added more context for duplicate lang item errors (fixes #60561)

src/librustc/middle/cstore.rs

@@ -126,10 +126,17 @@ pub struct ExternCrate {
     /// used to select the extern with the shortest path
     pub path_len: usize,
 
+    /// Crate that depends on this crate
+    pub dependency_of: CrateNum,
+}
+
+impl ExternCrate {
     /// If true, then this crate is the crate named by the extern
     /// crate referenced above. If false, then this crate is a dep
     /// of the crate.
-    pub direct: bool,
+    pub fn is_direct(&self) -> bool {
+        self.dependency_of == LOCAL_CRATE
+    }
 }
 
 #[derive(Copy, Clone, Debug, HashStable)]

src/librustc/middle/lang_items.rs

@@ -13,6 +13,7 @@ use crate::hir::def_id::DefId;
 use crate::hir::check_attr::Target;
 use crate::ty::{self, TyCtxt};
 use crate::middle::weak_lang_items;
+use crate::middle::cstore::ExternCrate;
 use crate::util::nodemap::FxHashMap;
 
 use syntax::ast;
@@ -182,16 +183,39 @@ impl LanguageItemCollector<'tcx> {
                         E0152,
                         "duplicate lang item found: `{}`.",
                         name),
-                    None => self.tcx.sess.struct_err(&format!(
-                            "duplicate lang item in crate `{}`: `{}`.",
-                            self.tcx.crate_name(item_def_id.krate),
-                            name)),
+                    None => {
+                        match self.tcx.extern_crate(item_def_id) {
+                            Some(ExternCrate {dependency_of, ..}) => {
+                                self.tcx.sess.struct_err(&format!(
+                                "duplicate lang item in crate `{}` (which `{}` depends on): `{}`.",
+                                self.tcx.crate_name(item_def_id.krate),
+                                self.tcx.crate_name(*dependency_of),
+                                name))
+                            },
+                            _ => {
+                                self.tcx.sess.struct_err(&format!(
+                                "duplicate lang item in crate `{}`: `{}`.",
+                                self.tcx.crate_name(item_def_id.krate),
+                                name))
+                            }
+                        }
+                    },
                 };
                 if let Some(span) = self.tcx.hir().span_if_local(original_def_id) {
                     span_note!(&mut err, span, "first defined here.");
                 } else {
-                    err.note(&format!("first defined in crate `{}`.",
+                    match self.tcx.extern_crate(original_def_id) {
+                        Some(ExternCrate {dependency_of, ..}) => {
+                            err.note(&format!(
+                            "first defined in crate `{}` (which `{}` depends on).",
+                                      self.tcx.crate_name(original_def_id.krate),
+                                      self.tcx.crate_name(*dependency_of)));
+                        },
+                        _ => {
+                            err.note(&format!("first defined in crate `{}`.",
                                       self.tcx.crate_name(original_def_id.krate)));
+                        }
+                    }
                 }
                 err.emit();
             }

src/librustc/ty/print/pretty.rs

@@ -278,7 +278,7 @@ pub trait PrettyPrinter<'tcx>:
             match self.tcx().extern_crate(def_id) {
                 Some(&ExternCrate {
                     src: ExternCrateSource::Extern(def_id),
-                    direct: true,
+                    dependency_of: LOCAL_CRATE,
                     span,
                     ..
                 }) => {

src/librustc_metadata/creader.rs

@@ -17,6 +17,7 @@ use rustc::middle::cstore::{ExternCrate, ExternCrateSource};
 use rustc::util::common::record_time;
 use rustc::util::nodemap::FxHashSet;
 use rustc::hir::map::Definitions;
+use rustc::hir::def_id::LOCAL_CRATE;
 
 use std::ops::Deref;
 use std::path::PathBuf;
@@ -430,7 +431,7 @@ impl<'a> CrateLoader<'a> {
                            mut extern_crate: ExternCrate,
                            visited: &mut FxHashSet<(CrateNum, bool)>)
     {
-        if !visited.insert((cnum, extern_crate.direct)) { return }
+        if !visited.insert((cnum, extern_crate.is_direct())) { return }
 
         let cmeta = self.cstore.get_crate_data(cnum);
         let mut old_extern_crate = cmeta.extern_crate.borrow_mut();
@@ -441,14 +442,14 @@ impl<'a> CrateLoader<'a> {
         // - shorter paths to longer (tuple.2).
         let new_rank = (
             true,
-            extern_crate.direct,
+            extern_crate.is_direct(),
             cmp::Reverse(extern_crate.path_len),
         );
         let old_rank = match *old_extern_crate {
             None => (false, false, cmp::Reverse(usize::max_value())),
             Some(ref c) => (
                 true,
-                c.direct,
+                c.is_direct(),
                 cmp::Reverse(c.path_len),
             ),
         };
@@ -460,7 +461,7 @@ impl<'a> CrateLoader<'a> {
         drop(old_extern_crate);
 
         // Propagate the extern crate info to dependencies.
-        extern_crate.direct = false;
+        extern_crate.dependency_of = cnum;
         for &dep_cnum in cmeta.dependencies.borrow().iter() {
             self.update_extern_crate(dep_cnum, extern_crate, visited);
         }
@@ -1030,7 +1031,7 @@ impl<'a> CrateLoader<'a> {
                         src: ExternCrateSource::Extern(def_id),
                         span: item.span,
                         path_len,
-                        direct: true,
+                        dependency_of: LOCAL_CRATE,
                     },
                     &mut FxHashSet::default(),
                 );
@@ -1057,7 +1058,7 @@ impl<'a> CrateLoader<'a> {
                 span,
                 // to have the least priority in `update_extern_crate`
                 path_len: usize::max_value(),
-                direct: true,
+                dependency_of: LOCAL_CRATE,
             },
             &mut FxHashSet::default(),
         );
@@ -1081,7 +1082,7 @@ impl<'a> CrateLoader<'a> {
                 span,
                 // to have the least priority in `update_extern_crate`
                 path_len: usize::max_value(),
-                direct: true,
+                dependency_of: LOCAL_CRATE,
             },
             &mut FxHashSet::default(),
         );

src/librustc_metadata/cstore_impl.rs

@@ -233,7 +233,7 @@ provide! { <'tcx> tcx, def_id, other, cdata,
 
     missing_extern_crate_item => {
         let r = match *cdata.extern_crate.borrow() {
-            Some(extern_crate) if !extern_crate.direct => true,
+            Some(extern_crate) if !extern_crate.is_direct() => true,
             _ => false,
         };
         r

src/test/ui/duplicate_entry_error.stderr

@@ -7,7 +7,7 @@ LL | |     loop {}
 LL | | }
    | |_^
    |
-   = note: first defined in crate `std`.
+   = note: first defined in crate `std` (which `duplicate_entry_error` depends on).
 
 error: aborting due to previous error
 

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

@@ -4,7 +4,7 @@ error[E0152]: duplicate lang item found: `arc`.
 LL | struct Foo;
    | ^^^^^^^^^^^
    |
-   = note: first defined in crate `alloc`.
+   = note: first defined in crate `alloc` (which `std` depends on).
 
 error: aborting due to previous error
 

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

@@ -6,7 +6,7 @@ LL | |     loop {}
 LL | | }
    | |_^
    |
-   = note: first defined in crate `std`.
+   = note: first defined in crate `std` (which `panic_handler_std` depends on).
 
 error: argument should be `&PanicInfo`
   --> $DIR/panic-handler-std.rs:7:16