Remove MirVisitable.

The `MirVisitable` trait is just a complicated way to visit either a
statement or a terminator. (And its impl for `Terminator` is unused.) It
has a single use.

This commit removes it, replacing it with an if/else, which is shorter
and simpler.

compiler/rustc_borrowck/src/diagnostics/find_use.rs

@@ -1,7 +1,7 @@
 use std::collections::VecDeque;
 
 use rustc_data_structures::fx::FxIndexSet;
-use rustc_middle::mir::visit::{MirVisitable, PlaceContext, Visitor};
+use rustc_middle::mir::visit::{PlaceContext, Visitor};
 use rustc_middle::mir::{self, Body, Local, Location};
 use rustc_middle::ty::{RegionVid, TyCtxt};
 
@@ -45,7 +45,22 @@ impl<'a, 'tcx> UseFinder<'a, 'tcx> {
 
             let block_data = &self.body[p.block];
 
-            match self.def_use(p, block_data.visitable(p.statement_index)) {
+            let mut visitor = DefUseVisitor {
+                body: self.body,
+                tcx: self.tcx,
+                region_vid: self.region_vid,
+                def_use_result: None,
+            };
+
+            let is_statement = p.statement_index < block_data.statements.len();
+
+            if is_statement {
+                visitor.visit_statement(&block_data.statements[p.statement_index], p);
+            } else {
+                visitor.visit_terminator(block_data.terminator.as_ref().unwrap(), p);
+            }
+
+            match visitor.def_use_result {
                 Some(DefUseResult::Def) => {}
 
                 Some(DefUseResult::UseLive { local }) => {
@@ -57,7 +72,7 @@ impl<'a, 'tcx> UseFinder<'a, 'tcx> {
                 }
 
                 None => {
-                    if p.statement_index < block_data.statements.len() {
+                    if is_statement {
                         queue.push_back(p.successor_within_block());
                     } else {
                         queue.extend(
@@ -77,19 +92,6 @@ impl<'a, 'tcx> UseFinder<'a, 'tcx> {
 
         None
     }
-
-    fn def_use(&self, location: Location, thing: &dyn MirVisitable<'tcx>) -> Option<DefUseResult> {
-        let mut visitor = DefUseVisitor {
-            body: self.body,
-            tcx: self.tcx,
-            region_vid: self.region_vid,
-            def_use_result: None,
-        };
-
-        thing.apply(location, &mut visitor);
-
-        visitor.def_use_result
-    }
 }
 
 struct DefUseVisitor<'a, 'tcx> {

compiler/rustc_middle/src/mir/mod.rs

@@ -32,7 +32,6 @@ use tracing::{debug, trace};
 pub use self::query::*;
 use self::visit::TyContext;
 use crate::mir::interpret::{AllocRange, Scalar};
-use crate::mir::visit::MirVisitable;
 use crate::ty::codec::{TyDecoder, TyEncoder};
 use crate::ty::print::{FmtPrinter, Printer, pretty_print_const, with_no_trimmed_paths};
 use crate::ty::visit::TypeVisitableExt;
@@ -1364,10 +1363,6 @@ impl<'tcx> BasicBlockData<'tcx> {
         self.terminator.as_mut().expect("invalid terminator state")
     }
 
-    pub fn visitable(&self, index: usize) -> &dyn MirVisitable<'tcx> {
-        if index < self.statements.len() { &self.statements[index] } else { &self.terminator }
-    }
-
     /// Does the block have no statements and an unreachable terminator?
     #[inline]
     pub fn is_empty_unreachable(&self) -> bool {

compiler/rustc_middle/src/mir/visit.rs

@@ -1265,28 +1265,6 @@ macro_rules! visit_place_fns {
 make_mir_visitor!(Visitor,);
 make_mir_visitor!(MutVisitor, mut);
 
-pub trait MirVisitable<'tcx> {
-    fn apply(&self, location: Location, visitor: &mut dyn Visitor<'tcx>);
-}
-
-impl<'tcx> MirVisitable<'tcx> for Statement<'tcx> {
-    fn apply(&self, location: Location, visitor: &mut dyn Visitor<'tcx>) {
-        visitor.visit_statement(self, location)
-    }
-}
-
-impl<'tcx> MirVisitable<'tcx> for Terminator<'tcx> {
-    fn apply(&self, location: Location, visitor: &mut dyn Visitor<'tcx>) {
-        visitor.visit_terminator(self, location)
-    }
-}
-
-impl<'tcx> MirVisitable<'tcx> for Option<Terminator<'tcx>> {
-    fn apply(&self, location: Location, visitor: &mut dyn Visitor<'tcx>) {
-        visitor.visit_terminator(self.as_ref().unwrap(), location)
-    }
-}
-
 /// Extra information passed to `visit_ty` and friends to give context
 /// about where the type etc appears.
 #[derive(Copy, Clone, Debug, Hash, Eq, PartialEq)]