Use more slice patterns inside the compiler

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 [stmt] = block.stmts.as_slice()
-            && let StmtKind::Expr(expr) = &block.stmts[0].kind
+            && 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
+        if let [stmt] = block.stmts.as_slice()
-            && let StmtKind::Expr(expr) = &block.stmts[0].kind
+            && 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 {
+                if let [line] = cmnt.lines.as_slice() {
-                    self.word(cmnt.lines[0].clone());
+                    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 {
+                } else if let [(item, _)] = items.as_slice() {
-                    self.print_use_tree(&items[0].0);
+                    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 {
+        let mut diag = if let [constraint] = suggested.as_slice() {
-            mbcx.dcx().struct_help(match suggested.last().unwrap() {
+            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() {
+    match unused_operands[..] {
-        0 => {}
+        [] => {}
-        1 => {
+        [(sp, msg)] => {
-            let (sp, msg) = unused_operands.into_iter().next().unwrap();
             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
+        } else if let [stmt] = self.0.as_slice()
-            && let ast::StmtKind::Expr(expr) = &self.0[0].kind
+            && let ast::StmtKind::Expr(expr) = &stmt.kind
             && self.1.is_none()
         {
             // There's only a single statement expression. Pull it out.
@@ -1273,7 +1273,7 @@ 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(
@@ -1281,7 +1281,7 @@ impl<'a> MethodDef<'a> {
                     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 [stmt] = block.stmts.as_slice()
-                                && let StmtKind::Expr(expr) = &block.stmts[0].kind
+                                && 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(),
-                                    _ => {
+                                    count => {
-                                        format!("{}{{}}", "{} ".repeat(args.explicit_args().len()))
+                                        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() {
+            match mubs[..] {
-                0 => return None,
+                [] => return None,
-                1 => return Some(mubs[0]),
+                [mub] => return Some(mub),
                 _ => {
                     let m = mubs.pop().unwrap();
                     let n = mubs.pop().unwrap();

compiler/rustc_driver_impl/src/lib.rs

@@ -335,12 +335,11 @@ fn run_compiler(
             config.input = input;
             true // has input: normal compilation
         }
-        Ok(None) => match matches.free.len() {
+        Ok(None) => match matches.free.as_slice() {
-            0 => false, // no input: we will exit early
+            [] => false, // no input: we will exit early
-            1 => panic!("make_input should have provided valid inputs"),
+            [_] => panic!("make_input should have provided valid inputs"),
-            _ => default_early_dcx.early_fatal(format!(
+            [fst, snd, ..] => default_early_dcx.early_fatal(format!(
-                "multiple input filenames provided (first two filenames are `{}` and `{}`)",
+                "multiple input filenames provided (first two filenames are `{fst}` and `{snd}`)"
-                matches.free[0], matches.free[1],
             )),
         },
     };
@@ -488,15 +487,14 @@ fn make_input(
     early_dcx: &EarlyDiagCtxt,
     free_matches: &[String],
 ) -> Result<Option<Input>, ErrorGuaranteed> {
-    if free_matches.len() == 1 {
+    let [ifile] = free_matches else { return Ok(None) };
-        let ifile = &free_matches[0];
     if ifile == "-" {
         let mut src = String::new();
         if io::stdin().read_to_string(&mut src).is_err() {
             // Immediately stop compilation if there was an issue reading
             // the input (for example if the input stream is not UTF-8).
-                let reported = early_dcx
+            let reported =
-                    .early_err("couldn't read from stdin, as it did not contain valid UTF-8");
+                early_dcx.early_err("couldn't read from stdin, as it did not contain valid UTF-8");
             return Err(reported);
         }
         if let Ok(path) = env::var("UNSTABLE_RUSTDOC_TEST_PATH") {
@@ -514,9 +512,6 @@ fn make_input(
     } else {
         Ok(Some(Input::File(PathBuf::from(ifile))))
     }
-    } else {
-        Ok(None)
-    }
 }
 
 /// Whether to stop or continue compilation.

compiler/rustc_errors/src/emitter.rs

@@ -231,17 +231,17 @@ pub trait Emitter: Translate {
     ) {
         if let Some((sugg, rest)) = suggestions.split_first() {
             let msg = self.translate_message(&sugg.msg, fluent_args).map_err(Report::new).unwrap();
-            if rest.is_empty() &&
+            if rest.is_empty()
                // ^ if there is only one suggestion
                // don't display multi-suggestions as labels
-               sugg.substitutions.len() == 1 &&
+               && let [substitution] = sugg.substitutions.as_slice()
                // don't display multipart suggestions as labels
-               sugg.substitutions[0].parts.len() == 1 &&
+               && let [part] = substitution.parts.as_slice()
                // don't display long messages as labels
-               msg.split_whitespace().count() < 10 &&
+               && msg.split_whitespace().count() < 10
                // don't display multiline suggestions as labels
-               !sugg.substitutions[0].parts[0].snippet.contains('\n') &&
+               && !part.snippet.contains('\n')
-               ![
+               && ![
                     // when this style is set we want the suggestion to be a message, not inline
                     SuggestionStyle::HideCodeAlways,
                     // trivial suggestion for tooling's sake, never shown
@@ -250,8 +250,8 @@ pub trait Emitter: Translate {
                     SuggestionStyle::ShowAlways,
                ].contains(&sugg.style)
             {
-                let substitution = &sugg.substitutions[0].parts[0].snippet.trim();
+                let snippet = part.snippet.trim();
-                let msg = if substitution.is_empty() || sugg.style.hide_inline() {
+                let msg = if snippet.is_empty() || sugg.style.hide_inline() {
                     // This substitution is only removal OR we explicitly don't want to show the
                     // code inline (`hide_inline`). Therefore, we don't show the substitution.
                     format!("help: {msg}")
@@ -260,19 +260,18 @@ pub trait Emitter: Translate {
                     format!(
                         "help: {}{}: `{}`",
                         msg,
-                        if self.source_map().is_some_and(|sm| is_case_difference(
+                        if self
-                            sm,
+                            .source_map()
-                            substitution,
+                            .is_some_and(|sm| is_case_difference(sm, snippet, part.span,))
-                            sugg.substitutions[0].parts[0].span,
+                        {
-                        )) {
                             " (notice the capitalization)"
                         } else {
                             ""
                         },
-                        substitution,
+                        snippet,
                     )
                 };
-                primary_span.push_span_label(sugg.substitutions[0].parts[0].span, msg);
+                primary_span.push_span_label(part.span, msg);
 
                 // We return only the modified primary_span
                 suggestions.clear();

compiler/rustc_errors/src/lib.rs

@@ -2024,11 +2024,11 @@ pub fn a_or_an(s: &str) -> &'static str {
 ///
 /// Take a list ["a", "b", "c"] and output a display friendly version "a, b and c"
 pub fn display_list_with_comma_and<T: std::fmt::Display>(v: &[T]) -> String {
-    match v.len() {
+    match v {
-        0 => "".to_string(),
+        [] => "".to_string(),
-        1 => v[0].to_string(),
+        [a] => a.to_string(),
-        2 => format!("{} and {}", v[0], v[1]),
+        [a, b] => format!("{a} and {b}"),
-        _ => format!("{}, {}", v[0], display_list_with_comma_and(&v[1..])),
+        [a, v @ ..] => format!("{a}, {}", display_list_with_comma_and(v)),
     }
 }
 

compiler/rustc_expand/src/base.rs

@@ -1306,12 +1306,12 @@ pub fn parse_macro_name_and_helper_attrs(
     // that it's of the form `#[proc_macro_derive(Foo)]` or
     // `#[proc_macro_derive(Foo, attributes(A, ..))]`
     let list = attr.meta_item_list()?;
-    if list.len() != 1 && list.len() != 2 {
+    let ([trait_attr] | [trait_attr, _]) = list.as_slice() else {
         dcx.emit_err(errors::AttrNoArguments { span: attr.span });
         return None;
-    }
+    };
-    let Some(trait_attr) = list[0].meta_item() else {
+    let Some(trait_attr) = trait_attr.meta_item() else {
-        dcx.emit_err(errors::NotAMetaItem { span: list[0].span() });
+        dcx.emit_err(errors::NotAMetaItem { span: trait_attr.span() });
         return None;
     };
     let trait_ident = match trait_attr.ident() {

compiler/rustc_hir_typeck/src/expr.rs

@@ -2734,15 +2734,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         } else if let ty::RawPtr(ptr_ty, _) = expr_t.kind()
             && let ty::Adt(adt_def, _) = ptr_ty.kind()
             && let ExprKind::Field(base_expr, _) = expr.kind
-            && adt_def.variants().len() == 1
+            && let [variant] = &adt_def.variants().raw
-            && adt_def
+            && variant.fields.iter().any(|f| f.ident(self.tcx) == field)
-                .variants()
-                .iter()
-                .next()
-                .unwrap()
-                .fields
-                .iter()
-                .any(|f| f.ident(self.tcx) == field)
         {
             err.multipart_suggestion(
                 "to access the field, dereference first",

compiler/rustc_hir_typeck/src/method/confirm.rs

@@ -654,17 +654,17 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
             traits::upcast_choices(self.tcx, source_trait_ref, target_trait_def_id);
 
         // must be exactly one trait ref or we'd get an ambig error etc
-        if upcast_trait_refs.len() != 1 {
+        let [upcast_trait_ref] = upcast_trait_refs.as_slice() else {
             span_bug!(
                 self.span,
                 "cannot uniquely upcast `{:?}` to `{:?}`: `{:?}`",
                 source_trait_ref,
                 target_trait_def_id,
                 upcast_trait_refs
-            );
+            )
-        }
+        };
 
-        upcast_trait_refs.into_iter().next().unwrap()
+        *upcast_trait_ref
     }
 
     fn instantiate_binder_with_fresh_vars<T>(&self, value: ty::Binder<'tcx, T>) -> T

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -321,19 +321,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                     );
                                 };
                             let suggest_for_privacy =
-                                |err: &mut Diag<'_>, mut msg: String, sugg: Vec<String>| {
+                                |err: &mut Diag<'_>, mut msg: String, suggs: Vec<String>| {
-                                    if sugg.len() == 1 {
+                                    if let [sugg] = suggs.as_slice() {
-                                        let msg = format!("\
+                                        err.help(format!("\
                                             trait `{}` provides `{item_name}` is implemented but not reachable",
-                                            sugg[0].trim()
+                                            sugg.trim(),
-                                        );
+                                        ));
-                                        err.help(msg);
                                     } else {
-                                        msg += &format!(" but {} not reachable", pluralize!("is", sugg.len()));
+                                        msg += &format!(" but {} not reachable", pluralize!("is", suggs.len()));
                                         err.span_suggestions(
                                             span,
                                             msg,
-                                            sugg,
+                                            suggs,
                                             Applicability::MaybeIncorrect,
                                         );
                                     }
@@ -2988,11 +2987,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         }
         if local_spans.primary_span().is_some() {
-            let msg = if local_preds.len() == 1 {
+            let msg = if let [local_pred] = local_preds.as_slice() {
                 format!(
                     "an implementation of `{}` might be missing for `{}`",
-                    local_preds[0].trait_ref.print_trait_sugared(),
+                    local_pred.trait_ref.print_trait_sugared(),
-                    local_preds[0].self_ty()
+                    local_pred.self_ty()
                 )
             } else {
                 format!(
@@ -3034,11 +3033,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         }
         if foreign_spans.primary_span().is_some() {
-            let msg = if foreign_preds.len() == 1 {
+            let msg = if let [foreign_pred] = foreign_preds.as_slice() {
                 format!(
                     "the foreign item type `{}` doesn't implement `{}`",
-                    foreign_preds[0].self_ty(),
+                    foreign_pred.self_ty(),
-                    foreign_preds[0].trait_ref.print_trait_sugared()
+                    foreign_pred.trait_ref.print_trait_sugared()
                 )
             } else {
                 format!(
@@ -3388,26 +3387,26 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             );
 
             self.suggest_use_candidates(candidates, |accessible_sugg, inaccessible_sugg, span| {
-                let suggest_for_access = |err: &mut Diag<'_>, mut msg: String, sugg: Vec<_>| {
+                let suggest_for_access = |err: &mut Diag<'_>, mut msg: String, suggs: Vec<_>| {
                     msg += &format!(
                         "; perhaps you want to import {one_of}",
-                        one_of = if sugg.len() == 1 { "it" } else { "one of them" },
+                        one_of = if suggs.len() == 1 { "it" } else { "one of them" },
                     );
-                    err.span_suggestions(span, msg, sugg, Applicability::MaybeIncorrect);
+                    err.span_suggestions(span, msg, suggs, Applicability::MaybeIncorrect);
                 };
-                let suggest_for_privacy = |err: &mut Diag<'_>, sugg: Vec<String>| {
+                let suggest_for_privacy = |err: &mut Diag<'_>, suggs: Vec<String>| {
                     let msg = format!(
                         "{this_trait_is} implemented but not reachable",
-                        this_trait_is = if sugg.len() == 1 {
+                        this_trait_is = if let [sugg] = suggs.as_slice() {
-                            format!("trait `{}` which provides `{item_name}` is", sugg[0].trim())
+                            format!("trait `{}` which provides `{item_name}` is", sugg.trim())
                         } else {
                             format!("the following traits which provide `{item_name}` are")
                         }
                     );
-                    if sugg.len() == 1 {
+                    if suggs.len() == 1 {
                         err.help(msg);
                     } else {
-                        err.span_suggestions(span, msg, sugg, Applicability::MaybeIncorrect);
+                        err.span_suggestions(span, msg, suggs, Applicability::MaybeIncorrect);
                     }
                 };
                 if accessible_sugg.is_empty() {

compiler/rustc_lint/src/nonstandard_style.rs

@@ -527,11 +527,11 @@ impl<'tcx> LateLintPass<'tcx> for NonUpperCaseGlobals {
         // Lint for constants that look like binding identifiers (#7526)
         if let PatKind::Path(hir::QPath::Resolved(None, path)) = p.kind {
             if let Res::Def(DefKind::Const, _) = path.res {
-                if path.segments.len() == 1 {
+                if let [segment] = path.segments {
                     NonUpperCaseGlobals::check_upper_case(
                         cx,
                         "constant in pattern",
-                        &path.segments[0].ident,
+                        &segment.ident,
                     );
                 }
             }

compiler/rustc_lint/src/types.rs

@@ -211,15 +211,12 @@ fn lint_overflowing_range_endpoint<'tcx>(
     if !is_range_literal(struct_expr) {
         return false;
     };
-    let ExprKind::Struct(_, eps, _) = &struct_expr.kind else { return false };
+    let ExprKind::Struct(_, [start, end], _) = &struct_expr.kind else { return false };
-    if eps.len() != 2 {
-        return false;
-    }
 
     // We can suggest using an inclusive range
     // (`..=`) instead only if it is the `end` that is
     // overflowing and only by 1.
-    if !(eps[1].expr.hir_id == expr.hir_id && lit_val - 1 == max) {
+    if !(end.expr.hir_id == expr.hir_id && lit_val - 1 == max) {
         return false;
     };
 
@@ -232,7 +229,7 @@ fn lint_overflowing_range_endpoint<'tcx>(
     };
 
     let sub_sugg = if expr.span.lo() == lit_span.lo() {
-        let Ok(start) = cx.sess().source_map().span_to_snippet(eps[0].span) else { return false };
+        let Ok(start) = cx.sess().source_map().span_to_snippet(start.span) else { return false };
         UseInclusiveRange::WithoutParen {
             sugg: struct_expr.span.shrink_to_lo().to(lit_span.shrink_to_hi()),
             start,

compiler/rustc_lint/src/unused.rs

@@ -801,7 +801,7 @@ trait UnusedDelimLint {
             return;
         }
         let spans = match value.kind {
-            ast::ExprKind::Block(ref block, None) if block.stmts.len() == 1 => block.stmts[0]
+            ast::ExprKind::Block(ref block, None) if let [stmt] = block.stmts.as_slice() => stmt
                 .span
                 .find_ancestor_inside(value.span)
                 .map(|span| (value.span.with_hi(span.lo()), value.span.with_lo(span.hi()))),
@@ -1537,14 +1537,12 @@ impl UnusedImportBraces {
             }
 
             // Trigger the lint only if there is one nested item
-            if items.len() != 1 {
+            let [(tree, _)] = items.as_slice() else { return };
-                return;
-            }
 
             // Trigger the lint if the nested item is a non-self single item
-            let node_name = match items[0].0.kind {
+            let node_name = match tree.kind {
                 ast::UseTreeKind::Simple(rename) => {
-                    let orig_ident = items[0].0.prefix.segments.last().unwrap().ident;
+                    let orig_ident = tree.prefix.segments.last().unwrap().ident;
                     if orig_ident.name == kw::SelfLower {
                         return;
                     }

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -483,9 +483,11 @@ impl<'p, 'tcx> MatchVisitor<'p, 'tcx> {
         // Check if the match is exhaustive.
         let witnesses = report.non_exhaustiveness_witnesses;
         if !witnesses.is_empty() {
-            if source == hir::MatchSource::ForLoopDesugar && arms.len() == 2 {
+            if source == hir::MatchSource::ForLoopDesugar
+                && let [_, snd_arm] = *arms
+            {
                 // the for loop pattern is not irrefutable
-                let pat = &self.thir[arms[1]].pattern;
+                let pat = &self.thir[snd_arm].pattern;
                 // `pat` should be `Some(<pat_field>)` from a desugared for loop.
                 debug_assert_eq!(pat.span.desugaring_kind(), Some(DesugaringKind::ForLoop));
                 let PatKind::Variant { ref subpatterns, .. } = pat.kind else { bug!() };

compiler/rustc_mir_transform/src/coverage/graph.rs

@@ -350,8 +350,8 @@ fn bcb_filtered_successors<'a, 'tcx>(terminator: &'a Terminator<'tcx>) -> Covera
         // An inline asm terminator can normally be chained, except when it diverges or uses asm
         // goto.
         InlineAsm { ref targets, .. } => {
-            if targets.len() == 1 {
+            if let [target] = targets[..] {
-                CoverageSuccessors::Chainable(targets[0])
+                CoverageSuccessors::Chainable(target)
             } else {
                 CoverageSuccessors::NotChainable(targets)
             }

compiler/rustc_mir_transform/src/early_otherwise_branch.rs

@@ -309,11 +309,11 @@ fn verify_candidate_branch<'tcx>(
 ) -> bool {
     // In order for the optimization to be correct, the branch must...
     // ...have exactly one statement
-    if branch.statements.len() != 1 {
+    let [statement] = branch.statements.as_slice() else {
         return false;
-    }
+    };
     // ...assign the discriminant of `place` in that statement
-    let StatementKind::Assign(boxed) = &branch.statements[0].kind else { return false };
+    let StatementKind::Assign(boxed) = &statement.kind else { return false };
     let (discr_place, Rvalue::Discriminant(from_place)) = &**boxed else { return false };
     if *from_place != place {
         return false;

compiler/rustc_mir_transform/src/instsimplify.rs

@@ -264,9 +264,7 @@ impl<'tcx> InstSimplifyContext<'tcx, '_> {
         };
 
         // It's definitely not a clone if there are multiple arguments
-        if args.len() != 1 {
+        let [arg] = &args[..] else { return };
-            return;
-        }
 
         let Some(destination_block) = *target else { return };
 
@@ -280,7 +278,7 @@ impl<'tcx> InstSimplifyContext<'tcx, '_> {
 
         // These types are easily available from locals, so check that before
         // doing DefId lookups to figure out what we're actually calling.
-        let arg_ty = args[0].node.ty(self.local_decls, self.tcx);
+        let arg_ty = arg.node.ty(self.local_decls, self.tcx);
 
         let ty::Ref(_region, inner_ty, Mutability::Not) = *arg_ty.kind() else { return };
 

compiler/rustc_mir_transform/src/validate.rs

@@ -898,8 +898,8 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                         self.param_env,
                         adt_def.non_enum_variant().fields[field].ty(self.tcx, args),
                     );
-                    if fields.len() == 1 {
+                    if let [field] = fields.raw.as_slice() {
-                        let src_ty = fields.raw[0].ty(self.body, self.tcx);
+                        let src_ty = field.ty(self.body, self.tcx);
                         if !self.mir_assign_valid_types(src_ty, dest_ty) {
                             self.fail(location, "union field has the wrong type");
                         }
@@ -967,11 +967,9 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                         self.fail(location, "RawPtr should be in runtime MIR only");
                     }
 
-                    if fields.len() != 2 {
+                    if let [data_ptr, metadata] = fields.raw.as_slice() {
-                        self.fail(location, "raw pointer aggregate must have 2 fields");
+                        let data_ptr_ty = data_ptr.ty(self.body, self.tcx);
-                    } else {
+                        let metadata_ty = metadata.ty(self.body, self.tcx);
-                        let data_ptr_ty = fields.raw[0].ty(self.body, self.tcx);
-                        let metadata_ty = fields.raw[1].ty(self.body, self.tcx);
                         if let ty::RawPtr(in_pointee, in_mut) = data_ptr_ty.kind() {
                             if *in_mut != mutability {
                                 self.fail(location, "input and output mutability must match");
@@ -998,6 +996,8 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                                 self.fail(location, "metadata for pointer-to-thin must be unit");
                             }
                         }
+                    } else {
+                        self.fail(location, "raw pointer aggregate must have 2 fields");
                     }
                 }
             },

compiler/rustc_parse/src/parser/expr.rs

@@ -694,12 +694,12 @@ impl<'a> Parser<'a> {
                         // `foo: `
                         ExprKind::Path(None, ast::Path { segments, .. }),
                         token::Ident(kw::For | kw::Loop | kw::While, IdentIsRaw::No),
-                    ) if segments.len() == 1 => {
+                    ) if let [segment] = segments.as_slice() => {
                         let snapshot = self.create_snapshot_for_diagnostic();
                         let label = Label {
                             ident: Ident::from_str_and_span(
-                                &format!("'{}", segments[0].ident),
+                                &format!("'{}", segment.ident),
-                                segments[0].ident.span,
+                                segment.ident.span,
                             ),
                         };
                         match self.parse_expr_labeled(label, false) {

compiler/rustc_parse/src/parser/item.rs

@@ -471,9 +471,8 @@ impl<'a> Parser<'a> {
             Err(mut err) => {
                 // Maybe the user misspelled `macro_rules` (issue #91227)
                 if self.token.is_ident()
-                    && path.segments.len() == 1
+                    && let [segment] = path.segments.as_slice()
-                    && edit_distance("macro_rules", &path.segments[0].ident.to_string(), 2)
+                    && edit_distance("macro_rules", &segment.ident.to_string(), 2).is_some()
-                        .is_some()
                 {
                     err.span_suggestion(
                         path.span,

compiler/rustc_parse/src/parser/path.rs

@@ -826,7 +826,8 @@ impl<'a> Parser<'a> {
             // We can only resolve single-segment paths at the moment, because multi-segment paths
             // require type-checking: see `visit_generic_arg` in `src/librustc_resolve/late.rs`.
             ast::ExprKind::Path(None, path)
-                if path.segments.len() == 1 && path.segments[0].args.is_none() =>
+                if let [segment] = path.segments.as_slice()
+                    && segment.args.is_none() =>
             {
                 true
             }

compiler/rustc_parse/src/parser/stmt.rs

@@ -672,7 +672,7 @@ impl<'a> Parser<'a> {
 
                             match &expr.kind {
                                 ExprKind::Path(None, ast::Path { segments, .. })
-                                    if segments.len() == 1 =>
+                                    if let [segment] = segments.as_slice() =>
                                 {
                                     if self.token == token::Colon
                                         && self.look_ahead(1, |token| {
@@ -689,8 +689,8 @@ impl<'a> Parser<'a> {
                                         let snapshot = self.create_snapshot_for_diagnostic();
                                         let label = Label {
                                             ident: Ident::from_str_and_span(
-                                                &format!("'{}", segments[0].ident),
+                                                &format!("'{}", segment.ident),
-                                                segments[0].ident.span,
+                                                segment.ident.span,
                                             ),
                                         };
                                         match self.parse_expr_labeled(label, false) {

compiler/rustc_passes/src/check_attr.rs

@@ -2207,8 +2207,8 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             attr.name_or_empty(),
             sym::allow | sym::warn | sym::deny | sym::forbid | sym::expect
         ) && let Some(meta) = attr.meta_item_list()
-            && meta.len() == 1
+            && let [meta] = meta.as_slice()
-            && let Some(item) = meta[0].meta_item()
+            && let Some(item) = meta.meta_item()
             && let MetaItemKind::NameValue(_) = &item.kind
             && item.path == sym::reason
         {

compiler/rustc_passes/src/debugger_visualizer.rs

@@ -19,9 +19,7 @@ impl DebuggerVisualizerCollector<'_> {
                 return;
             };
 
-            let hint = if hints.len() == 1 {
+            let [hint] = hints.as_slice() else {
-                &hints[0]
-            } else {
                 self.sess.dcx().emit_err(DebugVisualizerInvalid { span: attr.span });
                 return;
             };

compiler/rustc_query_system/src/dep_graph/debug.rs

@@ -46,15 +46,14 @@ pub struct EdgeFilter {
 
 impl EdgeFilter {
     pub fn new(test: &str) -> Result<EdgeFilter, Box<dyn Error>> {
-        let parts: Vec<_> = test.split("->").collect();
+        if let [source, target] = *test.split("->").collect::<Vec<_>>() {
-        if parts.len() != 2 {
-            Err(format!("expected a filter like `a&b -> c&d`, not `{test}`").into())
-        } else {
             Ok(EdgeFilter {
-                source: DepNodeFilter::new(parts[0]),
+                source: DepNodeFilter::new(source),
-                target: DepNodeFilter::new(parts[1]),
+                target: DepNodeFilter::new(target),
                 index_to_node: Lock::new(FxHashMap::default()),
             })
+        } else {
+            Err(format!("expected a filter like `a&b -> c&d`, not `{test}`").into())
         }
     }
 

compiler/rustc_resolve/src/late.rs

@@ -445,8 +445,8 @@ impl<'a> PathSource<'a> {
                 Some(ExprKind::Call(call_expr, _)) => match &call_expr.kind {
                     // the case of `::some_crate()`
                     ExprKind::Path(_, path)
-                        if path.segments.len() == 2
+                        if let [segment, _] = path.segments.as_slice()
-                            && path.segments[0].ident.name == kw::PathRoot =>
+                            && segment.ident.name == kw::PathRoot =>
                     {
                         "external crate"
                     }
@@ -2395,15 +2395,14 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
     }
 
     fn future_proof_import(&mut self, use_tree: &UseTree) {
-        let segments = &use_tree.prefix.segments;
+        if let [segment, rest @ ..] = use_tree.prefix.segments.as_slice() {
-        if !segments.is_empty() {
+            let ident = segment.ident;
-            let ident = segments[0].ident;
             if ident.is_path_segment_keyword() || ident.span.is_rust_2015() {
                 return;
             }
 
             let nss = match use_tree.kind {
-                UseTreeKind::Simple(..) if segments.len() == 1 => &[TypeNS, ValueNS][..],
+                UseTreeKind::Simple(..) if rest.is_empty() => &[TypeNS, ValueNS][..],
                 _ => &[TypeNS],
             };
             let report_error = |this: &Self, ns| {
@@ -3999,16 +3998,15 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
 
             if this.should_report_errs() {
                 if candidates.is_empty() {
-                    if path.len() == 2 && prefix_path.len() == 1 {
+                    if path.len() == 2
+                        && let [segment] = prefix_path
+                    {
                         // Delay to check whether methond name is an associated function or not
                         // ```
                         // let foo = Foo {};
                         // foo::bar(); // possibly suggest to foo.bar();
                         //```
-                        err.stash(
+                        err.stash(segment.ident.span, rustc_errors::StashKey::CallAssocMethod);
-                            prefix_path[0].ident.span,
-                            rustc_errors::StashKey::CallAssocMethod,
-                        );
                     } else {
                         // When there is no suggested imports, we can just emit the error
                         // and suggestions immediately. Note that we bypass the usually error

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -650,14 +650,14 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
         let typo_sugg = self
             .lookup_typo_candidate(path, following_seg, source.namespace(), is_expected)
             .to_opt_suggestion();
-        if path.len() == 1
+        if let [segment] = path
             && !matches!(source, PathSource::Delegation)
             && self.self_type_is_available()
         {
             if let Some(candidate) =
                 self.lookup_assoc_candidate(ident, ns, is_expected, source.is_call())
             {
-                let self_is_available = self.self_value_is_available(path[0].ident.span);
+                let self_is_available = self.self_value_is_available(segment.ident.span);
                 // Account for `Foo { field }` when suggesting `self.field` so we result on
                 // `Foo { field: self.field }`.
                 let pre = match source {
@@ -665,7 +665,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
                         if expr
                             .fields
                             .iter()
-                            .any(|f| f.ident == path[0].ident && f.is_shorthand) =>
+                            .any(|f| f.ident == segment.ident && f.is_shorthand) =>
                     {
                         format!("{path_str}: ")
                     }
@@ -1258,8 +1258,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
                             )
                         })
                         .collect();
-                if targets.len() == 1 {
+                if let [target] = targets.as_slice() {
-                    let target = targets[0];
                     return Some(TypoSuggestion::single_item_from_ident(target.0.ident, target.1));
                 }
             }
@@ -2104,8 +2103,8 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
         filter_fn: &impl Fn(Res) -> bool,
     ) -> TypoCandidate {
         let mut names = Vec::new();
-        if path.len() == 1 {
+        if let [segment] = path {
-            let mut ctxt = path.last().unwrap().ident.span.ctxt();
+            let mut ctxt = segment.ident.span.ctxt();
 
             // Search in lexical scope.
             // Walk backwards up the ribs in scope and collect candidates.

compiler/rustc_resolve/src/macros.rs

@@ -108,8 +108,8 @@ pub(crate) fn sub_namespace_match(
 // `format!("{}", path)`, because that tries to insert
 // line-breaks and is slow.
 fn fast_print_path(path: &ast::Path) -> Symbol {
-    if path.segments.len() == 1 {
+    if let [segment] = path.segments.as_slice() {
-        path.segments[0].ident.name
+        segment.ident.name
     } else {
         let mut path_str = String::with_capacity(64);
         for (i, segment) in path.segments.iter().enumerate() {
@@ -724,10 +724,10 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         // Possibly apply the macro helper hack
         if deleg_impl.is_none()
             && kind == Some(MacroKind::Bang)
-            && path.len() == 1
+            && let [segment] = path.as_slice()
-            && path[0].ident.span.ctxt().outer_expn_data().local_inner_macros
+            && segment.ident.span.ctxt().outer_expn_data().local_inner_macros
         {
-            let root = Ident::new(kw::DollarCrate, path[0].ident.span);
+            let root = Ident::new(kw::DollarCrate, segment.ident.span);
             path.insert(0, Segment::from_ident(root));
         }
 

compiler/rustc_session/src/options.rs

@@ -912,16 +912,9 @@ mod parse {
         match v {
             None => false,
             Some(s) => {
-                let parts = s.split('=').collect::<Vec<_>>();
+                let [crate_name, fuel] = *s.split('=').collect::<Vec<_>>() else { return false };
-                if parts.len() != 2 {
+                let Ok(fuel) = fuel.parse::<u64>() else { return false };
-                    return false;
+                *slot = Some((crate_name.to_string(), fuel));
-                }
-                let crate_name = parts[0].to_string();
-                let fuel = parts[1].parse::<u64>();
-                if fuel.is_err() {
-                    return false;
-                }
-                *slot = Some((crate_name, fuel.unwrap()));
                 true
             }
         }

compiler/rustc_target/src/spec/mod.rs

@@ -3149,11 +3149,10 @@ impl Target {
                     if let Some(a) = o.as_array() {
                         for o in a {
                             if let Some(s) = o.as_str() {
-                                let p = s.split('=').collect::<Vec<_>>();
+                                if let [k, v] = *s.split('=').collect::<Vec<_>>() {
-                                if p.len() == 2 {
+                                    base.$key_name
-                                    let k = p[0].to_string();
+                                        .to_mut()
-                                    let v = p[1].to_string();
+                                        .push((k.to_string().into(), v.to_string().into()))
-                                    base.$key_name.to_mut().push((k.into(), v.into()));
                                 }
                             }
                         }

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -943,8 +943,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 // The current method call returns `Result<_, ()>`
                 && self.can_eq(obligation.param_env, ty, found_ty)
                 // There's a single argument in the method call and it is a closure
-                && args.len() == 1
+                && let [arg] = args
-                && let Some(arg) = args.get(0)
                 && let hir::ExprKind::Closure(closure) = arg.kind
                 // The closure has a block for its body with no tail expression
                 && let body = self.tcx.hir().body(closure.body)

compiler/rustc_trait_selection/src/error_reporting/traits/on_unimplemented.rs

@@ -73,10 +73,10 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 }
             });
 
-            let impl_def_id_and_args = if self_match_impls.len() == 1 {
+            let impl_def_id_and_args = if let [impl_] = self_match_impls[..] {
-                self_match_impls[0]
+                impl_
-            } else if fuzzy_match_impls.len() == 1 {
+            } else if let [impl_] = fuzzy_match_impls[..] {
-                fuzzy_match_impls[0]
+                impl_
             } else {
                 return None;
             };

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -5300,7 +5300,8 @@ impl<'v> Visitor<'v> for FindTypeParam {
         match ty.kind {
             hir::TyKind::Ptr(_) | hir::TyKind::Ref(..) | hir::TyKind::TraitObject(..) => {}
             hir::TyKind::Path(hir::QPath::Resolved(None, path))
-                if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
+                if let [segment] = path.segments
+                    && segment.ident.name == self.param =>
             {
                 if !self.nested {
                     debug!(?ty, "FindTypeParam::visit_ty");