Rollup merge of rust-lang#37412 - eddyb:lazy-6, r=nikomatsakis

[6/n] rustc: transition HIR function bodies from Block to Expr.

_This is part of a series ([prev](rust-lang#37408) | [next](rust-lang#37676)) of patches designed to rework rustc into an out-of-order on-demand pipeline model for both better feature support (e.g. [MIR-based](https://github.com/solson/miri) early constant evaluation) and incremental execution of compiler passes (e.g. type-checking), with beneficial consequences to IDE support as well.
If any motivation is unclear, please ask for additional PR description clarifications or code comments._

<hr>

The main change here is that functions and closures both use `Expr` instead of `Block` for their bodies.
For closures this actually allows a honest representation of brace-less closure bodies, e.g. `|x| x + 1` is now distinguishable from `|x| { x + 1 }`, therefore this PR is `[syntax-breaking]` (cc @Manishearth).

Using `Expr` allows more logic to be shared between constant bodies and function bodies, with some small such changes already part of this PR, and eventually easing rust-lang#35078 and per-body type tables.

Incidentally, there used to be some corners cut here and there and as such I had to (re)write divergence tracking for type-checking so that it is capable of understanding basic structured control-flow:

``` rust
fn a(x: bool) -> i32 {
    // match also works (as long as all arms diverge)
    if x { panic!("true") } else { return 1; }
    0 // "unreachable expression" after this PR
}
```

And since liveness' "not all control paths return a value" moved to type-checking we can have nice things:

``` rust
// before & after:
fn b() -> i32 { 0; } // help: consider removing this semicolon

// only after this PR
fn c() -> i32 { { 0; } } // help: consider removing this semicolon
fn d() { let x: i32 = { 0; }; } // help: consider removing this semicolon
fn e() { f({ 0; }); } // help: consider removing this semicolon
```

Author: eddyb

src/libpanic_abort/lib.rs

@@ -53,7 +53,7 @@ pub unsafe extern fn __rust_maybe_catch_panic(f: fn(*mut u8),
 // now hopefully.
 #[no_mangle]
 pub unsafe extern fn __rust_start_panic(_data: usize, _vtable: usize) -> u32 {
-    return abort();
+    abort();
 
     #[cfg(unix)]
     unsafe fn abort() -> ! {

src/librustc/cfg/construct.rs

@@ -33,25 +33,25 @@ struct LoopScope {
 }
 
 pub fn construct<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
-                           blk: &hir::Block) -> CFG {
+                           body: &hir::Expr) -> CFG {
     let mut graph = graph::Graph::new();
     let entry = graph.add_node(CFGNodeData::Entry);
 
     // `fn_exit` is target of return exprs, which lies somewhere
-    // outside input `blk`. (Distinguishing `fn_exit` and `block_exit`
+    // outside input `body`. (Distinguishing `fn_exit` and `body_exit`
     // also resolves chicken-and-egg problem that arises if you try to
-    // have return exprs jump to `block_exit` during construction.)
+    // have return exprs jump to `body_exit` during construction.)
     let fn_exit = graph.add_node(CFGNodeData::Exit);
-    let block_exit;
+    let body_exit;
 
     let mut cfg_builder = CFGBuilder {
         graph: graph,
         fn_exit: fn_exit,
         tcx: tcx,
         loop_scopes: Vec::new()
     };
-    block_exit = cfg_builder.block(blk, entry);
-    cfg_builder.add_contained_edge(block_exit, fn_exit);
+    body_exit = cfg_builder.expr(body, entry);
+    cfg_builder.add_contained_edge(body_exit, fn_exit);
     let CFGBuilder {graph, ..} = cfg_builder;
     CFG {graph: graph,
          entry: entry,

src/librustc/cfg/mod.rs

@@ -59,8 +59,8 @@ pub type CFGEdge = graph::Edge<CFGEdgeData>;
 
 impl CFG {
     pub fn new<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
-                         blk: &hir::Block) -> CFG {
-        construct::construct(tcx, blk)
+                         body: &hir::Expr) -> CFG {
+        construct::construct(tcx, body)
     }
 
     pub fn node_is_reachable(&self, id: ast::NodeId) -> bool {

src/librustc/diagnostics.rs

@@ -672,120 +672,6 @@ extern "C" {
 ```
 "##,
 
-E0269: r##"
-A returned value was expected but not all control paths return one.
-
-Erroneous code example:
-
-```compile_fail,E0269
-fn abracada_FAIL() -> String {
-    "this won't work".to_string();
-    // error: not all control paths return a value
-}
-```
-
-In the previous code, the function is supposed to return a `String`, however,
-the code returns nothing (because of the ';'). Another erroneous code would be:
-
-```compile_fail
-fn abracada_FAIL(b: bool) -> u32 {
-    if b {
-        0
-    } else {
-        "a" // It fails because an `u32` was expected and something else is
-            // returned.
-    }
-}
-```
-
-It is advisable to find out what the unhandled cases are and check for them,
-returning an appropriate value or panicking if necessary. Check if you need
-to remove a semicolon from the last expression, like in the first erroneous
-code example.
-"##,
-
-E0270: r##"
-Rust lets you define functions which are known to never return, i.e. are
-'diverging', by marking its return type as `!`.
-
-For example, the following functions never return:
-
-```no_run
-fn foo() -> ! {
-    loop {}
-}
-
-fn bar() -> ! {
-    foo() // foo() is diverging, so this will diverge too
-}
-
-fn baz() -> ! {
-    panic!(); // this macro internally expands to a call to a diverging function
-}
-```
-
-Such functions can be used in a place where a value is expected without
-returning a value of that type, for instance:
-
-```no_run
-fn foo() -> ! {
-    loop {}
-}
-
-let x = 3;
-
-let y = match x {
-    1 => 1,
-    2 => 4,
-    _ => foo() // diverging function called here
-};
-
-println!("{}", y)
-```
-
-If the third arm of the match block is reached, since `foo()` doesn't ever
-return control to the match block, it is fine to use it in a place where an
-integer was expected. The `match` block will never finish executing, and any
-point where `y` (like the print statement) is needed will not be reached.
-
-However, if we had a diverging function that actually does finish execution:
-
-```ignore
-fn foo() -> ! {
-    loop {break;}
-}
-```
-
-Then we would have an unknown value for `y` in the following code:
-
-```no_run
-fn foo() -> ! {
-    loop {}
-}
-
-let x = 3;
-
-let y = match x {
-    1 => 1,
-    2 => 4,
-    _ => foo()
-};
-
-println!("{}", y);
-```
-
-In the previous example, the print statement was never reached when the
-wildcard match arm was hit, so we were okay with `foo()` not returning an
-integer that we could set to `y`. But in this example, `foo()` actually does
-return control, so the print statement will be executed with an uninitialized
-value.
-
-Obviously we cannot have functions which are allowed to be used in such
-positions and yet can return control. So, if you are defining a function that
-returns `!`, make sure that there is no way for it to actually finish
-executing.
-"##,
-
 E0271: r##"
 This is because of a type mismatch between the associated type of some
 trait (e.g. `T::Bar`, where `T` implements `trait Quux { type Bar; }`)

src/librustc/hir/intravisit.rs

@@ -138,7 +138,7 @@ pub trait Visitor<'v> : Sized {
     fn visit_where_predicate(&mut self, predicate: &'v WherePredicate) {
         walk_where_predicate(self, predicate)
     }
-    fn visit_fn(&mut self, fk: FnKind<'v>, fd: &'v FnDecl, b: &'v Block, s: Span, id: NodeId) {
+    fn visit_fn(&mut self, fk: FnKind<'v>, fd: &'v FnDecl, b: &'v Expr, s: Span, id: NodeId) {
         walk_fn(self, fk, fd, b, s, id)
     }
     fn visit_trait_item(&mut self, ti: &'v TraitItem) {
@@ -635,13 +635,13 @@ pub fn walk_fn_kind<'v, V: Visitor<'v>>(visitor: &mut V, function_kind: FnKind<'
 pub fn walk_fn<'v, V: Visitor<'v>>(visitor: &mut V,
                                    function_kind: FnKind<'v>,
                                    function_declaration: &'v FnDecl,
-                                   function_body: &'v Block,
+                                   function_body: &'v Expr,
                                    _span: Span,
                                    id: NodeId) {
     visitor.visit_id(id);
     walk_fn_decl(visitor, function_declaration);
     walk_fn_kind(visitor, function_kind);
-    visitor.visit_block(function_body)
+    visitor.visit_expr(function_body)
 }
 
 pub fn walk_trait_item<'v, V: Visitor<'v>>(visitor: &mut V, trait_item: &'v TraitItem) {
@@ -925,7 +925,7 @@ impl<'v> Visitor<'v> for IdRangeComputingVisitor {
 /// Computes the id range for a single fn body, ignoring nested items.
 pub fn compute_id_range_for_fn_body(fk: FnKind,
                                     decl: &FnDecl,
-                                    body: &Block,
+                                    body: &Expr,
                                     sp: Span,
                                     id: NodeId)
                                     -> IdRange {

src/librustc/hir/lowering.rs

@@ -595,12 +595,13 @@ impl<'a> LoweringContext<'a> {
                 hir::ItemConst(self.lower_ty(t), self.lower_expr(e))
             }
             ItemKind::Fn(ref decl, unsafety, constness, abi, ref generics, ref body) => {
+                let body = self.lower_block(body);
                 hir::ItemFn(self.lower_fn_decl(decl),
                             self.lower_unsafety(unsafety),
                             self.lower_constness(constness),
                             abi,
                             self.lower_generics(generics),
-                            self.lower_block(body))
+                            self.expr_block(body, ThinVec::new()))
             }
             ItemKind::Mod(ref m) => hir::ItemMod(self.lower_mod(m)),
             ItemKind::ForeignMod(ref nm) => hir::ItemForeignMod(self.lower_foreign_mod(nm)),
@@ -665,7 +666,10 @@ impl<'a> LoweringContext<'a> {
                     }
                     TraitItemKind::Method(ref sig, ref body) => {
                         hir::MethodTraitItem(this.lower_method_sig(sig),
-                                             body.as_ref().map(|x| this.lower_block(x)))
+                                             body.as_ref().map(|x| {
+                            let body = this.lower_block(x);
+                            this.expr_block(body, ThinVec::new())
+                        }))
                     }
                     TraitItemKind::Type(ref bounds, ref default) => {
                         hir::TypeTraitItem(this.lower_bounds(bounds),
@@ -691,8 +695,9 @@ impl<'a> LoweringContext<'a> {
                         hir::ImplItemKind::Const(this.lower_ty(ty), this.lower_expr(expr))
                     }
                     ImplItemKind::Method(ref sig, ref body) => {
+                        let body = this.lower_block(body);
                         hir::ImplItemKind::Method(this.lower_method_sig(sig),
-                                                  this.lower_block(body))
+                                                  this.expr_block(body, ThinVec::new()))
                     }
                     ImplItemKind::Type(ref ty) => hir::ImplItemKind::Type(this.lower_ty(ty)),
                     ImplItemKind::Macro(..) => panic!("Shouldn't exist any more"),
@@ -1110,7 +1115,7 @@ impl<'a> LoweringContext<'a> {
                     self.with_parent_def(e.id, |this| {
                         hir::ExprClosure(this.lower_capture_clause(capture_clause),
                                          this.lower_fn_decl(decl),
-                                         this.lower_block(body),
+                                         this.lower_expr(body),
                                          fn_decl_span)
                     })
                 }

src/librustc/hir/map/blocks.rs

@@ -21,11 +21,9 @@
 //! nested within a uniquely determined `FnLike`), and users can ask
 //! for the `Code` associated with a particular NodeId.
 
-pub use self::Code::*;
-
 use hir as ast;
 use hir::map::{self, Node};
-use hir::{Block, FnDecl};
+use hir::{Expr, FnDecl};
 use hir::intravisit::FnKind;
 use syntax::abi;
 use syntax::ast::{Attribute, Name, NodeId};
@@ -50,7 +48,7 @@ pub trait MaybeFnLike { fn is_fn_like(&self) -> bool; }
 /// Components shared by fn-like things (fn items, methods, closures).
 pub struct FnParts<'a> {
     pub decl: &'a FnDecl,
-    pub body: &'a Block,
+    pub body: &'a Expr,
     pub kind: FnKind<'a>,
     pub span: Span,
     pub id:   NodeId,
@@ -77,29 +75,32 @@ impl MaybeFnLike for ast::Expr {
     }
 }
 
-/// Carries either an FnLikeNode or a Block, as these are the two
+/// Carries either an FnLikeNode or a Expr, as these are the two
 /// constructs that correspond to "code" (as in, something from which
 /// we can construct a control-flow graph).
 #[derive(Copy, Clone)]
 pub enum Code<'a> {
-    FnLikeCode(FnLikeNode<'a>),
-    BlockCode(&'a Block),
+    FnLike(FnLikeNode<'a>),
+    Expr(&'a Expr),
 }
 
 impl<'a> Code<'a> {
     pub fn id(&self) -> NodeId {
         match *self {
-            FnLikeCode(node) => node.id(),
-            BlockCode(block) => block.id,
+            Code::FnLike(node) => node.id(),
+            Code::Expr(block) => block.id,
         }
     }
 
-    /// Attempts to construct a Code from presumed FnLike or Block node input.
-    pub fn from_node(node: Node) -> Option<Code> {
-        if let map::NodeBlock(block) = node {
-            Some(BlockCode(block))
-        } else {
-            FnLikeNode::from_node(node).map(|fn_like| FnLikeCode(fn_like))
+    /// Attempts to construct a Code from presumed FnLike or Expr node input.
+    pub fn from_node(map: &map::Map<'a>, id: NodeId) -> Option<Code<'a>> {
+        match map.get(id) {
+            map::NodeBlock(_) => {
+                //  Use the parent, hopefully an expression node.
+                Code::from_node(map, map.get_parent_node(id))
+            }
+            map::NodeExpr(expr) => Some(Code::Expr(expr)),
+            node => FnLikeNode::from_node(node).map(Code::FnLike)
         }
     }
 }
@@ -114,7 +115,7 @@ struct ItemFnParts<'a> {
     abi:      abi::Abi,
     vis:      &'a ast::Visibility,
     generics: &'a ast::Generics,
-    body:     &'a Block,
+    body:     &'a Expr,
     id:       NodeId,
     span:     Span,
     attrs:    &'a [Attribute],
@@ -124,14 +125,14 @@ struct ItemFnParts<'a> {
 /// for use when implementing FnLikeNode operations.
 struct ClosureParts<'a> {
     decl: &'a FnDecl,
-    body: &'a Block,
+    body: &'a Expr,
     id: NodeId,
     span: Span,
     attrs: &'a [Attribute],
 }
 
 impl<'a> ClosureParts<'a> {
-    fn new(d: &'a FnDecl, b: &'a Block, id: NodeId, s: Span, attrs: &'a [Attribute]) -> Self {
+    fn new(d: &'a FnDecl, b: &'a Expr, id: NodeId, s: Span, attrs: &'a [Attribute]) -> Self {
         ClosureParts {
             decl: d,
             body: b,
@@ -171,9 +172,9 @@ impl<'a> FnLikeNode<'a> {
         }
     }
 
-    pub fn body(self) -> &'a Block {
+    pub fn body(self) -> &'a Expr {
         self.handle(|i: ItemFnParts<'a>|  &*i.body,
-                    |_, _, _: &'a ast::MethodSig, _, body: &'a ast::Block, _, _|  body,
+                    |_, _, _: &'a ast::MethodSig, _, body: &'a ast::Expr, _, _|  body,
                     |c: ClosureParts<'a>| c.body)
     }
 
@@ -214,7 +215,7 @@ impl<'a> FnLikeNode<'a> {
                   Name,
                   &'a ast::MethodSig,
                   Option<&'a ast::Visibility>,
-                  &'a ast::Block,
+                  &'a ast::Expr,
                   Span,
                   &'a [Attribute])
                   -> A,

src/librustc/hir/map/collector.rs

@@ -211,7 +211,7 @@ impl<'ast> Visitor<'ast> for NodeCollector<'ast> {
     }
 
     fn visit_fn(&mut self, fk: intravisit::FnKind<'ast>, fd: &'ast FnDecl,
-                b: &'ast Block, s: Span, id: NodeId) {
+                b: &'ast Expr, s: Span, id: NodeId) {
         assert_eq!(self.parent_node, id);
         intravisit::walk_fn(self, fk, fd, b, s, id);
     }