Rename IndexVec::last → last_index

As I've been trying to replace a `Vec` with an `IndexVec`, having `last` exist on both but returning very different types makes the transition a bit awkward -- the errors are later, where you get things like "there's no `ty` method on `mir::Field`" rather than a more localized error like "hey, there's no `last` on `IndexVec`".

So I propose renaming `last` to `last_index` to help distinguish `Vec::last`, which returns an element, and `IndexVec::last_index`, which returns an index.

(Similarly, `Iterator::last` also returns an element, not an index.)

Author: scottmcm

compiler/rustc_const_eval/src/transform/promote_consts.rs

@@ -707,7 +707,7 @@ impl<'a, 'tcx> Promoter<'a, 'tcx> {
     }
 
     fn assign(&mut self, dest: Local, rvalue: Rvalue<'tcx>, span: Span) {
-        let last = self.promoted.basic_blocks.last().unwrap();
+        let last = self.promoted.basic_blocks.last_index().unwrap();
         let data = &mut self.promoted[last];
         data.statements.push(Statement {
             source_info: SourceInfo::outermost(span),
@@ -800,7 +800,7 @@ impl<'a, 'tcx> Promoter<'a, 'tcx> {
                         self.visit_operand(arg, loc);
                     }
 
-                    let last = self.promoted.basic_blocks.last().unwrap();
+                    let last = self.promoted.basic_blocks.last_index().unwrap();
                     let new_target = self.new_block();
 
                     *self.promoted[last].terminator_mut() = Terminator {

compiler/rustc_index/src/vec.rs

@@ -216,7 +216,7 @@ impl<I: Idx, T> IndexVec<I, T> {
     }
 
     #[inline]
-    pub fn last(&self) -> Option<I> {
+    pub fn last_index(&self) -> Option<I> {
         self.len().checked_sub(1).map(I::new)
     }
 

compiler/rustc_mir_transform/src/coverage/tests.rs

@@ -65,7 +65,7 @@ impl<'tcx> MockBlocks<'tcx> {
     }
 
     fn push(&mut self, kind: TerminatorKind<'tcx>) -> BasicBlock {
-        let next_lo = if let Some(last) = self.blocks.last() {
+        let next_lo = if let Some(last) = self.blocks.last_index() {
             self.blocks[last].terminator().source_info.span.hi()
         } else {
             BytePos(1)

compiler/rustc_trait_selection/src/solve/search_graph/mod.rs

@@ -54,7 +54,7 @@ impl<'tcx> SearchGraph<'tcx> {
     /// Whether we're currently in a cycle. This should only be used
     /// for debug assertions.
     pub(super) fn in_cycle(&self) -> bool {
-        if let Some(stack_depth) = self.stack.last() {
+        if let Some(stack_depth) = self.stack.last_index() {
             // Either the current goal on the stack is the root of a cycle...
             if self.stack[stack_depth].has_been_used {
                 return true;