From 698f8c2f01ea549d77d7dc3338a12e04c11057b9 Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Wed, 17 Apr 2024 14:02:58 +0200
Subject: Adding upstream version 1.64.0+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 compiler/rustc_ast_passes/src/ast_validation.rs | 1909 +++++++++++++++++++++++
 compiler/rustc_ast_passes/src/feature_gate.rs   |  901 +++++++++++
 compiler/rustc_ast_passes/src/lib.rs            |   18 +
 compiler/rustc_ast_passes/src/node_count.rs     |  135 ++
 compiler/rustc_ast_passes/src/show_span.rs      |   65 +
 5 files changed, 3028 insertions(+)
 create mode 100644 compiler/rustc_ast_passes/src/ast_validation.rs
 create mode 100644 compiler/rustc_ast_passes/src/feature_gate.rs
 create mode 100644 compiler/rustc_ast_passes/src/lib.rs
 create mode 100644 compiler/rustc_ast_passes/src/node_count.rs
 create mode 100644 compiler/rustc_ast_passes/src/show_span.rs

(limited to 'compiler/rustc_ast_passes/src')

diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
new file mode 100644
index 000000000..2d9d0073f
--- /dev/null
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -0,0 +1,1909 @@
+// Validate AST before lowering it to HIR.
+//
+// This pass is supposed to catch things that fit into AST data structures,
+// but not permitted by the language. It runs after expansion when AST is frozen,
+// so it can check for erroneous constructions produced by syntax extensions.
+// This pass is supposed to perform only simple checks not requiring name resolution
+// or type checking or some other kind of complex analysis.
+
+use itertools::{Either, Itertools};
+use rustc_ast::ptr::P;
+use rustc_ast::visit::{self, AssocCtxt, BoundKind, FnCtxt, FnKind, Visitor};
+use rustc_ast::walk_list;
+use rustc_ast::*;
+use rustc_ast_pretty::pprust::{self, State};
+use rustc_data_structures::fx::FxHashMap;
+use rustc_errors::{
+    error_code, pluralize, struct_span_err, Applicability, DiagnosticBuilder, ErrorGuaranteed,
+};
+use rustc_parse::validate_attr;
+use rustc_session::lint::builtin::{
+    DEPRECATED_WHERE_CLAUSE_LOCATION, MISSING_ABI, PATTERNS_IN_FNS_WITHOUT_BODY,
+};
+use rustc_session::lint::{BuiltinLintDiagnostics, LintBuffer};
+use rustc_session::Session;
+use rustc_span::source_map::Spanned;
+use rustc_span::symbol::{kw, sym, Ident};
+use rustc_span::Span;
+use rustc_target::spec::abi;
+use std::mem;
+use std::ops::{Deref, DerefMut};
+
+const MORE_EXTERN: &str =
+    "for more information, visit https://doc.rust-lang.org/std/keyword.extern.html";
+
+/// Is `self` allowed semantically as the first parameter in an `FnDecl`?
+enum SelfSemantic {
+    Yes,
+    No,
+}
+
+struct AstValidator<'a> {
+    session: &'a Session,
+
+    /// The span of the `extern` in an `extern { ... }` block, if any.
+    extern_mod: Option<&'a Item>,
+
+    /// Are we inside a trait impl?
+    in_trait_impl: bool,
+
+    in_const_trait_impl: bool,
+
+    has_proc_macro_decls: bool,
+
+    /// Used to ban nested `impl Trait`, e.g., `impl Into<impl Debug>`.
+    /// Nested `impl Trait` _is_ allowed in associated type position,
+    /// e.g., `impl Iterator<Item = impl Debug>`.
+    outer_impl_trait: Option<Span>,
+
+    is_tilde_const_allowed: bool,
+
+    /// Used to ban `impl Trait` in path projections like `<impl Iterator>::Item`
+    /// or `Foo::Bar<impl Trait>`
+    is_impl_trait_banned: bool,
+
+    /// Used to ban associated type bounds (i.e., `Type<AssocType: Bounds>`) in
+    /// certain positions.
+    is_assoc_ty_bound_banned: bool,
+
+    /// See [ForbiddenLetReason]
+    forbidden_let_reason: Option<ForbiddenLetReason>,
+
+    lint_buffer: &'a mut LintBuffer,
+}
+
+impl<'a> AstValidator<'a> {
+    fn with_in_trait_impl(
+        &mut self,
+        is_in: bool,
+        constness: Option<Const>,
+        f: impl FnOnce(&mut Self),
+    ) {
+        let old = mem::replace(&mut self.in_trait_impl, is_in);
+        let old_const =
+            mem::replace(&mut self.in_const_trait_impl, matches!(constness, Some(Const::Yes(_))));
+        f(self);
+        self.in_trait_impl = old;
+        self.in_const_trait_impl = old_const;
+    }
+
+    fn with_banned_impl_trait(&mut self, f: impl FnOnce(&mut Self)) {
+        let old = mem::replace(&mut self.is_impl_trait_banned, true);
+        f(self);
+        self.is_impl_trait_banned = old;
+    }
+
+    fn with_tilde_const(&mut self, allowed: bool, f: impl FnOnce(&mut Self)) {
+        let old = mem::replace(&mut self.is_tilde_const_allowed, allowed);
+        f(self);
+        self.is_tilde_const_allowed = old;
+    }
+
+    fn with_tilde_const_allowed(&mut self, f: impl FnOnce(&mut Self)) {
+        self.with_tilde_const(true, f)
+    }
+
+    fn with_banned_tilde_const(&mut self, f: impl FnOnce(&mut Self)) {
+        self.with_tilde_const(false, f)
+    }
+
+    fn with_let_management(
+        &mut self,
+        forbidden_let_reason: Option<ForbiddenLetReason>,
+        f: impl FnOnce(&mut Self, Option<ForbiddenLetReason>),
+    ) {
+        let old = mem::replace(&mut self.forbidden_let_reason, forbidden_let_reason);
+        f(self, old);
+        self.forbidden_let_reason = old;
+    }
+
+    /// Emits an error banning the `let` expression provided in the given location.
+    fn ban_let_expr(&self, expr: &'a Expr, forbidden_let_reason: ForbiddenLetReason) {
+        let sess = &self.session;
+        if sess.opts.unstable_features.is_nightly_build() {
+            let err = "`let` expressions are not supported here";
+            let mut diag = sess.struct_span_err(expr.span, err);
+            diag.note("only supported directly in conditions of `if` and `while` expressions");
+            match forbidden_let_reason {
+                ForbiddenLetReason::GenericForbidden => {}
+                ForbiddenLetReason::NotSupportedOr(span) => {
+                    diag.span_note(
+                        span,
+                        "`||` operators are not supported in let chain expressions",
+                    );
+                }
+                ForbiddenLetReason::NotSupportedParentheses(span) => {
+                    diag.span_note(
+                        span,
+                        "`let`s wrapped in parentheses are not supported in a context with let \
+                        chains",
+                    );
+                }
+            }
+            diag.emit();
+        } else {
+            sess.struct_span_err(expr.span, "expected expression, found statement (`let`)")
+                .note("variable declaration using `let` is a statement")
+                .emit();
+        }
+    }
+
+    fn check_gat_where(
+        &mut self,
+        id: NodeId,
+        before_predicates: &[WherePredicate],
+        where_clauses: (ast::TyAliasWhereClause, ast::TyAliasWhereClause),
+    ) {
+        if !before_predicates.is_empty() {
+            let mut state = State::new();
+            if !where_clauses.1.0 {
+                state.space();
+                state.word_space("where");
+            } else {
+                state.word_space(",");
+            }
+            let mut first = true;
+            for p in before_predicates.iter() {
+                if !first {
+                    state.word_space(",");
+                }
+                first = false;
+                state.print_where_predicate(p);
+            }
+            let suggestion = state.s.eof();
+            self.lint_buffer.buffer_lint_with_diagnostic(
+                DEPRECATED_WHERE_CLAUSE_LOCATION,
+                id,
+                where_clauses.0.1,
+                "where clause not allowed here",
+                BuiltinLintDiagnostics::DeprecatedWhereclauseLocation(
+                    where_clauses.1.1.shrink_to_hi(),
+                    suggestion,
+                ),
+            );
+        }
+    }
+
+    fn with_banned_assoc_ty_bound(&mut self, f: impl FnOnce(&mut Self)) {
+        let old = mem::replace(&mut self.is_assoc_ty_bound_banned, true);
+        f(self);
+        self.is_assoc_ty_bound_banned = old;
+    }
+
+    fn with_impl_trait(&mut self, outer: Option<Span>, f: impl FnOnce(&mut Self)) {
+        let old = mem::replace(&mut self.outer_impl_trait, outer);
+        if outer.is_some() {
+            self.with_banned_tilde_const(f);
+        } else {
+            f(self);
+        }
+        self.outer_impl_trait = old;
+    }
+
+    fn visit_assoc_constraint_from_generic_args(&mut self, constraint: &'a AssocConstraint) {
+        match constraint.kind {
+            AssocConstraintKind::Equality { .. } => {}
+            AssocConstraintKind::Bound { .. } => {
+                if self.is_assoc_ty_bound_banned {
+                    self.err_handler().span_err(
+                        constraint.span,
+                        "associated type bounds are not allowed within structs, enums, or unions",
+                    );
+                }
+            }
+        }
+        self.visit_assoc_constraint(constraint);
+    }
+
+    // Mirrors `visit::walk_ty`, but tracks relevant state.
+    fn walk_ty(&mut self, t: &'a Ty) {
+        match t.kind {
+            TyKind::ImplTrait(..) => {
+                self.with_impl_trait(Some(t.span), |this| visit::walk_ty(this, t))
+            }
+            TyKind::TraitObject(..) => self.with_banned_tilde_const(|this| visit::walk_ty(this, t)),
+            TyKind::Path(ref qself, ref path) => {
+                // We allow these:
+                //  - `Option<impl Trait>`
+                //  - `option::Option<impl Trait>`
+                //  - `option::Option<T>::Foo<impl Trait>
+                //
+                // But not these:
+                //  - `<impl Trait>::Foo`
+                //  - `option::Option<impl Trait>::Foo`.
+                //
+                // To implement this, we disallow `impl Trait` from `qself`
+                // (for cases like `<impl Trait>::Foo>`)
+                // but we allow `impl Trait` in `GenericArgs`
+                // iff there are no more PathSegments.
+                if let Some(ref qself) = *qself {
+                    // `impl Trait` in `qself` is always illegal
+                    self.with_banned_impl_trait(|this| this.visit_ty(&qself.ty));
+                }
+
+                // Note that there should be a call to visit_path here,
+                // so if any logic is added to process `Path`s a call to it should be
+                // added both in visit_path and here. This code mirrors visit::walk_path.
+                for (i, segment) in path.segments.iter().enumerate() {
+                    // Allow `impl Trait` iff we're on the final path segment
+                    if i == path.segments.len() - 1 {
+                        self.visit_path_segment(path.span, segment);
+                    } else {
+                        self.with_banned_impl_trait(|this| {
+                            this.visit_path_segment(path.span, segment)
+                        });
+                    }
+                }
+            }
+            _ => visit::walk_ty(self, t),
+        }
+    }
+
+    fn visit_struct_field_def(&mut self, field: &'a FieldDef) {
+        if let Some(ident) = field.ident {
+            if ident.name == kw::Underscore {
+                self.visit_vis(&field.vis);
+                self.visit_ident(ident);
+                self.visit_ty_common(&field.ty);
+                self.walk_ty(&field.ty);
+                walk_list!(self, visit_attribute, &field.attrs);
+                return;
+            }
+        }
+        self.visit_field_def(field);
+    }
+
+    fn err_handler(&self) -> &rustc_errors::Handler {
+        &self.session.diagnostic()
+    }
+
+    fn check_lifetime(&self, ident: Ident) {
+        let valid_names = [kw::UnderscoreLifetime, kw::StaticLifetime, kw::Empty];
+        if !valid_names.contains(&ident.name) && ident.without_first_quote().is_reserved() {
+            self.err_handler().span_err(ident.span, "lifetimes cannot use keyword names");
+        }
+    }
+
+    fn check_label(&self, ident: Ident) {
+        if ident.without_first_quote().is_reserved() {
+            self.err_handler()
+                .span_err(ident.span, &format!("invalid label name `{}`", ident.name));
+        }
+    }
+
+    fn invalid_visibility(&self, vis: &Visibility, note: Option<&str>) {
+        if let VisibilityKind::Inherited = vis.kind {
+            return;
+        }
+
+        let mut err =
+            struct_span_err!(self.session, vis.span, E0449, "unnecessary visibility qualifier");
+        if vis.kind.is_pub() {
+            err.span_label(vis.span, "`pub` not permitted here because it's implied");
+        }
+        if let Some(note) = note {
+            err.note(note);
+        }
+        err.emit();
+    }
+
+    fn check_decl_no_pat(decl: &FnDecl, mut report_err: impl FnMut(Span, Option<Ident>, bool)) {
+        for Param { pat, .. } in &decl.inputs {
+            match pat.kind {
+                PatKind::Ident(BindingMode::ByValue(Mutability::Not), _, None) | PatKind::Wild => {}
+                PatKind::Ident(BindingMode::ByValue(Mutability::Mut), ident, None) => {
+                    report_err(pat.span, Some(ident), true)
+                }
+                _ => report_err(pat.span, None, false),
+            }
+        }
+    }
+
+    fn check_trait_fn_not_async(&self, fn_span: Span, asyncness: Async) {
+        if let Async::Yes { span, .. } = asyncness {
+            struct_span_err!(
+                self.session,
+                fn_span,
+                E0706,
+                "functions in traits cannot be declared `async`"
+            )
+            .span_label(span, "`async` because of this")
+            .note("`async` trait functions are not currently supported")
+            .note("consider using the `async-trait` crate: https://crates.io/crates/async-trait")
+            .emit();
+        }
+    }
+
+    fn check_trait_fn_not_const(&self, constness: Const) {
+        if let Const::Yes(span) = constness {
+            struct_span_err!(
+                self.session,
+                span,
+                E0379,
+                "functions in traits cannot be declared const"
+            )
+            .span_label(span, "functions in traits cannot be const")
+            .emit();
+        }
+    }
+
+    fn check_late_bound_lifetime_defs(&self, params: &[GenericParam]) {
+        // Check only lifetime parameters are present and that the lifetime
+        // parameters that are present have no bounds.
+        let non_lt_param_spans: Vec<_> = params
+            .iter()
+            .filter_map(|param| match param.kind {
+                GenericParamKind::Lifetime { .. } => {
+                    if !param.bounds.is_empty() {
+                        let spans: Vec<_> = param.bounds.iter().map(|b| b.span()).collect();
+                        self.err_handler()
+                            .span_err(spans, "lifetime bounds cannot be used in this context");
+                    }
+                    None
+                }
+                _ => Some(param.ident.span),
+            })
+            .collect();
+        if !non_lt_param_spans.is_empty() {
+            self.err_handler().span_err(
+                non_lt_param_spans,
+                "only lifetime parameters can be used in this context",
+            );
+        }
+    }
+
+    fn check_fn_decl(&self, fn_decl: &FnDecl, self_semantic: SelfSemantic) {
+        self.check_decl_num_args(fn_decl);
+        self.check_decl_cvaradic_pos(fn_decl);
+        self.check_decl_attrs(fn_decl);
+        self.check_decl_self_param(fn_decl, self_semantic);
+    }
+
+    /// Emits fatal error if function declaration has more than `u16::MAX` arguments
+    /// Error is fatal to prevent errors during typechecking
+    fn check_decl_num_args(&self, fn_decl: &FnDecl) {
+        let max_num_args: usize = u16::MAX.into();
+        if fn_decl.inputs.len() > max_num_args {
+            let Param { span, .. } = fn_decl.inputs[0];
+            self.err_handler().span_fatal(
+                span,
+                &format!("function can not have more than {} arguments", max_num_args),
+            );
+        }
+    }
+
+    fn check_decl_cvaradic_pos(&self, fn_decl: &FnDecl) {
+        match &*fn_decl.inputs {
+            [Param { ty, span, .. }] => {
+                if let TyKind::CVarArgs = ty.kind {
+                    self.err_handler().span_err(
+                        *span,
+                        "C-variadic function must be declared with at least one named argument",
+                    );
+                }
+            }
+            [ps @ .., _] => {
+                for Param { ty, span, .. } in ps {
+                    if let TyKind::CVarArgs = ty.kind {
+                        self.err_handler().span_err(
+                            *span,
+                            "`...` must be the last argument of a C-variadic function",
+                        );
+                    }
+                }
+            }
+            _ => {}
+        }
+    }
+
+    fn check_decl_attrs(&self, fn_decl: &FnDecl) {
+        fn_decl
+            .inputs
+            .iter()
+            .flat_map(|i| i.attrs.as_ref())
+            .filter(|attr| {
+                let arr = [
+                    sym::allow,
+                    sym::cfg,
+                    sym::cfg_attr,
+                    sym::deny,
+                    sym::expect,
+                    sym::forbid,
+                    sym::warn,
+                ];
+                !arr.contains(&attr.name_or_empty()) && rustc_attr::is_builtin_attr(attr)
+            })
+            .for_each(|attr| {
+                if attr.is_doc_comment() {
+                    self.err_handler()
+                        .struct_span_err(
+                            attr.span,
+                            "documentation comments cannot be applied to function parameters",
+                        )
+                        .span_label(attr.span, "doc comments are not allowed here")
+                        .emit();
+                } else {
+                    self.err_handler().span_err(
+                        attr.span,
+                        "allow, cfg, cfg_attr, deny, expect, \
+                forbid, and warn are the only allowed built-in attributes in function parameters",
+                    );
+                }
+            });
+    }
+
+    fn check_decl_self_param(&self, fn_decl: &FnDecl, self_semantic: SelfSemantic) {
+        if let (SelfSemantic::No, [param, ..]) = (self_semantic, &*fn_decl.inputs) {
+            if param.is_self() {
+                self.err_handler()
+                    .struct_span_err(
+                        param.span,
+                        "`self` parameter is only allowed in associated functions",
+                    )
+                    .span_label(param.span, "not semantically valid as function parameter")
+                    .note("associated functions are those in `impl` or `trait` definitions")
+                    .emit();
+            }
+        }
+    }
+
+    fn check_defaultness(&self, span: Span, defaultness: Defaultness) {
+        if let Defaultness::Default(def_span) = defaultness {
+            let span = self.session.source_map().guess_head_span(span);
+            self.err_handler()
+                .struct_span_err(span, "`default` is only allowed on items in trait impls")
+                .span_label(def_span, "`default` because of this")
+                .emit();
+        }
+    }
+
+    fn error_item_without_body(&self, sp: Span, ctx: &str, msg: &str, sugg: &str) {
+        self.error_item_without_body_with_help(sp, ctx, msg, sugg, |_| ());
+    }
+
+    fn error_item_without_body_with_help(
+        &self,
+        sp: Span,
+        ctx: &str,
+        msg: &str,
+        sugg: &str,
+        help: impl FnOnce(&mut DiagnosticBuilder<'_, ErrorGuaranteed>),
+    ) {
+        let source_map = self.session.source_map();
+        let end = source_map.end_point(sp);
+        let replace_span = if source_map.span_to_snippet(end).map(|s| s == ";").unwrap_or(false) {
+            end
+        } else {
+            sp.shrink_to_hi()
+        };
+        let mut err = self.err_handler().struct_span_err(sp, msg);
+        err.span_suggestion(
+            replace_span,
+            &format!("provide a definition for the {}", ctx),
+            sugg,
+            Applicability::HasPlaceholders,
+        );
+        help(&mut err);
+        err.emit();
+    }
+
+    fn check_impl_item_provided<T>(&self, sp: Span, body: &Option<T>, ctx: &str, sugg: &str) {
+        if body.is_none() {
+            let msg = format!("associated {} in `impl` without body", ctx);
+            self.error_item_without_body(sp, ctx, &msg, sugg);
+        }
+    }
+
+    fn check_type_no_bounds(&self, bounds: &[GenericBound], ctx: &str) {
+        let span = match bounds {
+            [] => return,
+            [b0] => b0.span(),
+            [b0, .., bl] => b0.span().to(bl.span()),
+        };
+        self.err_handler()
+            .struct_span_err(span, &format!("bounds on `type`s in {} have no effect", ctx))
+            .emit();
+    }
+
+    fn check_foreign_ty_genericless(&self, generics: &Generics, where_span: Span) {
+        let cannot_have = |span, descr, remove_descr| {
+            self.err_handler()
+                .struct_span_err(
+                    span,
+                    &format!("`type`s inside `extern` blocks cannot have {}", descr),
+                )
+                .span_suggestion(
+                    span,
+                    &format!("remove the {}", remove_descr),
+                    "",
+                    Applicability::MaybeIncorrect,
+                )
+                .span_label(self.current_extern_span(), "`extern` block begins here")
+                .note(MORE_EXTERN)
+                .emit();
+        };
+
+        if !generics.params.is_empty() {
+            cannot_have(generics.span, "generic parameters", "generic parameters");
+        }
+
+        if !generics.where_clause.predicates.is_empty() {
+            cannot_have(where_span, "`where` clauses", "`where` clause");
+        }
+    }
+
+    fn check_foreign_kind_bodyless(&self, ident: Ident, kind: &str, body: Option<Span>) {
+        let Some(body) = body else {
+            return;
+        };
+        self.err_handler()
+            .struct_span_err(ident.span, &format!("incorrect `{}` inside `extern` block", kind))
+            .span_label(ident.span, "cannot have a body")
+            .span_label(body, "the invalid body")
+            .span_label(
+                self.current_extern_span(),
+                format!(
+                    "`extern` blocks define existing foreign {0}s and {0}s \
+                    inside of them cannot have a body",
+                    kind
+                ),
+            )
+            .note(MORE_EXTERN)
+            .emit();
+    }
+
+    /// An `fn` in `extern { ... }` cannot have a body `{ ... }`.
+    fn check_foreign_fn_bodyless(&self, ident: Ident, body: Option<&Block>) {
+        let Some(body) = body else {
+            return;
+        };
+        self.err_handler()
+            .struct_span_err(ident.span, "incorrect function inside `extern` block")
+            .span_label(ident.span, "cannot have a body")
+            .span_suggestion(
+                body.span,
+                "remove the invalid body",
+                ";",
+                Applicability::MaybeIncorrect,
+            )
+            .help(
+                "you might have meant to write a function accessible through FFI, \
+                which can be done by writing `extern fn` outside of the `extern` block",
+            )
+            .span_label(
+                self.current_extern_span(),
+                "`extern` blocks define existing foreign functions and functions \
+                inside of them cannot have a body",
+            )
+            .note(MORE_EXTERN)
+            .emit();
+    }
+
+    fn current_extern_span(&self) -> Span {
+        self.session.source_map().guess_head_span(self.extern_mod.unwrap().span)
+    }
+
+    /// An `fn` in `extern { ... }` cannot have qualifiers, e.g. `async fn`.
+    fn check_foreign_fn_headerless(&self, ident: Ident, span: Span, header: FnHeader) {
+        if header.has_qualifiers() {
+            self.err_handler()
+                .struct_span_err(ident.span, "functions in `extern` blocks cannot have qualifiers")
+                .span_label(self.current_extern_span(), "in this `extern` block")
+                .span_suggestion_verbose(
+                    span.until(ident.span.shrink_to_lo()),
+                    "remove the qualifiers",
+                    "fn ",
+                    Applicability::MaybeIncorrect,
+                )
+                .emit();
+        }
+    }
+
+    /// An item in `extern { ... }` cannot use non-ascii identifier.
+    fn check_foreign_item_ascii_only(&self, ident: Ident) {
+        if !ident.as_str().is_ascii() {
+            let n = 83942;
+            self.err_handler()
+                .struct_span_err(
+                    ident.span,
+                    "items in `extern` blocks cannot use non-ascii identifiers",
+                )
+                .span_label(self.current_extern_span(), "in this `extern` block")
+                .note(&format!(
+                    "this limitation may be lifted in the future; see issue #{} <https://github.com/rust-lang/rust/issues/{}> for more information",
+                    n, n,
+                ))
+                .emit();
+        }
+    }
+
+    /// Reject C-variadic type unless the function is foreign,
+    /// or free and `unsafe extern "C"` semantically.
+    fn check_c_variadic_type(&self, fk: FnKind<'a>) {
+        match (fk.ctxt(), fk.header()) {
+            (Some(FnCtxt::Foreign), _) => return,
+            (Some(FnCtxt::Free), Some(header)) => match header.ext {
+                Extern::Explicit(StrLit { symbol_unescaped: sym::C, .. }, _)
+                | Extern::Implicit(_)
+                    if matches!(header.unsafety, Unsafe::Yes(_)) =>
+                {
+                    return;
+                }
+                _ => {}
+            },
+            _ => {}
+        };
+
+        for Param { ty, span, .. } in &fk.decl().inputs {
+            if let TyKind::CVarArgs = ty.kind {
+                self.err_handler()
+                    .struct_span_err(
+                        *span,
+                        "only foreign or `unsafe extern \"C\"` functions may be C-variadic",
+                    )
+                    .emit();
+            }
+        }
+    }
+
+    fn check_item_named(&self, ident: Ident, kind: &str) {
+        if ident.name != kw::Underscore {
+            return;
+        }
+        self.err_handler()
+            .struct_span_err(ident.span, &format!("`{}` items in this context need a name", kind))
+            .span_label(ident.span, format!("`_` is not a valid name for this `{}` item", kind))
+            .emit();
+    }
+
+    fn check_nomangle_item_asciionly(&self, ident: Ident, item_span: Span) {
+        if ident.name.as_str().is_ascii() {
+            return;
+        }
+        let head_span = self.session.source_map().guess_head_span(item_span);
+        struct_span_err!(
+            self.session,
+            head_span,
+            E0754,
+            "`#[no_mangle]` requires ASCII identifier"
+        )
+        .emit();
+    }
+
+    fn check_mod_file_item_asciionly(&self, ident: Ident) {
+        if ident.name.as_str().is_ascii() {
+            return;
+        }
+        struct_span_err!(
+            self.session,
+            ident.span,
+            E0754,
+            "trying to load file for module `{}` with non-ascii identifier name",
+            ident.name
+        )
+        .help("consider using `#[path]` attribute to specify filesystem path")
+        .emit();
+    }
+
+    fn deny_generic_params(&self, generics: &Generics, ident_span: Span) {
+        if !generics.params.is_empty() {
+            struct_span_err!(
+                self.session,
+                generics.span,
+                E0567,
+                "auto traits cannot have generic parameters"
+            )
+            .span_label(ident_span, "auto trait cannot have generic parameters")
+            .span_suggestion(
+                generics.span,
+                "remove the parameters",
+                "",
+                Applicability::MachineApplicable,
+            )
+            .emit();
+        }
+    }
+
+    fn emit_e0568(&self, span: Span, ident_span: Span) {
+        struct_span_err!(
+            self.session,
+            span,
+            E0568,
+            "auto traits cannot have super traits or lifetime bounds"
+        )
+        .span_label(ident_span, "auto trait cannot have super traits or lifetime bounds")
+        .span_suggestion(
+            span,
+            "remove the super traits or lifetime bounds",
+            "",
+            Applicability::MachineApplicable,
+        )
+        .emit();
+    }
+
+    fn deny_super_traits(&self, bounds: &GenericBounds, ident_span: Span) {
+        if let [.., last] = &bounds[..] {
+            let span = ident_span.shrink_to_hi().to(last.span());
+            self.emit_e0568(span, ident_span);
+        }
+    }
+
+    fn deny_where_clause(&self, where_clause: &WhereClause, ident_span: Span) {
+        if !where_clause.predicates.is_empty() {
+            self.emit_e0568(where_clause.span, ident_span);
+        }
+    }
+
+    fn deny_items(&self, trait_items: &[P<AssocItem>], ident_span: Span) {
+        if !trait_items.is_empty() {
+            let spans: Vec<_> = trait_items.iter().map(|i| i.ident.span).collect();
+            let total_span = trait_items.first().unwrap().span.to(trait_items.last().unwrap().span);
+            struct_span_err!(
+                self.session,
+                spans,
+                E0380,
+                "auto traits cannot have associated items"
+            )
+            .span_suggestion(
+                total_span,
+                "remove these associated items",
+                "",
+                Applicability::MachineApplicable,
+            )
+            .span_label(ident_span, "auto trait cannot have associated items")
+            .emit();
+        }
+    }
+
+    fn correct_generic_order_suggestion(&self, data: &AngleBracketedArgs) -> String {
+        // Lifetimes always come first.
+        let lt_sugg = data.args.iter().filter_map(|arg| match arg {
+            AngleBracketedArg::Arg(lt @ GenericArg::Lifetime(_)) => {
+                Some(pprust::to_string(|s| s.print_generic_arg(lt)))
+            }
+            _ => None,
+        });
+        let args_sugg = data.args.iter().filter_map(|a| match a {
+            AngleBracketedArg::Arg(GenericArg::Lifetime(_)) | AngleBracketedArg::Constraint(_) => {
+                None
+            }
+            AngleBracketedArg::Arg(arg) => Some(pprust::to_string(|s| s.print_generic_arg(arg))),
+        });
+        // Constraints always come last.
+        let constraint_sugg = data.args.iter().filter_map(|a| match a {
+            AngleBracketedArg::Arg(_) => None,
+            AngleBracketedArg::Constraint(c) => {
+                Some(pprust::to_string(|s| s.print_assoc_constraint(c)))
+            }
+        });
+        format!(
+            "<{}>",
+            lt_sugg.chain(args_sugg).chain(constraint_sugg).collect::<Vec<String>>().join(", ")
+        )
+    }
+
+    /// Enforce generic args coming before constraints in `<...>` of a path segment.
+    fn check_generic_args_before_constraints(&self, data: &AngleBracketedArgs) {
+        // Early exit in case it's partitioned as it should be.
+        if data.args.iter().is_partitioned(|arg| matches!(arg, AngleBracketedArg::Arg(_))) {
+            return;
+        }
+        // Find all generic argument coming after the first constraint...
+        let (constraint_spans, arg_spans): (Vec<Span>, Vec<Span>) =
+            data.args.iter().partition_map(|arg| match arg {
+                AngleBracketedArg::Constraint(c) => Either::Left(c.span),
+                AngleBracketedArg::Arg(a) => Either::Right(a.span()),
+            });
+        let args_len = arg_spans.len();
+        let constraint_len = constraint_spans.len();
+        // ...and then error:
+        self.err_handler()
+            .struct_span_err(
+                arg_spans.clone(),
+                "generic arguments must come before the first constraint",
+            )
+            .span_label(constraint_spans[0], &format!("constraint{}", pluralize!(constraint_len)))
+            .span_label(
+                *arg_spans.iter().last().unwrap(),
+                &format!("generic argument{}", pluralize!(args_len)),
+            )
+            .span_labels(constraint_spans, "")
+            .span_labels(arg_spans, "")
+            .span_suggestion_verbose(
+                data.span,
+                &format!(
+                    "move the constraint{} after the generic argument{}",
+                    pluralize!(constraint_len),
+                    pluralize!(args_len)
+                ),
+                self.correct_generic_order_suggestion(&data),
+                Applicability::MachineApplicable,
+            )
+            .emit();
+    }
+
+    fn visit_ty_common(&mut self, ty: &'a Ty) {
+        match ty.kind {
+            TyKind::BareFn(ref bfty) => {
+                self.check_fn_decl(&bfty.decl, SelfSemantic::No);
+                Self::check_decl_no_pat(&bfty.decl, |span, _, _| {
+                    struct_span_err!(
+                        self.session,
+                        span,
+                        E0561,
+                        "patterns aren't allowed in function pointer types"
+                    )
+                    .emit();
+                });
+                self.check_late_bound_lifetime_defs(&bfty.generic_params);
+                if let Extern::Implicit(_) = bfty.ext {
+                    let sig_span = self.session.source_map().next_point(ty.span.shrink_to_lo());
+                    self.maybe_lint_missing_abi(sig_span, ty.id);
+                }
+            }
+            TyKind::TraitObject(ref bounds, ..) => {
+                let mut any_lifetime_bounds = false;
+                for bound in bounds {
+                    if let GenericBound::Outlives(ref lifetime) = *bound {
+                        if any_lifetime_bounds {
+                            struct_span_err!(
+                                self.session,
+                                lifetime.ident.span,
+                                E0226,
+                                "only a single explicit lifetime bound is permitted"
+                            )
+                            .emit();
+                            break;
+                        }
+                        any_lifetime_bounds = true;
+                    }
+                }
+            }
+            TyKind::ImplTrait(_, ref bounds) => {
+                if self.is_impl_trait_banned {
+                    struct_span_err!(
+                        self.session,
+                        ty.span,
+                        E0667,
+                        "`impl Trait` is not allowed in path parameters"
+                    )
+                    .emit();
+                }
+
+                if let Some(outer_impl_trait_sp) = self.outer_impl_trait {
+                    struct_span_err!(
+                        self.session,
+                        ty.span,
+                        E0666,
+                        "nested `impl Trait` is not allowed"
+                    )
+                    .span_label(outer_impl_trait_sp, "outer `impl Trait`")
+                    .span_label(ty.span, "nested `impl Trait` here")
+                    .emit();
+                }
+
+                if !bounds.iter().any(|b| matches!(b, GenericBound::Trait(..))) {
+                    self.err_handler().span_err(ty.span, "at least one trait must be specified");
+                }
+            }
+            _ => {}
+        }
+    }
+
+    fn maybe_lint_missing_abi(&mut self, span: Span, id: NodeId) {
+        // FIXME(davidtwco): This is a hack to detect macros which produce spans of the
+        // call site which do not have a macro backtrace. See #61963.
+        let is_macro_callsite = self
+            .session
+            .source_map()
+            .span_to_snippet(span)
+            .map(|snippet| snippet.starts_with("#["))
+            .unwrap_or(true);
+        if !is_macro_callsite {
+            self.lint_buffer.buffer_lint_with_diagnostic(
+                MISSING_ABI,
+                id,
+                span,
+                "extern declarations without an explicit ABI are deprecated",
+                BuiltinLintDiagnostics::MissingAbi(span, abi::Abi::FALLBACK),
+            )
+        }
+    }
+}
+
+/// Checks that generic parameters are in the correct order,
+/// which is lifetimes, then types and then consts. (`<'a, T, const N: usize>`)
+fn validate_generic_param_order(
+    handler: &rustc_errors::Handler,
+    generics: &[GenericParam],
+    span: Span,
+) {
+    let mut max_param: Option<ParamKindOrd> = None;
+    let mut out_of_order = FxHashMap::default();
+    let mut param_idents = Vec::with_capacity(generics.len());
+
+    for (idx, param) in generics.iter().enumerate() {
+        let ident = param.ident;
+        let (kind, bounds, span) = (&param.kind, &param.bounds, ident.span);
+        let (ord_kind, ident) = match &param.kind {
+            GenericParamKind::Lifetime => (ParamKindOrd::Lifetime, ident.to_string()),
+            GenericParamKind::Type { default: _ } => (ParamKindOrd::Type, ident.to_string()),
+            GenericParamKind::Const { ref ty, kw_span: _, default: _ } => {
+                let ty = pprust::ty_to_string(ty);
+                (ParamKindOrd::Const, format!("const {}: {}", ident, ty))
+            }
+        };
+        param_idents.push((kind, ord_kind, bounds, idx, ident));
+        match max_param {
+            Some(max_param) if max_param > ord_kind => {
+                let entry = out_of_order.entry(ord_kind).or_insert((max_param, vec![]));
+                entry.1.push(span);
+            }
+            Some(_) | None => max_param = Some(ord_kind),
+        };
+    }
+
+    if !out_of_order.is_empty() {
+        let mut ordered_params = "<".to_string();
+        param_idents.sort_by_key(|&(_, po, _, i, _)| (po, i));
+        let mut first = true;
+        for (kind, _, bounds, _, ident) in param_idents {
+            if !first {
+                ordered_params += ", ";
+            }
+            ordered_params += &ident;
+
+            if !bounds.is_empty() {
+                ordered_params += ": ";
+                ordered_params += &pprust::bounds_to_string(&bounds);
+            }
+
+            match kind {
+                GenericParamKind::Type { default: Some(default) } => {
+                    ordered_params += " = ";
+                    ordered_params += &pprust::ty_to_string(default);
+                }
+                GenericParamKind::Type { default: None } => (),
+                GenericParamKind::Lifetime => (),
+                GenericParamKind::Const { ty: _, kw_span: _, default: Some(default) } => {
+                    ordered_params += " = ";
+                    ordered_params += &pprust::expr_to_string(&*default.value);
+                }
+                GenericParamKind::Const { ty: _, kw_span: _, default: None } => (),
+            }
+            first = false;
+        }
+
+        ordered_params += ">";
+
+        for (param_ord, (max_param, spans)) in &out_of_order {
+            let mut err = handler.struct_span_err(
+                spans.clone(),
+                &format!(
+                    "{} parameters must be declared prior to {} parameters",
+                    param_ord, max_param,
+                ),
+            );
+            err.span_suggestion(
+                span,
+                "reorder the parameters: lifetimes, then consts and types",
+                &ordered_params,
+                Applicability::MachineApplicable,
+            );
+            err.emit();
+        }
+    }
+}
+
+impl<'a> Visitor<'a> for AstValidator<'a> {
+    fn visit_attribute(&mut self, attr: &Attribute) {
+        validate_attr::check_meta(&self.session.parse_sess, attr);
+    }
+
+    fn visit_expr(&mut self, expr: &'a Expr) {
+        self.with_let_management(Some(ForbiddenLetReason::GenericForbidden), |this, forbidden_let_reason| {
+            match &expr.kind {
+                ExprKind::Binary(Spanned { node: BinOpKind::Or, span }, lhs, rhs) => {
+                    let local_reason = Some(ForbiddenLetReason::NotSupportedOr(*span));
+                    this.with_let_management(local_reason, |this, _| this.visit_expr(lhs));
+                    this.with_let_management(local_reason, |this, _| this.visit_expr(rhs));
+                }
+                ExprKind::If(cond, then, opt_else) => {
+                    this.visit_block(then);
+                    walk_list!(this, visit_expr, opt_else);
+                    this.with_let_management(None, |this, _| this.visit_expr(cond));
+                    return;
+                }
+                ExprKind::Let(..) if let Some(elem) = forbidden_let_reason => {
+                    this.ban_let_expr(expr, elem);
+                },
+                ExprKind::Match(scrutinee, arms) => {
+                    this.visit_expr(scrutinee);
+                    for arm in arms {
+                        this.visit_expr(&arm.body);
+                        this.visit_pat(&arm.pat);
+                        walk_list!(this, visit_attribute, &arm.attrs);
+                        if let Some(guard) = &arm.guard && let ExprKind::Let(_, guard_expr, _) = &guard.kind {
+                            this.with_let_management(None, |this, _| {
+                                this.visit_expr(guard_expr)
+                            });
+                            return;
+                        }
+                    }
+                }
+                ExprKind::Paren(local_expr) => {
+                    fn has_let_expr(expr: &Expr) -> bool {
+                        match expr.kind {
+                            ExprKind::Binary(_, ref lhs, ref rhs) => has_let_expr(lhs) || has_let_expr(rhs),
+                            ExprKind::Let(..) => true,
+                            _ => false,
+                        }
+                    }
+                    let local_reason = if has_let_expr(local_expr) {
+                        Some(ForbiddenLetReason::NotSupportedParentheses(local_expr.span))
+                    }
+                    else {
+                        forbidden_let_reason
+                    };
+                    this.with_let_management(local_reason, |this, _| this.visit_expr(local_expr));
+                }
+                ExprKind::Binary(Spanned { node: BinOpKind::And, .. }, ..) => {
+                    this.with_let_management(forbidden_let_reason, |this, _| visit::walk_expr(this, expr));
+                    return;
+                }
+                ExprKind::While(cond, then, opt_label) => {
+                    walk_list!(this, visit_label, opt_label);
+                    this.visit_block(then);
+                    this.with_let_management(None, |this, _| this.visit_expr(cond));
+                    return;
+                }
+                _ => visit::walk_expr(this, expr),
+            }
+        });
+    }
+
+    fn visit_ty(&mut self, ty: &'a Ty) {
+        self.visit_ty_common(ty);
+        self.walk_ty(ty)
+    }
+
+    fn visit_label(&mut self, label: &'a Label) {
+        self.check_label(label.ident);
+        visit::walk_label(self, label);
+    }
+
+    fn visit_lifetime(&mut self, lifetime: &'a Lifetime, _: visit::LifetimeCtxt) {
+        self.check_lifetime(lifetime.ident);
+        visit::walk_lifetime(self, lifetime);
+    }
+
+    fn visit_field_def(&mut self, s: &'a FieldDef) {
+        visit::walk_field_def(self, s)
+    }
+
+    fn visit_item(&mut self, item: &'a Item) {
+        if item.attrs.iter().any(|attr| self.session.is_proc_macro_attr(attr)) {
+            self.has_proc_macro_decls = true;
+        }
+
+        if self.session.contains_name(&item.attrs, sym::no_mangle) {
+            self.check_nomangle_item_asciionly(item.ident, item.span);
+        }
+
+        match item.kind {
+            ItemKind::Impl(box Impl {
+                unsafety,
+                polarity,
+                defaultness: _,
+                constness,
+                ref generics,
+                of_trait: Some(ref t),
+                ref self_ty,
+                ref items,
+            }) => {
+                self.with_in_trait_impl(true, Some(constness), |this| {
+                    this.invalid_visibility(&item.vis, None);
+                    if let TyKind::Err = self_ty.kind {
+                        this.err_handler()
+                            .struct_span_err(
+                                item.span,
+                                "`impl Trait for .. {}` is an obsolete syntax",
+                            )
+                            .help("use `auto trait Trait {}` instead")
+                            .emit();
+                    }
+                    if let (Unsafe::Yes(span), ImplPolarity::Negative(sp)) = (unsafety, polarity) {
+                        struct_span_err!(
+                            this.session,
+                            sp.to(t.path.span),
+                            E0198,
+                            "negative impls cannot be unsafe"
+                        )
+                        .span_label(sp, "negative because of this")
+                        .span_label(span, "unsafe because of this")
+                        .emit();
+                    }
+
+                    this.visit_vis(&item.vis);
+                    this.visit_ident(item.ident);
+                    if let Const::Yes(_) = constness {
+                        this.with_tilde_const_allowed(|this| this.visit_generics(generics));
+                    } else {
+                        this.visit_generics(generics);
+                    }
+                    this.visit_trait_ref(t);
+                    this.visit_ty(self_ty);
+
+                    walk_list!(this, visit_assoc_item, items, AssocCtxt::Impl);
+                });
+                return; // Avoid visiting again.
+            }
+            ItemKind::Impl(box Impl {
+                unsafety,
+                polarity,
+                defaultness,
+                constness,
+                generics: _,
+                of_trait: None,
+                ref self_ty,
+                items: _,
+            }) => {
+                let error = |annotation_span, annotation| {
+                    let mut err = self.err_handler().struct_span_err(
+                        self_ty.span,
+                        &format!("inherent impls cannot be {}", annotation),
+                    );
+                    err.span_label(annotation_span, &format!("{} because of this", annotation));
+                    err.span_label(self_ty.span, "inherent impl for this type");
+                    err
+                };
+
+                self.invalid_visibility(
+                    &item.vis,
+                    Some("place qualifiers on individual impl items instead"),
+                );
+                if let Unsafe::Yes(span) = unsafety {
+                    error(span, "unsafe").code(error_code!(E0197)).emit();
+                }
+                if let ImplPolarity::Negative(span) = polarity {
+                    error(span, "negative").emit();
+                }
+                if let Defaultness::Default(def_span) = defaultness {
+                    error(def_span, "`default`")
+                        .note("only trait implementations may be annotated with `default`")
+                        .emit();
+                }
+                if let Const::Yes(span) = constness {
+                    error(span, "`const`")
+                        .note("only trait implementations may be annotated with `const`")
+                        .emit();
+                }
+            }
+            ItemKind::Fn(box Fn { defaultness, ref sig, ref generics, ref body }) => {
+                self.check_defaultness(item.span, defaultness);
+
+                if body.is_none() {
+                    let msg = "free function without a body";
+                    let ext = sig.header.ext;
+
+                    let f = |e: &mut DiagnosticBuilder<'_, _>| {
+                        if let Extern::Implicit(start_span) | Extern::Explicit(_, start_span) = &ext
+                        {
+                            let start_suggestion = if let Extern::Explicit(abi, _) = ext {
+                                format!("extern \"{}\" {{", abi.symbol_unescaped)
+                            } else {
+                                "extern {".to_owned()
+                            };
+
+                            let end_suggestion = " }".to_owned();
+                            let end_span = item.span.shrink_to_hi();
+
+                            e
+                            .multipart_suggestion(
+                                "if you meant to declare an externally defined function, use an `extern` block",
+                                vec![(*start_span, start_suggestion), (end_span, end_suggestion)],
+                                Applicability::MaybeIncorrect,
+                             );
+                        }
+                    };
+
+                    self.error_item_without_body_with_help(
+                        item.span,
+                        "function",
+                        msg,
+                        " { <body> }",
+                        f,
+                    );
+                }
+
+                self.visit_vis(&item.vis);
+                self.visit_ident(item.ident);
+                let kind =
+                    FnKind::Fn(FnCtxt::Free, item.ident, sig, &item.vis, generics, body.as_deref());
+                self.visit_fn(kind, item.span, item.id);
+                walk_list!(self, visit_attribute, &item.attrs);
+                return; // Avoid visiting again.
+            }
+            ItemKind::ForeignMod(ForeignMod { abi, unsafety, .. }) => {
+                let old_item = mem::replace(&mut self.extern_mod, Some(item));
+                self.invalid_visibility(
+                    &item.vis,
+                    Some("place qualifiers on individual foreign items instead"),
+                );
+                if let Unsafe::Yes(span) = unsafety {
+                    self.err_handler().span_err(span, "extern block cannot be declared unsafe");
+                }
+                if abi.is_none() {
+                    self.maybe_lint_missing_abi(item.span, item.id);
+                }
+                visit::walk_item(self, item);
+                self.extern_mod = old_item;
+                return; // Avoid visiting again.
+            }
+            ItemKind::Enum(ref def, _) => {
+                for variant in &def.variants {
+                    self.invalid_visibility(&variant.vis, None);
+                    for field in variant.data.fields() {
+                        self.invalid_visibility(&field.vis, None);
+                    }
+                }
+            }
+            ItemKind::Trait(box Trait { is_auto, ref generics, ref bounds, ref items, .. }) => {
+                if is_auto == IsAuto::Yes {
+                    // Auto traits cannot have generics, super traits nor contain items.
+                    self.deny_generic_params(generics, item.ident.span);
+                    self.deny_super_traits(bounds, item.ident.span);
+                    self.deny_where_clause(&generics.where_clause, item.ident.span);
+                    self.deny_items(items, item.ident.span);
+                }
+
+                // Equivalent of `visit::walk_item` for `ItemKind::Trait` that inserts a bound
+                // context for the supertraits.
+                self.visit_vis(&item.vis);
+                self.visit_ident(item.ident);
+                self.visit_generics(generics);
+                self.with_tilde_const_allowed(|this| {
+                    walk_list!(this, visit_param_bound, bounds, BoundKind::SuperTraits)
+                });
+                walk_list!(self, visit_assoc_item, items, AssocCtxt::Trait);
+                walk_list!(self, visit_attribute, &item.attrs);
+                return;
+            }
+            ItemKind::Mod(unsafety, ref mod_kind) => {
+                if let Unsafe::Yes(span) = unsafety {
+                    self.err_handler().span_err(span, "module cannot be declared unsafe");
+                }
+                // Ensure that `path` attributes on modules are recorded as used (cf. issue #35584).
+                if !matches!(mod_kind, ModKind::Loaded(_, Inline::Yes, _))
+                    && !self.session.contains_name(&item.attrs, sym::path)
+                {
+                    self.check_mod_file_item_asciionly(item.ident);
+                }
+            }
+            ItemKind::Struct(ref vdata, ref generics) => match vdata {
+                // Duplicating the `Visitor` logic allows catching all cases
+                // of `Anonymous(Struct, Union)` outside of a field struct or union.
+                //
+                // Inside `visit_ty` the validator catches every `Anonymous(Struct, Union)` it
+                // encounters, and only on `ItemKind::Struct` and `ItemKind::Union`
+                // it uses `visit_ty_common`, which doesn't contain that specific check.
+                VariantData::Struct(ref fields, ..) => {
+                    self.visit_vis(&item.vis);
+                    self.visit_ident(item.ident);
+                    self.visit_generics(generics);
+                    self.with_banned_assoc_ty_bound(|this| {
+                        walk_list!(this, visit_struct_field_def, fields);
+                    });
+                    walk_list!(self, visit_attribute, &item.attrs);
+                    return;
+                }
+                _ => {}
+            },
+            ItemKind::Union(ref vdata, ref generics) => {
+                if vdata.fields().is_empty() {
+                    self.err_handler().span_err(item.span, "unions cannot have zero fields");
+                }
+                match vdata {
+                    VariantData::Struct(ref fields, ..) => {
+                        self.visit_vis(&item.vis);
+                        self.visit_ident(item.ident);
+                        self.visit_generics(generics);
+                        self.with_banned_assoc_ty_bound(|this| {
+                            walk_list!(this, visit_struct_field_def, fields);
+                        });
+                        walk_list!(self, visit_attribute, &item.attrs);
+                        return;
+                    }
+                    _ => {}
+                }
+            }
+            ItemKind::Const(def, .., None) => {
+                self.check_defaultness(item.span, def);
+                let msg = "free constant item without body";
+                self.error_item_without_body(item.span, "constant", msg, " = <expr>;");
+            }
+            ItemKind::Static(.., None) => {
+                let msg = "free static item without body";
+                self.error_item_without_body(item.span, "static", msg, " = <expr>;");
+            }
+            ItemKind::TyAlias(box TyAlias {
+                defaultness,
+                where_clauses,
+                ref bounds,
+                ref ty,
+                ..
+            }) => {
+                self.check_defaultness(item.span, defaultness);
+                if ty.is_none() {
+                    let msg = "free type alias without body";
+                    self.error_item_without_body(item.span, "type", msg, " = <type>;");
+                }
+                self.check_type_no_bounds(bounds, "this context");
+                if where_clauses.1.0 {
+                    let mut err = self.err_handler().struct_span_err(
+                        where_clauses.1.1,
+                        "where clauses are not allowed after the type for type aliases",
+                    );
+                    err.note(
+                        "see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information",
+                    );
+                    err.emit();
+                }
+            }
+            _ => {}
+        }
+
+        visit::walk_item(self, item);
+    }
+
+    fn visit_foreign_item(&mut self, fi: &'a ForeignItem) {
+        match &fi.kind {
+            ForeignItemKind::Fn(box Fn { defaultness, sig, body, .. }) => {
+                self.check_defaultness(fi.span, *defaultness);
+                self.check_foreign_fn_bodyless(fi.ident, body.as_deref());
+                self.check_foreign_fn_headerless(fi.ident, fi.span, sig.header);
+                self.check_foreign_item_ascii_only(fi.ident);
+            }
+            ForeignItemKind::TyAlias(box TyAlias {
+                defaultness,
+                generics,
+                where_clauses,
+                bounds,
+                ty,
+                ..
+            }) => {
+                self.check_defaultness(fi.span, *defaultness);
+                self.check_foreign_kind_bodyless(fi.ident, "type", ty.as_ref().map(|b| b.span));
+                self.check_type_no_bounds(bounds, "`extern` blocks");
+                self.check_foreign_ty_genericless(generics, where_clauses.0.1);
+                self.check_foreign_item_ascii_only(fi.ident);
+            }
+            ForeignItemKind::Static(_, _, body) => {
+                self.check_foreign_kind_bodyless(fi.ident, "static", body.as_ref().map(|b| b.span));
+                self.check_foreign_item_ascii_only(fi.ident);
+            }
+            ForeignItemKind::MacCall(..) => {}
+        }
+
+        visit::walk_foreign_item(self, fi)
+    }
+
+    // Mirrors `visit::walk_generic_args`, but tracks relevant state.
+    fn visit_generic_args(&mut self, _: Span, generic_args: &'a GenericArgs) {
+        match *generic_args {
+            GenericArgs::AngleBracketed(ref data) => {
+                self.check_generic_args_before_constraints(data);
+
+                for arg in &data.args {
+                    match arg {
+                        AngleBracketedArg::Arg(arg) => self.visit_generic_arg(arg),
+                        // Type bindings such as `Item = impl Debug` in `Iterator<Item = Debug>`
+                        // are allowed to contain nested `impl Trait`.
+                        AngleBracketedArg::Constraint(constraint) => {
+                            self.with_impl_trait(None, |this| {
+                                this.visit_assoc_constraint_from_generic_args(constraint);
+                            });
+                        }
+                    }
+                }
+            }
+            GenericArgs::Parenthesized(ref data) => {
+                walk_list!(self, visit_ty, &data.inputs);
+                if let FnRetTy::Ty(ty) = &data.output {
+                    // `-> Foo` syntax is essentially an associated type binding,
+                    // so it is also allowed to contain nested `impl Trait`.
+                    self.with_impl_trait(None, |this| this.visit_ty(ty));
+                }
+            }
+        }
+    }
+
+    fn visit_generics(&mut self, generics: &'a Generics) {
+        let mut prev_param_default = None;
+        for param in &generics.params {
+            match param.kind {
+                GenericParamKind::Lifetime => (),
+                GenericParamKind::Type { default: Some(_), .. }
+                | GenericParamKind::Const { default: Some(_), .. } => {
+                    prev_param_default = Some(param.ident.span);
+                }
+                GenericParamKind::Type { .. } | GenericParamKind::Const { .. } => {
+                    if let Some(span) = prev_param_default {
+                        let mut err = self.err_handler().struct_span_err(
+                            span,
+                            "generic parameters with a default must be trailing",
+                        );
+                        err.emit();
+                        break;
+                    }
+                }
+            }
+        }
+
+        validate_generic_param_order(self.err_handler(), &generics.params, generics.span);
+
+        for predicate in &generics.where_clause.predicates {
+            if let WherePredicate::EqPredicate(ref predicate) = *predicate {
+                deny_equality_constraints(self, predicate, generics);
+            }
+        }
+        walk_list!(self, visit_generic_param, &generics.params);
+        for predicate in &generics.where_clause.predicates {
+            match predicate {
+                WherePredicate::BoundPredicate(bound_pred) => {
+                    // A type binding, eg `for<'c> Foo: Send+Clone+'c`
+                    self.check_late_bound_lifetime_defs(&bound_pred.bound_generic_params);
+
+                    // This is slightly complicated. Our representation for poly-trait-refs contains a single
+                    // binder and thus we only allow a single level of quantification. However,
+                    // the syntax of Rust permits quantification in two places in where clauses,
+                    // e.g., `T: for <'a> Foo<'a>` and `for <'a, 'b> &'b T: Foo<'a>`. If both are
+                    // defined, then error.
+                    if !bound_pred.bound_generic_params.is_empty() {
+                        for bound in &bound_pred.bounds {
+                            match bound {
+                                GenericBound::Trait(t, _) => {
+                                    if !t.bound_generic_params.is_empty() {
+                                        struct_span_err!(
+                                            self.err_handler(),
+                                            t.span,
+                                            E0316,
+                                            "nested quantification of lifetimes"
+                                        )
+                                        .emit();
+                                    }
+                                }
+                                GenericBound::Outlives(_) => {}
+                            }
+                        }
+                    }
+                }
+                _ => {}
+            }
+            self.visit_where_predicate(predicate);
+        }
+    }
+
+    fn visit_generic_param(&mut self, param: &'a GenericParam) {
+        if let GenericParamKind::Lifetime { .. } = param.kind {
+            self.check_lifetime(param.ident);
+        }
+        visit::walk_generic_param(self, param);
+    }
+
+    fn visit_param_bound(&mut self, bound: &'a GenericBound, ctxt: BoundKind) {
+        if let GenericBound::Trait(ref poly, modify) = *bound {
+            match (ctxt, modify) {
+                (BoundKind::SuperTraits, TraitBoundModifier::Maybe) => {
+                    let mut err = self
+                        .err_handler()
+                        .struct_span_err(poly.span, "`?Trait` is not permitted in supertraits");
+                    let path_str = pprust::path_to_string(&poly.trait_ref.path);
+                    err.note(&format!("traits are `?{}` by default", path_str));
+                    err.emit();
+                }
+                (BoundKind::TraitObject, TraitBoundModifier::Maybe) => {
+                    let mut err = self.err_handler().struct_span_err(
+                        poly.span,
+                        "`?Trait` is not permitted in trait object types",
+                    );
+                    err.emit();
+                }
+                (_, TraitBoundModifier::MaybeConst) => {
+                    if !self.is_tilde_const_allowed {
+                        self.err_handler()
+                            .struct_span_err(bound.span(), "`~const` is not allowed here")
+                            .note("only allowed on bounds on traits' associated types and functions, const fns, const impls and its associated functions")
+                            .emit();
+                    }
+                }
+                (_, TraitBoundModifier::MaybeConstMaybe) => {
+                    self.err_handler()
+                        .span_err(bound.span(), "`~const` and `?` are mutually exclusive");
+                }
+                _ => {}
+            }
+        }
+
+        visit::walk_param_bound(self, bound)
+    }
+
+    fn visit_poly_trait_ref(&mut self, t: &'a PolyTraitRef, m: &'a TraitBoundModifier) {
+        self.check_late_bound_lifetime_defs(&t.bound_generic_params);
+        visit::walk_poly_trait_ref(self, t, m);
+    }
+
+    fn visit_variant_data(&mut self, s: &'a VariantData) {
+        self.with_banned_assoc_ty_bound(|this| visit::walk_struct_def(this, s))
+    }
+
+    fn visit_enum_def(
+        &mut self,
+        enum_definition: &'a EnumDef,
+        generics: &'a Generics,
+        item_id: NodeId,
+        _: Span,
+    ) {
+        self.with_banned_assoc_ty_bound(|this| {
+            visit::walk_enum_def(this, enum_definition, generics, item_id)
+        })
+    }
+
+    fn visit_fn(&mut self, fk: FnKind<'a>, span: Span, id: NodeId) {
+        // Only associated `fn`s can have `self` parameters.
+        let self_semantic = match fk.ctxt() {
+            Some(FnCtxt::Assoc(_)) => SelfSemantic::Yes,
+            _ => SelfSemantic::No,
+        };
+        self.check_fn_decl(fk.decl(), self_semantic);
+
+        self.check_c_variadic_type(fk);
+
+        // Functions cannot both be `const async`
+        if let Some(FnHeader {
+            constness: Const::Yes(cspan),
+            asyncness: Async::Yes { span: aspan, .. },
+            ..
+        }) = fk.header()
+        {
+            self.err_handler()
+                .struct_span_err(
+                    vec![*cspan, *aspan],
+                    "functions cannot be both `const` and `async`",
+                )
+                .span_label(*cspan, "`const` because of this")
+                .span_label(*aspan, "`async` because of this")
+                .span_label(span, "") // Point at the fn header.
+                .emit();
+        }
+
+        if let FnKind::Closure(ClosureBinder::For { generic_params, .. }, ..) = fk {
+            self.check_late_bound_lifetime_defs(generic_params);
+        }
+
+        if let FnKind::Fn(
+            _,
+            _,
+            FnSig { span: sig_span, header: FnHeader { ext: Extern::Implicit(_), .. }, .. },
+            _,
+            _,
+            _,
+        ) = fk
+        {
+            self.maybe_lint_missing_abi(*sig_span, id);
+        }
+
+        // Functions without bodies cannot have patterns.
+        if let FnKind::Fn(ctxt, _, sig, _, _, None) = fk {
+            Self::check_decl_no_pat(&sig.decl, |span, ident, mut_ident| {
+                let (code, msg, label) = match ctxt {
+                    FnCtxt::Foreign => (
+                        error_code!(E0130),
+                        "patterns aren't allowed in foreign function declarations",
+                        "pattern not allowed in foreign function",
+                    ),
+                    _ => (
+                        error_code!(E0642),
+                        "patterns aren't allowed in functions without bodies",
+                        "pattern not allowed in function without body",
+                    ),
+                };
+                if mut_ident && matches!(ctxt, FnCtxt::Assoc(_)) {
+                    if let Some(ident) = ident {
+                        let diag = BuiltinLintDiagnostics::PatternsInFnsWithoutBody(span, ident);
+                        self.lint_buffer.buffer_lint_with_diagnostic(
+                            PATTERNS_IN_FNS_WITHOUT_BODY,
+                            id,
+                            span,
+                            msg,
+                            diag,
+                        )
+                    }
+                } else {
+                    self.err_handler()
+                        .struct_span_err(span, msg)
+                        .span_label(span, label)
+                        .code(code)
+                        .emit();
+                }
+            });
+        }
+
+        let tilde_const_allowed =
+            matches!(fk.header(), Some(FnHeader { constness: Const::Yes(_), .. }))
+                || matches!(fk.ctxt(), Some(FnCtxt::Assoc(_)));
+
+        self.with_tilde_const(tilde_const_allowed, |this| visit::walk_fn(this, fk, span));
+    }
+
+    fn visit_assoc_item(&mut self, item: &'a AssocItem, ctxt: AssocCtxt) {
+        if self.session.contains_name(&item.attrs, sym::no_mangle) {
+            self.check_nomangle_item_asciionly(item.ident, item.span);
+        }
+
+        if ctxt == AssocCtxt::Trait || !self.in_trait_impl {
+            self.check_defaultness(item.span, item.kind.defaultness());
+        }
+
+        if ctxt == AssocCtxt::Impl {
+            match &item.kind {
+                AssocItemKind::Const(_, _, body) => {
+                    self.check_impl_item_provided(item.span, body, "constant", " = <expr>;");
+                }
+                AssocItemKind::Fn(box Fn { body, .. }) => {
+                    self.check_impl_item_provided(item.span, body, "function", " { <body> }");
+                }
+                AssocItemKind::TyAlias(box TyAlias {
+                    generics,
+                    where_clauses,
+                    where_predicates_split,
+                    bounds,
+                    ty,
+                    ..
+                }) => {
+                    self.check_impl_item_provided(item.span, ty, "type", " = <type>;");
+                    self.check_type_no_bounds(bounds, "`impl`s");
+                    if ty.is_some() {
+                        self.check_gat_where(
+                            item.id,
+                            generics.where_clause.predicates.split_at(*where_predicates_split).0,
+                            *where_clauses,
+                        );
+                    }
+                }
+                _ => {}
+            }
+        }
+
+        if ctxt == AssocCtxt::Trait || self.in_trait_impl {
+            self.invalid_visibility(&item.vis, None);
+            if let AssocItemKind::Fn(box Fn { sig, .. }) = &item.kind {
+                self.check_trait_fn_not_const(sig.header.constness);
+                self.check_trait_fn_not_async(item.span, sig.header.asyncness);
+            }
+        }
+
+        if let AssocItemKind::Const(..) = item.kind {
+            self.check_item_named(item.ident, "const");
+        }
+
+        match item.kind {
+            AssocItemKind::TyAlias(box TyAlias { ref generics, ref bounds, ref ty, .. })
+                if ctxt == AssocCtxt::Trait =>
+            {
+                self.visit_vis(&item.vis);
+                self.visit_ident(item.ident);
+                walk_list!(self, visit_attribute, &item.attrs);
+                self.with_tilde_const_allowed(|this| {
+                    this.visit_generics(generics);
+                    walk_list!(this, visit_param_bound, bounds, BoundKind::Bound);
+                });
+                walk_list!(self, visit_ty, ty);
+            }
+            AssocItemKind::Fn(box Fn { ref sig, ref generics, ref body, .. })
+                if self.in_const_trait_impl
+                    || ctxt == AssocCtxt::Trait
+                    || matches!(sig.header.constness, Const::Yes(_)) =>
+            {
+                self.visit_vis(&item.vis);
+                self.visit_ident(item.ident);
+                let kind = FnKind::Fn(
+                    FnCtxt::Assoc(ctxt),
+                    item.ident,
+                    sig,
+                    &item.vis,
+                    generics,
+                    body.as_deref(),
+                );
+                self.visit_fn(kind, item.span, item.id);
+            }
+            _ => self
+                .with_in_trait_impl(false, None, |this| visit::walk_assoc_item(this, item, ctxt)),
+        }
+    }
+}
+
+/// When encountering an equality constraint in a `where` clause, emit an error. If the code seems
+/// like it's setting an associated type, provide an appropriate suggestion.
+fn deny_equality_constraints(
+    this: &mut AstValidator<'_>,
+    predicate: &WhereEqPredicate,
+    generics: &Generics,
+) {
+    let mut err = this.err_handler().struct_span_err(
+        predicate.span,
+        "equality constraints are not yet supported in `where` clauses",
+    );
+    err.span_label(predicate.span, "not supported");
+
+    // Given `<A as Foo>::Bar = RhsTy`, suggest `A: Foo<Bar = RhsTy>`.
+    if let TyKind::Path(Some(qself), full_path) = &predicate.lhs_ty.kind {
+        if let TyKind::Path(None, path) = &qself.ty.kind {
+            match &path.segments[..] {
+                [PathSegment { ident, args: None, .. }] => {
+                    for param in &generics.params {
+                        if param.ident == *ident {
+                            let param = ident;
+                            match &full_path.segments[qself.position..] {
+                                [PathSegment { ident, args, .. }] => {
+                                    // Make a new `Path` from `foo::Bar` to `Foo<Bar = RhsTy>`.
+                                    let mut assoc_path = full_path.clone();
+                                    // Remove `Bar` from `Foo::Bar`.
+                                    assoc_path.segments.pop();
+                                    let len = assoc_path.segments.len() - 1;
+                                    let gen_args = args.as_ref().map(|p| (**p).clone());
+                                    // Build `<Bar = RhsTy>`.
+                                    let arg = AngleBracketedArg::Constraint(AssocConstraint {
+                                        id: rustc_ast::node_id::DUMMY_NODE_ID,
+                                        ident: *ident,
+                                        gen_args,
+                                        kind: AssocConstraintKind::Equality {
+                                            term: predicate.rhs_ty.clone().into(),
+                                        },
+                                        span: ident.span,
+                                    });
+                                    // Add `<Bar = RhsTy>` to `Foo`.
+                                    match &mut assoc_path.segments[len].args {
+                                        Some(args) => match args.deref_mut() {
+                                            GenericArgs::Parenthesized(_) => continue,
+                                            GenericArgs::AngleBracketed(args) => {
+                                                args.args.push(arg);
+                                            }
+                                        },
+                                        empty_args => {
+                                            *empty_args = AngleBracketedArgs {
+                                                span: ident.span,
+                                                args: vec![arg],
+                                            }
+                                            .into();
+                                        }
+                                    }
+                                    err.span_suggestion_verbose(
+                                        predicate.span,
+                                        &format!(
+                                            "if `{}` is an associated type you're trying to set, \
+                                            use the associated type binding syntax",
+                                            ident
+                                        ),
+                                        format!(
+                                            "{}: {}",
+                                            param,
+                                            pprust::path_to_string(&assoc_path)
+                                        ),
+                                        Applicability::MaybeIncorrect,
+                                    );
+                                }
+                                _ => {}
+                            };
+                        }
+                    }
+                }
+                _ => {}
+            }
+        }
+    }
+    // Given `A: Foo, A::Bar = RhsTy`, suggest `A: Foo<Bar = RhsTy>`.
+    if let TyKind::Path(None, full_path) = &predicate.lhs_ty.kind {
+        if let [potential_param, potential_assoc] = &full_path.segments[..] {
+            for param in &generics.params {
+                if param.ident == potential_param.ident {
+                    for bound in &param.bounds {
+                        if let ast::GenericBound::Trait(trait_ref, TraitBoundModifier::None) = bound
+                        {
+                            if let [trait_segment] = &trait_ref.trait_ref.path.segments[..] {
+                                let assoc = pprust::path_to_string(&ast::Path::from_ident(
+                                    potential_assoc.ident,
+                                ));
+                                let ty = pprust::ty_to_string(&predicate.rhs_ty);
+                                let (args, span) = match &trait_segment.args {
+                                    Some(args) => match args.deref() {
+                                        ast::GenericArgs::AngleBracketed(args) => {
+                                            let Some(arg) = args.args.last() else {
+                                                continue;
+                                            };
+                                            (
+                                                format!(", {} = {}", assoc, ty),
+                                                arg.span().shrink_to_hi(),
+                                            )
+                                        }
+                                        _ => continue,
+                                    },
+                                    None => (
+                                        format!("<{} = {}>", assoc, ty),
+                                        trait_segment.span().shrink_to_hi(),
+                                    ),
+                                };
+                                err.multipart_suggestion(
+                                    &format!(
+                                        "if `{}::{}` is an associated type you're trying to set, \
+                                        use the associated type binding syntax",
+                                        trait_segment.ident, potential_assoc.ident,
+                                    ),
+                                    vec![(span, args), (predicate.span, String::new())],
+                                    Applicability::MaybeIncorrect,
+                                );
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+    err.note(
+        "see issue #20041 <https://github.com/rust-lang/rust/issues/20041> for more information",
+    );
+    err.emit();
+}
+
+pub fn check_crate(session: &Session, krate: &Crate, lints: &mut LintBuffer) -> bool {
+    let mut validator = AstValidator {
+        session,
+        extern_mod: None,
+        in_trait_impl: false,
+        in_const_trait_impl: false,
+        has_proc_macro_decls: false,
+        outer_impl_trait: None,
+        is_tilde_const_allowed: false,
+        is_impl_trait_banned: false,
+        is_assoc_ty_bound_banned: false,
+        forbidden_let_reason: Some(ForbiddenLetReason::GenericForbidden),
+        lint_buffer: lints,
+    };
+    visit::walk_crate(&mut validator, krate);
+
+    validator.has_proc_macro_decls
+}
+
+/// Used to forbid `let` expressions in certain syntactic locations.
+#[derive(Clone, Copy)]
+enum ForbiddenLetReason {
+    /// `let` is not valid and the source environment is not important
+    GenericForbidden,
+    /// A let chain with the `||` operator
+    NotSupportedOr(Span),
+    /// A let chain with invalid parentheses
+    ///
+    /// For exemple, `let 1 = 1 && (expr && expr)` is allowed
+    /// but `(let 1 = 1 && (let 1 = 1 && (let 1 = 1))) && let a = 1` is not
+    NotSupportedParentheses(Span),
+}
diff --git a/compiler/rustc_ast_passes/src/feature_gate.rs b/compiler/rustc_ast_passes/src/feature_gate.rs
new file mode 100644
index 000000000..789eca1f0
--- /dev/null
+++ b/compiler/rustc_ast_passes/src/feature_gate.rs
@@ -0,0 +1,901 @@
+use rustc_ast as ast;
+use rustc_ast::visit::{self, AssocCtxt, FnCtxt, FnKind, Visitor};
+use rustc_ast::{AssocConstraint, AssocConstraintKind, NodeId};
+use rustc_ast::{PatKind, RangeEnd, VariantData};
+use rustc_errors::{struct_span_err, Applicability};
+use rustc_feature::{AttributeGate, BuiltinAttribute, BUILTIN_ATTRIBUTE_MAP};
+use rustc_feature::{Features, GateIssue};
+use rustc_session::parse::{feature_err, feature_err_issue};
+use rustc_session::Session;
+use rustc_span::source_map::Spanned;
+use rustc_span::symbol::sym;
+use rustc_span::Span;
+
+use tracing::debug;
+
+macro_rules! gate_feature_fn {
+    ($visitor: expr, $has_feature: expr, $span: expr, $name: expr, $explain: expr, $help: expr) => {{
+        let (visitor, has_feature, span, name, explain, help) =
+            (&*$visitor, $has_feature, $span, $name, $explain, $help);
+        let has_feature: bool = has_feature(visitor.features);
+        debug!("gate_feature(feature = {:?}, span = {:?}); has? {}", name, span, has_feature);
+        if !has_feature && !span.allows_unstable($name) {
+            feature_err_issue(&visitor.sess.parse_sess, name, span, GateIssue::Language, explain)
+                .help(help)
+                .emit();
+        }
+    }};
+    ($visitor: expr, $has_feature: expr, $span: expr, $name: expr, $explain: expr) => {{
+        let (visitor, has_feature, span, name, explain) =
+            (&*$visitor, $has_feature, $span, $name, $explain);
+        let has_feature: bool = has_feature(visitor.features);
+        debug!("gate_feature(feature = {:?}, span = {:?}); has? {}", name, span, has_feature);
+        if !has_feature && !span.allows_unstable($name) {
+            feature_err_issue(&visitor.sess.parse_sess, name, span, GateIssue::Language, explain)
+                .emit();
+        }
+    }};
+}
+
+macro_rules! gate_feature_post {
+    ($visitor: expr, $feature: ident, $span: expr, $explain: expr, $help: expr) => {
+        gate_feature_fn!($visitor, |x: &Features| x.$feature, $span, sym::$feature, $explain, $help)
+    };
+    ($visitor: expr, $feature: ident, $span: expr, $explain: expr) => {
+        gate_feature_fn!($visitor, |x: &Features| x.$feature, $span, sym::$feature, $explain)
+    };
+}
+
+pub fn check_attribute(attr: &ast::Attribute, sess: &Session, features: &Features) {
+    PostExpansionVisitor { sess, features }.visit_attribute(attr)
+}
+
+struct PostExpansionVisitor<'a> {
+    sess: &'a Session,
+
+    // `sess` contains a `Features`, but this might not be that one.
+    features: &'a Features,
+}
+
+impl<'a> PostExpansionVisitor<'a> {
+    fn check_abi(&self, abi: ast::StrLit, constness: ast::Const) {
+        let ast::StrLit { symbol_unescaped, span, .. } = abi;
+
+        if let ast::Const::Yes(_) = constness {
+            match symbol_unescaped {
+                // Stable
+                sym::Rust | sym::C => {}
+                abi => gate_feature_post!(
+                    &self,
+                    const_extern_fn,
+                    span,
+                    &format!("`{}` as a `const fn` ABI is unstable", abi)
+                ),
+            }
+        }
+
+        match symbol_unescaped.as_str() {
+            // Stable
+            "Rust" | "C" | "cdecl" | "stdcall" | "fastcall" | "aapcs" | "win64" | "sysv64"
+            | "system" => {}
+            "rust-intrinsic" => {
+                gate_feature_post!(&self, intrinsics, span, "intrinsics are subject to change");
+            }
+            "platform-intrinsic" => {
+                gate_feature_post!(
+                    &self,
+                    platform_intrinsics,
+                    span,
+                    "platform intrinsics are experimental and possibly buggy"
+                );
+            }
+            "vectorcall" => {
+                gate_feature_post!(
+                    &self,
+                    abi_vectorcall,
+                    span,
+                    "vectorcall is experimental and subject to change"
+                );
+            }
+            "thiscall" => {
+                gate_feature_post!(
+                    &self,
+                    abi_thiscall,
+                    span,
+                    "thiscall is experimental and subject to change"
+                );
+            }
+            "rust-call" => {
+                gate_feature_post!(
+                    &self,
+                    unboxed_closures,
+                    span,
+                    "rust-call ABI is subject to change"
+                );
+            }
+            "rust-cold" => {
+                gate_feature_post!(
+                    &self,
+                    rust_cold_cc,
+                    span,
+                    "rust-cold is experimental and subject to change"
+                );
+            }
+            "ptx-kernel" => {
+                gate_feature_post!(
+                    &self,
+                    abi_ptx,
+                    span,
+                    "PTX ABIs are experimental and subject to change"
+                );
+            }
+            "unadjusted" => {
+                gate_feature_post!(
+                    &self,
+                    abi_unadjusted,
+                    span,
+                    "unadjusted ABI is an implementation detail and perma-unstable"
+                );
+            }
+            "msp430-interrupt" => {
+                gate_feature_post!(
+                    &self,
+                    abi_msp430_interrupt,
+                    span,
+                    "msp430-interrupt ABI is experimental and subject to change"
+                );
+            }
+            "x86-interrupt" => {
+                gate_feature_post!(
+                    &self,
+                    abi_x86_interrupt,
+                    span,
+                    "x86-interrupt ABI is experimental and subject to change"
+                );
+            }
+            "amdgpu-kernel" => {
+                gate_feature_post!(
+                    &self,
+                    abi_amdgpu_kernel,
+                    span,
+                    "amdgpu-kernel ABI is experimental and subject to change"
+                );
+            }
+            "avr-interrupt" | "avr-non-blocking-interrupt" => {
+                gate_feature_post!(
+                    &self,
+                    abi_avr_interrupt,
+                    span,
+                    "avr-interrupt and avr-non-blocking-interrupt ABIs are experimental and subject to change"
+                );
+            }
+            "efiapi" => {
+                gate_feature_post!(
+                    &self,
+                    abi_efiapi,
+                    span,
+                    "efiapi ABI is experimental and subject to change"
+                );
+            }
+            "C-cmse-nonsecure-call" => {
+                gate_feature_post!(
+                    &self,
+                    abi_c_cmse_nonsecure_call,
+                    span,
+                    "C-cmse-nonsecure-call ABI is experimental and subject to change"
+                );
+            }
+            "C-unwind" => {
+                gate_feature_post!(
+                    &self,
+                    c_unwind,
+                    span,
+                    "C-unwind ABI is experimental and subject to change"
+                );
+            }
+            "stdcall-unwind" => {
+                gate_feature_post!(
+                    &self,
+                    c_unwind,
+                    span,
+                    "stdcall-unwind ABI is experimental and subject to change"
+                );
+            }
+            "system-unwind" => {
+                gate_feature_post!(
+                    &self,
+                    c_unwind,
+                    span,
+                    "system-unwind ABI is experimental and subject to change"
+                );
+            }
+            "thiscall-unwind" => {
+                gate_feature_post!(
+                    &self,
+                    c_unwind,
+                    span,
+                    "thiscall-unwind ABI is experimental and subject to change"
+                );
+            }
+            "cdecl-unwind" => {
+                gate_feature_post!(
+                    &self,
+                    c_unwind,
+                    span,
+                    "cdecl-unwind ABI is experimental and subject to change"
+                );
+            }
+            "fastcall-unwind" => {
+                gate_feature_post!(
+                    &self,
+                    c_unwind,
+                    span,
+                    "fastcall-unwind ABI is experimental and subject to change"
+                );
+            }
+            "vectorcall-unwind" => {
+                gate_feature_post!(
+                    &self,
+                    c_unwind,
+                    span,
+                    "vectorcall-unwind ABI is experimental and subject to change"
+                );
+            }
+            "aapcs-unwind" => {
+                gate_feature_post!(
+                    &self,
+                    c_unwind,
+                    span,
+                    "aapcs-unwind ABI is experimental and subject to change"
+                );
+            }
+            "win64-unwind" => {
+                gate_feature_post!(
+                    &self,
+                    c_unwind,
+                    span,
+                    "win64-unwind ABI is experimental and subject to change"
+                );
+            }
+            "sysv64-unwind" => {
+                gate_feature_post!(
+                    &self,
+                    c_unwind,
+                    span,
+                    "sysv64-unwind ABI is experimental and subject to change"
+                );
+            }
+            "wasm" => {
+                gate_feature_post!(
+                    &self,
+                    wasm_abi,
+                    span,
+                    "wasm ABI is experimental and subject to change"
+                );
+            }
+            abi => {
+                if self.sess.opts.pretty.map_or(true, |ppm| ppm.needs_hir()) {
+                    self.sess.parse_sess.span_diagnostic.delay_span_bug(
+                        span,
+                        &format!("unrecognized ABI not caught in lowering: {}", abi),
+                    );
+                }
+            }
+        }
+    }
+
+    fn check_extern(&self, ext: ast::Extern, constness: ast::Const) {
+        if let ast::Extern::Explicit(abi, _) = ext {
+            self.check_abi(abi, constness);
+        }
+    }
+
+    fn maybe_report_invalid_custom_discriminants(&self, variants: &[ast::Variant]) {
+        let has_fields = variants.iter().any(|variant| match variant.data {
+            VariantData::Tuple(..) | VariantData::Struct(..) => true,
+            VariantData::Unit(..) => false,
+        });
+
+        let discriminant_spans = variants
+            .iter()
+            .filter(|variant| match variant.data {
+                VariantData::Tuple(..) | VariantData::Struct(..) => false,
+                VariantData::Unit(..) => true,
+            })
+            .filter_map(|variant| variant.disr_expr.as_ref().map(|c| c.value.span))
+            .collect::<Vec<_>>();
+
+        if !discriminant_spans.is_empty() && has_fields {
+            let mut err = feature_err(
+                &self.sess.parse_sess,
+                sym::arbitrary_enum_discriminant,
+                discriminant_spans.clone(),
+                "custom discriminant values are not allowed in enums with tuple or struct variants",
+            );
+            for sp in discriminant_spans {
+                err.span_label(sp, "disallowed custom discriminant");
+            }
+            for variant in variants.iter() {
+                match &variant.data {
+                    VariantData::Struct(..) => {
+                        err.span_label(variant.span, "struct variant defined here");
+                    }
+                    VariantData::Tuple(..) => {
+                        err.span_label(variant.span, "tuple variant defined here");
+                    }
+                    VariantData::Unit(..) => {}
+                }
+            }
+            err.emit();
+        }
+    }
+
+    fn check_gat(&self, generics: &ast::Generics, span: Span) {
+        if !generics.params.is_empty() {
+            gate_feature_post!(
+                &self,
+                generic_associated_types,
+                span,
+                "generic associated types are unstable"
+            );
+        }
+        if !generics.where_clause.predicates.is_empty() {
+            gate_feature_post!(
+                &self,
+                generic_associated_types,
+                span,
+                "where clauses on associated types are unstable"
+            );
+        }
+    }
+
+    /// Feature gate `impl Trait` inside `type Alias = $type_expr;`.
+    fn check_impl_trait(&self, ty: &ast::Ty) {
+        struct ImplTraitVisitor<'a> {
+            vis: &'a PostExpansionVisitor<'a>,
+        }
+        impl Visitor<'_> for ImplTraitVisitor<'_> {
+            fn visit_ty(&mut self, ty: &ast::Ty) {
+                if let ast::TyKind::ImplTrait(..) = ty.kind {
+                    gate_feature_post!(
+                        &self.vis,
+                        type_alias_impl_trait,
+                        ty.span,
+                        "`impl Trait` in type aliases is unstable"
+                    );
+                }
+                visit::walk_ty(self, ty);
+            }
+        }
+        ImplTraitVisitor { vis: self }.visit_ty(ty);
+    }
+}
+
+impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
+    fn visit_attribute(&mut self, attr: &ast::Attribute) {
+        let attr_info = attr.ident().and_then(|ident| BUILTIN_ATTRIBUTE_MAP.get(&ident.name));
+        // Check feature gates for built-in attributes.
+        if let Some(BuiltinAttribute {
+            gate: AttributeGate::Gated(_, name, descr, has_feature),
+            ..
+        }) = attr_info
+        {
+            gate_feature_fn!(self, has_feature, attr.span, *name, descr);
+        }
+        // Check unstable flavors of the `#[doc]` attribute.
+        if attr.has_name(sym::doc) {
+            for nested_meta in attr.meta_item_list().unwrap_or_default() {
+                macro_rules! gate_doc { ($($name:ident => $feature:ident)*) => {
+                    $(if nested_meta.has_name(sym::$name) {
+                        let msg = concat!("`#[doc(", stringify!($name), ")]` is experimental");
+                        gate_feature_post!(self, $feature, attr.span, msg);
+                    })*
+                }}
+
+                gate_doc!(
+                    cfg => doc_cfg
+                    cfg_hide => doc_cfg_hide
+                    masked => doc_masked
+                    notable_trait => doc_notable_trait
+                );
+
+                if nested_meta.has_name(sym::keyword) {
+                    let msg = "`#[doc(keyword)]` is meant for internal use only";
+                    gate_feature_post!(self, rustdoc_internals, attr.span, msg);
+                }
+
+                if nested_meta.has_name(sym::fake_variadic) {
+                    let msg = "`#[doc(fake_variadic)]` is meant for internal use only";
+                    gate_feature_post!(self, rustdoc_internals, attr.span, msg);
+                }
+            }
+        }
+
+        // Emit errors for non-staged-api crates.
+        if !self.features.staged_api {
+            if attr.has_name(sym::unstable)
+                || attr.has_name(sym::stable)
+                || attr.has_name(sym::rustc_const_unstable)
+                || attr.has_name(sym::rustc_const_stable)
+            {
+                struct_span_err!(
+                    self.sess,
+                    attr.span,
+                    E0734,
+                    "stability attributes may not be used outside of the standard library",
+                )
+                .emit();
+            }
+        }
+    }
+
+    fn visit_item(&mut self, i: &'a ast::Item) {
+        match i.kind {
+            ast::ItemKind::ForeignMod(ref foreign_module) => {
+                if let Some(abi) = foreign_module.abi {
+                    self.check_abi(abi, ast::Const::No);
+                }
+            }
+
+            ast::ItemKind::Fn(..) => {
+                if self.sess.contains_name(&i.attrs, sym::start) {
+                    gate_feature_post!(
+                        &self,
+                        start,
+                        i.span,
+                        "`#[start]` functions are experimental \
+                         and their signature may change \
+                         over time"
+                    );
+                }
+            }
+
+            ast::ItemKind::Struct(..) => {
+                for attr in self.sess.filter_by_name(&i.attrs, sym::repr) {
+                    for item in attr.meta_item_list().unwrap_or_else(Vec::new) {
+                        if item.has_name(sym::simd) {
+                            gate_feature_post!(
+                                &self,
+                                repr_simd,
+                                attr.span,
+                                "SIMD types are experimental and possibly buggy"
+                            );
+                        }
+                    }
+                }
+            }
+
+            ast::ItemKind::Enum(ast::EnumDef { ref variants, .. }, ..) => {
+                for variant in variants {
+                    match (&variant.data, &variant.disr_expr) {
+                        (ast::VariantData::Unit(..), _) => {}
+                        (_, Some(disr_expr)) => gate_feature_post!(
+                            &self,
+                            arbitrary_enum_discriminant,
+                            disr_expr.value.span,
+                            "discriminants on non-unit variants are experimental"
+                        ),
+                        _ => {}
+                    }
+                }
+
+                let has_feature = self.features.arbitrary_enum_discriminant;
+                if !has_feature && !i.span.allows_unstable(sym::arbitrary_enum_discriminant) {
+                    self.maybe_report_invalid_custom_discriminants(&variants);
+                }
+            }
+
+            ast::ItemKind::Impl(box ast::Impl { polarity, defaultness, ref of_trait, .. }) => {
+                if let ast::ImplPolarity::Negative(span) = polarity {
+                    gate_feature_post!(
+                        &self,
+                        negative_impls,
+                        span.to(of_trait.as_ref().map_or(span, |t| t.path.span)),
+                        "negative trait bounds are not yet fully implemented; \
+                         use marker types for now"
+                    );
+                }
+
+                if let ast::Defaultness::Default(_) = defaultness {
+                    gate_feature_post!(&self, specialization, i.span, "specialization is unstable");
+                }
+            }
+
+            ast::ItemKind::Trait(box ast::Trait { is_auto: ast::IsAuto::Yes, .. }) => {
+                gate_feature_post!(
+                    &self,
+                    auto_traits,
+                    i.span,
+                    "auto traits are experimental and possibly buggy"
+                );
+            }
+
+            ast::ItemKind::TraitAlias(..) => {
+                gate_feature_post!(&self, trait_alias, i.span, "trait aliases are experimental");
+            }
+
+            ast::ItemKind::MacroDef(ast::MacroDef { macro_rules: false, .. }) => {
+                let msg = "`macro` is experimental";
+                gate_feature_post!(&self, decl_macro, i.span, msg);
+            }
+
+            ast::ItemKind::TyAlias(box ast::TyAlias { ty: Some(ref ty), .. }) => {
+                self.check_impl_trait(&ty)
+            }
+
+            _ => {}
+        }
+
+        visit::walk_item(self, i);
+    }
+
+    fn visit_foreign_item(&mut self, i: &'a ast::ForeignItem) {
+        match i.kind {
+            ast::ForeignItemKind::Fn(..) | ast::ForeignItemKind::Static(..) => {
+                let link_name = self.sess.first_attr_value_str_by_name(&i.attrs, sym::link_name);
+                let links_to_llvm =
+                    link_name.map_or(false, |val| val.as_str().starts_with("llvm."));
+                if links_to_llvm {
+                    gate_feature_post!(
+                        &self,
+                        link_llvm_intrinsics,
+                        i.span,
+                        "linking to LLVM intrinsics is experimental"
+                    );
+                }
+            }
+            ast::ForeignItemKind::TyAlias(..) => {
+                gate_feature_post!(&self, extern_types, i.span, "extern types are experimental");
+            }
+            ast::ForeignItemKind::MacCall(..) => {}
+        }
+
+        visit::walk_foreign_item(self, i)
+    }
+
+    fn visit_ty(&mut self, ty: &'a ast::Ty) {
+        match ty.kind {
+            ast::TyKind::BareFn(ref bare_fn_ty) => {
+                // Function pointers cannot be `const`
+                self.check_extern(bare_fn_ty.ext, ast::Const::No);
+            }
+            ast::TyKind::Never => {
+                gate_feature_post!(&self, never_type, ty.span, "the `!` type is experimental");
+            }
+            _ => {}
+        }
+        visit::walk_ty(self, ty)
+    }
+
+    fn visit_fn_ret_ty(&mut self, ret_ty: &'a ast::FnRetTy) {
+        if let ast::FnRetTy::Ty(ref output_ty) = *ret_ty {
+            if let ast::TyKind::Never = output_ty.kind {
+                // Do nothing.
+            } else {
+                self.visit_ty(output_ty)
+            }
+        }
+    }
+
+    fn visit_stmt(&mut self, stmt: &'a ast::Stmt) {
+        if let ast::StmtKind::Semi(expr) = &stmt.kind
+            && let ast::ExprKind::Assign(lhs, _, _) = &expr.kind
+            && let ast::ExprKind::Type(..) = lhs.kind
+            && self.sess.parse_sess.span_diagnostic.err_count() == 0
+            && !self.features.type_ascription
+            && !lhs.span.allows_unstable(sym::type_ascription)
+        {
+            // When we encounter a statement of the form `foo: Ty = val;`, this will emit a type
+            // ascription error, but the likely intention was to write a `let` statement. (#78907).
+            feature_err_issue(
+                &self.sess.parse_sess,
+                sym::type_ascription,
+                lhs.span,
+                GateIssue::Language,
+                "type ascription is experimental",
+            ).span_suggestion_verbose(
+                lhs.span.shrink_to_lo(),
+                "you might have meant to introduce a new binding",
+                "let ".to_string(),
+                Applicability::MachineApplicable,
+            ).emit();
+        }
+        visit::walk_stmt(self, stmt);
+    }
+
+    fn visit_expr(&mut self, e: &'a ast::Expr) {
+        match e.kind {
+            ast::ExprKind::Box(_) => {
+                gate_feature_post!(
+                    &self,
+                    box_syntax,
+                    e.span,
+                    "box expression syntax is experimental; you can call `Box::new` instead"
+                );
+            }
+            ast::ExprKind::Type(..) => {
+                // To avoid noise about type ascription in common syntax errors, only emit if it
+                // is the *only* error.
+                if self.sess.parse_sess.span_diagnostic.err_count() == 0 {
+                    gate_feature_post!(
+                        &self,
+                        type_ascription,
+                        e.span,
+                        "type ascription is experimental"
+                    );
+                }
+            }
+            ast::ExprKind::TryBlock(_) => {
+                gate_feature_post!(&self, try_blocks, e.span, "`try` expression is experimental");
+            }
+            ast::ExprKind::Block(_, Some(label)) => {
+                gate_feature_post!(
+                    &self,
+                    label_break_value,
+                    label.ident.span,
+                    "labels on blocks are unstable"
+                );
+            }
+            _ => {}
+        }
+        visit::walk_expr(self, e)
+    }
+
+    fn visit_pat(&mut self, pattern: &'a ast::Pat) {
+        match &pattern.kind {
+            PatKind::Slice(pats) => {
+                for pat in pats {
+                    let inner_pat = match &pat.kind {
+                        PatKind::Ident(.., Some(pat)) => pat,
+                        _ => pat,
+                    };
+                    if let PatKind::Range(Some(_), None, Spanned { .. }) = inner_pat.kind {
+                        gate_feature_post!(
+                            &self,
+                            half_open_range_patterns,
+                            pat.span,
+                            "`X..` patterns in slices are experimental"
+                        );
+                    }
+                }
+            }
+            PatKind::Box(..) => {
+                gate_feature_post!(
+                    &self,
+                    box_patterns,
+                    pattern.span,
+                    "box pattern syntax is experimental"
+                );
+            }
+            PatKind::Range(_, Some(_), Spanned { node: RangeEnd::Excluded, .. }) => {
+                gate_feature_post!(
+                    &self,
+                    exclusive_range_pattern,
+                    pattern.span,
+                    "exclusive range pattern syntax is experimental"
+                );
+            }
+            _ => {}
+        }
+        visit::walk_pat(self, pattern)
+    }
+
+    fn visit_fn(&mut self, fn_kind: FnKind<'a>, span: Span, _: NodeId) {
+        if let Some(header) = fn_kind.header() {
+            // Stability of const fn methods are covered in `visit_assoc_item` below.
+            self.check_extern(header.ext, header.constness);
+        }
+
+        if fn_kind.ctxt() != Some(FnCtxt::Foreign) && fn_kind.decl().c_variadic() {
+            gate_feature_post!(&self, c_variadic, span, "C-variadic functions are unstable");
+        }
+
+        visit::walk_fn(self, fn_kind, span)
+    }
+
+    fn visit_assoc_constraint(&mut self, constraint: &'a AssocConstraint) {
+        if let AssocConstraintKind::Bound { .. } = constraint.kind {
+            gate_feature_post!(
+                &self,
+                associated_type_bounds,
+                constraint.span,
+                "associated type bounds are unstable"
+            )
+        }
+        visit::walk_assoc_constraint(self, constraint)
+    }
+
+    fn visit_assoc_item(&mut self, i: &'a ast::AssocItem, ctxt: AssocCtxt) {
+        let is_fn = match i.kind {
+            ast::AssocItemKind::Fn(_) => true,
+            ast::AssocItemKind::TyAlias(box ast::TyAlias { ref generics, ref ty, .. }) => {
+                if let (Some(_), AssocCtxt::Trait) = (ty, ctxt) {
+                    gate_feature_post!(
+                        &self,
+                        associated_type_defaults,
+                        i.span,
+                        "associated type defaults are unstable"
+                    );
+                }
+                if let Some(ty) = ty {
+                    self.check_impl_trait(ty);
+                }
+                self.check_gat(generics, i.span);
+                false
+            }
+            _ => false,
+        };
+        if let ast::Defaultness::Default(_) = i.kind.defaultness() {
+            // Limit `min_specialization` to only specializing functions.
+            gate_feature_fn!(
+                &self,
+                |x: &Features| x.specialization || (is_fn && x.min_specialization),
+                i.span,
+                sym::specialization,
+                "specialization is unstable"
+            );
+        }
+        visit::walk_assoc_item(self, i, ctxt)
+    }
+}
+
+pub fn check_crate(krate: &ast::Crate, sess: &Session) {
+    maybe_stage_features(sess, krate);
+    check_incompatible_features(sess);
+    let mut visitor = PostExpansionVisitor { sess, features: &sess.features_untracked() };
+
+    let spans = sess.parse_sess.gated_spans.spans.borrow();
+    macro_rules! gate_all {
+        ($gate:ident, $msg:literal, $help:literal) => {
+            if let Some(spans) = spans.get(&sym::$gate) {
+                for span in spans {
+                    gate_feature_post!(&visitor, $gate, *span, $msg, $help);
+                }
+            }
+        };
+        ($gate:ident, $msg:literal) => {
+            if let Some(spans) = spans.get(&sym::$gate) {
+                for span in spans {
+                    gate_feature_post!(&visitor, $gate, *span, $msg);
+                }
+            }
+        };
+    }
+    gate_all!(
+        if_let_guard,
+        "`if let` guards are experimental",
+        "you can write `if matches!(<expr>, <pattern>)` instead of `if let <pattern> = <expr>`"
+    );
+    gate_all!(let_chains, "`let` expressions in this position are unstable");
+    gate_all!(
+        async_closure,
+        "async closures are unstable",
+        "to use an async block, remove the `||`: `async {`"
+    );
+    gate_all!(
+        closure_lifetime_binder,
+        "`for<...>` binders for closures are experimental",
+        "consider removing `for<...>`"
+    );
+    gate_all!(more_qualified_paths, "usage of qualified paths in this context is experimental");
+    gate_all!(generators, "yield syntax is experimental");
+    gate_all!(raw_ref_op, "raw address of syntax is experimental");
+    gate_all!(const_trait_impl, "const trait impls are experimental");
+    gate_all!(half_open_range_patterns, "half-open range patterns are unstable");
+    gate_all!(inline_const, "inline-const is experimental");
+    gate_all!(inline_const_pat, "inline-const in pattern position is experimental");
+    gate_all!(associated_const_equality, "associated const equality is incomplete");
+    gate_all!(yeet_expr, "`do yeet` expression is experimental");
+
+    // All uses of `gate_all!` below this point were added in #65742,
+    // and subsequently disabled (with the non-early gating readded).
+    macro_rules! gate_all {
+        ($gate:ident, $msg:literal) => {
+            // FIXME(eddyb) do something more useful than always
+            // disabling these uses of early feature-gatings.
+            if false {
+                for span in spans.get(&sym::$gate).unwrap_or(&vec![]) {
+                    gate_feature_post!(&visitor, $gate, *span, $msg);
+                }
+            }
+        };
+    }
+
+    gate_all!(trait_alias, "trait aliases are experimental");
+    gate_all!(associated_type_bounds, "associated type bounds are unstable");
+    gate_all!(decl_macro, "`macro` is experimental");
+    gate_all!(box_patterns, "box pattern syntax is experimental");
+    gate_all!(exclusive_range_pattern, "exclusive range pattern syntax is experimental");
+    gate_all!(try_blocks, "`try` blocks are unstable");
+    gate_all!(label_break_value, "labels on blocks are unstable");
+    gate_all!(box_syntax, "box expression syntax is experimental; you can call `Box::new` instead");
+    // To avoid noise about type ascription in common syntax errors,
+    // only emit if it is the *only* error. (Also check it last.)
+    if sess.parse_sess.span_diagnostic.err_count() == 0 {
+        gate_all!(type_ascription, "type ascription is experimental");
+    }
+
+    visit::walk_crate(&mut visitor, krate);
+}
+
+fn maybe_stage_features(sess: &Session, krate: &ast::Crate) {
+    // checks if `#![feature]` has been used to enable any lang feature
+    // does not check the same for lib features unless there's at least one
+    // declared lang feature
+    if !sess.opts.unstable_features.is_nightly_build() {
+        let lang_features = &sess.features_untracked().declared_lang_features;
+        if lang_features.len() == 0 {
+            return;
+        }
+        for attr in krate.attrs.iter().filter(|attr| attr.has_name(sym::feature)) {
+            let mut err = struct_span_err!(
+                sess.parse_sess.span_diagnostic,
+                attr.span,
+                E0554,
+                "`#![feature]` may not be used on the {} release channel",
+                option_env!("CFG_RELEASE_CHANNEL").unwrap_or("(unknown)")
+            );
+            let mut all_stable = true;
+            for ident in
+                attr.meta_item_list().into_iter().flatten().flat_map(|nested| nested.ident())
+            {
+                let name = ident.name;
+                let stable_since = lang_features
+                    .iter()
+                    .flat_map(|&(feature, _, since)| if feature == name { since } else { None })
+                    .next();
+                if let Some(since) = stable_since {
+                    err.help(&format!(
+                        "the feature `{}` has been stable since {} and no longer requires \
+                                  an attribute to enable",
+                        name, since
+                    ));
+                } else {
+                    all_stable = false;
+                }
+            }
+            if all_stable {
+                err.span_suggestion(
+                    attr.span,
+                    "remove the attribute",
+                    "",
+                    Applicability::MachineApplicable,
+                );
+            }
+            err.emit();
+        }
+    }
+}
+
+fn check_incompatible_features(sess: &Session) {
+    let features = sess.features_untracked();
+
+    let declared_features = features
+        .declared_lang_features
+        .iter()
+        .copied()
+        .map(|(name, span, _)| (name, span))
+        .chain(features.declared_lib_features.iter().copied());
+
+    for (f1, f2) in rustc_feature::INCOMPATIBLE_FEATURES
+        .iter()
+        .filter(|&&(f1, f2)| features.enabled(f1) && features.enabled(f2))
+    {
+        if let Some((f1_name, f1_span)) = declared_features.clone().find(|(name, _)| name == f1) {
+            if let Some((f2_name, f2_span)) = declared_features.clone().find(|(name, _)| name == f2)
+            {
+                let spans = vec![f1_span, f2_span];
+                sess.struct_span_err(
+                    spans.clone(),
+                    &format!(
+                        "features `{}` and `{}` are incompatible, using them at the same time \
+                        is not allowed",
+                        f1_name, f2_name
+                    ),
+                )
+                .help("remove one of these features")
+                .emit();
+            }
+        }
+    }
+}
diff --git a/compiler/rustc_ast_passes/src/lib.rs b/compiler/rustc_ast_passes/src/lib.rs
new file mode 100644
index 000000000..9d52c3288
--- /dev/null
+++ b/compiler/rustc_ast_passes/src/lib.rs
@@ -0,0 +1,18 @@
+//! The `rustc_ast_passes` crate contains passes which validate the AST in `syntax`
+//! parsed by `rustc_parse` and then lowered, after the passes in this crate,
+//! by `rustc_ast_lowering`.
+//!
+//! The crate also contains other misc AST visitors, e.g. `node_count` and `show_span`.
+
+#![allow(rustc::potential_query_instability)]
+#![feature(box_patterns)]
+#![feature(if_let_guard)]
+#![feature(iter_is_partitioned)]
+#![feature(let_chains)]
+#![feature(let_else)]
+#![recursion_limit = "256"]
+
+pub mod ast_validation;
+pub mod feature_gate;
+pub mod node_count;
+pub mod show_span;
diff --git a/compiler/rustc_ast_passes/src/node_count.rs b/compiler/rustc_ast_passes/src/node_count.rs
new file mode 100644
index 000000000..9c7369c83
--- /dev/null
+++ b/compiler/rustc_ast_passes/src/node_count.rs
@@ -0,0 +1,135 @@
+// Simply gives a rough count of the number of nodes in an AST.
+
+use rustc_ast::visit::*;
+use rustc_ast::*;
+use rustc_span::symbol::Ident;
+use rustc_span::Span;
+
+pub struct NodeCounter {
+    pub count: usize,
+}
+
+impl NodeCounter {
+    pub fn new() -> NodeCounter {
+        NodeCounter { count: 0 }
+    }
+}
+
+impl<'ast> Visitor<'ast> for NodeCounter {
+    fn visit_ident(&mut self, _ident: Ident) {
+        self.count += 1;
+    }
+    fn visit_foreign_item(&mut self, i: &ForeignItem) {
+        self.count += 1;
+        walk_foreign_item(self, i)
+    }
+    fn visit_item(&mut self, i: &Item) {
+        self.count += 1;
+        walk_item(self, i)
+    }
+    fn visit_local(&mut self, l: &Local) {
+        self.count += 1;
+        walk_local(self, l)
+    }
+    fn visit_block(&mut self, b: &Block) {
+        self.count += 1;
+        walk_block(self, b)
+    }
+    fn visit_stmt(&mut self, s: &Stmt) {
+        self.count += 1;
+        walk_stmt(self, s)
+    }
+    fn visit_arm(&mut self, a: &Arm) {
+        self.count += 1;
+        walk_arm(self, a)
+    }
+    fn visit_pat(&mut self, p: &Pat) {
+        self.count += 1;
+        walk_pat(self, p)
+    }
+    fn visit_expr(&mut self, ex: &Expr) {
+        self.count += 1;
+        walk_expr(self, ex)
+    }
+    fn visit_ty(&mut self, t: &Ty) {
+        self.count += 1;
+        walk_ty(self, t)
+    }
+    fn visit_generic_param(&mut self, param: &GenericParam) {
+        self.count += 1;
+        walk_generic_param(self, param)
+    }
+    fn visit_generics(&mut self, g: &Generics) {
+        self.count += 1;
+        walk_generics(self, g)
+    }
+    fn visit_fn(&mut self, fk: visit::FnKind<'_>, s: Span, _: NodeId) {
+        self.count += 1;
+        walk_fn(self, fk, s)
+    }
+    fn visit_assoc_item(&mut self, ti: &AssocItem, ctxt: AssocCtxt) {
+        self.count += 1;
+        walk_assoc_item(self, ti, ctxt);
+    }
+    fn visit_trait_ref(&mut self, t: &TraitRef) {
+        self.count += 1;
+        walk_trait_ref(self, t)
+    }
+    fn visit_param_bound(&mut self, bounds: &GenericBound, _ctxt: BoundKind) {
+        self.count += 1;
+        walk_param_bound(self, bounds)
+    }
+    fn visit_poly_trait_ref(&mut self, t: &PolyTraitRef, m: &TraitBoundModifier) {
+        self.count += 1;
+        walk_poly_trait_ref(self, t, m)
+    }
+    fn visit_variant_data(&mut self, s: &VariantData) {
+        self.count += 1;
+        walk_struct_def(self, s)
+    }
+    fn visit_field_def(&mut self, s: &FieldDef) {
+        self.count += 1;
+        walk_field_def(self, s)
+    }
+    fn visit_enum_def(
+        &mut self,
+        enum_definition: &EnumDef,
+        generics: &Generics,
+        item_id: NodeId,
+        _: Span,
+    ) {
+        self.count += 1;
+        walk_enum_def(self, enum_definition, generics, item_id)
+    }
+    fn visit_variant(&mut self, v: &Variant) {
+        self.count += 1;
+        walk_variant(self, v)
+    }
+    fn visit_lifetime(&mut self, lifetime: &Lifetime, _: visit::LifetimeCtxt) {
+        self.count += 1;
+        walk_lifetime(self, lifetime)
+    }
+    fn visit_mac_call(&mut self, mac: &MacCall) {
+        self.count += 1;
+        walk_mac(self, mac)
+    }
+    fn visit_path(&mut self, path: &Path, _id: NodeId) {
+        self.count += 1;
+        walk_path(self, path)
+    }
+    fn visit_use_tree(&mut self, use_tree: &UseTree, id: NodeId, _nested: bool) {
+        self.count += 1;
+        walk_use_tree(self, use_tree, id)
+    }
+    fn visit_generic_args(&mut self, path_span: Span, generic_args: &GenericArgs) {
+        self.count += 1;
+        walk_generic_args(self, path_span, generic_args)
+    }
+    fn visit_assoc_constraint(&mut self, constraint: &AssocConstraint) {
+        self.count += 1;
+        walk_assoc_constraint(self, constraint)
+    }
+    fn visit_attribute(&mut self, _attr: &Attribute) {
+        self.count += 1;
+    }
+}
diff --git a/compiler/rustc_ast_passes/src/show_span.rs b/compiler/rustc_ast_passes/src/show_span.rs
new file mode 100644
index 000000000..27637e311
--- /dev/null
+++ b/compiler/rustc_ast_passes/src/show_span.rs
@@ -0,0 +1,65 @@
+//! Span debugger
+//!
+//! This module shows spans for all expressions in the crate
+//! to help with compiler debugging.
+
+use std::str::FromStr;
+
+use rustc_ast as ast;
+use rustc_ast::visit;
+use rustc_ast::visit::Visitor;
+
+enum Mode {
+    Expression,
+    Pattern,
+    Type,
+}
+
+impl FromStr for Mode {
+    type Err = ();
+    fn from_str(s: &str) -> Result<Mode, ()> {
+        let mode = match s {
+            "expr" => Mode::Expression,
+            "pat" => Mode::Pattern,
+            "ty" => Mode::Type,
+            _ => return Err(()),
+        };
+        Ok(mode)
+    }
+}
+
+struct ShowSpanVisitor<'a> {
+    span_diagnostic: &'a rustc_errors::Handler,
+    mode: Mode,
+}
+
+impl<'a> Visitor<'a> for ShowSpanVisitor<'a> {
+    fn visit_expr(&mut self, e: &'a ast::Expr) {
+        if let Mode::Expression = self.mode {
+            self.span_diagnostic.span_warn(e.span, "expression");
+        }
+        visit::walk_expr(self, e);
+    }
+
+    fn visit_pat(&mut self, p: &'a ast::Pat) {
+        if let Mode::Pattern = self.mode {
+            self.span_diagnostic.span_warn(p.span, "pattern");
+        }
+        visit::walk_pat(self, p);
+    }
+
+    fn visit_ty(&mut self, t: &'a ast::Ty) {
+        if let Mode::Type = self.mode {
+            self.span_diagnostic.span_warn(t.span, "type");
+        }
+        visit::walk_ty(self, t);
+    }
+}
+
+pub fn run(span_diagnostic: &rustc_errors::Handler, mode: &str, krate: &ast::Crate) {
+    let Ok(mode) = mode.parse() else {
+        return;
+    };
+    let mut v = ShowSpanVisitor { span_diagnostic, mode };
+    visit::walk_crate(&mut v, krate);
+}
-- 
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