_expression, ref if_block, ref optional_else) => {
             visitor.visit_expr(head_expression);
-            visitor.visit_block(if_block);
+            visitor.visit_expr(if_block);
             walk_list!(visitor, visit_expr, optional_else);
         }
         ExprWhile(ref subexpression, ref block, ref opt_sp_name) => {

src/librustc/hir/lowering.rs

@@ -1156,7 +1156,7 @@ impl<'a> LoweringContext<'a> {
         bounds.iter().map(|bound| self.lower_ty_param_bound(bound)).collect()
     }
 
-    fn lower_block(&mut self, b: &Block, break_to: Option<NodeId>) -> P<hir::Block> {
+    fn lower_block(&mut self, b: &Block, targeted_by_break: bool) -> P<hir::Block> {
         let mut expr = None;
 
         let mut stmts = vec![];
@@ -1179,7 +1179,7 @@ impl<'a> LoweringContext<'a> {
             expr: expr,
             rules: self.lower_block_check_mode(&b.rules),
             span: b.span,
-            break_to_expr_id: break_to,
+            targeted_by_break: targeted_by_break,
         })
     }
 
@@ -1274,7 +1274,7 @@ impl<'a> LoweringContext<'a> {
             }
             ItemKind::Fn(ref decl, unsafety, constness, abi, ref generics, ref body) => {
                 self.with_new_scopes(|this| {
-                    let body = this.lower_block(body, None);
+                    let body = this.lower_block(body, false);
                     let body = this.expr_block(body, ThinVec::new());
                     let body_id = this.record_body(body, Some(decl));
                     hir::ItemFn(this.lower_fn_decl(decl),
@@ -1368,7 +1368,7 @@ impl<'a> LoweringContext<'a> {
                                                    hir::TraitMethod::Required(names))
                     }
                     TraitItemKind::Method(ref sig, Some(ref body)) => {
-                        let body = this.lower_block(body, None);
+                        let body = this.lower_block(body, false);
                         let expr = this.expr_block(body, ThinVec::new());
                         let body_id = this.record_body(expr, Some(&sig.decl));
                         hir::TraitItemKind::Method(this.lower_method_sig(sig),
@@ -1424,7 +1424,7 @@ impl<'a> LoweringContext<'a> {
                         hir::ImplItemKind::Const(this.lower_ty(ty), body_id)
                     }
                     ImplItemKind::Method(ref sig, ref body) => {
-                        let body = this.lower_block(body, None);
+                        let body = this.lower_block(body, false);
                         let expr = this.expr_block(body, ThinVec::new());
                         let body_id = this.record_body(expr, Some(&sig.decl));
                         hir::ImplItemKind::Method(this.lower_method_sig(sig), body_id)
@@ -1848,32 +1848,35 @@ impl<'a> LoweringContext<'a> {
                                 id: id,
                                 rules: hir::DefaultBlock,
                                 span: span,
-                                break_to_expr_id: None,
+                                targeted_by_break: false,
                             });
                             P(self.expr_block(blk, ThinVec::new()))
                         }
                         _ => P(self.lower_expr(els)),
                     }
                 });
 
-                hir::ExprIf(P(self.lower_expr(cond)), self.lower_block(blk, None), else_opt)
+                let then_blk = self.lower_block(blk, false);
+                let then_expr = self.expr_block(then_blk, ThinVec::new());
+
+                hir::ExprIf(P(self.lower_expr(cond)), P(then_expr), else_opt)
             }
             ExprKind::While(ref cond, ref body, opt_ident) => {
                 self.with_loop_scope(e.id, |this|
                     hir::ExprWhile(
                         this.with_loop_condition_scope(|this| P(this.lower_expr(cond))),
-                        this.lower_block(body, None),
+                        this.lower_block(body, false),
                         this.lower_opt_sp_ident(opt_ident)))
             }
             ExprKind::Loop(ref body, opt_ident) => {
                 self.with_loop_scope(e.id, |this|
-                    hir::ExprLoop(this.lower_block(body, None),
+                    hir::ExprLoop(this.lower_block(body, false),
                                   this.lower_opt_sp_ident(opt_ident),
                                   hir::LoopSource::Loop))
             }
             ExprKind::Catch(ref body) => {
-                self.with_catch_scope(e.id, |this|
-                    hir::ExprBlock(this.lower_block(body, Some(e.id))))
+                self.with_catch_scope(body.id, |this|
+                    hir::ExprBlock(this.lower_block(body, true)))
             }
             ExprKind::Match(ref expr, ref arms) => {
                 hir::ExprMatch(P(self.lower_expr(expr)),
@@ -1891,7 +1894,7 @@ impl<'a> LoweringContext<'a> {
                     })
                 })
             }
-            ExprKind::Block(ref blk) => hir::ExprBlock(self.lower_block(blk, None)),
+            ExprKind::Block(ref blk) => hir::ExprBlock(self.lower_block(blk, false)),
             ExprKind::Assign(ref el, ref er) => {
                 hir::ExprAssign(P(self.lower_expr(el)), P(self.lower_expr(er)))
             }
@@ -2037,7 +2040,7 @@ impl<'a> LoweringContext<'a> {
 
                 // `<pat> => <body>`
                 {
-                    let body = self.lower_block(body, None);
+                    let body = self.lower_block(body, false);
                     let body_expr = P(self.expr_block(body, ThinVec::new()));
                     let pat = self.lower_pat(pat);
                     arms.push(self.arm(hir_vec![pat], body_expr));
@@ -2109,7 +2112,7 @@ impl<'a> LoweringContext<'a> {
                             let (guard, body) = if let ExprKind::If(ref cond,
                                                                     ref then,
                                                                     _) = else_expr.node {
-                                let then = self.lower_block(then, None);
+                                let then = self.lower_block(then, false);
                                 (Some(cond),
                                  self.expr_block(then, ThinVec::new()))
                             } else {
@@ -2159,7 +2162,7 @@ impl<'a> LoweringContext<'a> {
                 // Note that the block AND the condition are evaluated in the loop scope.
                 // This is done to allow `break` from inside the condition of the loop.
                 let (body, break_expr, sub_expr) = self.with_loop_scope(e.id, |this| (
-                    this.lower_block(body, None),
+                    this.lower_block(body, false),
                     this.expr_break(e.span, ThinVec::new()),
                     this.with_loop_condition_scope(|this| P(this.lower_expr(sub_expr))),
                 ));
@@ -2220,7 +2223,7 @@ impl<'a> LoweringContext<'a> {
                 // `::std::option::Option::Some(<pat>) => <body>`
                 let pat_arm = {
                     let body_block = self.with_loop_scope(e.id,
-                                                          |this| this.lower_block(body, None));
+                                                          |this| this.lower_block(body, false));
                     let body_expr = P(self.expr_block(body_block, ThinVec::new()));
                     let pat = self.lower_pat(pat);
                     let some_pat = self.pat_some(e.span, pat);
@@ -2652,7 +2655,7 @@ impl<'a> LoweringContext<'a> {
             id: self.next_id(),
             rules: hir::DefaultBlock,
             span: span,
-            break_to_expr_id: None,
+            targeted_by_break: false,
         }
     }
 
@@ -2760,7 +2763,7 @@ impl<'a> LoweringContext<'a> {
             id: id,
             stmts: stmts,
             expr: Some(expr),
-            break_to_expr_id: None,
+            targeted_by_break: false,
         });
         self.expr_block(block, attrs)
     }

src/librustc/hir/mod.rs

@@ -549,9 +549,11 @@ pub struct Block {
     /// Distinguishes between `unsafe { ... }` and `{ ... }`
     pub rules: BlockCheckMode,
     pub span: Span,
-    /// The id of the expression that `break` breaks to if the block can be broken out of.
-    /// Currently only `Some(_)` for `catch {}` blocks
-    pub break_to_expr_id: Option<NodeId>,
+    /// If true, then there may exist `break 'a` values that aim to
+    /// break out of this block early. As of this writing, this is not
+    /// currently permitted in Rust itself, but it is generated as
+    /// part of `catch` statements.
+    pub targeted_by_break: bool,
 }
 
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash)]
@@ -993,8 +995,8 @@ pub enum Expr_ {
     ExprType(P<Expr>, P<Ty>),
     /// An `if` block, with an optional else block
     ///
-    /// `if expr { block } else { expr }`
-    ExprIf(P<Expr>, P<Block>, Option<P<Expr>>),
+    /// `if expr { expr } else { expr }`
+    ExprIf(P<Expr>, P<Expr>, Option<P<Expr>>),
     /// A while loop, with an optional label
     ///
     /// `'label: while expr { block }`

src/librustc/hir/print.rs

@@ -1036,7 +1036,7 @@ impl<'a> State<'a> {
                         word(&mut self.s, " else if ")?;
                         self.print_expr(&i)?;
                         space(&mut self.s)?;
-                        self.print_block(&then)?;
+                        self.print_expr(&then)?;
                         self.print_else(e.as_ref().map(|e| &**e))
                     }
                     // "final else"
@@ -1058,13 +1058,13 @@ impl<'a> State<'a> {
 
     pub fn print_if(&mut self,
                     test: &hir::Expr,
-                    blk: &hir::Block,
+                    blk: &hir::Expr,
                     elseopt: Option<&hir::Expr>)
                     -> io::Result<()> {
         self.head("if")?;
         self.print_expr(test)?;
         space(&mut self.s)?;
-        self.print_block(blk)?;
+        self.print_expr(blk)?;
         self.print_else(elseopt)
     }
 

src/librustc/infer/type_variable.rs

@@ -30,6 +30,7 @@ pub struct TypeVariableTable<'tcx> {
 }
 
 /// Reasons to create a type inference variable
+#[derive(Debug)]
 pub enum TypeVariableOrigin {
     MiscVariable(Span),
     NormalizeProjectionType(Span),
@@ -41,6 +42,7 @@ pub enum TypeVariableOrigin {
     AdjustmentType(Span),
     DivergingStmt(Span),
     DivergingBlockExpr(Span),
+    DivergingFn(Span),
     LatticeVariable(Span),
 }
 
@@ -196,14 +198,15 @@ impl<'tcx> TypeVariableTable<'tcx> {
                    diverging: bool,
                    origin: TypeVariableOrigin,
                    default: Option<Default<'tcx>>,) -> ty::TyVid {
+        debug!("new_var(diverging={:?}, origin={:?})", diverging, origin);
         self.eq_relations.new_key(());
         let index = self.values.push(TypeVariableData {
             value: Bounded { relations: vec![], default: default },
             origin: origin,
             diverging: diverging
         });
         let v = ty::TyVid { index: index as u32 };
-        debug!("new_var() -> {:?}", v);
+        debug!("new_var: diverging={:?} index={:?}", diverging, v);
         v
     }
 

src/librustc/middle/expr_use_visitor.rs

@@ -414,9 +414,9 @@ impl<'a, 'gcx, 'tcx> ExprUseVisitor<'a, 'gcx, 'tcx> {
                 self.consume_exprs(exprs);
             }
 
-            hir::ExprIf(ref cond_expr, ref then_blk, ref opt_else_expr) => {
+            hir::ExprIf(ref cond_expr, ref then_expr, ref opt_else_expr) => {
                 self.consume_expr(&cond_expr);
-                self.walk_block(&then_blk);
+                self.walk_expr(&then_expr);
                 if let Some(ref else_expr) = *opt_else_expr {
                     self.consume_expr(&else_expr);
                 }

src/librustc/middle/liveness.rs

@@ -821,8 +821,8 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
 
     fn propagate_through_block(&mut self, blk: &hir::Block, succ: LiveNode)
                                -> LiveNode {
-        if let Some(break_to_expr_id) = blk.break_to_expr_id {
-            self.breakable_block_ln.insert(break_to_expr_id, succ);
+        if blk.targeted_by_break {
+            self.breakable_block_ln.insert(blk.id, succ);
         }
         let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ);
         blk.stmts.iter().rev().fold(succ, |succ, stmt| {
@@ -951,7 +951,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             //   (  succ  )
             //
             let else_ln = self.propagate_through_opt_expr(els.as_ref().map(|e| &**e), succ);
-            let then_ln = self.propagate_through_block(&then, succ);
+            let then_ln = self.propagate_through_expr(&then, succ);
             let ln = self.live_node(expr.id, expr.span);
             self.init_from_succ(ln, else_ln);
             self.merge_from_succ(ln, then_ln, false);

src/librustc/traits/error_reporting.rs

@@ -904,6 +904,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             ObligationCauseCode::StartFunctionType |
             ObligationCauseCode::IntrinsicType |
             ObligationCauseCode::MethodReceiver |
+            ObligationCauseCode::ReturnNoExpression |
             ObligationCauseCode::MiscObligation => {
             }
             ObligationCauseCode::SliceOrArrayElem => {

src/librustc/traits/mod.rs

@@ -173,6 +173,9 @@ pub enum ObligationCauseCode<'tcx> {
 
     // method receiver
     MethodReceiver,
+
+    // `return` with no expression
+    ReturnNoExpression,
 }
 
 #[derive(Clone, Debug, PartialEq, Eq)]

src/librustc/traits/structural_impls.rs

@@ -167,6 +167,7 @@ impl<'a, 'tcx> Lift<'tcx> for traits::ObligationCauseCode<'a> {
     type Lifted = traits::ObligationCauseCode<'tcx>;
     fn lift_to_tcx<'b, 'gcx>(&self, tcx: TyCtxt<'b, 'gcx, 'tcx>) -> Option<Self::Lifted> {
         match *self {
+            super::ReturnNoExpression => Some(super::ReturnNoExpression),
             super::MiscObligation => Some(super::MiscObligation),
             super::SliceOrArrayElem => Some(super::SliceOrArrayElem),
             super::TupleElem => Some(super::TupleElem),
@@ -489,6 +490,7 @@ impl<'tcx> TypeFoldable<'tcx> for traits::ObligationCauseCode<'tcx> {
             super::StructInitializerSized |
             super::VariableType(_) |
             super::ReturnType |
+            super::ReturnNoExpression |
             super::RepeatVec |
             super::FieldSized |
             super::ConstSized |
@@ -533,6 +535,7 @@ impl<'tcx> TypeFoldable<'tcx> for traits::ObligationCauseCode<'tcx> {
             super::StructInitializerSized |
             super::VariableType(_) |
             super::ReturnType |
+            super::ReturnNoExpression |
             super::RepeatVec |
             super::FieldSized |
             super::ConstSized |