Auto merge of rust-lang#122258 - RalfJung:required-fns, r=<try>

Draft: monomorphize things from dead code, too

This is another attempt at fixing rust-lang#107503. The previous attempt at rust-lang#112879 seems stuck in figuring out where the perf regression comes from. So here I want to take baby steps to see the impact of each step.

r? `@ghost`

compiler/rustc_middle/src/mir/mod.rs

@@ -9,7 +9,7 @@ use crate::ty::fold::{FallibleTypeFolder, TypeFoldable};
 use crate::ty::print::{pretty_print_const, with_no_trimmed_paths};
 use crate::ty::print::{FmtPrinter, Printer};
 use crate::ty::visit::TypeVisitableExt;
-use crate::ty::{self, List, Ty, TyCtxt};
+use crate::ty::{self, Instance, List, Ty, TyCtxt};
 use crate::ty::{AdtDef, InstanceDef, UserTypeAnnotationIndex};
 use crate::ty::{GenericArg, GenericArgsRef};
 
@@ -375,6 +375,9 @@ pub struct Body<'tcx> {
     /// We hold in this field all the constants we are not able to evaluate yet.
     pub required_consts: Vec<ConstOperand<'tcx>>,
 
+    /// Functions that need to monomorphize successfully for this MIR to be well-formed.
+    pub required_fns: Vec<Instance<'tcx>>,
+
     /// Does this body use generic parameters. This is used for the `ConstEvaluatable` check.
     ///
     /// Note that this does not actually mean that this body is not computable right now.
@@ -445,6 +448,7 @@ impl<'tcx> Body<'tcx> {
             var_debug_info,
             span,
             required_consts: Vec::new(),
+            required_fns: Vec::new(),
             is_polymorphic: false,
             injection_phase: None,
             tainted_by_errors,
@@ -473,6 +477,7 @@ impl<'tcx> Body<'tcx> {
             spread_arg: None,
             span: DUMMY_SP,
             required_consts: Vec::new(),
+            required_fns: Vec::new(),
             var_debug_info: Vec::new(),
             is_polymorphic: false,
             injection_phase: None,

compiler/rustc_mir_build/src/build/custom/mod.rs

@@ -56,6 +56,7 @@ pub(super) fn build_custom_mir<'tcx>(
         var_debug_info: Vec::new(),
         span,
         required_consts: Vec::new(),
+        required_fns: Vec::new(),
         is_polymorphic: false,
         tainted_by_errors: None,
         injection_phase: None,

compiler/rustc_mir_transform/src/lib.rs

@@ -344,11 +344,14 @@ fn mir_promoted(
     }
 
     let mut required_consts = Vec::new();
-    let mut required_consts_visitor = RequiredConstsVisitor::new(&mut required_consts);
+    let mut required_fns = Vec::new();
+    let mut required_consts_visitor =
+        RequiredConstsVisitor::new(tcx, &body, &mut required_consts, &mut required_fns);
     for (bb, bb_data) in traversal::reverse_postorder(&body) {
         required_consts_visitor.visit_basic_block_data(bb, bb_data);
     }
     body.required_consts = required_consts;
+    body.required_fns = required_fns;
 
     // What we need to run borrowck etc.
     let promote_pass = promote_consts::PromoteTemps::default();

compiler/rustc_mir_transform/src/required_consts.rs

@@ -1,14 +1,23 @@
+use rustc_hir::LangItem;
 use rustc_middle::mir::visit::Visitor;
-use rustc_middle::mir::{Const, ConstOperand, Location};
-use rustc_middle::ty::ConstKind;
+use rustc_middle::mir::{self, Const, ConstOperand, Location};
+use rustc_middle::ty::{self, ConstKind, Instance, InstanceDef, TyCtxt};
 
 pub struct RequiredConstsVisitor<'a, 'tcx> {
+    tcx: TyCtxt<'tcx>,
+    body: &'a mir::Body<'tcx>,
     required_consts: &'a mut Vec<ConstOperand<'tcx>>,
+    required_fns: &'a mut Vec<Instance<'tcx>>,
 }
 
 impl<'a, 'tcx> RequiredConstsVisitor<'a, 'tcx> {
-    pub fn new(required_consts: &'a mut Vec<ConstOperand<'tcx>>) -> Self {
-        RequiredConstsVisitor { required_consts }
+    pub fn new(
+        tcx: TyCtxt<'tcx>,
+        body: &'a mir::Body<'tcx>,
+        required_consts: &'a mut Vec<ConstOperand<'tcx>>,
+        required_fns: &'a mut Vec<Instance<'tcx>>,
+    ) -> Self {
+        RequiredConstsVisitor { tcx, body, required_consts, required_fns }
     }
 }
 
@@ -21,7 +30,34 @@ impl<'tcx> Visitor<'tcx> for RequiredConstsVisitor<'_, 'tcx> {
                 _ => bug!("only ConstKind::Param/Value should be encountered here, got {:#?}", c),
             },
             Const::Unevaluated(..) => self.required_consts.push(*constant),
-            Const::Val(..) => {}
+            Const::Val(_val, ty) => {
+                // This is how function items get referenced: via zero-sized constants of `FnDef` type
+                if let ty::FnDef(def_id, args) = ty.kind() {
+                    debug!("adding to required_fns: {def_id:?}");
+                    // FIXME maybe we shouldn't use `Instance`? We can't use `Instance::new`, it is
+                    // for codegen. But `Instance` feels like the right representation... Check what
+                    // the regular collector does.
+                    self.required_fns.push(Instance { def: InstanceDef::Item(*def_id), args });
+                }
+            }
+        }
+    }
+
+    fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, location: Location) {
+        self.super_terminator(terminator, location);
+
+        match terminator.kind {
+            // We don't need to handle `Call` as we already handled all function type operands in
+            // `visit_constant`. But we do need to handle `Drop`.
+            mir::TerminatorKind::Drop { place, .. } => {
+                let ty = place.ty(self.body, self.tcx).ty;
+                let def_id = self.tcx.require_lang_item(LangItem::DropInPlace, None);
+                let args = self.tcx.mk_args(&[ty.into()]);
+                // FIXME: same as above (we cannot use `Instance::resolve_drop_in_place` as this is
+                // still generic).
+                self.required_fns.push(Instance { def: InstanceDef::Item(def_id), args });
+            }
+            _ => {}
         }
     }
 }