Split explain_why_borrow_contains_point into two functions

Allows callers to change other parts of their message based on the
explanation

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

@@ -11,11 +11,28 @@
 use borrow_check::borrow_set::BorrowData;
 use borrow_check::nll::region_infer::Cause;
 use borrow_check::{Context, MirBorrowckCtxt, WriteKind};
-use rustc::mir::{Location, Place, TerminatorKind};
+use rustc::mir::{Local, Location, Place, TerminatorKind};
 use rustc_errors::DiagnosticBuilder;
+use rustc::ty::Region;
 
 mod find_use;
 
+#[derive(Copy, Clone, Debug)]
+pub enum BorrowContainsPointReason<'tcx> {
+    Liveness {
+        local: Local,
+        location: Location,
+        in_loop: bool,
+    },
+    DropLiveness {
+        local: Local,
+        location: Location,
+    },
+    OutlivesFreeRegion {
+        outlived_region: Option<Region<'tcx>>,
+    },
+}
+
 impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
     /// Adds annotations to `err` explaining *why* the borrow contains the
     /// point from `context`. This is key for the "3-point errors"
@@ -32,15 +49,30 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
     ///
     /// [d]: https://rust-lang.github.io/rfcs/2094-nll.html#leveraging-intuition-framing-errors-in-terms-of-points
     pub(in borrow_check) fn explain_why_borrow_contains_point(
-        &mut self,
+        &self,
         context: Context,
         borrow: &BorrowData<'tcx>,
         kind_place: Option<(WriteKind, &Place<'tcx>)>,
         err: &mut DiagnosticBuilder<'_>,
     ) {
+        let reason = self.find_why_borrow_contains_point(context, borrow);
+        self.report_why_borrow_contains_point(err, reason, kind_place);
+    }
+
+    /// Finds the reason that [explain_why_borrow_contains_point] will report
+    /// but doesn't add it to any message. This is a separate function in case
+    /// the caller wants to change the error they report based on the reason
+    /// that will be reported.
+    pub(in borrow_check) fn find_why_borrow_contains_point(
+        &self,
+        context: Context,
+        borrow: &BorrowData<'tcx>
+    ) -> BorrowContainsPointReason<'tcx> {
+        use self::BorrowContainsPointReason::*;
+
         debug!(
-            "explain_why_borrow_contains_point(context={:?}, borrow={:?}, kind_place={:?})",
-            context, borrow, kind_place,
+            "find_why_borrow_contains_point(context={:?}, borrow={:?})",
+            context, borrow,
         );
 
         let regioncx = &self.nonlexical_regioncx;
@@ -62,11 +94,45 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         );
 
         match find_use::find(mir, regioncx, tcx, region_sub, context.loc) {
-            Some(Cause::LiveVar(local, location)) => {
+            Some(Cause::LiveVar(local, location)) => Liveness {
+                local,
+                location,
+                in_loop: self.is_borrow_location_in_loop(context.loc),
+            },
+            Some(Cause::DropVar(local, location)) => DropLiveness {
+                local,
+                location,
+            },
+            None => OutlivesFreeRegion {
+                outlived_region: regioncx.to_error_region(region_sub),
+            },
+        }
+    }
+
+    /// Adds annotations to `err` for the explanation `reason`. This is a
+    /// separate method so that the caller can change their error message based
+    /// on the reason that is going to be reported.
+    pub (in borrow_check) fn report_why_borrow_contains_point(
+        &self,
+        err: &mut DiagnosticBuilder,
+        reason: BorrowContainsPointReason<'tcx>,
+        kind_place: Option<(WriteKind, &Place<'tcx>)>,
+    ) {
+        use self::BorrowContainsPointReason::*;
+
+        debug!(
+            "find_why_borrow_contains_point(reason={:?}, kind_place={:?})",
+            reason, kind_place,
+        );
+
+        let mir = self.mir;
+
+        match reason {
+            Liveness { local, location, in_loop } => {
                 let span = mir.source_info(location).span;
                 let spans = self.move_spans(&Place::Local(local), location)
                     .or_else(|| self.borrow_spans(span, location));
-                let message = if self.is_borrow_location_in_loop(context.loc) {
+                let message = if in_loop {
                     if spans.for_closure() {
                         "borrow captured here by closure in later iteration of loop"
                     } else {
@@ -81,8 +147,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
                 };
                 err.span_label(spans.var_or_use(), message);
             }
-
-            Some(Cause::DropVar(local, location)) => match &mir.local_decls[local].name {
+            DropLiveness { local, location } => match &mir.local_decls[local].name {
                 Some(local_name) => {
                     err.span_label(
                         mir.source_info(location).span,
@@ -93,12 +158,12 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
                         if let Place::Local(borrowed_local) = place {
                             let dropped_local_scope = mir.local_decls[local].visibility_scope;
                             let borrowed_local_scope =
-                                mir.local_decls[*borrowed_local].visibility_scope;
+                            mir.local_decls[*borrowed_local].visibility_scope;
 
                             if mir.is_sub_scope(borrowed_local_scope, dropped_local_scope) {
                                 err.note(
-                                    "values in a scope are dropped \
-                                     in the opposite order they are defined",
+                                "values in a scope are dropped \
+                                                     in the opposite order they are defined",
                                 );
                             }
                         }
@@ -106,18 +171,16 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
                 }
 
                 None => {}
-            },
-
-            None => {
-                if let Some(region) = regioncx.to_error_region(region_sub) {
-                    self.tcx.note_and_explain_free_region(
-                        err,
-                        "borrowed value must be valid for ",
-                        region,
-                        "...",
-                    );
-                }
             }
+            OutlivesFreeRegion { outlived_region: Some(region) } => {
+                self.tcx.note_and_explain_free_region(
+                    err,
+                    "borrowed value must be valid for ",
+                    region,
+                    "...",
+                );
+            }
+            OutlivesFreeRegion { outlived_region: None } => (),
         }
     }
 
@@ -193,3 +256,4 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         false
     }
 }
+