Moved mir-borrowck pass down to where comments say it should be.

Added two fixmes: The `SimplifyBranches` pass cannot stay where it is,
and `BorrowckMir` should be a query, not a pass. But I am going to
leave those changes to a future PR.

Author: pnkfelix

src/librustc_driver/driver.rs

@@ -970,15 +970,23 @@ pub fn phase_3_run_analysis_passes<'tcx, F, R>(sess: &'tcx Session,
     // We compute "constant qualifications" between MIR_CONST and MIR_VALIDATED.
 
     // What we need to run borrowck etc.
-    passes.push_pass(MIR_CONST, mir::transform::borrow_check::BorrowckMir);
+
     passes.push_pass(MIR_VALIDATED, mir::transform::qualify_consts::QualifyAndPromoteConstants);
+
+    // FIXME: ariel points SimplifyBranches should run after
+    // mir-borrowck; otherwise code within `if false { ... }` would
+    // not be checked.
     passes.push_pass(MIR_VALIDATED,
                      mir::transform::simplify_branches::SimplifyBranches::new("initial"));
     passes.push_pass(MIR_VALIDATED, mir::transform::simplify::SimplifyCfg::new("qualify-consts"));
     passes.push_pass(MIR_VALIDATED, mir::transform::nll::NLL);
 
     // borrowck runs between MIR_VALIDATED and MIR_OPTIMIZED.
 
+    // FIXME: niko says this should be a query (see rustc::ty::maps)
+    // instead of a pass.
+    passes.push_pass(MIR_VALIDATED, mir::transform::borrow_check::BorrowckMir);
+
     // These next passes must be executed together
     passes.push_pass(MIR_OPTIMIZED, mir::transform::no_landing_pads::NoLandingPads);
     passes.push_pass(MIR_OPTIMIZED, mir::transform::add_call_guards::CriticalCallEdges);