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Make post-monomorphization error detection work with fatal errors #153194

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Make post-monomorphization error detection work with fatal errors #153194

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82998d0
Make post-monomorphization error detection work with fatal errors
Zoxc Feb 27, 2026
0052e11
Update tests
Zoxc Feb 27, 2026
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273 changes: 143 additions & 130 deletions compiler/rustc_monomorphize/src/collector.rs
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Expand Up @@ -210,6 +210,7 @@ mod autodiff;
use std::cell::OnceCell;
use std::ops::ControlFlow;

use rustc_data_structures::defer;
use rustc_data_structures::fx::FxIndexMap;
use rustc_data_structures::sync::{MTLock, par_for_each_in};
use rustc_data_structures::unord::{UnordMap, UnordSet};
Expand Down Expand Up @@ -419,153 +420,165 @@ fn collect_items_rec<'tcx>(
// current step of mono items collection.
//
// FIXME: don't rely on global state, instead bubble up errors. Note: this is very hard to do.
// FIXME: This detection can have false positives with the parallel compiler.
// FIXME: The notes may not be right after the original error with the parallel compiler.
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@nnethercote nnethercote Mar 3, 2026

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Are you documenting existing behaviours, or are these new behaviours?

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@Zoxc Zoxc Mar 6, 2026

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These are existing.

let error_count = tcx.dcx().err_count();

// In `mentioned_items` we collect items that were mentioned in this MIR but possibly do not
// need to be monomorphized. This is done to ensure that optimizing away function calls does not
// hide const-eval errors that those calls would otherwise have triggered.
match starting_item.node {
MonoItem::Static(def_id) => {
recursion_depth_reset = None;

// Statics always get evaluated (which is possible because they can't be generic), so for
// `MentionedItems` collection there's nothing to do here.
if mode == CollectionMode::UsedItems {
let instance = Instance::mono(tcx, def_id);
{
// Use `defer` so detection works with fatal errors too.
let _guard = defer(|| {
// Check for PMEs and emit a diagnostic if one happened. To try to show relevant edges of the
// mono item graph.
if tcx.dcx().err_count() > error_count
&& starting_item.node.is_generic_fn()
&& starting_item.node.is_user_defined()
{
match starting_item.node {
MonoItem::Fn(instance) => {
tcx.dcx().emit_note(EncounteredErrorWhileInstantiating {
span: starting_item.span,
kind: "fn",
instance,
})
}
MonoItem::Static(def_id) => {
tcx.dcx().emit_note(EncounteredErrorWhileInstantiating {
span: starting_item.span,
kind: "static",
instance: Instance::new_raw(def_id, GenericArgs::empty()),
})
}
MonoItem::GlobalAsm(_) => {
tcx.dcx().emit_note(EncounteredErrorWhileInstantiatingGlobalAsm {
span: starting_item.span,
})
}
}
}
});

// Sanity check whether this ended up being collected accidentally
debug_assert!(tcx.should_codegen_locally(instance));
// In `mentioned_items` we collect items that were mentioned in this MIR but possibly do not
// need to be monomorphized. This is done to ensure that optimizing away function calls does not
// hide const-eval errors that those calls would otherwise have triggered.
match starting_item.node {
MonoItem::Static(def_id) => {
recursion_depth_reset = None;

// Statics always get evaluated (which is possible because they can't be generic), so for
// `MentionedItems` collection there's nothing to do here.
if mode == CollectionMode::UsedItems {
let instance = Instance::mono(tcx, def_id);

// Sanity check whether this ended up being collected accidentally
debug_assert!(tcx.should_codegen_locally(instance));

let DefKind::Static { nested, .. } = tcx.def_kind(def_id) else { bug!() };
// Nested statics have no type.
if !nested {
let ty = instance.ty(tcx, ty::TypingEnv::fully_monomorphized());
visit_drop_use(tcx, ty, true, starting_item.span, &mut used_items);
}

let DefKind::Static { nested, .. } = tcx.def_kind(def_id) else { bug!() };
// Nested statics have no type.
if !nested {
let ty = instance.ty(tcx, ty::TypingEnv::fully_monomorphized());
visit_drop_use(tcx, ty, true, starting_item.span, &mut used_items);
}
if let Ok(alloc) = tcx.eval_static_initializer(def_id) {
for &prov in alloc.inner().provenance().ptrs().values() {
collect_alloc(tcx, prov.alloc_id(), &mut used_items);
}
}

if let Ok(alloc) = tcx.eval_static_initializer(def_id) {
for &prov in alloc.inner().provenance().ptrs().values() {
collect_alloc(tcx, prov.alloc_id(), &mut used_items);
if tcx.needs_thread_local_shim(def_id) {
used_items.push(respan(
starting_item.span,
MonoItem::Fn(Instance {
def: InstanceKind::ThreadLocalShim(def_id),
args: GenericArgs::empty(),
}),
));
}
}

if tcx.needs_thread_local_shim(def_id) {
used_items.push(respan(
starting_item.span,
MonoItem::Fn(Instance {
def: InstanceKind::ThreadLocalShim(def_id),
args: GenericArgs::empty(),
}),
));
}
// mentioned_items stays empty since there's no codegen for statics. statics don't get
// optimized, and if they did then the const-eval interpreter would have to worry about
// mentioned_items.
}
MonoItem::Fn(instance) => {
// Sanity check whether this ended up being collected accidentally
debug_assert!(tcx.should_codegen_locally(instance));

// mentioned_items stays empty since there's no codegen for statics. statics don't get
// optimized, and if they did then the const-eval interpreter would have to worry about
// mentioned_items.
}
MonoItem::Fn(instance) => {
// Sanity check whether this ended up being collected accidentally
debug_assert!(tcx.should_codegen_locally(instance));

// Keep track of the monomorphization recursion depth
recursion_depth_reset = Some(check_recursion_limit(
tcx,
instance,
starting_item.span,
recursion_depths,
recursion_limit,
));

rustc_data_structures::stack::ensure_sufficient_stack(|| {
let Ok((used, mentioned)) = tcx.items_of_instance((instance, mode)) else {
// Normalization errors here are usually due to trait solving overflow.
// FIXME: I assume that there are few type errors at post-analysis stage, but not
// entirely sure.
// We have to emit the error outside of `items_of_instance` to access the
// span of the `starting_item`.
let def_id = instance.def_id();
let def_span = tcx.def_span(def_id);
let def_path_str = tcx.def_path_str(def_id);
tcx.dcx().emit_fatal(RecursionLimit {
span: starting_item.span,
instance,
def_span,
def_path_str,
});
};
used_items.extend(used.into_iter().copied());
mentioned_items.extend(mentioned.into_iter().copied());
});
}
MonoItem::GlobalAsm(item_id) => {
assert!(
mode == CollectionMode::UsedItems,
"should never encounter global_asm when collecting mentioned items"
);
recursion_depth_reset = None;

let item = tcx.hir_item(item_id);
if let hir::ItemKind::GlobalAsm { asm, .. } = item.kind {
for (op, op_sp) in asm.operands {
match *op {
hir::InlineAsmOperand::Const { .. } => {
// Only constants which resolve to a plain integer
// are supported. Therefore the value should not
// depend on any other items.
}
hir::InlineAsmOperand::SymFn { expr } => {
let fn_ty = tcx.typeck(item_id.owner_id).expr_ty(expr);
visit_fn_use(tcx, fn_ty, false, *op_sp, &mut used_items);
}
hir::InlineAsmOperand::SymStatic { path: _, def_id } => {
let instance = Instance::mono(tcx, def_id);
if tcx.should_codegen_locally(instance) {
trace!("collecting static {:?}", def_id);
used_items.push(dummy_spanned(MonoItem::Static(def_id)));
// Keep track of the monomorphization recursion depth
recursion_depth_reset = Some(check_recursion_limit(
tcx,
instance,
starting_item.span,
recursion_depths,
recursion_limit,
));

rustc_data_structures::stack::ensure_sufficient_stack(|| {
let Ok((used, mentioned)) = tcx.items_of_instance((instance, mode)) else {
// Normalization errors here are usually due to trait solving overflow.
// FIXME: I assume that there are few type errors at post-analysis stage, but not
// entirely sure.
// We have to emit the error outside of `items_of_instance` to access the
// span of the `starting_item`.
let def_id = instance.def_id();
let def_span = tcx.def_span(def_id);
let def_path_str = tcx.def_path_str(def_id);
tcx.dcx().emit_fatal(RecursionLimit {
span: starting_item.span,
instance,
def_span,
def_path_str,
});
};
used_items.extend(used.into_iter().copied());
mentioned_items.extend(mentioned.into_iter().copied());
});
}
MonoItem::GlobalAsm(item_id) => {
assert!(
mode == CollectionMode::UsedItems,
"should never encounter global_asm when collecting mentioned items"
);
recursion_depth_reset = None;

let item = tcx.hir_item(item_id);
if let hir::ItemKind::GlobalAsm { asm, .. } = item.kind {
for (op, op_sp) in asm.operands {
match *op {
hir::InlineAsmOperand::Const { .. } => {
// Only constants which resolve to a plain integer
// are supported. Therefore the value should not
// depend on any other items.
}
hir::InlineAsmOperand::SymFn { expr } => {
let fn_ty = tcx.typeck(item_id.owner_id).expr_ty(expr);
visit_fn_use(tcx, fn_ty, false, *op_sp, &mut used_items);
}
hir::InlineAsmOperand::SymStatic { path: _, def_id } => {
let instance = Instance::mono(tcx, def_id);
if tcx.should_codegen_locally(instance) {
trace!("collecting static {:?}", def_id);
used_items.push(dummy_spanned(MonoItem::Static(def_id)));
}
}
hir::InlineAsmOperand::In { .. }
| hir::InlineAsmOperand::Out { .. }
| hir::InlineAsmOperand::InOut { .. }
| hir::InlineAsmOperand::SplitInOut { .. }
| hir::InlineAsmOperand::Label { .. } => {
span_bug!(*op_sp, "invalid operand type for global_asm!")
}
}
hir::InlineAsmOperand::In { .. }
| hir::InlineAsmOperand::Out { .. }
| hir::InlineAsmOperand::InOut { .. }
| hir::InlineAsmOperand::SplitInOut { .. }
| hir::InlineAsmOperand::Label { .. } => {
span_bug!(*op_sp, "invalid operand type for global_asm!")
}
}
} else {
span_bug!(item.span, "Mismatch between hir::Item type and MonoItem type")
}
} else {
span_bug!(item.span, "Mismatch between hir::Item type and MonoItem type")
}

// mention_items stays empty as nothing gets optimized here.
}
};

// Check for PMEs and emit a diagnostic if one happened. To try to show relevant edges of the
// mono item graph.
if tcx.dcx().err_count() > error_count
&& starting_item.node.is_generic_fn()
&& starting_item.node.is_user_defined()
{
match starting_item.node {
MonoItem::Fn(instance) => tcx.dcx().emit_note(EncounteredErrorWhileInstantiating {
span: starting_item.span,
kind: "fn",
instance,
}),
MonoItem::Static(def_id) => tcx.dcx().emit_note(EncounteredErrorWhileInstantiating {
span: starting_item.span,
kind: "static",
instance: Instance::new_raw(def_id, GenericArgs::empty()),
}),
MonoItem::GlobalAsm(_) => {
tcx.dcx().emit_note(EncounteredErrorWhileInstantiatingGlobalAsm {
span: starting_item.span,
})
// mention_items stays empty as nothing gets optimized here.
}
}
};
}

// Only updating `usage_map` for used items as otherwise we may be inserting the same item
// multiple times (if it is first 'mentioned' and then later actually used), and the usage map
// logic does not like that.
Expand Down
10 changes: 10 additions & 0 deletions tests/ui/async-await/async-closures/post-mono-higher-ranked-hang-2.stderr
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Expand Up @@ -14,5 +14,15 @@ note: `recur` defined here
LL | fn recur<'l>(closure: &'l impl AsyncFn()) -> Pin<Box<dyn Future<Output = ()> + 'l>> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: the above error was encountered while instantiating `fn recur::<{async closure@$DIR/post-mono-higher-ranked-hang-2.rs:13:24: 13:32}>`
--> $DIR/post-mono-higher-ranked-hang-2.rs:13:17
|
LL | let _ = recur(&async || {
| _________________^
LL | |
LL | | let _ = closure();
LL | | });
| |__________^

error: aborting due to 1 previous error

9 changes: 9 additions & 0 deletions tests/ui/async-await/async-closures/post-mono-higher-ranked-hang.current.stderr
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Expand Up @@ -17,5 +17,14 @@ LL | | yield_chain: &'l mut impl AsyncFnMut(&SymPerm<'db>),
LL | | ) -> Pin<Box<dyn std::future::Future<Output = ()> + 'l>> {
| |____________________________________________________________^

note: the above error was encountered while instantiating `fn ToChain::<'_, '_>::perm_pairs::<{async closure@$DIR/post-mono-higher-ranked-hang.rs:47:45: 47:67}>`
--> $DIR/post-mono-higher-ranked-hang.rs:47:21
|
LL | / self.perm_pairs(l, &mut async move |left_pair| {
LL | |
LL | | self.perm_pairs(r, yield_chain).await
LL | | })
| |______________________^

error: aborting due to 1 previous error

9 changes: 9 additions & 0 deletions tests/ui/async-await/async-closures/post-mono-higher-ranked-hang.next.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -17,5 +17,14 @@ LL | | yield_chain: &'l mut impl AsyncFnMut(&SymPerm<'db>),
LL | | ) -> Pin<Box<dyn std::future::Future<Output = ()> + 'l>> {
| |____________________________________________________________^

note: the above error was encountered while instantiating `fn ToChain::<'_, '_>::perm_pairs::<{async closure@$DIR/post-mono-higher-ranked-hang.rs:47:45: 47:67}>`
--> $DIR/post-mono-higher-ranked-hang.rs:47:21
|
LL | / self.perm_pairs(l, &mut async move |left_pair| {
LL | |
LL | | self.perm_pairs(r, yield_chain).await
LL | | })
| |______________________^

error: aborting due to 1 previous error

6 changes: 6 additions & 0 deletions tests/ui/codegen/normalization-overflow/recursion-issue-105275.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -10,5 +10,11 @@ note: `encode_num` defined here
LL | pub fn encode_num<Writer: ExampleWriter>(n: u32, mut writer: Writer) -> Result<(), Writer::Error> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: the above error was encountered while instantiating `fn encode_num::<&mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut Error>`
--> $DIR/recursion-issue-105275.rs:6:9
|
LL | encode_num(n / 16, &mut writer)?;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error

6 changes: 6 additions & 0 deletions tests/ui/codegen/normalization-overflow/recursion-issue-105937.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -10,5 +10,11 @@ note: `encode_num` defined here
LL | pub fn encode_num<Writer: ExampleWriter>(n: u32, mut writer: Writer) -> Result<(), Writer::Error> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: the above error was encountered while instantiating `fn encode_num::<&mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut Error>`
--> $DIR/recursion-issue-105937.rs:6:9
|
LL | encode_num(n / 16, &mut writer)?;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error

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