track intercrate ambiguity only when there is a coherence error

[rust.git] / src / librustc / traits / coherence.rs

diff --git a/src/librustc/traits/coherence.rs b/src/librustc/traits/coherence.rs
index 77bb78e4ee0783c59846cb44ac4ab92564950699..81bec308a8981849cc210f4e34442bff680b78b9 100644 (file)
--- a/src/librustc/traits/coherence.rs
+++ b/src/librustc/traits/coherence.rs
@@ -19,7 +19,7 @@
 use ty::fold::TypeFoldable;
 use ty::subst::Subst;
 
-use infer::{InferCtxt, InferOk};
+use infer::{InferOk};
 
 /// Whether we do the orphan check relative to this crate or
 /// to some remote crate.
@@ -43,8 +43,8 @@ pub struct OverlapResult<'tcx> {
 /// If there are types that satisfy both impls, invokes `on_overlap`
 /// with a suitably-freshened `ImplHeader` with those types
 /// substituted. Otherwise, invokes `no_overlap`.
-pub fn overlapping_impls<F1, F2, R>(
-    infcx: &InferCtxt<'_, '_, '_>,
+pub fn overlapping_impls<'gcx, F1, F2, R>(
+    tcx: TyCtxt<'_, 'gcx, 'gcx>,
     impl1_def_id: DefId,
     impl2_def_id: DefId,
     intercrate_mode: IntercrateMode,
@@ -63,12 +63,23 @@ pub fn overlapping_impls<F1, F2, R>(
            impl2_def_id,
            intercrate_mode);
 
-    let selcx = &mut SelectionContext::intercrate(infcx, intercrate_mode);
-    if let Some(r) = overlap(selcx, impl1_def_id, impl2_def_id) {
-        on_overlap(r)
-    } else {
-        no_overlap()
+    let overlaps = tcx.infer_ctxt().enter(|infcx| {
+        let selcx = &mut SelectionContext::intercrate(&infcx, intercrate_mode);
+        overlap(selcx, impl1_def_id, impl2_def_id).is_some()
+    });
+
+    if !overlaps {
+        return no_overlap();
     }
+
+    // In the case where we detect an error, run the check again, but
+    // this time tracking intercrate ambuiguity causes for better
+    // diagnostics. (These take time and can lead to false errors.)
+    tcx.infer_ctxt().enter(|infcx| {
+        let selcx = &mut SelectionContext::intercrate(&infcx, intercrate_mode);
+        selcx.enable_tracking_intercrate_ambiguity_causes();
+        on_overlap(overlap(selcx, impl1_def_id, impl2_def_id).unwrap())
+    })
 }
 
 fn with_fresh_ty_vars<'cx, 'gcx, 'tcx>(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
@@ -146,10 +157,10 @@ fn overlap<'cx, 'gcx, 'tcx>(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
         return None
     }
 
-    Some(OverlapResult {
-        impl_header: selcx.infcx().resolve_type_vars_if_possible(&a_impl_header),
-        intercrate_ambiguity_causes: selcx.intercrate_ambiguity_causes().to_vec(),
-    })
+    let impl_header =  selcx.infcx().resolve_type_vars_if_possible(&a_impl_header);
+    let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
+    debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
+    Some(OverlapResult { impl_header, intercrate_ambiguity_causes })
 }
 
 pub fn trait_ref_is_knowable<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,