Auto merge of #49870 - pnkfelix:issue-27282-immut-borrow-all-pat-ids-…

…in-guards, r=nikomatsakis

Immutably and implicitly borrow all pattern ids for their guards (NLL only)

This is an important piece of #27282.

It applies only to NLL mode. It is a change to MIR codegen that is currently toggled on only when NLL is turned on. It thus affect MIR-borrowck but not the earlier static analyses (such as the type checker).

This change makes it so that any pattern bindings of type T for a match arm will map to a `&T` within the context of the guard expression for that arm, but will continue to map to a `T` in the context of the arm body.

To avoid surfacing this type distinction in the user source code (which would be a severe change to the language and would also require far more revision to the compiler internals), any occurrence of such an identifier in the guard expression will automatically get a deref op applied to it.

So an input like:
```rust
let place = (1, Foo::new());
match place {
  (1, foo) if inspect(foo) => feed(foo),
  ...
}
```
will be treated as if it were really something like:
 ```rust
let place = (1, Foo::new());
match place {
    (1, Foo { .. }) if { let tmp1 = &place.1; inspect(*tmp1) }
                    => { let tmp2 = place.1; feed(tmp2) },
    ...
}
```

And an input like:
```rust
let place = (2, Foo::new());
match place {
    (2, ref mut foo) if inspect(foo) => feed(foo),
    ...
}
```
will be treated as if it were really something like:

```rust
let place = (2, Foo::new());
match place {
    (2, Foo { .. }) if { let tmp1 = & &mut place.1; inspect(*tmp1) }
                    => { let tmp2 = &mut place.1; feed(tmp2) },
    ...
}
```

In short, any pattern binding will always look like *some* kind of `&T` within the guard at least in terms of how the MIR-borrowck views it, and this will ensure that guard expressions cannot mutate their the match inputs via such bindings. (It also ensures that guard expressions can at most *copy* values from such bindings; non-Copy things cannot be moved via these pattern bindings in guard expressions, since one cannot move out of a `&T`.)

src/librustc/ty/context.rs

@@ -1467,6 +1467,14 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         self.borrowck_mode().use_mir()
     }
 
+    /// If true, pattern variables for use in guards on match arms
+    /// will be bound as references to the data, and occurrences of
+    /// those variables in the guard expression will implicitly
+    /// dereference those bindings. (See rust-lang/rust#27282.)
+    pub fn all_pat_vars_are_implicit_refs_within_guards(self) -> bool {
+        self.borrowck_mode().use_mir()
+    }
+
     /// If true, we should enable two-phase borrows checks. This is
     /// done with either `-Ztwo-phase-borrows` or with
     /// `#![feature(nll)]`.

src/librustc_mir/borrow_check/error_reporting.rs

@@ -471,7 +471,8 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
             | (RegionKind::ReClosureBound(_), _)
             | (RegionKind::ReCanonical(_), _)
             | (RegionKind::ReErased, _) => {
-                span_bug!(drop_span, "region does not make sense in this context");
+                span_bug!(drop_span, "region {:?} does not make sense in this context",
+                          borrow.region);
             }
         }
     }

src/librustc_mir/build/block.rs

@@ -9,6 +9,8 @@
 // except according to those terms.
 
 use build::{BlockAnd, BlockAndExtension, Builder};
+use build::ForGuard::OutsideGuard;
+use build::matches::ArmHasGuard;
 use hair::*;
 use rustc::mir::*;
 use rustc::hir;
@@ -113,7 +115,8 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                     // Declare the bindings, which may create a visibility scope.
                     let remainder_span = remainder_scope.span(this.hir.tcx(),
                                                               &this.hir.region_scope_tree);
-                    let scope = this.declare_bindings(None, remainder_span, lint_level, &pattern);
+                    let scope = this.declare_bindings(None, remainder_span, lint_level, &pattern,
+                                                      ArmHasGuard(false));
 
                     // Evaluate the initializer, if present.
                     if let Some(init) = initializer {
@@ -135,8 +138,8 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                         }
 
                         this.visit_bindings(&pattern, &mut |this, _, _, node, span, _| {
-                            this.storage_live_binding(block, node, span);
-                            this.schedule_drop_for_binding(node, span);
+                            this.storage_live_binding(block, node, span, OutsideGuard);
+                            this.schedule_drop_for_binding(node, span, OutsideGuard);
                         })
                     }
 

src/librustc_mir/build/expr/as_place.rs

@@ -11,6 +11,7 @@
 //! See docs in build/expr/mod.rs
 
 use build::{BlockAnd, BlockAndExtension, Builder};
+use build::ForGuard::{OutsideGuard, WithinGuard};
 use build::expr::category::Category;
 use hair::*;
 use rustc::mir::*;
@@ -86,8 +87,18 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                 block.and(Place::Local(Local::new(1)))
             }
             ExprKind::VarRef { id } => {
-                let index = this.var_indices[&id];
-                block.and(Place::Local(index))
+                let place = if this.is_bound_var_in_guard(id) {
+                    let index = this.var_local_id(id, WithinGuard);
+                    if this.hir.tcx().all_pat_vars_are_implicit_refs_within_guards() {
+                        Place::Local(index).deref()
+                    } else {
+                        Place::Local(index)
+                    }
+                } else {
+                    let index = this.var_local_id(id, OutsideGuard);
+                    Place::Local(index)
+                };
+                block.and(place)
             }
             ExprKind::StaticRef { id } => {
                 block.and(Place::Static(Box::new(Static { def_id: id, ty: expr.ty })))