Auto merge of #47503 - arielb1:check-size, r=eddyb

avoid double-unsizing arrays in bytestring match lowering

The match lowering code, when lowering matches against bytestrings,
works by coercing both the scrutinee and the pattern to `&[u8]` and
then comparing them using `<[u8] as Eq>::eq`.

If the scrutinee is already of type `&[u8]`, then unsizing it is both
unneccessary and a trait error caught by the new and updated MIR typeck,
so this PR changes lowering to avoid doing that (match lowering tried to
avoid that before, but that attempt was quite broken).

Fixes #46920.

r? @eddyb

src/librustc_mir/build/matches/test.rs

@@ -174,12 +174,50 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
         }
     }
 
+    /// Convert a byte array or byte slice to a byte slice.
+    fn to_slice_operand(&mut self,
+                        block: BasicBlock,
+                        source_info: SourceInfo,
+                        operand: Operand<'tcx>)
+                        -> Operand<'tcx>
+    {
+        let tcx = self.hir.tcx();
+        let ty = operand.ty(&self.local_decls, tcx);
+        debug!("to_slice_operand({:?}, {:?}: {:?})", block, operand, ty);
+        match ty.sty {
+            ty::TyRef(region, mt) => match mt.ty.sty {
+                ty::TyArray(ety, _) => {
+                    let ty = tcx.mk_imm_ref(region, tcx.mk_slice(ety));
+                    let temp = self.temp(ty, source_info.span);
+                    self.cfg.push_assign(block, source_info, &temp,
+                                         Rvalue::Cast(CastKind::Unsize, operand, ty));
+                    Operand::Move(temp)
+                }
+                ty::TySlice(_) => operand,
+                _ => {
+                    span_bug!(source_info.span,
+                              "bad operand {:?}: {:?} to `to_slice_operand`", operand, ty)
+                }
+            }
+            _ => {
+                span_bug!(source_info.span,
+                          "bad operand {:?}: {:?} to `to_slice_operand`", operand, ty)
+            }
+        }
+
+    }
+
     /// Generates the code to perform a test.
     pub fn perform_test(&mut self,
                         block: BasicBlock,
                         place: &Place<'tcx>,
                         test: &Test<'tcx>)
                         -> Vec<BasicBlock> {
+        debug!("perform_test({:?}, {:?}: {:?}, {:?})",
+               block,
+               place,
+               place.ty(&self.local_decls, self.hir.tcx()),
+               test);
         let source_info = self.source_info(test.span);
         match test.kind {
             TestKind::Switch { adt_def, ref variants } => {
@@ -258,45 +296,35 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                 ret
             }
 
-            TestKind::Eq { value, mut ty } => {
+            TestKind::Eq { value, ty } => {
+                let tcx = self.hir.tcx();
                 let mut val = Operand::Copy(place.clone());
 
                 // If we're using b"..." as a pattern, we need to insert an
                 // unsizing coercion, as the byte string has the type &[u8; N].
-                let expect = if let ConstVal::ByteStr(bytes) = value.val {
-                    let tcx = self.hir.tcx();
-
-                    // Unsize the place to &[u8], too, if necessary.
-                    if let ty::TyRef(region, mt) = ty.sty {
-                        if let ty::TyArray(_, _) = mt.ty.sty {
-                            ty = tcx.mk_imm_ref(region, tcx.mk_slice(tcx.types.u8));
-                            let val_slice = self.temp(ty, test.span);
-                            self.cfg.push_assign(block, source_info, &val_slice,
-                                                 Rvalue::Cast(CastKind::Unsize, val, ty));
-                            val = Operand::Move(val_slice);
-                        }
-                    }
-
-                    assert!(ty.is_slice());
-
+                //
+                // We want to do this even when the scrutinee is a reference to an
+                // array, so we can call `<[u8]>::eq` rather than having to find an
+                // `<[u8; N]>::eq`.
+                let (expect, val) = if let ConstVal::ByteStr(bytes) = value.val {
                     let array_ty = tcx.mk_array(tcx.types.u8, bytes.data.len() as u64);
                     let array_ref = tcx.mk_imm_ref(tcx.types.re_static, array_ty);
                     let array = self.literal_operand(test.span, array_ref, Literal::Value {
                         value
                     });
 
-                    let slice = self.temp(ty, test.span);
-                    self.cfg.push_assign(block, source_info, &slice,
-                                         Rvalue::Cast(CastKind::Unsize, array, ty));
-                    Operand::Move(slice)
+                    let val = self.to_slice_operand(block, source_info, val);
+                    let slice = self.to_slice_operand(block, source_info, array);
+                    (slice, val)
                 } else {
-                    self.literal_operand(test.span, ty, Literal::Value {
+                    (self.literal_operand(test.span, ty, Literal::Value {
                         value
-                    })
+                    }), val)
                 };
 
                 // Use PartialEq::eq for &str and &[u8] slices, instead of BinOp::Eq.
                 let fail = self.cfg.start_new_block();
+                let ty = expect.ty(&self.local_decls, tcx);
                 if let ty::TyRef(_, mt) = ty.sty {
                     assert!(ty.is_slice());
                     let eq_def_id = self.hir.tcx().lang_items().eq_trait().unwrap();

src/test/run-pass/issue-46920-byte-array-patterns.rs

@@ -0,0 +1,37 @@
+// Copyright 2017 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.
+
+const CURSOR_PARTITION_LABEL: &'static [u8] = b"partition";
+const CURSOR_EVENT_TYPE_LABEL: &'static [u8] = b"event_type";
+const BYTE_PATTERN: &'static [u8; 5] = b"hello";
+
+fn match_slice(x: &[u8]) -> u32 {
+    match x {
+        CURSOR_PARTITION_LABEL => 0,
+        CURSOR_EVENT_TYPE_LABEL => 1,
+        _ => 2,
+    }
+}
+
+fn match_array(x: &[u8; 5]) -> bool {
+    match x {
+        BYTE_PATTERN => true,
+        _ => false
+    }
+}
+
+fn main() {
+    assert_eq!(match_slice(b"abcde"), 2);
+    assert_eq!(match_slice(b"event_type"), 1);
+    assert_eq!(match_slice(b"partition"), 0);
+
+    assert_eq!(match_array(b"hello"), true);
+    assert_eq!(match_array(b"hella"), false);
+}