Explicitly export core and std macros

compiler/rustc_builtin_macros/src/standard_library_imports.rs

@@ -43,7 +43,7 @@ pub fn inject(
 
     let item = cx.item(
         span,
-        thin_vec![cx.attr_word(sym::macro_use, span)],
+        ast::AttrVec::new(),
         ast::ItemKind::ExternCrate(None, Ident::new(name, ident_span)),
     );
 

compiler/rustc_lint/src/early/diagnostics.rs

@@ -257,8 +257,8 @@ pub fn decorate_builtin_lint(
 
             lints::ExternCrateNotIdiomatic { span: suggestion_span, code }.decorate_lint(diag);
         }
-        BuiltinLintDiag::AmbiguousGlobImports { diag: ambiguity } => {
-            lints::AmbiguousGlobImports { ambiguity }.decorate_lint(diag);
+        BuiltinLintDiag::AmbiguousImports { diag: ambiguity } => {
+            lints::AmbiguousImports { ambiguity }.decorate_lint(diag);
         }
         BuiltinLintDiag::AmbiguousGlobReexports {
             name,

compiler/rustc_lint/src/lints.rs

@@ -2870,11 +2870,11 @@ pub(crate) struct ExternCrateNotIdiomatic {
 }
 
 // FIXME: make this translatable
-pub(crate) struct AmbiguousGlobImports {
+pub(crate) struct AmbiguousImports {
     pub ambiguity: AmbiguityErrorDiag,
 }
 
-impl<'a, G: EmissionGuarantee> LintDiagnostic<'a, G> for AmbiguousGlobImports {
+impl<'a, G: EmissionGuarantee> LintDiagnostic<'a, G> for AmbiguousImports {
     fn decorate_lint<'b>(self, diag: &'b mut Diag<'a, G>) {
         diag.primary_message(self.ambiguity.msg.clone());
         rustc_errors::report_ambiguity_error(diag, self.ambiguity);

compiler/rustc_lint_defs/src/builtin.rs

@@ -21,6 +21,7 @@ declare_lint_pass! {
         AMBIGUOUS_ASSOCIATED_ITEMS,
         AMBIGUOUS_GLOB_IMPORTS,
         AMBIGUOUS_GLOB_REEXPORTS,
+        AMBIGUOUS_PANIC_IMPORTS,
         ARITHMETIC_OVERFLOW,
         ASM_SUB_REGISTER,
         BAD_ASM_STYLE,
@@ -4488,6 +4489,43 @@ declare_lint! {
     };
 }
 
+declare_lint! {
+    /// The `ambiguous_panic_imports` lint detects ambiguous core and std panic imports, but
+    /// previously didn't do that due to `#[macro_use]` prelude macro import.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![deny(ambiguous_panic_imports)]
+    /// #![no_std]
+    ///
+    /// extern crate std;
+    /// use std::prelude::v1::*;
+    ///
+    /// fn xx() {
+    ///     panic!(); // resolves to core::panic
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Future versions of Rust will no longer accept the ambiguous resolution.
+    ///
+    /// This is a [future-incompatible] lint to transition this to a hard error in the future.
+    ///
+    /// [future-incompatible]: ../index.md#future-incompatible-lints
+    pub AMBIGUOUS_PANIC_IMPORTS,
+    Warn,
+    "detects ambiguous core and std panic imports",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::FutureReleaseError,
+        reference: "issue #147319 <https://github.com/rust-lang/rust/issues/147319>",
+        report_in_deps: false,
+    };
+}
+
 declare_lint! {
     /// The `refining_impl_trait_reachable` lint detects `impl Trait` return
     /// types in method signatures that are refined by a publically reachable

compiler/rustc_lint_defs/src/lib.rs

@@ -682,7 +682,7 @@ pub enum BuiltinLintDiag {
         vis_span: Span,
         ident_span: Span,
     },
-    AmbiguousGlobImports {
+    AmbiguousImports {
         diag: AmbiguityErrorDiag,
     },
     AmbiguousGlobReexports {

compiler/rustc_resolve/src/diagnostics.rs

@@ -21,7 +21,7 @@ use rustc_middle::bug;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::Session;
 use rustc_session::lint::builtin::{
-    ABSOLUTE_PATHS_NOT_STARTING_WITH_CRATE, AMBIGUOUS_GLOB_IMPORTS,
+    ABSOLUTE_PATHS_NOT_STARTING_WITH_CRATE, AMBIGUOUS_GLOB_IMPORTS, AMBIGUOUS_PANIC_IMPORTS,
     MACRO_EXPANDED_MACRO_EXPORTS_ACCESSED_BY_ABSOLUTE_PATHS,
 };
 use rustc_session::lint::{AmbiguityErrorDiag, BuiltinLintDiag};
@@ -42,9 +42,9 @@ use crate::errors::{
 use crate::imports::{Import, ImportKind};
 use crate::late::{DiagMetadata, PatternSource, Rib};
 use crate::{
-    AmbiguityError, AmbiguityErrorMisc, AmbiguityKind, BindingError, BindingKey, Finalize,
-    ForwardGenericParamBanReason, HasGenericParams, LexicalScopeBinding, MacroRulesScope, Module,
-    ModuleKind, ModuleOrUniformRoot, NameBinding, NameBindingKind, ParentScope, PathResult,
+    AmbiguityError, AmbiguityErrorMisc, AmbiguityKind, AmbiguityWarning, BindingError, BindingKey,
+    Finalize, ForwardGenericParamBanReason, HasGenericParams, LexicalScopeBinding, MacroRulesScope,
+    Module, ModuleKind, ModuleOrUniformRoot, NameBinding, NameBindingKind, ParentScope, PathResult,
     PrivacyError, ResolutionError, Resolver, Scope, ScopeSet, Segment, UseError, Used,
     VisResolutionError, errors as errs, path_names_to_string,
 };
@@ -144,15 +144,21 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
 
         for ambiguity_error in &self.ambiguity_errors {
             let diag = self.ambiguity_diagnostics(ambiguity_error);
-            if ambiguity_error.warning {
+            if let Some(ambiguity_warning) = ambiguity_error.warning {
                 let NameBindingKind::Import { import, .. } = ambiguity_error.b1.0.kind else {
                     unreachable!()
                 };
+
+                let lint = match ambiguity_warning {
+                    AmbiguityWarning::GlobImport => AMBIGUOUS_GLOB_IMPORTS,
+                    AmbiguityWarning::PanicImport => AMBIGUOUS_PANIC_IMPORTS,
+                };
+
                 self.lint_buffer.buffer_lint(
-                    AMBIGUOUS_GLOB_IMPORTS,
+                    lint,
                     import.root_id,
                     ambiguity_error.ident.span,
-                    BuiltinLintDiag::AmbiguousGlobImports { diag },
+                    BuiltinLintDiag::AmbiguousImports { diag },
                 );
             } else {
                 let mut err = struct_span_code_err!(self.dcx(), diag.span, E0659, "{}", diag.msg);

compiler/rustc_resolve/src/ident.rs

@@ -6,6 +6,7 @@ use rustc_hir::def::{DefKind, MacroKinds, Namespace, NonMacroAttrKind, PartialRe
 use rustc_middle::{bug, span_bug};
 use rustc_session::lint::builtin::PROC_MACRO_DERIVE_RESOLUTION_FALLBACK;
 use rustc_session::parse::feature_err;
+use rustc_span::edition::Edition;
 use rustc_span::hygiene::{ExpnId, ExpnKind, LocalExpnId, MacroKind, SyntaxContext};
 use rustc_span::{Ident, Span, kw, sym};
 use tracing::{debug, instrument};
@@ -17,10 +18,10 @@ use crate::late::{
 };
 use crate::macros::{MacroRulesScope, sub_namespace_match};
 use crate::{
-    AmbiguityError, AmbiguityErrorMisc, AmbiguityKind, BindingKey, CmResolver, Determinacy,
-    Finalize, ImportKind, LexicalScopeBinding, Module, ModuleKind, ModuleOrUniformRoot,
-    NameBinding, NameBindingKind, ParentScope, PathResult, PrivacyError, Res, ResolutionError,
-    Resolver, Scope, ScopeSet, Segment, Stage, Used, Weak, errors,
+    AmbiguityError, AmbiguityErrorMisc, AmbiguityKind, AmbiguityWarning, BindingKey, CmResolver,
+    Determinacy, Finalize, ImportKind, LexicalScopeBinding, Module, ModuleKind,
+    ModuleOrUniformRoot, NameBinding, NameBindingKind, ParentScope, PathResult, PrivacyError, Res,
+    ResolutionError, Resolver, Scope, ScopeSet, Segment, Stage, Used, Weak, errors,
 };
 
 #[derive(Copy, Clone)]
@@ -644,11 +645,20 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                             return None;
                         }
 
+                        // preemptively look for ambiguities for panic macros to ensure we don't
+                        // speculatively resolve to a non-prelude item when an ambiguity that we'd
+                        // downgrade is present
+                        let is_issue_147319_hack = || ctxt.edition()
+                            <= Edition::Edition2024
+                            && matches!(orig_ident.name, sym::panic)
+                            && (this.is_specific_builtin_macro(binding.res(), sym::std_panic) || this.is_specific_builtin_macro(binding.res(), sym::core_panic));
+                        let finalizing = !matches!(finalize, None | Some(Finalize { stage: Stage::Late, .. }));
+
                         // Below we report various ambiguity errors.
                         // We do not need to report them if we are either in speculative resolution,
                         // or in late resolution when everything is already imported and expanded
                         // and no ambiguities exist.
-                        if matches!(finalize, None | Some(Finalize { stage: Stage::Late, .. })) {
+                        if !finalizing && !is_issue_147319_hack() {
                             return Some(Ok(binding));
                         }
 
@@ -705,13 +715,41 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                                 // Skip ambiguity errors for extern flag bindings "overridden"
                                 // by extern item bindings.
                                 // FIXME: Remove with lang team approval.
-                                let issue_145575_hack = Some(binding)
-                                    == extern_prelude_flag_binding
-                                    && extern_prelude_item_binding.is_some()
-                                    && extern_prelude_item_binding != Some(innermost_binding);
+                                let is_issue_145575_hack = || {
+                                    Some(binding) == extern_prelude_flag_binding
+                                        && extern_prelude_item_binding.is_some()
+                                        && extern_prelude_item_binding != Some(innermost_binding)
+                                };
+
                                 if let Some(kind) = ambiguity_error_kind
-                                    && !issue_145575_hack
+                                    && !is_issue_145575_hack()
                                 {
+                                    // Turn ambiguity errors for core vs std panic into warnings.
+                                    // FIXME: Remove with lang team approval.
+                                    let is_issue_147319_hack = ctxt.edition()
+                                        <= Edition::Edition2024
+                                        && matches!(orig_ident.name, sym::panic)
+                                        && matches!(scope, Scope::StdLibPrelude)
+                                        && ((this.is_specific_builtin_macro(
+                                            binding.res(),
+                                            sym::std_panic,
+                                        ) && this.is_specific_builtin_macro(
+                                            innermost_binding.res(),
+                                            sym::core_panic,
+                                        )) || (this.is_specific_builtin_macro(
+                                            binding.res(),
+                                            sym::core_panic,
+                                        ) && this.is_specific_builtin_macro(
+                                            innermost_binding.res(),
+                                            sym::std_panic,
+                                        )));
+
+                                    let warning = if is_issue_147319_hack {
+                                        Some(AmbiguityWarning::PanicImport)
+                                    } else {
+                                        None
+                                    };
+
                                     let misc = |f: Flags| {
                                         if f.contains(Flags::MISC_SUGGEST_CRATE) {
                                             AmbiguityErrorMisc::SuggestCrate
@@ -723,16 +761,22 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                                             AmbiguityErrorMisc::None
                                         }
                                     };
-                                    this.get_mut().ambiguity_errors.push(AmbiguityError {
-                                        kind,
-                                        ident: orig_ident,
-                                        b1: innermost_binding,
-                                        b2: binding,
-                                        warning: false,
-                                        misc1: misc(innermost_flags),
-                                        misc2: misc(flags),
-                                    });
-                                    return Some(Ok(innermost_binding));
+                                    if finalizing {
+                                        this.get_mut().ambiguity_errors.push(AmbiguityError {
+                                            kind,
+                                            ident: orig_ident,
+                                            b1: innermost_binding,
+                                            b2: binding,
+                                            warning,
+                                            misc1: misc(innermost_flags),
+                                            misc2: misc(flags),
+                                        });
+                                    }
+                                    if is_issue_147319_hack {
+                                        return Some(Ok(binding));
+                                    } else {
+                                        return Some(Ok(innermost_binding));
+                                    }
                                 }
                             }
                         } else {
@@ -1072,7 +1116,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                 ident,
                 b1: binding,
                 b2: shadowed_glob,
-                warning: false,
+                warning: None,
                 misc1: AmbiguityErrorMisc::None,
                 misc2: AmbiguityErrorMisc::None,
             });

compiler/rustc_resolve/src/imports.rs

@@ -945,8 +945,9 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             ImportKind::Single { bindings, .. } => bindings[TypeNS].get().binding(),
             _ => None,
         };
-        let ambiguity_errors_len =
-            |errors: &Vec<AmbiguityError<'_>>| errors.iter().filter(|error| !error.warning).count();
+        let ambiguity_errors_len = |errors: &Vec<AmbiguityError<'_>>| {
+            errors.iter().filter(|error| error.warning.is_none()).count()
+        };
         let prev_ambiguity_errors_len = ambiguity_errors_len(&self.ambiguity_errors);
         let finalize = Finalize::with_root_span(import.root_id, import.span, import.root_span);
 
@@ -1161,7 +1162,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                         });
                         let res = binding.res();
                         let has_ambiguity_error =
-                            this.ambiguity_errors.iter().any(|error| !error.warning);
+                            this.ambiguity_errors.iter().any(|error| error.warning.is_none());
                         if res == Res::Err || has_ambiguity_error {
                             this.dcx()
                                 .span_delayed_bug(import.span, "some error happened for an import");

compiler/rustc_resolve/src/lib.rs

@@ -907,14 +907,20 @@ enum AmbiguityErrorMisc {
     None,
 }
 
+#[derive(Clone, Copy, PartialEq)]
+enum AmbiguityWarning {
+    GlobImport,
+    PanicImport,
+}
+
 struct AmbiguityError<'ra> {
     kind: AmbiguityKind,
     ident: Ident,
     b1: NameBinding<'ra>,
     b2: NameBinding<'ra>,
     misc1: AmbiguityErrorMisc,
     misc2: AmbiguityErrorMisc,
-    warning: bool,
+    warning: Option<AmbiguityWarning>,
 }
 
 impl<'ra> NameBindingData<'ra> {
@@ -1851,6 +1857,10 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         self.get_macro(res).is_some_and(|macro_data| macro_data.ext.builtin_name.is_some())
     }
 
+    fn is_specific_builtin_macro(&self, res: Res, symbol: Symbol) -> bool {
+        self.get_macro(res).is_some_and(|macro_data| macro_data.ext.builtin_name == Some(symbol))
+    }
+
     fn macro_def(&self, mut ctxt: SyntaxContext) -> DefId {
         loop {
             match ctxt.outer_expn_data().macro_def_id {
@@ -2042,7 +2052,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                 b2,
                 misc1: AmbiguityErrorMisc::None,
                 misc2: AmbiguityErrorMisc::None,
-                warning: warn_ambiguity,
+                warning: if warn_ambiguity { Some(AmbiguityWarning::GlobImport) } else { None },
             };
             if !self.matches_previous_ambiguity_error(&ambiguity_error) {
                 // avoid duplicated span information to be emit out

compiler/rustc_resolve/src/macros.rs

@@ -838,10 +838,19 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                                  res: Res| {
             if let Some(initial_res) = initial_res {
                 if res != initial_res {
-                    // Make sure compilation does not succeed if preferred macro resolution
-                    // has changed after the macro had been expanded. In theory all such
-                    // situations should be reported as errors, so this is a bug.
-                    this.dcx().span_delayed_bug(span, "inconsistent resolution for a macro");
+                    // Delayed versions of the #147319 workaround are inconsistent, but not really
+                    // problematic and already seen as a bug.
+                    // FIXME: Remove with lang team approval.
+                    if !this
+                        .ambiguity_errors
+                        .iter()
+                        .all(|ambiguity_error| ambiguity_error.warning.is_some())
+                    {
+                        // Make sure compilation does not succeed if preferred macro resolution
+                        // has changed after the macro had been expanded. In theory all such
+                        // situations should be reported as errors, so this is a bug.
+                        this.dcx().span_delayed_bug(span, "inconsistent resolution for a macro");
+                    }
                 }
             } else if this.tcx.dcx().has_errors().is_none() && this.privacy_errors.is_empty() {
                 // It's possible that the macro was unresolved (indeterminate) and silently

library/alloctests/lib.rs

@@ -59,6 +59,7 @@
 #![feature(negative_impls)]
 #![feature(never_type)]
 #![feature(optimize_attribute)]
+#![feature(prelude_import)]
 #![feature(rustc_allow_const_fn_unstable)]
 #![feature(rustc_attrs)]
 #![feature(staged_api)]
@@ -68,11 +69,17 @@
 
 // Allow testing this library
 extern crate alloc as realalloc;
-#[macro_use]
+
+// This is needed to provide macros to the directly imported alloc modules below.
 extern crate std;
+#[prelude_import]
+#[allow(unused_imports)]
+use std::prelude::rust_2024::*;
+
 #[cfg(test)]
 extern crate test;
 mod testing;
+
 use realalloc::*;
 
 // We are directly including collections, raw_vec, and wtf8 here as they use non-public
@@ -96,8 +103,7 @@ pub(crate) mod test_helpers {
         let mut hasher = std::hash::RandomState::new().build_hasher();
         std::panic::Location::caller().hash(&mut hasher);
         let hc64 = hasher.finish();
-        let seed_vec =
-            hc64.to_le_bytes().into_iter().chain(0u8..8).collect::<crate::vec::Vec<u8>>();
+        let seed_vec = hc64.to_le_bytes().into_iter().chain(0u8..8).collect::<std::vec::Vec<u8>>();
         let seed: [u8; 16] = seed_vec.as_slice().try_into().unwrap();
         rand::SeedableRng::from_seed(seed)
     }