document and tweak the nll, use_mir, etc helpers

In particular, -Znll might as well imply -Zborrowck=mir by default,
just like `#![feature(nll)]` does.

Also, if NLL is in use, no reason to emit end regions. The NLL pass
just strips them out anyway.

src/librustc/session/mod.rs

@@ -437,25 +437,42 @@ impl Session {
     pub fn print_llvm_passes(&self) -> bool {
         self.opts.debugging_opts.print_llvm_passes
     }
+
+    /// If true, we should use NLL-style region checking instead of
+    /// lexical style.
     pub fn nll(&self) -> bool {
         self.features.borrow().nll || self.opts.debugging_opts.nll
     }
+
+    /// If true, we should use the MIR-based borrowck (we may *also* use
+    /// the AST-based borrowck).
     pub fn use_mir(&self) -> bool {
-        self.features.borrow().nll || self.opts.borrowck_mode.use_mir()
+        self.borrowck_mode().use_mir()
     }
+
+    /// If true, we should gather causal information during NLL
+    /// checking. This will eventually be the normal thing, but right
+    /// now it is too unoptimized.
     pub fn nll_dump_cause(&self) -> bool {
         self.opts.debugging_opts.nll_dump_cause
     }
+
+    /// If true, we should enable two-phase borrows checks. This is
+    /// done with either `-Ztwo-phase-borrows` or with
+    /// `#![feature(nll)]`.
     pub fn two_phase_borrows(&self) -> bool {
         self.features.borrow().nll || self.opts.debugging_opts.two_phase_borrows
     }
+
+    /// What mode(s) of borrowck should we run? AST? MIR? both?
+    /// (Also considers the `#![feature(nll)]` setting.)
     pub fn borrowck_mode(&self) -> BorrowckMode {
         match self.opts.borrowck_mode {
             mode @ BorrowckMode::Mir |
             mode @ BorrowckMode::Compare => mode,
 
             mode @ BorrowckMode::Ast => {
-                if self.features.borrow().nll {
+                if self.nll() {
                     BorrowckMode::Mir
                 } else {
                     mode
@@ -464,11 +481,18 @@ impl Session {
 
         }
     }
+
+    /// Should we emit EndRegion MIR statements? These are consumed by
+    /// MIR borrowck, but not when NLL is used. They are also consumed
+    /// by the validation stuff.
     pub fn emit_end_regions(&self) -> bool {
+        // FIXME(#46875) -- we should not emit end regions when NLL is enabled,
+        // but for now we can't stop doing so because it causes false positives
         self.opts.debugging_opts.emit_end_regions ||
-            (self.opts.debugging_opts.mir_emit_validate > 0) ||
+            self.opts.debugging_opts.mir_emit_validate > 0 ||
             self.use_mir()
     }
+
     pub fn lto(&self) -> bool {
         self.opts.cg.lto || self.target.target.options.requires_lto
     }

src/librustc_mir/borrow_check/mod.rs

@@ -72,10 +72,7 @@ fn mir_borrowck<'a, 'tcx>(
     let input_mir = tcx.mir_validated(def_id);
     debug!("run query mir_borrowck: {}", tcx.item_path_str(def_id));
 
-    if {
-        !tcx.has_attr(def_id, "rustc_mir_borrowck") && !tcx.sess.use_mir()
-            && !tcx.sess.nll()
-    } {
+    if !tcx.has_attr(def_id, "rustc_mir_borrowck") && !tcx.sess.use_mir() {
         return None;
     }