integrate the edition code.

As a driveby change, I made `#![feature(nll)]` *always* take
precedence over `-Z borrowck`.  The main effect this had is that it
means tests with `#![feature(nll)]` will ignore uses of `-Z
borrowck=compare`. This affected only one test as far as I can tell,
and I think that test used `-Z borrowck=compare` only as a historical
accident.

src/librustc/ty/context.rs

@@ -74,6 +74,7 @@
 use syntax::ast::{self, NodeId};
 use syntax::attr;
 use syntax::codemap::MultiSpan;
+use syntax::edition::Edition;
 use syntax::feature_gate;
 use syntax::symbol::{Symbol, keywords, InternedString};
 use syntax_pos::Span;
@@ -1403,34 +1404,51 @@ pub fn two_phase_borrows(self) -> bool {
     /// What mode(s) of borrowck should we run? AST? MIR? both?
     /// (Also considers the `#![feature(nll)]` setting.)
     pub fn borrowck_mode(&self) -> BorrowckMode {
+        // Here are the main constraints we need to deal with:
+        //
+        // 1. An opts.borrowck_mode of `BorrowckMode::Ast` is
+        //    synonymous with no `-Z borrowck=...` flag at all.
+        //    (This is arguably a historical accident.)
+        //
+        // 2. `BorrowckMode::Migrate` is the limited migration to
+        //    NLL that we are deploying with the 2018 edition.
+        //
+        // 3. We want to allow developers on the Nightly channel
+        //    to opt back into the "hard error" mode for NLL,
+        //    (which they can do via specifying `#![feature(nll)]`
+        //    explicitly in their crate).
+        //
+        // So, this precedence list is how pnkfelix chose to work with
+        // the above constraints:
+        //
+        // * `#![feature(nll)]` *always* means use NLL with hard
+        //   errors. (To simplify the code here, it now even overrides
+        //   a user's attempt to specify `-Z borrowck=compare`, which
+        //   we arguably do not need anymore and should remove.)
+        //
+        // * Otherwise, if no `-Z borrowck=...` flag was given (or
+        //   if `borrowck=ast` was specified), then use the default
+        //   as required by the edition.
+        //
+        // * Otherwise, use the behavior requested via `-Z borrowck=...`
+
+        if self.features().nll { return BorrowckMode::Mir; }
+
         match self.sess.opts.borrowck_mode {
             mode @ BorrowckMode::Mir |
-            mode @ BorrowckMode::Compare => mode,
-
-            // `BorrowckMode::Ast` is synonymous with no `-Z
-            // borrowck=...` flag at all. Therefore, we definitely
-            // want `#![feature(nll)]` to override it.
-            mode @ BorrowckMode::Ast => {
-                if self.features().nll {
-                    BorrowckMode::Mir
-                } else {
-                    mode
-                }
-            }
-
-            // `BorrowckMode::Migrate` is modelling the behavior one
-            // will eventually specify via `--edition 2018`. We want
-            // to allow developers on the Nightly channel to opt back
-            // into the "hard error" mode for NLL, which they can do
-            // via specifying `#![feature(nll)]` explicitly in their
-            // crate.
-            mode @ BorrowckMode::Migrate => {
-                if self.features().nll {
-                    BorrowckMode::Mir
-                } else {
-                    mode
-                }
-            }
+            mode @ BorrowckMode::Compare |
+            mode @ BorrowckMode::Migrate => mode,
+
+            BorrowckMode::Ast => match self.sess.edition() {
+                Edition::Edition2015 => BorrowckMode::Ast,
+                Edition::Edition2018 => BorrowckMode::Migrate,
+
+                // For now, future editions mean Migrate. (But it
+                // would make a lot of sense for it to be changed to
+                // `BorrowckMode::Mir`, depending on how we plan to
+                // time the forcing of full migration to NLL.)
+                _ => BorrowckMode::Migrate,
+            },
         }
     }
 

src/test/ui/generator/generator-with-nll.rs

@@ -8,17 +8,15 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// compile-flags: -Z borrowck=compare
-
 #![feature(generators)]
 #![feature(nll)]
 
 fn main() {
     || {
         // The reference in `_a` is a Legal with NLL since it ends before the yield
-        let _a = &mut true; //~ ERROR borrow may still be in use when generator yields (Ast)
-        let b = &mut true; //~ ERROR borrow may still be in use when generator yields (Ast)
-        //~^ borrow may still be in use when generator yields (Mir)
+        let _a = &mut true;
+        let b = &mut true;
+        //~^ borrow may still be in use when generator yields
         yield ();
         println!("{}", b);
     };

src/test/ui/generator/generator-with-nll.stderr

-error[E0626]: borrow may still be in use when generator yields (Ast)
-  --> $DIR/generator-with-nll.rs:19:23
+error[E0626]: borrow may still be in use when generator yields
+  --> $DIR/generator-with-nll.rs:18:17
    |
-LL |         let _a = &mut true; //~ ERROR borrow may still be in use when generator yields (Ast)
-   |                       ^^^^
-...
-LL |         yield ();
-   |         -------- possible yield occurs here
-
-error[E0626]: borrow may still be in use when generator yields (Ast)
-  --> $DIR/generator-with-nll.rs:20:22
-   |
-LL |         let b = &mut true; //~ ERROR borrow may still be in use when generator yields (Ast)
-   |                      ^^^^
-LL |         //~^ borrow may still be in use when generator yields (Mir)
-LL |         yield ();
-   |         -------- possible yield occurs here
-
-error[E0626]: borrow may still be in use when generator yields (Mir)
-  --> $DIR/generator-with-nll.rs:20:17
-   |
-LL |         let b = &mut true; //~ ERROR borrow may still be in use when generator yields (Ast)
+LL |         let b = &mut true;
    |                 ^^^^^^^^^
-LL |         //~^ borrow may still be in use when generator yields (Mir)
+LL |         //~^ borrow may still be in use when generator yields
 LL |         yield ();
    |         -------- possible yield occurs here
 
-error: aborting due to 3 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0626`.