Small mir-opt refactor

Hopefully-non-controversial changes from some not-ready-yet work that I'd figured I'd submit on their own.

compiler/rustc_middle/src/mir/mod.rs

@@ -1504,6 +1504,7 @@ impl Statement<'_> {
     }
 
     /// Changes a statement to a nop and returns the original statement.
+    #[must_use = "If you don't need the statement, use `make_nop` instead"]
     pub fn replace_nop(&mut self) -> Self {
         Statement {
             source_info: self.source_info,

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/const_goto.rs

@@ -82,20 +82,7 @@ impl<'a, 'tcx> Visitor<'tcx> for ConstGotoOptimizationFinder<'a, 'tcx> {
                     // Now find which value in the Switch matches the const value.
                     let const_value =
                         _const.literal.try_eval_bits(self.tcx, self.param_env, switch_ty)?;
-                    let found_value_idx_option = targets
-                        .iter()
-                        .enumerate()
-                        .find(|(_, (value, _))| const_value == *value)
-                        .map(|(idx, _)| idx);
-
-                    let target_to_use_in_goto =
-                        if let Some(found_value_idx) = found_value_idx_option {
-                            targets.iter().nth(found_value_idx).unwrap().1
-                        } else {
-                            // If we did not find the const value in values, it must be the otherwise case
-                            targets.otherwise()
-                        };
-
+                    let target_to_use_in_goto = targets.target_for_value(const_value);
                     self.optimizations.push(OptimizationToApply {
                         bb_with_goto: location.block,
                         target_to_use_in_goto,

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;
                     }