Skip those blocks in codegen too, to avoid the linker errors

And hey, maybe even be faster because it means less IR emitted.

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -956,6 +956,12 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         self.codegen_terminator(bx, bb, data.terminator());
     }
 
+    pub fn codegen_block_as_unreachable(&mut self, bb: mir::BasicBlock) {
+        let mut bx = self.build_block(bb);
+        debug!("codegen_block_as_unreachable({:?})", bb);
+        bx.unreachable();
+    }
+
     fn codegen_terminator(
         &mut self,
         mut bx: Bx,

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -240,10 +240,19 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // Apply debuginfo to the newly allocated locals.
     fx.debug_introduce_locals(&mut bx);
 
+    let reachable_blocks = mir.reachable_blocks_in_mono(cx.tcx(), instance);
+
     // Codegen the body of each block using reverse postorder
     // FIXME(eddyb) reuse RPO iterator between `analysis` and this.
     for (bb, _) in traversal::reverse_postorder(&mir) {
-        fx.codegen_block(bb);
+        if reachable_blocks.contains(bb) {
+            fx.codegen_block(bb);
+        } else {
+            // This may have references to things we didn't monomorphize, so we
+            // don't actually codegen the body.  We still create the block so
+            // terminators in other blocks can reference it without worry.
+            fx.codegen_block_as_unreachable(bb);
+        }
     }
 
     // For backends that support CFI using type membership (i.e., testing whether a given  pointer

compiler/rustc_middle/src/mir/mod.rs

@@ -11,7 +11,7 @@ use crate::ty::fold::{TypeFoldable, TypeFolder, TypeVisitor};
 use crate::ty::print::{FmtPrinter, Printer};
 use crate::ty::subst::{Subst, SubstsRef};
 use crate::ty::{self, List, Ty, TyCtxt};
-use crate::ty::{AdtDef, InstanceDef, Region, ScalarInt, UserTypeAnnotationIndex};
+use crate::ty::{AdtDef, Instance, InstanceDef, Region, ScalarInt, UserTypeAnnotationIndex};
 use rustc_hir::def::{CtorKind, Namespace};
 use rustc_hir::def_id::{DefId, CRATE_DEF_INDEX};
 use rustc_hir::{self, GeneratorKind};
@@ -23,7 +23,7 @@ pub use rustc_ast::Mutability;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::graph::dominators::{dominators, Dominators};
 use rustc_data_structures::graph::{self, GraphSuccessors};
-use rustc_index::bit_set::BitMatrix;
+use rustc_index::bit_set::{BitMatrix, BitSet};
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_serialize::{Decodable, Encodable};
 use rustc_span::symbol::Symbol;
@@ -517,6 +517,73 @@ impl<'tcx> Body<'tcx> {
     pub fn generator_kind(&self) -> Option<GeneratorKind> {
         self.generator.as_ref().map(|generator| generator.generator_kind)
     }
+
+    /// Finds which basic blocks are actually reachable for a specific
+    /// monomorphization of this body.
+    ///
+    /// This is allowed to have false positives; just because this says a block
+    /// is reachable doesn't mean that's necessarily true.  It's thus always
+    /// legal for this to return a filled set.
+    ///
+    /// Regardless, the [`BitSet::domain_size`] of the returned set will always
+    /// exactly match the number of blocks in the body so that `contains`
+    /// checks can be done without worrying about panicking.
+    ///
+    /// The main case this supports is filtering out `if <T as Trait>::CONST`
+    /// bodies that can't be removed in generic MIR, but *can* be removed once
+    /// the specific `T` is known.
+    ///
+    /// This is used in the monomorphization collector as well as in codegen.
+    pub fn reachable_blocks_in_mono(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        instance: Instance<'tcx>,
+    ) -> BitSet<BasicBlock> {
+        if instance.substs.is_noop() {
+            // If it's non-generic, then mir-opt const prop has already run, meaning it's
+            // probably not worth doing any further filtering.  So call everything reachable.
+            return BitSet::new_filled(self.basic_blocks().len());
+        }
+
+        let mut set = BitSet::new_empty(self.basic_blocks().len());
+        self.reachable_blocks_in_mono_from(tcx, instance, &mut set, START_BLOCK);
+        set
+    }
+
+    fn reachable_blocks_in_mono_from(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        instance: Instance<'tcx>,
+        set: &mut BitSet<BasicBlock>,
+        bb: BasicBlock,
+    ) {
+        if !set.insert(bb) {
+            return;
+        }
+
+        let data = &self.basic_blocks()[bb];
+
+        if let TerminatorKind::SwitchInt {
+            discr: Operand::Constant(constant),
+            switch_ty,
+            targets,
+        } = &data.terminator().kind
+        {
+            let env = ty::ParamEnv::reveal_all();
+            let mono_literal =
+                instance.subst_mir_and_normalize_erasing_regions(tcx, env, constant.literal);
+            if let Some(bits) = mono_literal.try_eval_bits(tcx, env, switch_ty) {
+                let target = targets.target_for_value(bits);
+                return self.reachable_blocks_in_mono_from(tcx, instance, set, target);
+            } else {
+                bug!("Couldn't evaluate constant {:?} in mono {:?}", constant, instance);
+            }
+        }
+
+        for &target in data.terminator().successors() {
+            self.reachable_blocks_in_mono_from(tcx, instance, set, target);
+        }
+    }
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug, TyEncodable, TyDecodable, HashStable)]

compiler/rustc_middle/src/mir/terminator.rs

@@ -78,6 +78,13 @@ impl SwitchTargets {
     pub fn all_targets_mut(&mut self) -> &mut [BasicBlock] {
         &mut self.targets
     }
+
+    /// Finds the `BasicBlock` to which this `SwitchInt` will branch given the
+    /// specific value.  This cannot fail, as it'll return the `otherwise`
+    /// branch if there's not a specific match for the value.
+    pub fn target_for_value(&self, value: u128) -> BasicBlock {
+        self.iter().find_map(|(v, t)| (v == value).then_some(t)).unwrap_or_else(|| self.otherwise())
+    }
 }
 
 pub struct SwitchTargetsIter<'a> {

compiler/rustc_mir_transform/src/simplify_branches.rs

@@ -34,15 +34,8 @@ impl<'tcx> MirPass<'tcx> for SimplifyBranches {
                 } => {
                     let constant = c.literal.try_eval_bits(tcx, param_env, switch_ty);
                     if let Some(constant) = constant {
-                        let otherwise = targets.otherwise();
-                        let mut ret = TerminatorKind::Goto { target: otherwise };
-                        for (v, t) in targets.iter() {
-                            if v == constant {
-                                ret = TerminatorKind::Goto { target: t };
-                                break;
-                            }
-                        }
-                        ret
+                        let target = targets.target_for_value(constant);
+                        TerminatorKind::Goto { target }
                     } else {
                         continue;
                     }

compiler/rustc_monomorphize/src/collector.rs

@@ -623,92 +623,14 @@ impl<'a, 'tcx> MirNeighborCollector<'a, 'tcx> {
             value,
         )
     }
-
-    fn find_reachable_blocks(&mut self, bb: mir::BasicBlock) {
-        if !self.reachable_blocks.insert(bb) {
-            return;
-        }
-
-        use mir::TerminatorKind::*;
-        let data = &self.body.basic_blocks()[bb];
-        match data.terminator().kind {
-            Goto { target } => self.find_reachable_blocks(target),
-            Resume | Abort | Return | Unreachable | GeneratorDrop => (),
-            Drop { place: _, target, unwind }
-            | DropAndReplace { place: _, value: _, target, unwind }
-            | Assert { cond: _, expected: _, msg: _, target, cleanup: unwind }
-            | Yield { value: _, resume: target, resume_arg: _, drop: unwind } => {
-                self.find_reachable_blocks(target);
-                unwind.map(|b| self.find_reachable_blocks(b));
-            }
-            Call { func: _, args: _, destination, cleanup, from_hir_call: _, fn_span: _} => {
-                destination.map(|(_, b)| self.find_reachable_blocks(b));
-                cleanup.map(|b| self.find_reachable_blocks(b));
-            }
-            FalseEdge { .. } | FalseUnwind { .. } => {
-                bug!("Expected false edges to already be gone when collecting neighbours for {:?}", self.instance);
-            }
-            InlineAsm { template: _, operands: _, options: _, line_spans: _, destination} => {
-                destination.map(|b| self.find_reachable_blocks(b));
-            }
-            SwitchInt { ref discr, switch_ty, ref targets } => {
-                if let mir::Operand::Constant(constant) = discr {
-                    if let Some(raw_value) = self.try_eval_constant_to_bits(constant, switch_ty) {
-                        // We know what this is going to be,
-                        // so we can ignore all the other blocks.
-                        for (test_value, target) in targets.iter() {
-                            if test_value == raw_value {
-                                return self.find_reachable_blocks(target);
-                            }
-                        }
-
-                        return self.find_reachable_blocks(targets.otherwise());
-                    }
-                }
-
-                // If it's not a constant or we can't evaluate it,
-                // then we have to consider them all as reachable.
-                for &b in targets.all_targets() {
-                    self.find_reachable_blocks(b)
-                }
-            }
-        }
-    }
-
-    fn try_eval_constant_to_bits(&self, constant: &mir::Constant<'tcx>, ty: Ty<'tcx>) -> Option<u128> {
-        let env = ty::ParamEnv::reveal_all();
-        let ct = self.monomorphize(constant.literal);
-        let value = match ct {
-            mir::ConstantKind::Val(value, _) => value,
-            mir::ConstantKind::Ty(ct) => {
-                match ct.val {
-                    ty::ConstKind::Unevaluated(ct) => self
-                        .tcx
-                        .const_eval_resolve(env, ct, None).ok()?,
-                    ty::ConstKind::Value(value) => value,
-                    other => span_bug!(
-                        constant.span,
-                        "encountered bad ConstKind after monomorphizing: {:?}",
-                        other
-                    ),
-                }
-            }
-        };
-        value.try_to_bits_for_ty(self.tcx, env, ty)
-    }
 }
 
 impl<'a, 'tcx> MirVisitor<'tcx> for MirNeighborCollector<'a, 'tcx> {
-    fn visit_body(&mut self, body: &mir::Body<'tcx>) {
-        self.find_reachable_blocks(mir::START_BLOCK);
-        self.super_body(body);
-    }
-
     fn visit_basic_block_data(&mut self, block: mir::BasicBlock, data: &mir::BasicBlockData<'tcx>) {
         if self.reachable_blocks.contains(block) {
             self.super_basic_block_data(block, data);
         } else {
-            debug!("skipping basic block {:?}", block);
+            debug!("skipping mono-unreachable basic block {:?}", block);
         }
     }
 
@@ -1482,15 +1404,9 @@ fn collect_neighbours<'tcx>(
     debug!("collect_neighbours: {:?}", instance.def_id());
     let body = tcx.instance_mir(instance.def);
 
-    let reachable_blocks =
-        if instance.substs.is_noop() {
-            // If it's non-generic, then it's already filtered out
-            // any blocks that are unreachable, so don't re-do the work.
-            BitSet::new_filled(body.basic_blocks().len())
-        } else {
-            BitSet::new_empty(body.basic_blocks().len())
-        };
-    let mut collector = MirNeighborCollector { tcx, body: &body, output, instance, reachable_blocks };
+    let reachable_blocks = body.reachable_blocks_in_mono(tcx, instance);
+    let mut collector =
+        MirNeighborCollector { tcx, body: &body, output, instance, reachable_blocks };
     collector.visit_body(&body);
 }
 

src/test/codegen/skip-mono-inside-if-false.rs

@@ -7,19 +7,21 @@ pub fn demo_for_i32() {
     generic_impl::<i32>();
 }
 
+// Two important things here:
+// - We replace the "then" block with `unreachable` to avoid linking problems
+// - We neither declare nor define the `big_impl` that said block "calls".
+
 // CHECK-LABEL: ; skip_mono_inside_if_false::generic_impl
 // CHECK: start:
 // CHECK-NEXT: br i1 false, label %[[THEN_BRANCH:bb[0-9]+]], label %[[ELSE_BRANCH:bb[0-9]+]]
 // CHECK: [[ELSE_BRANCH]]:
 // CHECK-NEXT: call skip_mono_inside_if_false::small_impl
 // CHECK: [[THEN_BRANCH]]:
-// CHECK-NEXT: call skip_mono_inside_if_false::big_impl
-
-// Then despite there being calls to both of them, only the ones that's used has a definition.
-// The other is only forward-declared, and its use will disappear with LLVM's simplifycfg.
+// CHECK-NEXT: unreachable
 
+// CHECK-NOT: @_ZN25skip_mono_inside_if_false8big_impl
 // CHECK: define internal void @_ZN25skip_mono_inside_if_false10small_impl
-// CHECK: declare hidden void @_ZN25skip_mono_inside_if_false8big_impl
+// CHECK-NOT: @_ZN25skip_mono_inside_if_false8big_impl
 
 fn generic_impl<T>() {
     trait MagicTrait {

src/test/mir-opt/const_prop/control-flow-simplification.rs

@@ -1,7 +1,7 @@
 // compile-flags: -Zmir-opt-level=1
 
 trait HasSize: Sized {
-    const BYTES:usize = std::mem::size_of::<Self>();
+    const BYTES: usize = std::mem::size_of::<Self>();
 }
 
 impl<This> HasSize for This{}

src/test/mir-opt/simplify_if_generic_constant.rs

@@ -1,14 +1,14 @@
 #![crate_type = "lib"]
 
 pub trait HasBoolConst {
-	const B: bool;
+    const B: bool;
 }
 
 // EMIT_MIR simplify_if_generic_constant.use_associated_const.SimplifyIfConst.diff
 pub fn use_associated_const<T: HasBoolConst>() -> u8 {
-	if <T as HasBoolConst>::B {
-		13
-	} else {
-		42
-	}
+    if <T as HasBoolConst>::B {
+        13
+    } else {
+        42
+    }
 }

src/test/mir-opt/simplify_if_generic_constant.use_associated_const.SimplifyIfConst.diff

@@ -3,27 +3,27 @@
 
   fn use_associated_const() -> u8 {
       let mut _0: u8;                      // return place in scope 0 at $DIR/simplify_if_generic_constant.rs:8:51: 8:53
-      let mut _1: bool;                    // in scope 0 at $DIR/simplify_if_generic_constant.rs:9:5: 9:27
+      let mut _1: bool;                    // in scope 0 at $DIR/simplify_if_generic_constant.rs:9:8: 9:30
 
       bb0: {
-          StorageLive(_1);                 // scope 0 at $DIR/simplify_if_generic_constant.rs:9:5: 9:27
--         _1 = const <T as HasBoolConst>::B; // scope 0 at $DIR/simplify_if_generic_constant.rs:9:5: 9:27
--         switchInt(move _1) -> [false: bb2, otherwise: bb1]; // scope 0 at $DIR/simplify_if_generic_constant.rs:9:5: 9:27
-+         switchInt(const <T as HasBoolConst>::B) -> [false: bb2, otherwise: bb1]; // scope 0 at $DIR/simplify_if_generic_constant.rs:9:5: 9:27
+          StorageLive(_1);                 // scope 0 at $DIR/simplify_if_generic_constant.rs:9:8: 9:30
+-         _1 = const <T as HasBoolConst>::B; // scope 0 at $DIR/simplify_if_generic_constant.rs:9:8: 9:30
+-         switchInt(move _1) -> [false: bb2, otherwise: bb1]; // scope 0 at $DIR/simplify_if_generic_constant.rs:9:8: 9:30
++         switchInt(const <T as HasBoolConst>::B) -> [false: bb2, otherwise: bb1]; // scope 0 at $DIR/simplify_if_generic_constant.rs:9:8: 9:30
       }
 
       bb1: {
-          _0 = const 13_u8;                // scope 0 at $DIR/simplify_if_generic_constant.rs:10:3: 10:5
-          goto -> bb3;                     // scope 0 at $DIR/simplify_if_generic_constant.rs:9:2: 13:3
+          _0 = const 13_u8;                // scope 0 at $DIR/simplify_if_generic_constant.rs:10:9: 10:11
+          goto -> bb3;                     // scope 0 at $DIR/simplify_if_generic_constant.rs:9:5: 13:6
       }
 
       bb2: {
-          _0 = const 42_u8;                // scope 0 at $DIR/simplify_if_generic_constant.rs:12:3: 12:5
-          goto -> bb3;                     // scope 0 at $DIR/simplify_if_generic_constant.rs:9:2: 13:3
+          _0 = const 42_u8;                // scope 0 at $DIR/simplify_if_generic_constant.rs:12:9: 12:11
+          goto -> bb3;                     // scope 0 at $DIR/simplify_if_generic_constant.rs:9:5: 13:6
       }
 
       bb3: {
-          StorageDead(_1);                 // scope 0 at $DIR/simplify_if_generic_constant.rs:13:2: 13:3
+          StorageDead(_1);                 // scope 0 at $DIR/simplify_if_generic_constant.rs:13:5: 13:6
           return;                          // scope 0 at $DIR/simplify_if_generic_constant.rs:14:2: 14:2
       }
   }

src/test/ui/consts/control-flow/drop-fail.precise.stderr

@@ -5,7 +5,7 @@ LL |     let x = Some(Vec::new());
    |         ^ constants cannot evaluate destructors
 
 error[E0493]: destructors cannot be evaluated at compile-time
-  --> $DIR/drop-fail.rs:39:9
+  --> $DIR/drop-fail.rs:44:9
    |
 LL |     let mut tmp = None;
    |         ^^^^^^^ constants cannot evaluate destructors

src/test/ui/consts/control-flow/drop-fail.rs

@@ -8,13 +8,18 @@ const _: Option<Vec<i32>> = {
     let x = Some(Vec::new());
     //[stock,precise]~^ ERROR destructors cannot be evaluated at compile-time
 
-    if true {
+    if unknown() {
         x
     } else {
         y
     }
 };
 
+#[inline(never)]
+const fn unknown() -> bool {
+    true
+}
+
 // We only clear `NeedsDrop` if a local is moved from in entirely. This is a shortcoming of the
 // existing analysis.
 const _: Vec<i32> = {