Add lint for recursive default impls

compiler/rustc_mir_build/messages.ftl

@@ -265,6 +265,8 @@ mir_build_pointer_pattern = function pointers and raw pointers not derived from
 
 mir_build_privately_uninhabited = pattern `{$witness_1}` is currently uninhabited, but this variant contains private fields which may become inhabited in the future
 
+mir_build_recursive_default_impl = calling `..Default::default()` in a `Default` implementation leads to infinite recursion, and does not initialize the fields of a struct or enum
+
 mir_build_rust_2024_incompatible_pat = patterns are not allowed to reset the default binding mode in edition 2024
 
 mir_build_rustc_box_attribute_error = `#[rustc_box]` attribute used incorrectly

compiler/rustc_mir_build/src/errors.rs

@@ -18,6 +18,8 @@ use crate::fluent_generated as fluent;
 pub(crate) struct UnconditionalRecursion {
     #[label]
     pub(crate) span: Span,
+    #[help(mir_build_recursive_default_impl)]
+    pub(crate) default_impl_note: Option<()>,
     #[label(mir_build_unconditional_recursion_call_site_label)]
     pub(crate) call_sites: Vec<Span>,
 }

compiler/rustc_mir_build/src/lints.rs

@@ -7,7 +7,8 @@ use rustc_hir::def::DefKind;
 use rustc_middle::mir::{self, BasicBlock, BasicBlocks, Body, Terminator, TerminatorKind};
 use rustc_middle::ty::{self, GenericArg, GenericArgs, Instance, Ty, TyCtxt};
 use rustc_session::lint::builtin::UNCONDITIONAL_RECURSION;
-use rustc_span::Span;
+use rustc_span::def_id::LocalDefId;
+use rustc_span::{Span, sym};
 
 use crate::errors::UnconditionalRecursion;
 
@@ -32,6 +33,19 @@ fn check_call_recursion<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
     }
 }
 
+fn is_default_impl<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> bool {
+    // Check if this is a trait impl and get the defid of the trait if it is
+    if tcx.def_kind(def_id) == DefKind::AssocFn
+        && let Some(parent_def_id) = tcx.opt_parent(def_id.into())
+        && tcx.def_kind(parent_def_id) == (DefKind::Impl { of_trait: true })
+        && let Some(trait_def_id) = tcx.trait_id_of_impl(parent_def_id)
+    {
+        // check if it is a default impl
+        return tcx.get_diagnostic_name(trait_def_id) == Some(sym::Default);
+    }
+    false
+}
+
 fn check_recursion<'tcx>(
     tcx: TyCtxt<'tcx>,
     body: &Body<'tcx>,
@@ -57,6 +71,7 @@ fn check_recursion<'tcx>(
         tcx.emit_node_span_lint(UNCONDITIONAL_RECURSION, hir_id, sp, UnconditionalRecursion {
             span: sp,
             call_sites: vis.reachable_recursive_calls,
+            default_impl_note: is_default_impl(tcx, def_id).then(|| ()),
         });
     }
 }

tests/ui/lint/lint-unconditional-recursion.stderr

@@ -122,6 +122,7 @@ LL |         let x = Default::default();
    |                 ------------------ recursive call site
    |
    = help: a `loop` may express intention better if this is on purpose
+   = help: calling `..Default::default()` in a `Default` implementation leads to infinite recursion, and does not initialize the fields of a struct or enum
 
 error: function cannot return without recursing
   --> $DIR/lint-unconditional-recursion.rs:102:5
@@ -196,6 +197,7 @@ LL |             ..Default::default()
    |               ------------------ recursive call site
    |
    = help: a `loop` may express intention better if this is on purpose
+   = help: calling `..Default::default()` in a `Default` implementation leads to infinite recursion, and does not initialize the fields of a struct or enum
 
 error: aborting due to 18 previous errors