Rollup merge of rust-lang#128762 - fmease:use-more-slice-pats, r=compiler-errors

Use more slice patterns inside the compiler

Nothing super noteworthy. Just replacing the common 'fragile' pattern of "length check followed by indexing or unwrap" with slice patterns for legibility and 'robustness'.

r? ghost

Author: matthiaskrgr

compiler/rustc_ast/src/ast.rs

@@ -585,7 +585,9 @@ impl Pat {
             }
             // A slice/array pattern `[P]` can be reparsed as `[T]`, an unsized array,
             // when `P` can be reparsed as a type `T`.
-            PatKind::Slice(pats) if pats.len() == 1 => pats[0].to_ty().map(TyKind::Slice)?,
+            PatKind::Slice(pats) if let [pat] = pats.as_slice() => {
+                pat.to_ty().map(TyKind::Slice)?
+            }
             // A tuple pattern `(P0, .., Pn)` can be reparsed as `(T0, .., Tn)`
             // assuming `T0` to `Tn` are all syntactically valid as types.
             PatKind::Tuple(pats) => {
@@ -1187,8 +1189,8 @@ impl Expr {
     /// 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 {
         let this = if let ExprKind::Block(block, None) = &self.kind
-            && block.stmts.len() == 1
-            && let StmtKind::Expr(expr) = &block.stmts[0].kind
+            && let [stmt] = block.stmts.as_slice()
+            && let StmtKind::Expr(expr) = &stmt.kind
         {
             expr
         } else {
@@ -1248,7 +1250,9 @@ impl Expr {
                 expr.to_ty().map(|ty| TyKind::Array(ty, expr_len.clone()))?
             }
 
-            ExprKind::Array(exprs) if exprs.len() == 1 => exprs[0].to_ty().map(TyKind::Slice)?,
+            ExprKind::Array(exprs) if let [expr] = exprs.as_slice() => {
+                expr.to_ty().map(TyKind::Slice)?
+            }
 
             ExprKind::Tup(exprs) => {
                 let tys = exprs.iter().map(|expr| expr.to_ty()).collect::<Option<ThinVec<_>>>()?;

compiler/rustc_ast_lowering/src/delegation.rs

@@ -275,8 +275,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
     // FIXME(fn_delegation): Alternatives for target expression lowering:
     // https://github.com/rust-lang/rfcs/pull/3530#issuecomment-2197170600.
     fn lower_target_expr(&mut self, block: &Block) -> hir::Expr<'hir> {
-        if block.stmts.len() == 1
-            && let StmtKind::Expr(expr) = &block.stmts[0].kind
+        if let [stmt] = block.stmts.as_slice()
+            && let StmtKind::Expr(expr) = &stmt.kind
         {
             return self.lower_expr_mut(expr);
         }

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -502,8 +502,8 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
                 if !self.is_beginning_of_line() {
                     self.word(" ");
                 }
-                if cmnt.lines.len() == 1 {
-                    self.word(cmnt.lines[0].clone());
+                if let [line] = cmnt.lines.as_slice() {
+                    self.word(line.clone());
                     self.hardbreak()
                 } else {
                     self.visual_align();

compiler/rustc_ast_pretty/src/pprust/state/item.rs

@@ -783,8 +783,8 @@ impl<'a> State<'a> {
                 }
                 if items.is_empty() {
                     self.word("{}");
-                } else if items.len() == 1 {
-                    self.print_use_tree(&items[0].0);
+                } else if let [(item, _)] = items.as_slice() {
+                    self.print_use_tree(item);
                 } else {
                     self.cbox(INDENT_UNIT);
                     self.word("{");

compiler/rustc_attr/src/builtin.rs

@@ -665,12 +665,12 @@ pub fn eval_condition(
                         res & eval_condition(mi.meta_item().unwrap(), sess, features, eval)
                     }),
                 sym::not => {
-                    if mis.len() != 1 {
+                    let [mi] = mis.as_slice() else {
                         dcx.emit_err(session_diagnostics::ExpectedOneCfgPattern { span: cfg.span });
                         return false;
-                    }
+                    };
 
-                    !eval_condition(mis[0].meta_item().unwrap(), sess, features, eval)
+                    !eval_condition(mi.meta_item().unwrap(), sess, features, eval)
                 }
                 sym::target => {
                     if let Some(features) = features
@@ -1051,10 +1051,10 @@ pub fn parse_repr_attr(sess: &Session, attr: &Attribute) -> Vec<ReprAttr> {
                     MetaItemKind::List(nested_items) => {
                         if meta_item.has_name(sym::align) {
                             recognised = true;
-                            if nested_items.len() == 1 {
+                            if let [nested_item] = nested_items.as_slice() {
                                 sess.dcx().emit_err(
                                     session_diagnostics::IncorrectReprFormatExpectInteger {
-                                        span: nested_items[0].span(),
+                                        span: nested_item.span(),
                                     },
                                 );
                             } else {
@@ -1066,10 +1066,10 @@ pub fn parse_repr_attr(sess: &Session, attr: &Attribute) -> Vec<ReprAttr> {
                             }
                         } else if meta_item.has_name(sym::packed) {
                             recognised = true;
-                            if nested_items.len() == 1 {
+                            if let [nested_item] = nested_items.as_slice() {
                                 sess.dcx().emit_err(
                                     session_diagnostics::IncorrectReprFormatPackedExpectInteger {
-                                        span: nested_items[0].span(),
+                                        span: nested_item.span(),
                                     },
                                 );
                             } else {

compiler/rustc_borrowck/src/diagnostics/outlives_suggestion.rs

@@ -206,8 +206,8 @@ impl OutlivesSuggestionBuilder {
 
         // If there is exactly one suggestable constraints, then just suggest it. Otherwise, emit a
         // list of diagnostics.
-        let mut diag = if suggested.len() == 1 {
-            mbcx.dcx().struct_help(match suggested.last().unwrap() {
+        let mut diag = if let [constraint] = suggested.as_slice() {
+            mbcx.dcx().struct_help(match constraint {
                 SuggestedConstraint::Outlives(a, bs) => {
                     let bs: SmallVec<[String; 2]> = bs.iter().map(|r| r.to_string()).collect();
                     format!("add bound `{a}: {}`", bs.join(" + "))

compiler/rustc_builtin_macros/src/asm.rs

@@ -745,10 +745,9 @@ fn expand_preparsed_asm(
             unused_operands.push((args.operands[idx].1, msg));
         }
     }
-    match unused_operands.len() {
-        0 => {}
-        1 => {
-            let (sp, msg) = unused_operands.into_iter().next().unwrap();
+    match unused_operands[..] {
+        [] => {}
+        [(sp, msg)] => {
             ecx.dcx()
                 .struct_span_err(sp, msg)
                 .with_span_label(sp, msg)

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -378,8 +378,8 @@ impl BlockOrExpr {
                 None => cx.expr_block(cx.block(span, ThinVec::new())),
                 Some(expr) => expr,
             }
-        } else if self.0.len() == 1
-            && let ast::StmtKind::Expr(expr) = &self.0[0].kind
+        } else if let [stmt] = self.0.as_slice()
+            && let ast::StmtKind::Expr(expr) = &stmt.kind
             && self.1.is_none()
         {
             // There's only a single statement expression. Pull it out.
@@ -1273,15 +1273,15 @@ impl<'a> MethodDef<'a> {
                     }
                     FieldlessVariantsStrategy::Default => (),
                 }
-            } else if variants.len() == 1 {
+            } else if let [variant] = variants.as_slice() {
                 // If there is a single variant, we don't need an operation on
                 // the discriminant(s). Just use the most degenerate result.
                 return self.call_substructure_method(
                     cx,
                     trait_,
                     type_ident,
                     nonselflike_args,
-                    &EnumMatching(0, &variants[0], Vec::new()),
+                    &EnumMatching(0, variant, Vec::new()),
                 );
             }
         }

compiler/rustc_builtin_macros/src/format.rs

@@ -180,8 +180,8 @@ fn make_format_args(
                     Ok((mut err, suggested)) => {
                         if !suggested {
                             if let ExprKind::Block(block, None) = &efmt.kind
-                                && block.stmts.len() == 1
-                                && let StmtKind::Expr(expr) = &block.stmts[0].kind
+                                && let [stmt] = block.stmts.as_slice()
+                                && let StmtKind::Expr(expr) = &stmt.kind
                                 && let ExprKind::Path(None, path) = &expr.kind
                                 && path.is_potential_trivial_const_arg()
                             {
@@ -196,8 +196,8 @@ fn make_format_args(
                             } else {
                                 let sugg_fmt = match args.explicit_args().len() {
                                     0 => "{}".to_string(),
-                                    _ => {
-                                        format!("{}{{}}", "{} ".repeat(args.explicit_args().len()))
+                                    count => {
+                                        format!("{}{{}}", "{} ".repeat(count))
                                     }
                                 };
                                 err.span_suggestion(

compiler/rustc_data_structures/src/transitive_relation.rs

@@ -203,9 +203,9 @@ impl<T: Eq + Hash + Copy> TransitiveRelation<T> {
     /// exists). See `postdom_upper_bound` for details.
     pub fn mutual_immediate_postdominator(&self, mut mubs: Vec<T>) -> Option<T> {
         loop {
-            match mubs.len() {
-                0 => return None,
-                1 => return Some(mubs[0]),
+            match mubs[..] {
+                [] => return None,
+                [mub] => return Some(mub),
                 _ => {
                     let m = mubs.pop().unwrap();
                     let n = mubs.pop().unwrap();