optimize `let x: T = ..` to avoid a temporary

For some weird reason this fixes `intrinsic-move-val`. It also affects
various test heuristics. I removed one test (`reborrow_basic`) that
didn't seem to really be testing anything in particular anymore,
compared to all the other tests we've got.

src/librustc_mir/build/matches/mod.rs

@@ -253,8 +253,8 @@ pub fn expr_into_pattern(
         irrefutable_pat: Pattern<'tcx>,
         initializer: ExprRef<'tcx>,
     ) -> BlockAnd<()> {
-        // optimize the case of `let x = ...`
         match *irrefutable_pat.kind {
+            // Optimize the case of `let x = ...` to write directly into `x`
             PatternKind::Binding {
                 mode: BindingMode::ByValue,
                 var,
@@ -267,6 +267,47 @@ pub fn expr_into_pattern(
                 self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
                 block.unit()
             }
+
+            // Optimize the case of `let x: T = ...` to write directly
+            // into `x` and then require that `T == typeof(x)`.
+            //
+            // Weirdly, this is needed to prevent the
+            // `intrinsic-move-val.rs` test case from crashing. That
+            // test works with uninitialized values in a rather
+            // dubious way, so it may be that the test is kind of
+            // broken.
+            PatternKind::AscribeUserType {
+                subpattern: Pattern {
+                    kind: box PatternKind::Binding {
+                        mode: BindingMode::ByValue,
+                        var,
+                        subpattern: None,
+                        ..
+                    },
+                    ..
+                },
+                user_ty: ascription_user_ty,
+            } => {
+                let place =
+                    self.storage_live_binding(block, var, irrefutable_pat.span, OutsideGuard);
+                unpack!(block = self.into(&place, block, initializer));
+
+                let source_info = self.source_info(irrefutable_pat.span);
+                self.cfg.push(
+                    block,
+                    Statement {
+                        source_info,
+                        kind: StatementKind::AscribeUserType(
+                            place.clone(),
+                            ty::Variance::Invariant,
+                            ascription_user_ty,
+                        ),
+                    },
+                );
+
+                self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
+                block.unit()
+            }
             _ => {
                 let place = unpack!(block = self.as_place(block, initializer));
                 self.place_into_pattern(block, irrefutable_pat, &place, true)

src/test/mir-opt/basic_assignment.rs

@@ -19,9 +19,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// check that codegen of assignment expressions is sane. Assignments
-// tend to be absent in simple code, so subtle breakage in them can
-// leave a quite hard-to-find trail of destruction.
+// Check codegen for assignments (`a = b`) where the left-hand-side is
+// not yet initialized. Assignments tend to be absent in simple code,
+// so subtle breakage in them can leave a quite hard-to-find trail of
+// destruction.
 
 // ignore-tidy-linelength
 
@@ -29,60 +30,42 @@ fn main() {
     let nodrop_x = false;
     let nodrop_y;
 
+    // Since boolean does not require drop, this can be a simple
+    // assignment:
     nodrop_y = nodrop_x;
 
     let drop_x : Option<Box<u32>> = None;
     let drop_y;
 
+    // Since the type of `drop_y` has drop, we generate a `replace`
+    // terminator:
     drop_y = drop_x;
 }
 
 // END RUST SOURCE
 // START rustc.main.SimplifyCfg-initial.after.mir
-//     bb0: {
-//         StorageLive(_1);
-//         _1 = const false;
-//         StorageLive(_2);
-//         StorageLive(_3);
-//         _3 = _1;
-//         _2 = move _3;
-//         StorageDead(_3);
-//         StorageLive(_4);
-//         AscribeUserType(_4, Canonical { variables: [], value: std::option::Option<std::boxed::Box<u32>> });
-//         _4 = std::option::Option<std::boxed::Box<u32>>::None;
-//         StorageLive(_5);
-//         StorageLive(_6);
-//         _6 = move _4;
-//         replace(_5 <-move _6) -> [return: bb2, unwind: bb5];
-//     }
-//     bb1: {
-//         resume;
-//     }
-//     bb2: {
-//         drop(_6) -> [return: bb6, unwind: bb4];
-//     }
-//     bb3: {
-//         drop(_4) -> bb1;
-//     }
-//     bb4: {
-//         drop(_5) -> bb3;
-//     }
-//     bb5: {
-//         drop(_6) -> bb4;
-//     }
-//     bb6: {
-//         StorageDead(_6);
-//         _0 = ();
-//         drop(_5) -> [return: bb7, unwind: bb3];
-//     }
-//     bb7: {
-//         StorageDead(_5);
-//         drop(_4) -> [return: bb8, unwind: bb1];
-//     }
-//     bb8: {
-//         StorageDead(_4);
-//         StorageDead(_2);
-//         StorageDead(_1);
-//         return;
-//     }
+//    bb0: {
+//        StorageLive(_1);
+//        _1 = const false;
+//        StorageLive(_2);
+//        StorageLive(_3);
+//        _3 = _1;
+//        _2 = move _3;
+//        StorageDead(_3);
+//        StorageLive(_4);
+//        _4 = std::option::Option<std::boxed::Box<u32>>::None;
+//        AscribeUserType(_4, o, Canonical { variables: [], value: std::option::Option<std::boxed::Box<u32>> });
+//        StorageLive(_5);
+//        StorageLive(_6);
+//        _6 = move _4;
+//        replace(_5 <- move _6) -> [return: bb2, unwind: bb5];
+//    }
+//    ...
+//    bb2: {
+//        drop(_6) -> [return: bb6, unwind: bb4];
+//    }
+//    ...
+//    bb5: {
+//        drop(_6) -> bb4;
+//    }
 // END rustc.main.SimplifyCfg-initial.after.mir

src/test/mir-opt/nll/reborrow-basic.rs

-// Copyright 2012-2016 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.
-
-// Basic test for reborrow constraints: the region (`R5`) that appears
-// in the type of `r_a` must outlive the region (`R7`) that appears in
-// the type of `r_b`
-
-// compile-flags:-Zborrowck=mir -Zverbose
-//                              ^^^^^^^^^ force compiler to dump more region information
-
-#![allow(warnings)]
-
-fn use_x(_: &mut i32) -> bool { true }
-
-fn main() {
-    let mut foo: i32     = 22;
-    let r_a: &mut i32 = &mut foo;
-    let r_b: &mut i32 = &mut *r_a;
-    use_x(r_b);
-}
-
-// END RUST SOURCE
-// START rustc.main.nll.0.mir
-// | '_#7r    | U0 | {bb0[4], bb0[8..=17]}
-// ...
-// | '_#9r    | U0 | {bb0[10], bb0[14..=17]}
-// ...
-// let _4: &'_#9r mut i32;
-// ...
-// let _2: &'_#7r mut i32;
-// END rustc.main.nll.0.mir

src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn-body.nll.stderr

 error: unsatisfied lifetime constraints
-  --> $DIR/associated-types-project-from-hrtb-in-fn-body.rs:32:19
+  --> $DIR/associated-types-project-from-hrtb-in-fn-body.rs:32:29
    |
 LL | fn bar<'a, 'b, I : for<'x> Foo<&'x isize>>(
    |        --  -- lifetime `'b` defined here
@@ -7,10 +7,10 @@ LL | fn bar<'a, 'b, I : for<'x> Foo<&'x isize>>(
    |        lifetime `'a` defined here
 ...
 LL |     let z: I::A = if cond { x } else { y };
-   |                   ^^^^^^^^^^^^^^^^^^^^^^^^ requires that `'a` must outlive `'b`
+   |                             ^ assignment requires that `'a` must outlive `'b`
 
 error: unsatisfied lifetime constraints
-  --> $DIR/associated-types-project-from-hrtb-in-fn-body.rs:32:19
+  --> $DIR/associated-types-project-from-hrtb-in-fn-body.rs:32:40
    |
 LL | fn bar<'a, 'b, I : for<'x> Foo<&'x isize>>(
    |        --  -- lifetime `'b` defined here
@@ -18,7 +18,7 @@ LL | fn bar<'a, 'b, I : for<'x> Foo<&'x isize>>(
    |        lifetime `'a` defined here
 ...
 LL |     let z: I::A = if cond { x } else { y };
-   |                   ^^^^^^^^^^^^^^^^^^^^^^^^ requires that `'b` must outlive `'a`
+   |                                        ^ assignment requires that `'b` must outlive `'a`
 
 error: aborting due to 2 previous errors
 

src/test/ui/borrowck/borrowck-access-permissions.ast.nll.stderr

@@ -35,7 +35,7 @@ error[E0596]: cannot borrow `*ptr_x` as mutable, as it is behind a `*const` poin
   --> $DIR/borrowck-access-permissions.rs:56:23
    |
 LL |         let ptr_x : *const _ = &x;
-   |             ----- help: consider changing this to be a mutable pointer: `*mut i32`
+   |                                -- help: consider changing this to be a mutable pointer: `&mut x`
 ...
 LL |             let _y1 = &mut *ptr_x; //[ast]~ ERROR [E0596]
    |                       ^^^^^^^^^^^ `ptr_x` is a `*const` pointer, so the data it refers to cannot be borrowed as mutable

src/test/ui/borrowck/borrowck-access-permissions.mir.stderr

@@ -35,7 +35,7 @@ error[E0596]: cannot borrow `*ptr_x` as mutable, as it is behind a `*const` poin
   --> $DIR/borrowck-access-permissions.rs:56:23
    |
 LL |         let ptr_x : *const _ = &x;
-   |             ----- help: consider changing this to be a mutable pointer: `*mut i32`
+   |                                -- help: consider changing this to be a mutable pointer: `&mut x`
 ...
 LL |             let _y1 = &mut *ptr_x; //[ast]~ ERROR [E0596]
    |                       ^^^^^^^^^^^ `ptr_x` is a `*const` pointer, so the data it refers to cannot be borrowed as mutable

src/test/ui/nll/relate_tys/hr-fn-aaa-as-aba.rs

@@ -20,9 +20,17 @@ fn make_it() -> for<'a> fn(&'a u32, &'a u32) -> &'a u32 {
     panic!()
 }
 
-fn main() {
+fn foo() {
     let a: for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32 = make_it();
     //~^ ERROR higher-ranked subtype error
-    //~| ERROR higher-ranked subtype error
     drop(a);
 }
+
+fn bar() {
+    // The code path for patterns is mildly different, so go ahead and
+    // test that too:
+    let _: for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32 = make_it();
+    //~^ ERROR higher-ranked subtype error
+}
+
+fn main() { }

src/test/ui/nll/relate_tys/hr-fn-aaa-as-aba.stderr

@@ -5,10 +5,10 @@ LL |     let a: for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32 = make_it();
    |                                                          ^^^^^^^^^
 
 error: higher-ranked subtype error
-  --> $DIR/hr-fn-aaa-as-aba.rs:24:9
+  --> $DIR/hr-fn-aaa-as-aba.rs:32:58
    |
-LL |     let a: for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32 = make_it();
-   |         ^
+LL |     let _: for<'a, 'b> fn(&'a u32, &'b u32) -> &'a u32 = make_it();
+   |                                                          ^^^^^^^^^
 
 error: aborting due to 2 previous errors
 

src/test/ui/nll/relate_tys/universe-violation.stderr

 error: higher-ranked subtype error
-  --> $DIR/universe-violation.rs:14:9
-   |
-LL |     let a: fn(_) -> _ = make_it();
-   |         ^
-
-error: higher-ranked subtype error
-  --> $DIR/universe-violation.rs:15:9
+  --> $DIR/universe-violation.rs:15:31
    |
 LL |     let b: fn(&u32) -> &u32 = a;
-   |         ^
+   |                               ^
 
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 

src/test/ui/regions/region-object-lifetime-in-coercion.nll.stderr

 error[E0621]: explicit lifetime required in the type of `v`
-  --> $DIR/region-object-lifetime-in-coercion.rs:20:5
+  --> $DIR/region-object-lifetime-in-coercion.rs:18:33
    |
 LL | fn a(v: &[u8]) -> Box<Foo + 'static> {
    |         ----- help: add explicit lifetime `'static` to the type of `v`: `&'static [u8]`
-...
-LL |     x
-   |     ^ lifetime `'static` required
+LL |     let x: Box<Foo + 'static> = Box::new(v);
+   |                                 ^^^^^^^^^^^ lifetime `'static` required
 
 error[E0621]: explicit lifetime required in the type of `v`
   --> $DIR/region-object-lifetime-in-coercion.rs:24:5

src/test/ui/slice-mut-2.nll.stderr

@@ -2,7 +2,7 @@ error[E0596]: cannot borrow `*x` as mutable, as it is behind a `&` reference
   --> $DIR/slice-mut-2.rs:17:18
    |
 LL |     let x: &[isize] = &[1, 2, 3, 4, 5];
-   |         - help: consider changing this to be a mutable reference: `&mut [isize]`
+   |                       ---------------- help: consider changing this to be a mutable reference: `&mut [1, 2, 3, 4, 5]`
 ...
 LL |     let _ = &mut x[2..4]; //~ERROR cannot borrow immutable borrowed content `*x` as mutable
    |                  ^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable

src/test/ui/unboxed-closures/unboxed-closures-failed-recursive-fn-1.nll.stderr

@@ -38,12 +38,9 @@ LL |         let g = factorial.as_ref().unwrap();
    |                 ^^^^^^^^^ borrowed value does not live long enough
 ...
 LL | }
-   | -
-   | |
-   | `factorial` dropped here while still borrowed
-   | borrow later used here, when `factorial` is dropped
+   | - `factorial` dropped here while still borrowed
    |
-   = note: values in a scope are dropped in the opposite order they are defined
+   = note: borrowed value must be valid for the static lifetime...
 
 error[E0506]: cannot assign to `factorial` because it is borrowed
   --> $DIR/unboxed-closures-failed-recursive-fn-1.rs:42:5
@@ -55,10 +52,9 @@ LL |         let g = factorial.as_ref().unwrap();
    |                 --------- borrow occurs due to use in closure
 ...
 LL |     factorial = Some(Box::new(f));
-   |     ^^^^^^^^^
-   |     |
-   |     assignment to borrowed `factorial` occurs here
-   |     borrow later used here
+   |     ^^^^^^^^^ assignment to borrowed `factorial` occurs here
+   |
+   = note: borrowed value must be valid for the static lifetime...
 
 error: aborting due to 4 previous errors