Provide more information on duplicate lang item error.

src/librustc_metadata/rmeta/decoder/cstore_impl.rs

@@ -27,6 +27,7 @@ use rustc_span::symbol::{Ident, Symbol};
 use rustc_data_structures::sync::Lrc;
 use smallvec::SmallVec;
 use std::any::Any;
+use std::path::PathBuf;
 
 macro_rules! provide {
     (<$lt:tt> $tcx:ident, $def_id:ident, $other:ident, $cdata:ident,
@@ -513,4 +514,8 @@ impl CrateStore for CStore {
     fn allocator_kind(&self) -> Option<AllocatorKind> {
         self.allocator_kind()
     }
+
+    fn crate_extern_paths(&self, cnum: CrateNum) -> Vec<PathBuf> {
+        self.get_crate_data(cnum).source().paths().cloned().collect()
+    }
 }

src/librustc_middle/middle/cstore.rs

@@ -203,6 +203,7 @@ pub trait CrateStore {
     fn encode_metadata(&self, tcx: TyCtxt<'_>) -> EncodedMetadata;
     fn metadata_encoding_version(&self) -> &[u8];
     fn allocator_kind(&self) -> Option<AllocatorKind>;
+    fn crate_extern_paths(&self, cnum: CrateNum) -> Vec<PathBuf>;
 }
 
 pub type CrateStoreDyn = dyn CrateStore + sync::Sync;

src/librustc_middle/ty/context.rs

@@ -62,6 +62,7 @@ use std::hash::{Hash, Hasher};
 use std::iter;
 use std::mem;
 use std::ops::{Bound, Deref};
+use std::path::PathBuf;
 use std::sync::Arc;
 
 type InternedSet<'tcx, T> = ShardedHashMap<Interned<'tcx, T>, ()>;
@@ -1252,6 +1253,14 @@ impl<'tcx> TyCtxt<'tcx> {
         if cnum == LOCAL_CRATE { false } else { self.cstore.crate_is_private_dep_untracked(cnum) }
     }
 
+    pub fn crate_extern_paths(&self, cnum: CrateNum) -> Vec<PathBuf> {
+        if cnum == LOCAL_CRATE {
+            self.sess.local_crate_source_file.iter().cloned().collect()
+        } else {
+            self.cstore.crate_extern_paths(cnum)
+        }
+    }
+
     #[inline]
     pub fn def_path_hash(self, def_id: DefId) -> rustc_hir::definitions::DefPathHash {
         if let Some(def_id) = def_id.as_local() {

src/librustc_passes/lang_items.rs

@@ -146,6 +146,27 @@ impl LanguageItemCollector<'tcx> {
                             ));
                         }
                     }
+                    let mut note_def = |which, def_id: DefId| {
+                        let location = if def_id.is_local() {
+                            "the local crate".to_string()
+                        } else {
+                            let paths: Vec<_> = self
+                                .tcx
+                                .crate_extern_paths(def_id.krate)
+                                .iter()
+                                .map(|p| p.display().to_string())
+                                .collect();
+                            paths.join(", ")
+                        };
+                        err.note(&format!(
+                            "{} definition in `{}` loaded from {}",
+                            which,
+                            self.tcx.crate_name(def_id.krate),
+                            location
+                        ));
+                    };
+                    note_def("first", original_def_id);
+                    note_def("second", item_def_id);
                 }
                 err.emit();
             }

src/test/ui/duplicate_entry_error.rs

@@ -1,3 +1,4 @@
+// normalize-stderr-test "loaded from .*libstd-.*.rlib" -> "loaded from SYSROOT/libstd-*.rlib"
 // note-pattern: first defined in crate `std`.
 
 // Test for issue #31788 and E0152

src/test/ui/duplicate_entry_error.stderr

@@ -1,5 +1,5 @@
 error[E0152]: found duplicate lang item `panic_impl`
-  --> $DIR/duplicate_entry_error.rs:10:1
+  --> $DIR/duplicate_entry_error.rs:11:1
    |
 LL | / fn panic_impl(info: &PanicInfo) -> ! {
 LL | |
@@ -8,6 +8,8 @@ LL | | }
    | |_^
    |
    = note: the lang item is first defined in crate `std` (which `duplicate_entry_error` depends on)
+   = note: first definition in `std` loaded from SYSROOT/libstd-*.rlib
+   = note: second definition in `duplicate_entry_error` loaded from the local crate
 
 error: aborting due to previous error
 

src/test/ui/error-codes/E0152.rs

@@ -1,3 +1,4 @@
+// normalize-stderr-test "loaded from .*liballoc-.*.rlib" -> "loaded from SYSROOT/liballoc-*.rlib"
 #![feature(lang_items)]
 
 #[lang = "owned_box"]

src/test/ui/error-codes/E0152.stderr

@@ -1,10 +1,12 @@
 error[E0152]: found duplicate lang item `owned_box`
-  --> $DIR/E0152.rs:4:1
+  --> $DIR/E0152.rs:5:1
    |
 LL | struct Foo;
    | ^^^^^^^^^^^
    |
    = note: the lang item is first defined in crate `alloc` (which `std` depends on)
+   = note: first definition in `alloc` loaded from SYSROOT/liballoc-*.rlib
+   = note: second definition in `E0152` loaded from the local crate
 
 error: aborting due to previous error
 

src/test/ui/panic-handler/panic-handler-std.rs

@@ -1,3 +1,4 @@
+// normalize-stderr-test "loaded from .*libstd-.*.rlib" -> "loaded from SYSROOT/libstd-*.rlib"
 // error-pattern: found duplicate lang item `panic_impl`
 
 

src/test/ui/panic-handler/panic-handler-std.stderr

@@ -1,5 +1,5 @@
 error[E0152]: found duplicate lang item `panic_impl`
-  --> $DIR/panic-handler-std.rs:7:1
+  --> $DIR/panic-handler-std.rs:8:1
    |
 LL | / fn panic(info: PanicInfo) -> ! {
 LL | |     loop {}
@@ -7,9 +7,11 @@ LL | | }
    | |_^
    |
    = note: the lang item is first defined in crate `std` (which `panic_handler_std` depends on)
+   = note: first definition in `std` loaded from SYSROOT/libstd-*.rlib
+   = note: second definition in `panic_handler_std` loaded from the local crate
 
 error: argument should be `&PanicInfo`
-  --> $DIR/panic-handler-std.rs:7:16
+  --> $DIR/panic-handler-std.rs:8:16
    |
 LL | fn panic(info: PanicInfo) -> ! {
    |                ^^^^^^^^^