Auto merge of #38833 - arielb1:constant-mir-overflow, r=eddyb

fix promotion of MIR terminators

promotion of MIR terminators used to try to promote the destination it
is trying to promote, leading to stack overflow.

Also clean up the code in `promote_temp` a bit to make it more understandable.

Fixes #37991.

cc @nikomatsakis
r? @eddyb

3 files changed, 87 insertions, 79 deletions

diff --git a/src/librustc_mir/transform/promote_consts.rs b/src/librustc_mir/transform/promote_consts.rs
index 41698574e0f..2a4b2b515cc 100644
--- a/src/librustc_mir/transform/promote_consts.rs
+++ b/src/librustc_mir/transform/promote_consts.rs
@@ -66,6 +66,7 @@ impl TempState {
 /// A "root candidate" for promotion, which will become the
 /// returned value in a promoted MIR, unless it's a subset
 /// of a larger candidate.
+#[derive(Debug)]
 pub enum Candidate {
     /// Borrow of a constant temporary.
     Ref(Location),
@@ -190,15 +191,12 @@ impl<'a, 'tcx> Promoter<'a, 'tcx> {
     /// promoted MIR, recursing through temps.
     fn promote_temp(&mut self, temp: Local) -> Local {
         let old_keep_original = self.keep_original;
-        let (bb, stmt_idx) = match self.temps[temp] {
-            TempState::Defined {
-                location: Location { block, statement_index },
-                uses
-            } if uses > 0 => {
+        let loc = match self.temps[temp] {
+            TempState::Defined { location, uses } if uses > 0 => {
                 if uses > 1 {
                     self.keep_original = true;
                 }
-                (block, statement_index)
+                location
             }
             state =>  {
                 span_bug!(self.promoted.span, "{:?} not promotable: {:?}",
@@ -209,91 +207,80 @@ impl<'a, 'tcx> Promoter<'a, 'tcx> {
             self.temps[temp] = TempState::PromotedOut;
         }
 
-        let no_stmts = self.source[bb].statements.len();
+        let no_stmts = self.source[loc.block].statements.len();
+        let new_temp = self.promoted.local_decls.push(
+            LocalDecl::new_temp(self.source.local_decls[temp].ty));
+
+        debug!("promote({:?} @ {:?}/{:?}, {:?})",
+               temp, loc, no_stmts, self.keep_original);
 
         // First, take the Rvalue or Call out of the source MIR,
         // or duplicate it, depending on keep_original.
-        let (mut rvalue, mut call) = (None, None);
-        let source_info = if stmt_idx < no_stmts {
-            let statement = &mut self.source[bb].statements[stmt_idx];
-            let rhs = match statement.kind {
-                StatementKind::Assign(_, ref mut rhs) => rhs,
-                _ => {
-                    span_bug!(statement.source_info.span, "{:?} is not an assignment",
-                              statement);
-                }
+        if loc.statement_index < no_stmts {
+            let (mut rvalue, source_info) = {
+                let statement = &mut self.source[loc.block].statements[loc.statement_index];
+                let rhs = match statement.kind {
+                    StatementKind::Assign(_, ref mut rhs) => rhs,
+                    _ => {
+                        span_bug!(statement.source_info.span, "{:?} is not an assignment",
+                                  statement);
+                    }
+                };
+
+                (if self.keep_original {
+                    rhs.clone()
+                } else {
+                    let unit = Rvalue::Aggregate(AggregateKind::Tuple, vec![]);
+                    mem::replace(rhs, unit)
+                }, statement.source_info)
             };
-            if self.keep_original {
-                rvalue = Some(rhs.clone());
-            } else {
-                let unit = Rvalue::Aggregate(AggregateKind::Tuple, vec![]);
-                rvalue = Some(mem::replace(rhs, unit));
-            }
-            statement.source_info
-        } else if self.keep_original {
-            let terminator = self.source[bb].terminator().clone();
-            call = Some(terminator.kind);
-            terminator.source_info
+
+            self.visit_rvalue(&mut rvalue, loc);
+            self.assign(new_temp, rvalue, source_info.span);
         } else {
-            let terminator = self.source[bb].terminator_mut();
-            let target = match terminator.kind {
-                TerminatorKind::Call {
-                    destination: ref mut dest @ Some(_),
-                    ref mut cleanup, ..
-                } => {
-                    // No cleanup necessary.
-                    cleanup.take();
-
-                    // We'll put a new destination in later.
-                    dest.take().unwrap().1
-                }
-                ref kind => {
-                    span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
+            let mut terminator = if self.keep_original {
+                self.source[loc.block].terminator().clone()
+            } else {
+                let terminator = self.source[loc.block].terminator_mut();
+                let target = match terminator.kind {
+                    TerminatorKind::Call { destination: Some((_, target)), .. } => target,
+                    ref kind => {
+                        span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
+                    }
+                };
+                Terminator {
+                    source_info: terminator.source_info,
+                    kind: mem::replace(&mut terminator.kind, TerminatorKind::Goto {
+                        target: target
+                    })
                 }
             };
-            call = Some(mem::replace(&mut terminator.kind, TerminatorKind::Goto {
-                target: target
-            }));
-            terminator.source_info
-        };
 
-        // Then, recurse for components in the Rvalue or Call.
-        if stmt_idx < no_stmts {
-            self.visit_rvalue(rvalue.as_mut().unwrap(), Location {
-                block: bb,
-                statement_index: stmt_idx
-            });
-        } else {
-            self.visit_terminator_kind(bb, call.as_mut().unwrap(), Location {
-                block: bb,
-                statement_index: no_stmts
-            });
-        }
-
-        let new_temp = self.promoted.local_decls.push(
-            LocalDecl::new_temp(self.source.local_decls[temp].ty));
-
-        // Inject the Rvalue or Call into the promoted MIR.
-        if stmt_idx < no_stmts {
-            self.assign(new_temp, rvalue.unwrap(), source_info.span);
-        } else {
             let last = self.promoted.basic_blocks().last().unwrap();
             let new_target = self.new_block();
-            let mut call = call.unwrap();
-            match call {
-                TerminatorKind::Call { ref mut destination, ..}  => {
-                    *destination = Some((Lvalue::Local(new_temp), new_target));
+
+            terminator.kind = match terminator.kind {
+                TerminatorKind::Call { mut func, mut args, .. } => {
+                    self.visit_operand(&mut func, loc);
+                    for arg in &mut args {
+                        self.visit_operand(arg, loc);
+                    }
+                    TerminatorKind::Call {
+                        func: func,
+                        args: args,
+                        cleanup: None,
+                        destination: Some((Lvalue::Local(new_temp), new_target))
+                    }
                 }
-                _ => bug!()
-            }
-            let terminator = self.promoted[last].terminator_mut();
-            terminator.source_info.span = source_info.span;
-            terminator.kind = call;
-        }
+                ref kind => {
+                    span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
+                }
+            };
 
-        // Restore the old duplication state.
-        self.keep_original = old_keep_original;
+            *self.promoted[last].terminator_mut() = terminator;
+        };
 
+        self.keep_original = old_keep_original;
         new_temp
     }
 
@@ -355,6 +342,7 @@ pub fn promote_candidates<'a, 'tcx>(mir: &mut Mir<'tcx>,
                                     mut temps: IndexVec<Local, TempState>,
                                     candidates: Vec<Candidate>) {
     // Visit candidates in reverse, in case they're nested.
+    debug!("promote_candidates({:?})", candidates);
     for candidate in candidates.into_iter().rev() {
         let (span, ty) = match candidate {
             Candidate::Ref(Location { block: bb, statement_index: stmt_idx }) => {
diff --git a/src/librustc_mir/transform/qualify_consts.rs b/src/librustc_mir/transform/qualify_consts.rs
index 893478a9331..d144651fb7d 100644
--- a/src/librustc_mir/transform/qualify_consts.rs
+++ b/src/librustc_mir/transform/qualify_consts.rs
@@ -993,9 +993,9 @@ impl<'tcx> MirPass<'tcx> for QualifyAndPromoteConstants {
                     Entry::Vacant(entry) => {
                         // Guard against `const` recursion.
                         entry.insert(Qualif::RECURSIVE);
+                        Mode::Const
                     }
                 }
-                Mode::Const
             }
             MirSource::Static(_, hir::MutImmutable) => Mode::Static,
             MirSource::Static(_, hir::MutMutable) => Mode::StaticMut,
diff --git a/src/test/run-pass/issue-37991.rs b/src/test/run-pass/issue-37991.rs
new file mode 100644
index 00000000000..cacc653871a
--- /dev/null
+++ b/src/test/run-pass/issue-37991.rs
@@ -0,0 +1,20 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(const_fn)]
+
+const fn foo() -> i64 {
+    3
+}
+
+fn main() {
+    let val = &(foo() % 2);
+    assert_eq!(*val, 1);
+}