add check and note for recursive default impl

Author: mj10021

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 = ..default() in the Default impl does not apply a default for each struct field
+
 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

@@ -18,10 +18,16 @@ use crate::fluent_generated as fluent;
 pub(crate) struct UnconditionalRecursion {
     #[label]
     pub(crate) span: Span,
+    #[subdiagnostic]
+    pub(crate) default_impl_note: Option<RecursiveDefaultImpl>,
     #[label(mir_build_unconditional_recursion_call_site_label)]
     pub(crate) call_sites: Vec<Span>,
 }
 
+#[derive(Subdiagnostic)]
+#[help(mir_build_recursive_default_impl)]
+pub(crate) struct RecursiveDefaultImpl {}
+
 #[derive(LintDiagnostic)]
 #[diag(mir_build_call_to_deprecated_safe_fn_requires_unsafe)]
 pub(crate) struct CallToDeprecatedSafeFnRequiresUnsafe {

compiler/rustc_mir_build/src/lints.rs

@@ -9,7 +9,7 @@ use rustc_middle::ty::{self, GenericArg, GenericArgs, Instance, Ty, TyCtxt};
 use rustc_session::lint::builtin::UNCONDITIONAL_RECURSION;
 use rustc_span::Span;
 
-use crate::errors::UnconditionalRecursion;
+use crate::errors::{RecursiveDefaultImpl, UnconditionalRecursion};
 
 pub(crate) fn check<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
     check_call_recursion(tcx, body);
@@ -31,6 +31,24 @@ fn check_call_recursion<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
         check_recursion(tcx, body, CallRecursion { trait_args })
     }
 }
+use rustc_span::def_id::LocalDefId;
+use rustc_span::sym;
+fn is_default_impl<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> bool {
+    if tcx.def_kind(def_id) == DefKind::AssocFn {
+        // Check if this is a trait impl and get the defid of the trait if it is
+        if let Some(parent_def_id) = tcx.opt_parent(def_id.into()) {
+            if tcx.def_kind(parent_def_id) == (DefKind::Impl { of_trait: true }) {
+                if let Some(trait_def_id) = tcx.trait_id_of_impl(parent_def_id) {
+                    // check if it is a default impl
+                    if tcx.get_diagnostic_name(trait_def_id) == Some(sym::Default) {
+                        return true;
+                    }
+                }
+            }
+        }
+    }
+    false
+}
 
 fn check_recursion<'tcx>(
     tcx: TyCtxt<'tcx>,
@@ -54,9 +72,12 @@ fn check_recursion<'tcx>(
 
         let sp = tcx.def_span(def_id);
         let hir_id = tcx.local_def_id_to_hir_id(def_id);
+        let default_impl_note =
+            { if is_default_impl(tcx, def_id) { Some(RecursiveDefaultImpl {}) } else { None } };
         tcx.emit_node_span_lint(UNCONDITIONAL_RECURSION, hir_id, sp, UnconditionalRecursion {
             span: sp,
             call_sites: vis.reachable_recursive_calls,
+            default_impl_note,
         });
     }
 }

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: ..default() in the Default impl does not apply a default for each struct field
 
 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: ..default() in the Default impl does not apply a default for each struct field
 
 error: aborting due to 18 previous errors