Auto merge of #54125 - varkor:less-conservative-uninhabitedness-check, r=nikomatsakis

Less conservative uninhabitedness check

Extends the uninhabitedness check to structs, non-empty enums, tuples and arrays.

Pulled out of #47291 and #50262.

Fixes #54586.

r? @nikomatsakis

Author: bors

src/librustc/cfg/construct.rs

@@ -415,8 +415,8 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
             args: I) -> CFGIndex {
         let func_or_rcvr_exit = self.expr(func_or_rcvr, pred);
         let ret = self.straightline(call_expr, func_or_rcvr_exit, args);
-        // FIXME(canndrew): This is_never should probably be an is_uninhabited.
-        if self.tables.expr_ty(call_expr).is_never() {
+        let m = self.tcx.hir().get_module_parent(call_expr.id);
+        if self.tcx.is_ty_uninhabited_from(m, self.tables.expr_ty(call_expr)) {
             self.add_unreachable_node()
         } else {
             ret

src/librustc/middle/liveness.rs

@@ -1197,8 +1197,8 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             }
 
             hir::ExprKind::Call(ref f, ref args) => {
-                // FIXME(canndrew): This is_never should really be an is_uninhabited
-                let succ = if self.tables.expr_ty(expr).is_never() {
+                let m = self.ir.tcx.hir().get_module_parent(expr.id);
+                let succ = if self.ir.tcx.is_ty_uninhabited_from(m, self.tables.expr_ty(expr)) {
                     self.s.exit_ln
                 } else {
                     succ
@@ -1208,8 +1208,8 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             }
 
             hir::ExprKind::MethodCall(.., ref args) => {
-                // FIXME(canndrew): This is_never should really be an is_uninhabited
-                let succ = if self.tables.expr_ty(expr).is_never() {
+                let m = self.ir.tcx.hir().get_module_parent(expr.id);
+                let succ = if self.ir.tcx.is_ty_uninhabited_from(m, self.tables.expr_ty(expr)) {
                     self.s.exit_ln
                 } else {
                     succ

src/librustc/ty/layout.rs

@@ -191,7 +191,14 @@ fn layout_raw<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 
         ty::tls::enter_context(&icx, |_| {
             let cx = LayoutCx { tcx, param_env };
-            cx.layout_raw_uncached(ty)
+            let layout = cx.layout_raw_uncached(ty);
+            // Type-level uninhabitedness should always imply ABI uninhabitedness.
+            if let Ok(layout) = layout {
+                if ty.conservative_is_privately_uninhabited(tcx) {
+                    assert!(layout.abi.is_uninhabited());
+                }
+            }
+            layout
         })
     })
 }
@@ -205,12 +212,11 @@ pub fn provide(providers: &mut ty::query::Providers<'_>) {
 
 pub struct LayoutCx<'tcx, C> {
     pub tcx: C,
-    pub param_env: ty::ParamEnv<'tcx>
+    pub param_env: ty::ParamEnv<'tcx>,
 }
 
 impl<'a, 'tcx> LayoutCx<'tcx, TyCtxt<'a, 'tcx, 'tcx>> {
-    fn layout_raw_uncached(&self, ty: Ty<'tcx>)
-                           -> Result<&'tcx LayoutDetails, LayoutError<'tcx>> {
+    fn layout_raw_uncached(&self, ty: Ty<'tcx>) -> Result<&'tcx LayoutDetails, LayoutError<'tcx>> {
         let tcx = self.tcx;
         let param_env = self.param_env;
         let dl = self.data_layout();
@@ -551,13 +557,19 @@ impl<'a, 'tcx> LayoutCx<'tcx, TyCtxt<'a, 'tcx, 'tcx>> {
                 let size = element.size.checked_mul(count, dl)
                     .ok_or(LayoutError::SizeOverflow(ty))?;
 
+                let abi = if count != 0 && ty.conservative_is_privately_uninhabited(tcx) {
+                    Abi::Uninhabited
+                } else {
+                    Abi::Aggregate { sized: true }
+                };
+
                 tcx.intern_layout(LayoutDetails {
                     variants: Variants::Single { index: VariantIdx::new(0) },
                     fields: FieldPlacement::Array {
                         stride: element.size,
                         count
                     },
-                    abi: Abi::Aggregate { sized: true },
+                    abi,
                     align: element.align,
                     size
                 })

src/librustc/ty/sty.rs

@@ -1543,6 +1543,51 @@ impl<'a, 'gcx, 'tcx> TyS<'tcx> {
         }
     }
 
+    /// Checks whether a type is definitely uninhabited. This is
+    /// conservative: for some types that are uninhabited we return `false`,
+    /// but we only return `true` for types that are definitely uninhabited.
+    /// `ty.conservative_is_privately_uninhabited` implies that any value of type `ty`
+    /// will be `Abi::Uninhabited`. (Note that uninhabited types may have nonzero
+    /// size, to account for partial initialisation. See #49298 for details.)
+    pub fn conservative_is_privately_uninhabited(&self, tcx: TyCtxt<'a, 'gcx, 'tcx>) -> bool {
+        // FIXME(varkor): we can make this less conversative by substituting concrete
+        // type arguments.
+        match self.sty {
+            ty::Never => true,
+            ty::Adt(def, _) if def.is_union() => {
+                // For now, `union`s are never considered uninhabited.
+                false
+            }
+            ty::Adt(def, _) => {
+                // Any ADT is uninhabited if either:
+                // (a) It has no variants (i.e. an empty `enum`);
+                // (b) Each of its variants (a single one in the case of a `struct`) has at least
+                //     one uninhabited field.
+                def.variants.iter().all(|var| {
+                    var.fields.iter().any(|field| {
+                        tcx.type_of(field.did).conservative_is_privately_uninhabited(tcx)
+                    })
+                })
+            }
+            ty::Tuple(tys) => tys.iter().any(|ty| ty.conservative_is_privately_uninhabited(tcx)),
+            ty::Array(ty, len) => {
+                match len.assert_usize(tcx) {
+                    // If the array is definitely non-empty, it's uninhabited if
+                    // the type of its elements is uninhabited.
+                    Some(n) if n != 0 => ty.conservative_is_privately_uninhabited(tcx),
+                    _ => false
+                }
+            }
+            ty::Ref(..) => {
+                // References to uninitialised memory is valid for any type, including
+                // uninhabited types, in unsafe code, so we treat all references as
+                // inhabited.
+                false
+            }
+            _ => false,
+        }
+    }
+
     pub fn is_primitive(&self) -> bool {
         match self.sty {
             Bool | Char | Int(_) | Uint(_) | Float(_) => true,

src/librustc_mir/borrow_check/nll/type_check/mod.rs

@@ -1545,8 +1545,7 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
                 }
             }
             None => {
-                // FIXME(canndrew): This is_never should probably be an is_uninhabited
-                if !sig.output().is_never() {
+                if !sig.output().conservative_is_privately_uninhabited(self.tcx()) {
                     span_mirbug!(self, term, "call to converging function {:?} w/o dest", sig);
                 }
             }

src/librustc_mir/build/expr/into.rs

@@ -275,8 +275,6 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                 exit_block.unit()
             }
             ExprKind::Call { ty, fun, args, from_hir_call } => {
-                // FIXME(canndrew): This is_never should probably be an is_uninhabited
-                let diverges = expr.ty.is_never();
                 let intrinsic = match ty.sty {
                     ty::FnDef(def_id, _) => {
                         let f = ty.fn_sig(this.hir.tcx());
@@ -332,7 +330,10 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                             func: fun,
                             args,
                             cleanup: Some(cleanup),
-                            destination: if diverges {
+                            // FIXME(varkor): replace this with an uninhabitedness-based check.
+                            // This requires getting access to the current module to call
+                            // `tcx.is_ty_uninhabited_from`, which is currently tricky to do.
+                            destination: if expr.ty.is_never() {
                                 None
                             } else {
                                 Some((destination.clone(), success))
@@ -421,8 +422,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                 });
 
                 let rvalue = unpack!(block = this.as_local_rvalue(block, expr));
-                this.cfg
-                    .push_assign(block, source_info, destination, rvalue);
+                this.cfg.push_assign(block, source_info, destination, rvalue);
                 block.unit()
             }
         };

src/librustc_mir/hair/pattern/check_match.rs

@@ -230,7 +230,11 @@ impl<'a, 'tcx> MatchVisitor<'a, 'tcx> {
                 let scrutinee_is_uninhabited = if self.tcx.features().exhaustive_patterns {
                     self.tcx.is_ty_uninhabited_from(module, pat_ty)
                 } else {
-                    self.conservative_is_uninhabited(pat_ty)
+                    match pat_ty.sty {
+                        ty::Never => true,
+                        ty::Adt(def, _) => def.variants.is_empty(),
+                        _ => false
+                    }
                 };
                 if !scrutinee_is_uninhabited {
                     // We know the type is inhabited, so this must be wrong
@@ -258,15 +262,6 @@ impl<'a, 'tcx> MatchVisitor<'a, 'tcx> {
         })
     }
 
-    fn conservative_is_uninhabited(&self, scrutinee_ty: Ty<'tcx>) -> bool {
-        // "rustc-1.0-style" uncontentious uninhabitableness check
-        match scrutinee_ty.sty {
-            ty::Never => true,
-            ty::Adt(def, _) => def.variants.is_empty(),
-            _ => false
-        }
-    }
-
     fn check_irrefutable(&self, pat: &'tcx Pat, origin: &str) {
         let module = self.tcx.hir().get_module_parent(pat.id);
         MatchCheckCtxt::create_and_enter(self.tcx, self.param_env, module, |ref mut cx| {

src/libstd/net/tcp.rs

@@ -729,6 +729,9 @@ impl TcpListener {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn accept(&self) -> io::Result<(TcpStream, SocketAddr)> {
+        // On WASM, `TcpStream` is uninhabited (as it's unsupported) and so
+        // the `a` variable here is technically unused.
+        #[cfg_attr(target_arch = "wasm32", allow(unused_variables))]
         self.0.accept().map(|(a, b)| (TcpStream(a), b))
     }
 

src/test/debuginfo/nil-enum.rs

@@ -0,0 +1,37 @@
+#![feature(never_type)]
+
+pub enum Void {}
+
+#[no_mangle]
+pub fn process_never(input: *const !) {
+   let _input = unsafe { &*input };
+}
+
+#[no_mangle]
+pub fn process_void(input: *const Void) {
+   let _input = unsafe { &*input };
+   // In the future, this should end with `unreachable`, but we currently only do
+   // unreachability analysis for `!`.
+}
+
+fn main() {}
+
+// END RUST SOURCE
+//
+// START rustc.process_never.SimplifyLocals.after.mir
+// bb0: {
+//     StorageLive(_2);
+//     _2 = &(*_1);
+//     StorageDead(_2);
+//     unreachable;
+// }
+// END rustc.process_never.SimplifyLocals.after.mir
+//
+// START rustc.process_void.SimplifyLocals.after.mir
+// bb0: {
+//     StorageLive(_2);
+//     _2 = &(*_1);
+//     StorageDead(_2);
+//     return;
+// }
+// END rustc.process_void.SimplifyLocals.after.mir