NLL: Suggest extraction of subcomputation into a new binding

src/librustc_mir/borrow_check/error_reporting.rs

@@ -13,15 +13,15 @@ use rustc::middle::region::ScopeTree;
 use rustc::mir::VarBindingForm;
 use rustc::mir::{BindingForm, BorrowKind, ClearCrossCrate, Field, Local};
 use rustc::mir::{LocalDecl, LocalKind, Location, Operand, Place};
-use rustc::mir::{ProjectionElem, Rvalue, Statement, StatementKind};
+use rustc::mir::{ProjectionElem, Rvalue, Statement, StatementKind, TerminatorKind};
 use rustc::ty;
 use rustc_data_structures::indexed_vec::Idx;
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::DiagnosticBuilder;
 use syntax_pos::Span;
 
 use super::borrow_set::BorrowData;
-use super::{Context, MirBorrowckCtxt};
+use super::{Context, ContextKind, MirBorrowckCtxt};
 use super::{InitializationRequiringAction, PrefixSet};
 
 use dataflow::move_paths::MovePathIndex;
@@ -372,6 +372,31 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
 
         self.explain_why_borrow_contains_point(context, issued_borrow, None, &mut err);
 
+        let bb = &self.mir[context.loc.block];
+        if let (&TerminatorKind::Call { ref args, .. }, ContextKind::AssignRhs) =
+            (&bb.terminator().kind, context.kind)
+        {
+            let stmt = &bb.statements[context.loc.statement_index];
+            let assigned_place = match stmt.kind {
+                StatementKind::Assign(ref place, _) => place,
+                _ => unreachable!(),
+            };
+
+            for arg in args {
+                match arg {
+                    Operand::Copy(ref operand) | Operand::Move(ref operand) => {
+                        if assigned_place == operand {
+                            err.span_note(
+                                bb.terminator().source_info.span,
+                                &format!("consider extracting this into a `let`-binding")
+                            );
+                        }
+                    }
+                    Operand::Constant(..) => {}
+                }
+            }
+        }
+
         err.buffer(&mut self.errors_buffer);
     }
 

src/test/ui/borrowck/borrowck-suggest-extraction-of-subcomputation.nll.stderr

@@ -0,0 +1,19 @@
+error[E0502]: cannot borrow `buckets` as immutable because it is also borrowed as mutable
+  --> $DIR/borrowck-suggest-extraction-of-subcomputation.rs:25:70
+   |
+LL |     buckets.slice_mut()[(key as usize).wrapping_add(22).wrapping_rem(buckets.sweep())] = 22;
+   |     -----------------------------------------------------------------^^^^^^^----------
+   |     |                                                                |
+   |     |                                                                immutable borrow occurs here
+   |     mutable borrow occurs here
+   |     borrow later used here
+   |
+note: consider extracting this into a `let`-binding
+  --> $DIR/borrowck-suggest-extraction-of-subcomputation.rs:25:70
+   |
+LL |     buckets.slice_mut()[(key as usize).wrapping_add(22).wrapping_rem(buckets.sweep())] = 22;
+   |                                                                      ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0502`.

src/test/ui/borrowck/borrowck-suggest-extraction-of-subcomputation.rs

@@ -0,0 +1,29 @@
+// Copyright 2012 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.
+
+trait Sweep {
+    fn sweep(&self) -> usize;
+}
+
+trait SliceWrapper<T> {
+    fn slice(&self) -> &[T];
+}
+
+trait SliceWrapperMut<T> {
+    fn slice_mut(&mut self) -> &mut [T];
+}
+
+fn foo<B: SliceWrapper<u32> + SliceWrapperMut<u32> + Sweep>(mut buckets: B) {
+    let key = 0u32;
+    buckets.slice_mut()[(key as usize).wrapping_add(22).wrapping_rem(buckets.sweep())] = 22;
+    //~^ ERROR cannot borrow `buckets` as immutable
+}
+
+fn main() { }

src/test/ui/borrowck/borrowck-suggest-extraction-of-subcomputation.stderr

@@ -0,0 +1,11 @@
+error[E0502]: cannot borrow `buckets` as immutable because it is also borrowed as mutable
+  --> $DIR/borrowck-suggest-extraction-of-subcomputation.rs:25:70
+   |
+LL |     buckets.slice_mut()[(key as usize).wrapping_add(22).wrapping_rem(buckets.sweep())] = 22;
+   |     -------                                                          ^^^^^^^         - mutable borrow ends here
+   |     |                                                                |
+   |     mutable borrow occurs here                                       immutable borrow occurs here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0502`.