refactor(clippy_utils::mir::visit_local_usage): use const generics (rust-lang#16637)

Encodes the direct relationship between the lengths of `locals` and the
returned list, making the code more type-safe

changelog: none

Author: samueltardieu

clippy_lints/src/methods/readonly_write_lock.rs

@@ -38,15 +38,14 @@ pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, receiver
             .local_decls
             .iter_enumerated()
             .find(|(_, decl)| local.span.contains(decl.source_info.span))
-        && let Some(usages) = visit_local_usage(
-            &[local],
+        && let Some([usage]) = visit_local_usage(
+            [local],
             mir,
             Location {
                 block: START_BLOCK,
                 statement_index: 0,
             },
         )
-        && let [usage] = usages.as_slice()
     {
         let writer_never_mutated = usage.local_consume_or_mutate_locs.is_empty();
 

clippy_lints/src/redundant_clone.rs

@@ -368,25 +368,25 @@ struct CloneUsage {
 }
 
 fn visit_clone_usage(cloned: mir::Local, clone: mir::Local, mir: &mir::Body<'_>, bb: mir::BasicBlock) -> CloneUsage {
-    if let Some((
-        LocalUsage {
-            local_use_locs: cloned_use_locs,
-            local_consume_or_mutate_locs: cloned_consume_or_mutate_locs,
-        },
-        LocalUsage {
-            local_use_locs: _,
-            local_consume_or_mutate_locs: clone_consume_or_mutate_locs,
-        },
-    )) = visit_local_usage(
-        &[cloned, clone],
+    if let Some(
+        [
+            LocalUsage {
+                local_use_locs: cloned_use_locs,
+                local_consume_or_mutate_locs: cloned_consume_or_mutate_locs,
+            },
+            LocalUsage {
+                local_use_locs: _,
+                local_consume_or_mutate_locs: clone_consume_or_mutate_locs,
+            },
+        ],
+    ) = visit_local_usage(
+        [cloned, clone],
         mir,
         mir::Location {
             block: bb,
             statement_index: mir.basic_blocks[bb].statements.len(),
         },
-    )
-    .map(|mut vec| (vec.remove(0), vec.remove(0)))
-    {
+    ) {
         CloneUsage {
             cloned_use_loc: cloned_use_locs.first().copied().into(),
             cloned_consume_or_mutate_loc: cloned_consume_or_mutate_locs.first().copied(),

clippy_utils/src/mir/mod.rs

@@ -1,3 +1,5 @@
+use std::iter;
+
 use rustc_data_structures::either::Either;
 use rustc_hir::{Expr, HirId};
 use rustc_index::bit_set::DenseBitSet;
@@ -22,14 +24,17 @@ pub struct LocalUsage {
     pub local_consume_or_mutate_locs: Vec<Location>,
 }
 
-pub fn visit_local_usage(locals: &[Local], mir: &Body<'_>, location: Location) -> Option<Vec<LocalUsage>> {
-    let init = vec![
+pub fn visit_local_usage<const N: usize>(
+    locals: [Local; N],
+    mir: &Body<'_>,
+    location: Location,
+) -> Option<[LocalUsage; N]> {
+    let init = [const {
         LocalUsage {
             local_use_locs: Vec::new(),
             local_consume_or_mutate_locs: Vec::new(),
-        };
-        locals.len()
-    ];
+        }
+    }; N];
 
     traversal::Postorder::new(&mir.basic_blocks, location.block, None)
         .collect::<Vec<_>>()
@@ -44,7 +49,7 @@ pub fn visit_local_usage(locals: &[Local], mir: &Body<'_>, location: Location) -
             }
 
             let mut v = V {
-                locals,
+                locals: &locals,
                 location,
                 results: usage,
             };
@@ -53,34 +58,34 @@ pub fn visit_local_usage(locals: &[Local], mir: &Body<'_>, location: Location) -
         })
 }
 
-struct V<'a> {
-    locals: &'a [Local],
+struct V<'a, const N: usize> {
+    locals: &'a [Local; N],
     location: Location,
-    results: Vec<LocalUsage>,
+    results: [LocalUsage; N],
 }
 
-impl<'tcx> Visitor<'tcx> for V<'_> {
+impl<'tcx, const N: usize> Visitor<'tcx> for V<'_, N> {
     fn visit_place(&mut self, place: &Place<'tcx>, ctx: PlaceContext, loc: Location) {
         if loc.block == self.location.block && loc.statement_index <= self.location.statement_index {
             return;
         }
 
         let local = place.local;
 
-        for (i, self_local) in self.locals.iter().enumerate() {
+        for (self_local, result) in iter::zip(self.locals, &mut self.results) {
             if local == *self_local {
                 if !matches!(
                     ctx,
                     PlaceContext::MutatingUse(MutatingUseContext::Drop) | PlaceContext::NonUse(_)
                 ) {
-                    self.results[i].local_use_locs.push(loc);
+                    result.local_use_locs.push(loc);
                 }
                 if matches!(
                     ctx,
                     PlaceContext::NonMutatingUse(NonMutatingUseContext::Move | NonMutatingUseContext::Inspect)
                         | PlaceContext::MutatingUse(MutatingUseContext::Borrow)
                 ) {
-                    self.results[i].local_consume_or_mutate_locs.push(loc);
+                    result.local_consume_or_mutate_locs.push(loc);
                 }
             }
         }
@@ -114,16 +119,16 @@ pub fn block_in_cycle(body: &Body<'_>, block: BasicBlock) -> bool {
 /// Convenience wrapper around `visit_local_usage`.
 pub fn used_exactly_once(mir: &Body<'_>, local: Local) -> Option<bool> {
     visit_local_usage(
-        &[local],
+        [local],
         mir,
         Location {
             block: START_BLOCK,
             statement_index: 0,
         },
     )
-    .map(|mut vec| {
-        let LocalUsage { local_use_locs, .. } = vec.remove(0);
-        let mut locations = local_use_locs
+    .map(|[local_usage]| {
+        let mut locations = local_usage
+            .local_use_locs
             .into_iter()
             .filter(|&location| !is_local_assignment(mir, local, location));
         if let Some(location) = locations.next() {