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 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/default.rs

@@ -181,20 +181,20 @@ fn validate_default_attribute(
         attr::filter_by_name(&default_variant.attrs, kw::Default).collect();
 
     let attr = match attrs.as_slice() {
-        [attr] => attr,
         [] => cx.dcx().bug(
             "this method must only be called with a variant that has a `#[default]` attribute",
         ),
-        [first, rest @ ..] => {
+        [attr] => attr,
+        [first, second, rest @ ..] => {
             let sugg = errors::MultipleDefaultAttrsSugg {
                 spans: rest.iter().map(|attr| attr.span).collect(),
             };
             let guar = cx.dcx().emit_err(errors::MultipleDefaultAttrs {
                 span: default_variant.ident.span,
                 first: first.span,
-                first_rest: rest[0].span,
+                first_rest: second.span,
                 rest: rest.iter().map(|attr| attr.span).collect::<Vec<_>>().into(),
-                only_one: rest.len() == 1,
+                only_one: rest.is_empty(),
                 sugg,
             });
 

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();

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() {
-            0 => false, // no input: we will exit early
-            1 => panic!("make_input should have provided valid inputs"),
-            _ => default_early_dcx.early_fatal(format!(
-                "multiple input filenames provided (first two filenames are `{}` and `{}`)",
-                matches.free[0], matches.free[1],
+        Ok(None) => match matches.free.as_slice() {
+            [] => false, // no input: we will exit early
+            [_] => panic!("make_input should have provided valid inputs"),
+            [fst, snd, ..] => default_early_dcx.early_fatal(format!(
+                "multiple input filenames provided (first two filenames are `{fst}` and `{snd}`)"
             )),
         },
     };
@@ -488,34 +487,30 @@ fn make_input(
     early_dcx: &EarlyDiagCtxt,
     free_matches: &[String],
 ) -> Result<Option<Input>, ErrorGuaranteed> {
-    if free_matches.len() == 1 {
-        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
-                    .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") {
-                let line = env::var("UNSTABLE_RUSTDOC_TEST_LINE").expect(
-                    "when UNSTABLE_RUSTDOC_TEST_PATH is set \
+    let [ifile] = free_matches else { return Ok(None) };
+    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.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") {
+            let line = env::var("UNSTABLE_RUSTDOC_TEST_LINE").expect(
+                "when UNSTABLE_RUSTDOC_TEST_PATH is set \
                                     UNSTABLE_RUSTDOC_TEST_LINE also needs to be set",
-                );
-                let line = isize::from_str_radix(&line, 10)
-                    .expect("UNSTABLE_RUSTDOC_TEST_LINE needs to be an number");
-                let file_name = FileName::doc_test_source_code(PathBuf::from(path), line);
-                Ok(Some(Input::Str { name: file_name, input: src }))
-            } else {
-                Ok(Some(Input::Str { name: FileName::anon_source_code(&src), input: src }))
-            }
+            );
+            let line = isize::from_str_radix(&line, 10)
+                .expect("UNSTABLE_RUSTDOC_TEST_LINE needs to be an number");
+            let file_name = FileName::doc_test_source_code(PathBuf::from(path), line);
+            Ok(Some(Input::Str { name: file_name, input: src }))
         } else {
-            Ok(Some(Input::File(PathBuf::from(ifile))))
+            Ok(Some(Input::Str { name: FileName::anon_source_code(&src), input: src }))
         }
     } else {
-        Ok(None)
+        Ok(Some(Input::File(PathBuf::from(ifile))))
     }
 }
 

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 msg = if substitution.is_empty() || sugg.style.hide_inline() {
+                let snippet = part.snippet.trim();
+                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(
-                            sm,
-                            substitution,
-                            sugg.substitutions[0].parts[0].span,
-                        )) {
+                        if self
+                            .source_map()
+                            .is_some_and(|sm| is_case_difference(sm, snippet, part.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() {
-        0 => "".to_string(),
-        1 => v[0].to_string(),
-        2 => format!("{} and {}", v[0], v[1]),
-        _ => format!("{}, {}", v[0], display_list_with_comma_and(&v[1..])),
+    match v {
+        [] => "".to_string(),
+        [a] => a.to_string(),
+        [a, b] => format!("{a} and {b}"),
+        [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 {
-        dcx.emit_err(errors::NotAMetaItem { span: list[0].span() });
+    };
+    let Some(trait_attr) = trait_attr.meta_item() else {
+        dcx.emit_err(errors::NotAMetaItem { span: trait_attr.span() });
         return None;
     };
     let trait_ident = match trait_attr.ident() {