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Support weak definitions #4414

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222f5a0
Support weak definitions
bjorn3 Jun 26, 2025
2402b40
Add test for weak definitions
bjorn3 Jun 26, 2025
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86 changes: 59 additions & 27 deletions src/shims/foreign_items.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,7 @@ use rustc_hir::def::DefKind;
use rustc_hir::def_id::CrateNum;
use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
use rustc_middle::mir::interpret::AllocInit;
use rustc_middle::mir::mono::Linkage;
use rustc_middle::ty::{Instance, Ty};
use rustc_middle::{mir, ty};
use rustc_span::Symbol;
Expand Down Expand Up @@ -138,7 +139,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
Entry::Occupied(e) => e.into_mut(),
Entry::Vacant(e) => {
// Find it if it was not cached.
let mut instance_and_crate: Option<(ty::Instance<'_>, CrateNum)> = None;
let mut instance_and_crate: Option<(ty::Instance<'_>, CrateNum, bool)> = None;
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Please explain what the bool means, or use a self-explaining type. It might be worth using a struct instead of a tuple.

helpers::iter_exported_symbols(tcx, |cnum, def_id| {
let attrs = tcx.codegen_fn_attrs(def_id);
// Skip over imports of items.
Expand All @@ -155,39 +156,70 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {

let instance = Instance::mono(tcx, def_id);
let symbol_name = tcx.symbol_name(instance).name;
let is_weak = attrs.linkage == Some(Linkage::WeakAny);
if symbol_name == link_name.as_str() {
if let Some((original_instance, original_cnum)) = instance_and_crate {
// Make sure we are consistent wrt what is 'first' and 'second'.
let original_span = tcx.def_span(original_instance.def_id()).data();
let span = tcx.def_span(def_id).data();
if original_span < span {
throw_machine_stop!(TerminationInfo::MultipleSymbolDefinitions {
link_name,
first: original_span,
first_crate: tcx.crate_name(original_cnum),
second: span,
second_crate: tcx.crate_name(cnum),
});
} else {
throw_machine_stop!(TerminationInfo::MultipleSymbolDefinitions {
link_name,
first: span,
first_crate: tcx.crate_name(cnum),
second: original_span,
second_crate: tcx.crate_name(original_cnum),
});
if let Some((original_instance, original_cnum, original_is_weak)) =
instance_and_crate
{
match (is_weak, original_is_weak) {
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Suggested change
match (is_weak, original_is_weak) {
// There is more than one definition with this name. What we do now
// depends on whether one or both definitions are weak.
match (is_weak, original_is_weak) {

(false, true) => {
// Original definition is a weak definition. Override it.

instance_and_crate =
Some((ty::Instance::mono(tcx, def_id), cnum, is_weak));
}
(true, false) => {
// Current definition is a weak definition. Keep the original one.
}
(true, true) | (false, false) => {
// Either both definitions are non-weak or both are weak. In
// either case return an error. For weak definitions we error
// because it is undefined which definition would have been
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Suggested change
// because it is undefined which definition would have been
// because it is unspecified which definition would have been

// picked by the linker.

// Make sure we are consistent wrt what is 'first' and 'second'.
let original_span =
tcx.def_span(original_instance.def_id()).data();
let span = tcx.def_span(def_id).data();
if original_span < span {
throw_machine_stop!(
TerminationInfo::MultipleSymbolDefinitions {
link_name,
first: original_span,
first_crate: tcx.crate_name(original_cnum),
second: span,
second_crate: tcx.crate_name(cnum),
}
);
} else {
throw_machine_stop!(
TerminationInfo::MultipleSymbolDefinitions {
link_name,
first: span,
first_crate: tcx.crate_name(cnum),
second: original_span,
second_crate: tcx.crate_name(original_cnum),
}
);
}
}
}
} else {
instance_and_crate =
Some((ty::Instance::mono(tcx, def_id), cnum, is_weak));
}
if !matches!(tcx.def_kind(def_id), DefKind::Fn | DefKind::AssocFn) {
throw_ub_format!(
"attempt to call an exported symbol that is not defined as a function"
);
}
instance_and_crate = Some((ty::Instance::mono(tcx, def_id), cnum));
}
interp_ok(())
})?;

if let Some((instance, _, _)) = instance_and_crate {
if !matches!(tcx.def_kind(instance.def_id()), DefKind::Fn | DefKind::AssocFn) {
throw_ub_format!(
"attempt to call an exported symbol that is not defined as a function"
);
}
}

Comment on lines +215 to +222
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I guess this logic can live here as well, but is there a specific reason you moved it?

e.insert(instance_and_crate.map(|ic| ic.0))
}
};
Expand Down
39 changes: 39 additions & 0 deletions tests/pass/function_calls/weak_definition.rs
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Please rename this to exported_symbol_weak for better test grouping.

Original file line number Diff line number Diff line change
@@ -0,0 +1,39 @@
#![feature(linkage)]

// FIXME move this module to a separate crate once aux-build is allowed
// This currently depends on the fact that miri skips the codegen check
// that denies multiple symbols with the same name.
mod first {
#[no_mangle]
#[linkage = "weak"]
extern "C" fn foo() -> i32 {
1
}

#[no_mangle]
#[linkage = "weak"]
extern "C" fn bar() -> i32 {
2
}
}

mod second {
#[no_mangle]
extern "C" fn bar() -> i32 {
3
}
}

extern "C" {
fn foo() -> i32;
fn bar() -> i32;
}

fn main() {
unsafe {
// If there is no non-weak definition, the weak definition will be used.
assert_eq!(foo(), 1);
// Non-weak definition takes presedence.
assert_eq!(bar(), 3);
}
}