Auto merge of #79070 - jonas-schievink:rollup-wacn2b8, r=jonas-schievink

Rollup of 13 pull requests

Successful merges:

 - #77802 (Allow making `RUSTC_BOOTSTRAP` conditional on the crate name)
 - #79004 (Add `--color` support to bootstrap)
 - #79005 (cleanup: Remove `ParseSess::injected_crate_name`)
 - #79016 (Make `_` an expression, to discard values in destructuring assignments)
 - #79019 (astconv: extract closures into a separate trait)
 - #79026 (Implement BTreeMap::retain and BTreeSet::retain)
 - #79031 (Validate that locals have a corresponding `LocalDecl`)
 - #79034 (rustc_resolve: Make `macro_rules` scope chain compression lazy)
 - #79036 (Move Steal to rustc_data_structures.)
 - #79041 (Rename clean::{ItemEnum -> ItemKind}, clean::Item::{inner -> kind})
 - #79058 (Move likely/unlikely argument outside of invisible unsafe block)
 - #79059 (Print 'checking cranelift artifacts' to easily separate it from other artifacts)
 - #79063 (Update rustfmt to v1.4.26)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

Author: bors

Cargo.lock

@@ -4337,7 +4337,7 @@ dependencies = [
 
 [[package]]
 name = "rustfmt-nightly"
-version = "1.4.25"
+version = "1.4.26"
 dependencies = [
  "annotate-snippets 0.6.1",
  "anyhow",

compiler/rustc_ast/src/ast.rs

@@ -1192,6 +1192,7 @@ impl Expr {
             ExprKind::Field(..) => ExprPrecedence::Field,
             ExprKind::Index(..) => ExprPrecedence::Index,
             ExprKind::Range(..) => ExprPrecedence::Range,
+            ExprKind::Underscore => ExprPrecedence::Path,
             ExprKind::Path(..) => ExprPrecedence::Path,
             ExprKind::AddrOf(..) => ExprPrecedence::AddrOf,
             ExprKind::Break(..) => ExprPrecedence::Break,
@@ -1324,6 +1325,8 @@ pub enum ExprKind {
     Index(P<Expr>, P<Expr>),
     /// A range (e.g., `1..2`, `1..`, `..2`, `1..=2`, `..=2`; and `..` in destructuring assingment).
     Range(Option<P<Expr>>, Option<P<Expr>>, RangeLimits),
+    /// An underscore, used in destructuring assignment to ignore a value.
+    Underscore,
 
     /// Variable reference, possibly containing `::` and/or type
     /// parameters (e.g., `foo::bar::<baz>`).

compiler/rustc_ast/src/mut_visit.rs

@@ -1232,6 +1232,7 @@ pub fn noop_visit_expr<T: MutVisitor>(
             visit_opt(e1, |e1| vis.visit_expr(e1));
             visit_opt(e2, |e2| vis.visit_expr(e2));
         }
+        ExprKind::Underscore => {}
         ExprKind::Path(qself, path) => {
             vis.visit_qself(qself);
             vis.visit_path(path);

compiler/rustc_ast/src/visit.rs

@@ -806,6 +806,7 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
             walk_list!(visitor, visit_expr, start);
             walk_list!(visitor, visit_expr, end);
         }
+        ExprKind::Underscore => {}
         ExprKind::Path(ref maybe_qself, ref path) => {
             if let Some(ref qself) = *maybe_qself {
                 visitor.visit_ty(&qself.ty);

compiler/rustc_ast_lowering/src/expr.rs

@@ -164,6 +164,16 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 ExprKind::Range(ref e1, ref e2, lims) => {
                     self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
                 }
+                ExprKind::Underscore => {
+                    self.sess
+                        .struct_span_err(
+                            e.span,
+                            "in expressions, `_` can only be used on the left-hand side of an assignment",
+                        )
+                        .span_label(e.span, "`_` not allowed here")
+                        .emit();
+                    hir::ExprKind::Err
+                }
                 ExprKind::Path(ref qself, ref path) => {
                     let qpath = self.lower_qpath(
                         e.id,
@@ -863,7 +873,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
         // Return early in case of an ordinary assignment.
         fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
             match &lhs.kind {
-                ExprKind::Array(..) | ExprKind::Struct(..) | ExprKind::Tup(..) => false,
+                ExprKind::Array(..)
+                | ExprKind::Struct(..)
+                | ExprKind::Tup(..)
+                | ExprKind::Underscore => false,
                 // Check for tuple struct constructor.
                 ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
                 ExprKind::Paren(e) => {
@@ -943,6 +956,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
         assignments: &mut Vec<hir::Stmt<'hir>>,
     ) -> &'hir hir::Pat<'hir> {
         match &lhs.kind {
+            // Underscore pattern.
+            ExprKind::Underscore => {
+                return self.pat_without_dbm(lhs.span, hir::PatKind::Wild);
+            }
             // Slice patterns.
             ExprKind::Array(elements) => {
                 let (pats, rest) =

compiler/rustc_ast_lowering/src/lib.rs

@@ -53,7 +53,6 @@ use rustc_hir::definitions::{DefKey, DefPathData, Definitions};
 use rustc_hir::intravisit;
 use rustc_hir::{ConstArg, GenericArg, ParamName};
 use rustc_index::vec::{Idx, IndexVec};
-use rustc_session::config::nightly_options;
 use rustc_session::lint::{builtin::BARE_TRAIT_OBJECTS, BuiltinLintDiagnostics, LintBuffer};
 use rustc_session::parse::ParseSess;
 use rustc_session::Session;
@@ -1398,8 +1397,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             "`impl Trait` not allowed outside of {}",
                             allowed_in,
                         );
-                        if pos == ImplTraitPosition::Binding && nightly_options::is_nightly_build()
-                        {
+                        if pos == ImplTraitPosition::Binding && self.sess.is_nightly_build() {
                             err.help(
                                 "add `#![feature(impl_trait_in_bindings)]` to the crate \
                                    attributes to enable",

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -630,7 +630,11 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
     gate_all!(const_trait_impl, "const trait impls are experimental");
     gate_all!(half_open_range_patterns, "half-open range patterns are unstable");
     gate_all!(inline_const, "inline-const is experimental");
-    gate_all!(destructuring_assignment, "destructuring assignments are unstable");
+    if sess.parse_sess.span_diagnostic.err_count() == 0 {
+        // Errors for `destructuring_assignment` can get quite noisy, especially where `_` is
+        // involved, so we only emit errors where there are no other parsing errors.
+        gate_all!(destructuring_assignment, "destructuring assignments are unstable");
+    }
 
     // All uses of `gate_all!` below this point were added in #65742,
     // and subsequently disabled (with the non-early gating readded).

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -109,7 +109,6 @@ pub fn print_crate<'a>(
     ann: &'a dyn PpAnn,
     is_expanded: bool,
     edition: Edition,
-    has_injected_crate: bool,
 ) -> String {
     let mut s = State {
         s: pp::mk_printer(),
@@ -119,7 +118,7 @@ pub fn print_crate<'a>(
         insert_extra_parens: true,
     };
 
-    if is_expanded && has_injected_crate {
+    if is_expanded && !krate.attrs.iter().any(|attr| attr.has_name(sym::no_core)) {
         // We need to print `#![no_std]` (and its feature gate) so that
         // compiling pretty-printed source won't inject libstd again.
         // However, we don't want these attributes in the AST because
@@ -2068,6 +2067,7 @@ impl<'a> State<'a> {
                     self.print_expr_maybe_paren(e, fake_prec);
                 }
             }
+            ast::ExprKind::Underscore => self.s.word("_"),
             ast::ExprKind::Path(None, ref path) => self.print_path(path, true, 0),
             ast::ExprKind::Path(Some(ref qself), ref path) => self.print_qpath(path, qself, true),
             ast::ExprKind::Break(opt_label, ref opt_expr) => {

compiler/rustc_builtin_macros/src/standard_library_imports.rs

@@ -13,12 +13,12 @@ pub fn inject(
     resolver: &mut dyn ResolverExpand,
     sess: &Session,
     alt_std_name: Option<Symbol>,
-) -> (ast::Crate, Option<Symbol>) {
+) -> ast::Crate {
     let rust_2018 = sess.parse_sess.edition >= Edition::Edition2018;
 
     // the first name in this list is the crate name of the crate with the prelude
     let names: &[Symbol] = if sess.contains_name(&krate.attrs, sym::no_core) {
-        return (krate, None);
+        return krate;
     } else if sess.contains_name(&krate.attrs, sym::no_std) {
         if sess.contains_name(&krate.attrs, sym::compiler_builtins) {
             &[sym::core]
@@ -81,5 +81,5 @@ pub fn inject(
 
     krate.module.items.insert(0, use_item);
 
-    (krate, Some(name))
+    krate
 }

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -3,7 +3,6 @@ use crate::llvm;
 use libc::c_int;
 use rustc_codegen_ssa::target_features::supported_target_features;
 use rustc_data_structures::fx::FxHashSet;
-use rustc_feature::UnstableFeatures;
 use rustc_middle::bug;
 use rustc_session::config::PrintRequest;
 use rustc_session::Session;
@@ -147,13 +146,11 @@ pub fn target_features(sess: &Session) -> Vec<Symbol> {
     let target_machine = create_informational_target_machine(sess);
     supported_target_features(sess)
         .iter()
-        .filter_map(|&(feature, gate)| {
-            if UnstableFeatures::from_environment().is_nightly_build() || gate.is_none() {
-                Some(feature)
-            } else {
-                None
-            }
-        })
+        .filter_map(
+            |&(feature, gate)| {
+                if sess.is_nightly_build() || gate.is_none() { Some(feature) } else { None }
+            },
+        )
         .filter(|feature| {
             let llvm_feature = to_llvm_feature(sess, feature);
             let cstr = CString::new(llvm_feature).unwrap();