Separate RemoveFalseEdges from SimplifyBranches

Otherwise dataflow state will propagate along false edges and cause
things to be marked as maybe init unnecessarily. These should be
separate, since `SimplifyBranches` also makes `if true {} else {}` into
a `goto`, which means we wouldn't lint anything in the `else` block.

compiler/rustc_mir_transform/src/lib.rs

@@ -60,6 +60,7 @@ mod match_branches;
 mod multiple_return_terminators;
 mod normalize_array_len;
 mod nrvo;
+mod remove_false_edges;
 mod remove_noop_landing_pads;
 mod remove_storage_markers;
 mod remove_unneeded_drops;
@@ -456,7 +457,7 @@ fn run_post_borrowck_cleanup_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tc
 
     let post_borrowck_cleanup: &[&dyn MirPass<'tcx>] = &[
         // Remove all things only needed by analysis
-        &simplify_branches::SimplifyBranches::new("initial"),
+        &simplify_branches::SimplifyConstCondition::new("initial"),
         &remove_noop_landing_pads::RemoveNoopLandingPads,
         &cleanup_post_borrowck::CleanupNonCodegenStatements,
         &simplify::SimplifyCfg::new("early-opt"),
@@ -515,13 +516,13 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         &instcombine::InstCombine,
         &separate_const_switch::SeparateConstSwitch,
         &const_prop::ConstProp,
-        &simplify_branches::SimplifyBranches::new("after-const-prop"),
+        &simplify_branches::SimplifyConstCondition::new("after-const-prop"),
         &early_otherwise_branch::EarlyOtherwiseBranch,
         &simplify_comparison_integral::SimplifyComparisonIntegral,
         &simplify_try::SimplifyArmIdentity,
         &simplify_try::SimplifyBranchSame,
         &dest_prop::DestinationPropagation,
-        &simplify_branches::SimplifyBranches::new("final"),
+        &simplify_branches::SimplifyConstCondition::new("final"),
         &remove_noop_landing_pads::RemoveNoopLandingPads,
         &simplify::SimplifyCfg::new("final"),
         &nrvo::RenameReturnPlace,

compiler/rustc_mir_transform/src/remove_false_edges.rs

@@ -0,0 +1,29 @@
+use rustc_middle::mir::{Body, TerminatorKind};
+use rustc_middle::ty::TyCtxt;
+
+use crate::MirPass;
+
+/// Removes `FalseEdge` and `FalseUnwind` terminators from the MIR.
+///
+/// These are only needed for borrow checking, and can be removed afterwards.
+///
+/// FIXME: This should probably have its own MIR phase.
+pub struct RemoveFalseEdges;
+
+impl<'tcx> MirPass<'tcx> for RemoveFalseEdges {
+    fn run_pass(&self, _: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        for block in body.basic_blocks_mut() {
+            let terminator = block.terminator_mut();
+            terminator.kind = match terminator.kind {
+                TerminatorKind::FalseEdge { real_target, .. } => {
+                    TerminatorKind::Goto { target: real_target }
+                }
+                TerminatorKind::FalseUnwind { real_target, .. } => {
+                    TerminatorKind::Goto { target: real_target }
+                }
+
+                _ => continue,
+            }
+        }
+    }
+}

compiler/rustc_mir_transform/src/simplify_branches.rs

@@ -1,22 +1,21 @@
-//! A pass that simplifies branches when their condition is known.
-
 use crate::MirPass;
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
 
 use std::borrow::Cow;
 
-pub struct SimplifyBranches {
+/// A pass that replaces a branch with a goto when its condition is known.
+pub struct SimplifyConstCondition {
     label: String,
 }
 
-impl SimplifyBranches {
+impl SimplifyConstCondition {
     pub fn new(label: &str) -> Self {
-        SimplifyBranches { label: format!("SimplifyBranches-{}", label) }
+        SimplifyConstCondition { label: format!("SimplifyConstCondition-{}", label) }
     }
 }
 
-impl<'tcx> MirPass<'tcx> for SimplifyBranches {
+impl<'tcx> MirPass<'tcx> for SimplifyConstCondition {
     fn name(&self) -> Cow<'_, str> {
         Cow::Borrowed(&self.label)
     }
@@ -53,12 +52,6 @@ impl<'tcx> MirPass<'tcx> for SimplifyBranches {
                     Some(v) if v == expected => TerminatorKind::Goto { target },
                     _ => continue,
                 },
-                TerminatorKind::FalseEdge { real_target, .. } => {
-                    TerminatorKind::Goto { target: real_target }
-                }
-                TerminatorKind::FalseUnwind { real_target, .. } => {
-                    TerminatorKind::Goto { target: real_target }
-                }
                 _ => continue,
             };
         }