rustc_ast: replace some len-checks + indexing with slice patterns etc.

compiler/rustc_ast/src/ast.rs

@@ -100,7 +100,7 @@ pub struct Path {
 impl PartialEq<Symbol> for Path {
     #[inline]
     fn eq(&self, symbol: &Symbol) -> bool {
-        self.segments.len() == 1 && { self.segments[0].ident.name == *symbol }
+        matches!(&self.segments[..], [segment] if segment.ident.name == *symbol)
     }
 }
 
@@ -121,13 +121,13 @@ impl Path {
     }
 
     pub fn is_global(&self) -> bool {
-        !self.segments.is_empty() && self.segments[0].ident.name == kw::PathRoot
+        self.segments.first().is_some_and(|segment| segment.ident.name == kw::PathRoot)
     }
 
     /// If this path is a single identifier with no arguments, does not ensure
     /// that the path resolves to a const param, the caller should check this.
     pub fn is_potential_trivial_const_arg(&self) -> bool {
-        self.segments.len() == 1 && self.segments[0].args.is_none()
+        matches!(self.segments[..], [PathSegment { args: None, .. }])
     }
 }
 

compiler/rustc_ast/src/attr/mod.rs

@@ -302,7 +302,7 @@ impl AttrItem {
 impl MetaItem {
     /// For a single-segment meta item, returns its name; otherwise, returns `None`.
     pub fn ident(&self) -> Option<Ident> {
-        if self.path.segments.len() == 1 { Some(self.path.segments[0].ident) } else { None }
+        if let [PathSegment { ident, .. }] = self.path.segments[..] { Some(ident) } else { None }
     }
 
     pub fn name_or_empty(&self) -> Symbol {

compiler/rustc_ast/src/mut_visit.rs

@@ -1813,10 +1813,10 @@ pub fn walk_flat_map_stmt<T: MutVisitor>(
         .into_iter()
         .map(|kind| Stmt { id, kind, span })
         .collect();
-    match stmts.len() {
-        0 => {}
-        1 => vis.visit_span(&mut stmts[0].span),
-        2.. => panic!(
+    match &mut stmts[..] {
+        [] => {}
+        [stmt] => vis.visit_span(&mut stmt.span),
+        _ => panic!(
             "cloning statement `NodeId`s is prohibited by default, \
              the visitor should implement custom statement visiting"
         ),

compiler/rustc_ast/src/util/comments.rs

@@ -39,7 +39,7 @@ pub fn beautify_doc_string(data: Symbol, kind: CommentKind) -> Symbol {
         let mut i = 0;
         let mut j = lines.len();
         // first line of all-stars should be omitted
-        if !lines.is_empty() && lines[0].chars().all(|c| c == '*') {
+        if lines.first().is_some_and(|line| line.chars().all(|c| c == '*')) {
             i += 1;
         }
 
@@ -97,7 +97,7 @@ pub fn beautify_doc_string(data: Symbol, kind: CommentKind) -> Symbol {
                 return None;
             }
         }
-        if lines.is_empty() { None } else { Some(lines[0][..i].into()) }
+        Some(lines.first()?[..i].to_string())
     }
 
     let data_s = data.as_str();