Discard anonymous locals from tracking.

Author: cjgillot

compiler/rustc_mir_transform/src/liveness.rs

@@ -16,7 +16,7 @@ use rustc_mir_dataflow::fmt::DebugWithContext;
 use rustc_mir_dataflow::{Analysis, Backward, ResultsCursor};
 use rustc_session::lint;
 use rustc_span::Span;
-use rustc_span::symbol::{Symbol, kw};
+use rustc_span::symbol::{Symbol, kw, sym};
 
 use crate::errors;
 
@@ -497,15 +497,32 @@ impl<'tcx> PlaceSet<'tcx> {
     }
 
     fn record_debuginfo(&mut self, var_debug_info: &Vec<VarDebugInfo<'tcx>>) {
+        let ignore_name = |name: Symbol| {
+            name == sym::empty || name == kw::SelfLower || name.as_str().starts_with('_')
+        };
         for var_debug_info in var_debug_info {
             if let VarDebugInfoContents::Place(place) = var_debug_info.value
                 && let Some(index) = self.locals[place.local]
+                && !ignore_name(var_debug_info.name)
             {
                 self.names.get_or_insert_with(index, || {
                     (var_debug_info.name, var_debug_info.source_info.span)
                 });
             }
         }
+
+        // Discard places that will not result in a diagnostic.
+        for index_opt in self.locals.iter_mut() {
+            if let Some(index) = *index_opt {
+                let remove = match self.names[index] {
+                    None => true,
+                    Some((name, _)) => ignore_name(name),
+                };
+                if remove {
+                    *index_opt = None;
+                }
+            }
+        }
     }
 
     fn get(&self, place: PlaceRef<'tcx>) -> Option<(PlaceIndex, &'tcx [PlaceElem<'tcx>])> {
@@ -794,11 +811,6 @@ impl AssignmentResult {
                 continue;
             }
 
-            let Some((name, def_span)) = checked_places.names[index] else { continue };
-            if name.is_empty() || name.as_str().starts_with('_') || name == kw::SelfLower {
-                continue;
-            }
-
             let local = place.local;
             let decl = &body.local_decls[local];
 
@@ -814,6 +826,8 @@ impl AssignmentResult {
 
             let introductions = &binding.introductions;
 
+            let Some((name, def_span)) = checked_places.names[index] else { continue };
+
             // #117284, when `ident_span` and `def_span` have different contexts
             // we can't provide a good suggestion, instead we pointed out the spans from macro
             let from_macro = def_span.from_expansion()
@@ -933,9 +947,6 @@ impl AssignmentResult {
             }
 
             let Some((name, decl_span)) = checked_places.names[index] else { continue };
-            if name.is_empty() || name.as_str().starts_with('_') || name == kw::SelfLower {
-                continue;
-            }
 
             // We have outstanding assignments and with non-trivial drop.
             // This is probably a drop-guard, so we do not issue a warning there.