Make explain borrow work for Universal lifetimes

Author: spastorino

src/librustc/infer/error_reporting/mod.rs

@@ -165,12 +165,19 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
                 );
             }
         };
-        let message = format!("{}{}{}", prefix, description, suffix);
-        if let Some(span) = span {
-            err.span_note(span, &message);
-        } else {
-            err.note(&message);
-        }
+
+        TyCtxt::emit_msg_span(err, prefix, description, span, suffix);
+    }
+
+    pub fn note_and_explain_free_region(self,
+                                        err: &mut DiagnosticBuilder,
+                                        prefix: &str,
+                                        region: ty::Region<'tcx>,
+                                        suffix: &str) {
+        let (description, span) = self.msg_span_from_free_region(region);
+
+
+        TyCtxt::emit_msg_span(err, prefix, description, span, suffix);
     }
 
     fn msg_span_from_free_region(self,
@@ -224,6 +231,20 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         (format!("{} {}", prefix, msg), opt_span)
     }
 
+    fn emit_msg_span(err: &mut DiagnosticBuilder,
+                     prefix: &str,
+                     description: String,
+                     span: Option<Span>,
+                     suffix: &str) {
+        let message = format!("{}{}{}", prefix, description, suffix);
+
+        if let Some(span) = span {
+            err.span_note(span, &message);
+        } else {
+            err.note(&message);
+        }
+    }
+
     fn item_scope_tag(item: &hir::Item) -> &'static str {
         match item.node {
             hir::ItemImpl(..) => "impl",

src/librustc_mir/borrow_check/error_reporting.rs

@@ -233,7 +233,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         context: Context,
         (place, span): (&Place<'tcx>, Span),
         gen_borrow_kind: BorrowKind,
-        issued_borrow: &BorrowData,
+        issued_borrow: &BorrowData<'tcx>,
         end_issued_loan_span: Option<Span>,
     ) {
         let issued_span = self.retrieve_borrow_span(issued_borrow);
@@ -574,7 +574,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         &mut self,
         context: Context,
         (place, span): (&Place<'tcx>, Span),
-        loan: &BorrowData,
+        loan: &BorrowData<'tcx>,
     ) {
         let mut err = self.tcx.cannot_assign_to_borrowed(
             span,

src/librustc_mir/borrow_check/nll/explain_borrow/mod.rs

@@ -21,7 +21,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
     pub(in borrow_check) fn explain_why_borrow_contains_point(
         &self,
         context: Context,
-        borrow: &BorrowData<'_>,
+        borrow: &BorrowData<'tcx>,
         err: &mut DiagnosticBuilder<'_>,
     ) {
         if let Some(regioncx) = &self.nonlexical_regioncx {
@@ -70,6 +70,18 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
                         }
                     }
 
+                    Cause::UniversalRegion(region_vid) => {
+                        if let Some(region) = regioncx.to_error_region(region_vid) {
+
+                            self.tcx.note_and_explain_free_region(
+                                err,
+                                "borrowed value must be valid for ",
+                                region,
+                                "...",
+                            );
+                        }
+                    }
+
                     _ => {
                     }
                 }

src/librustc_mir/borrow_check/nll/region_infer/mod.rs

@@ -588,7 +588,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     /// existentially bound, then we check its inferred value and try
     /// to find a good name from that. Returns `None` if we can't find
     /// one (e.g., this is just some random part of the CFG).
-    fn to_error_region(&self, r: RegionVid) -> Option<ty::Region<'tcx>> {
+    pub fn to_error_region(&self, r: RegionVid) -> Option<ty::Region<'tcx>> {
         if self.universal_regions.is_universal_region(r) {
             return self.definitions[r].external_name;
         } else {

src/test/ui/nll/borrowed-universal-error-2.rs

@@ -0,0 +1,22 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -Znll-dump-cause
+
+#![feature(nll)]
+#![allow(warnings)]
+
+fn foo<'a>(x: &'a (u32,)) -> &'a u32 {
+    let v = 22;
+    &v
+    //~^ ERROR `v` does not live long enough [E0597]
+}
+
+fn main() {}

src/test/ui/nll/borrowed-universal-error-2.stderr

@@ -0,0 +1,18 @@
+error[E0597]: `v` does not live long enough
+  --> $DIR/borrowed-universal-error-2.rs:18:5
+   |
+LL |     &v
+   |     ^^ borrowed value does not live long enough
+LL |     //~^ ERROR `v` does not live long enough [E0597]
+LL | }
+   | - borrowed value only lives until here
+   |
+note: borrowed value must be valid for the lifetime 'a as defined on the function body at 16:1...
+  --> $DIR/borrowed-universal-error-2.rs:16:1
+   |
+LL | fn foo<'a>(x: &'a (u32,)) -> &'a u32 {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+If you want more information on this error, try using "rustc --explain E0597"

src/test/ui/nll/borrowed-universal-error.rs

@@ -0,0 +1,26 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -Znll-dump-cause
+
+#![feature(nll)]
+#![allow(warnings)]
+
+fn gimme(x: &(u32,)) -> &u32 {
+    &x.0
+}
+
+fn foo<'a>(x: &'a (u32,)) -> &'a u32 {
+    let v = 22;
+    gimme(&(v,))
+    //~^ ERROR borrowed value does not live long enough [E0597]
+}
+
+fn main() {}

src/test/ui/nll/borrowed-universal-error.stderr

@@ -0,0 +1,18 @@
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/borrowed-universal-error.rs:22:12
+   |
+LL |     gimme(&(v,))
+   |            ^^^^ temporary value does not live long enough
+LL |     //~^ ERROR borrowed value does not live long enough [E0597]
+LL | }
+   | - temporary value only lives until here
+   |
+note: borrowed value must be valid for the lifetime 'a as defined on the function body at 20:1...
+  --> $DIR/borrowed-universal-error.rs:20:1
+   |
+LL | fn foo<'a>(x: &'a (u32,)) -> &'a u32 {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+If you want more information on this error, try using "rustc --explain E0597"

src/test/ui/nll/capture-ref-in-struct.stderr

@@ -9,8 +9,6 @@ LL |     }
 LL | 
 LL |     deref(p);
    |           - borrow later used here
-   |
-   = note: borrowed value must be valid for lifetime '_#5r...
 
 error: aborting due to previous error
 

src/test/ui/nll/closure-requirements/escape-argument.stderr

@@ -34,8 +34,6 @@ LL |     }
 LL | 
 LL |     deref(p);
    |           - borrow later used here
-   |
-   = note: borrowed value must be valid for lifetime '_#6r...
 
 error: aborting due to previous error