Reword trying to operate in immutable fields

The previous message ("cannot assign/mutably borrow immutable field")
when trying to modify a field of an immutable binding gave the
(incorrect) impression that fields can be mutable independently of their
ADT's binding. Slightly reword the message to read "cannot
assign/mutably borrow field of immutable binding".

src/librustc_borrowck/borrowck/mod.rs

@@ -484,54 +484,60 @@ impl<'a, 'tcx> LoanPath<'tcx> {
     }
 }
 
-pub fn opt_loan_path<'tcx>(cmt: &mc::cmt<'tcx>) -> Option<Rc<LoanPath<'tcx>>> {
-    //! Computes the `LoanPath` (if any) for a `cmt`.
-    //! Note that this logic is somewhat duplicated in
-    //! the method `compute()` found in `gather_loans::restrictions`,
-    //! which allows it to share common loan path pieces as it
-    //! traverses the CMT.
-
+// Avoid "cannot borrow immutable field `self.x` as mutable" as that implies that a field *can* be
+// mutable independently of the struct it belongs to. (#35937)
+pub fn opt_loan_path_is_field<'tcx>(cmt: &mc::cmt<'tcx>) -> (Option<Rc<LoanPath<'tcx>>>, bool) {
     let new_lp = |v: LoanPathKind<'tcx>| Rc::new(LoanPath::new(v, cmt.ty));
 
     match cmt.cat {
         Categorization::Rvalue(..) |
         Categorization::StaticItem => {
-            None
+            (None, false)
         }
 
         Categorization::Local(id) => {
-            Some(new_lp(LpVar(id)))
+            (Some(new_lp(LpVar(id))), false)
         }
 
         Categorization::Upvar(mc::Upvar { id, .. }) => {
-            Some(new_lp(LpUpvar(id)))
+            (Some(new_lp(LpUpvar(id))), false)
         }
 
         Categorization::Deref(ref cmt_base, pk) => {
-            opt_loan_path(cmt_base).map(|lp| {
+            let lp = opt_loan_path_is_field(cmt_base);
+            (lp.0.map(|lp| {
                 new_lp(LpExtend(lp, cmt.mutbl, LpDeref(pk)))
-            })
+            }), lp.1)
         }
 
         Categorization::Interior(ref cmt_base, ik) => {
-            opt_loan_path(cmt_base).map(|lp| {
+            (opt_loan_path(cmt_base).map(|lp| {
                 let opt_variant_id = match cmt_base.cat {
                     Categorization::Downcast(_, did) =>  Some(did),
                     _ => None
                 };
                 new_lp(LpExtend(lp, cmt.mutbl, LpInterior(opt_variant_id, ik.cleaned())))
-            })
+            }), true)
         }
 
-        Categorization::Downcast(ref cmt_base, variant_def_id) =>
-            opt_loan_path(cmt_base)
-            .map(|lp| {
+        Categorization::Downcast(ref cmt_base, variant_def_id) => {
+            let lp = opt_loan_path_is_field(cmt_base);
+            (lp.0.map(|lp| {
                 new_lp(LpDowncast(lp, variant_def_id))
-            }),
-
+            }), lp.1)
+        }
     }
 }
 
+/// Computes the `LoanPath` (if any) for a `cmt`.
+/// Note that this logic is somewhat duplicated in
+/// the method `compute()` found in `gather_loans::restrictions`,
+/// which allows it to share common loan path pieces as it
+/// traverses the CMT.
+pub fn opt_loan_path<'tcx>(cmt: &mc::cmt<'tcx>) -> Option<Rc<LoanPath<'tcx>>> {
+    opt_loan_path_is_field(cmt).0
+}
+
 ///////////////////////////////////////////////////////////////////////////
 // Errors
 
@@ -787,14 +793,26 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                     mc::NoteClosureEnv(_) | mc::NoteUpvarRef(_) => {
                         self.cmt_to_string(&err.cmt)
                     }
-                    _ => match opt_loan_path(&err.cmt) {
-                        None => {
+                    _ => match opt_loan_path_is_field(&err.cmt) {
+                        (None, true) => {
+                            format!("{} of {} binding",
+                                    self.cmt_to_string(&err.cmt),
+                                    err.cmt.mutbl.to_user_str())
+
+                        }
+                        (None, false) => {
                             format!("{} {}",
                                     err.cmt.mutbl.to_user_str(),
                                     self.cmt_to_string(&err.cmt))
 
                         }
-                        Some(lp) => {
+                        (Some(lp), true) => {
+                            format!("{} `{}` of {} binding",
+                                    self.cmt_to_string(&err.cmt),
+                                    self.loan_path_to_string(&lp),
+                                    err.cmt.mutbl.to_user_str())
+                        }
+                        (Some(lp), false) => {
                             format!("{} {} `{}`",
                                     err.cmt.mutbl.to_user_str(),
                                     self.cmt_to_string(&err.cmt),
@@ -1326,7 +1344,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                 } else if let Categorization::Interior(ref cmt, _) = err.cmt.cat {
                     if let mc::MutabilityCategory::McImmutable = cmt.mutbl {
                         db.span_label(*error_span,
-                                      "cannot mutably borrow immutable field");
+                                      "cannot mutably borrow field of immutable binding");
                     }
                 }
             }

src/test/compile-fail/issue-5500-1.rs

@@ -18,8 +18,8 @@ struct TrieMapIterator<'a> {
 fn main() {
     let a = 5;
     let _iter = TrieMapIterator{node: &a};
-    _iter.node = & //[ast]~ ERROR cannot assign to immutable field `_iter.node`
-                   //[mir]~^ ERROR cannot assign to immutable field `_iter.node` (Ast)
+    _iter.node = & //[ast]~ ERROR cannot assign to field `_iter.node` of immutable binding
+                   //[mir]~^ ERROR cannot assign to field `_iter.node` of immutable binding (Ast)
                    // MIR doesn't generate an error because the code isn't reachable. This is OK
                    // because the test is here to check that the compiler doesn't ICE (cf. #5500).
     panic!()

src/test/ui/did_you_mean/issue-35937.stderr

@@ -1,26 +1,26 @@
-error[E0596]: cannot borrow immutable field `f.v` as mutable
+error[E0596]: cannot borrow field `f.v` of immutable binding as mutable
   --> $DIR/issue-35937.rs:17:5
    |
 16 |     let f = Foo { v: Vec::new() };
    |         - consider changing this to `mut f`
 17 |     f.v.push("cat".to_string()); //~ ERROR cannot borrow
-   |     ^^^ cannot mutably borrow immutable field
+   |     ^^^ cannot mutably borrow field of immutable binding
 
-error[E0594]: cannot assign to immutable field `s.x`
+error[E0594]: cannot assign to field `s.x` of immutable binding
   --> $DIR/issue-35937.rs:26:5
    |
 25 |     let s = S { x: 42 };
    |         - consider changing this to `mut s`
 26 |     s.x += 1; //~ ERROR cannot assign
-   |     ^^^^^^^^ cannot mutably borrow immutable field
+   |     ^^^^^^^^ cannot mutably borrow field of immutable binding
 
-error[E0594]: cannot assign to immutable field `s.x`
+error[E0594]: cannot assign to field `s.x` of immutable binding
   --> $DIR/issue-35937.rs:30:5
    |
 29 | fn bar(s: S) {
    |        - consider changing this to `mut s`
 30 |     s.x += 1; //~ ERROR cannot assign
-   |     ^^^^^^^^ cannot mutably borrow immutable field
+   |     ^^^^^^^^ cannot mutably borrow field of immutable binding
 
 error: aborting due to 3 previous errors
 

src/test/ui/did_you_mean/issue-38147-2.rs

@@ -14,7 +14,8 @@ struct Bar<'a> {
 
 impl<'a> Bar<'a> {
     fn f(&mut self) {
-        self.s.push('x'); //~ ERROR cannot borrow immutable borrowed
+        self.s.push('x');
+        //~^ ERROR cannot borrow borrowed content `*self.s` of immutable binding as mutable
     }
 }
 

src/test/ui/did_you_mean/issue-38147-2.stderr

@@ -1,10 +1,10 @@
-error[E0596]: cannot borrow immutable borrowed content `*self.s` as mutable
+error[E0596]: cannot borrow borrowed content `*self.s` of immutable binding as mutable
   --> $DIR/issue-38147-2.rs:17:9
    |
 12 |     s: &'a String
    |        ---------- use `&'a mut String` here to make mutable
 ...
-17 |         self.s.push('x'); //~ ERROR cannot borrow immutable borrowed
+17 |         self.s.push('x');
    |         ^^^^^^ cannot borrow as mutable
 
 error: aborting due to previous error

src/test/ui/did_you_mean/issue-38147-3.rs

@@ -14,7 +14,8 @@ struct Qux<'a> {
 
 impl<'a> Qux<'a> {
     fn f(&self) {
-        self.s.push('x'); //~ ERROR cannot borrow immutable borrowed
+        self.s.push('x');
+        //~^ ERROR cannot borrow borrowed content `*self.s` of immutable binding as mutable
     }
 }
 

src/test/ui/did_you_mean/issue-38147-3.stderr

@@ -1,10 +1,10 @@
-error[E0596]: cannot borrow immutable borrowed content `*self.s` as mutable
+error[E0596]: cannot borrow borrowed content `*self.s` of immutable binding as mutable
   --> $DIR/issue-38147-3.rs:17:9
    |
 12 |     s: &'a String
    |        ---------- use `&'a mut String` here to make mutable
 ...
-17 |         self.s.push('x'); //~ ERROR cannot borrow immutable borrowed
+17 |         self.s.push('x');
    |         ^^^^^^ cannot borrow as mutable
 
 error: aborting due to previous error

src/test/ui/did_you_mean/issue-39544.rs

@@ -55,5 +55,6 @@ pub fn with_arg(z: Z, w: &Z) {
 pub fn with_tuple() {
     let mut y = 0;
     let x = (&y,);
-    *x.0 = 1; //~ ERROR cannot assign to immutable borrowed content
+    *x.0 = 1;
+    //~^ ERROR cannot assign to borrowed content `*x.0` of immutable binding
 }

src/test/ui/did_you_mean/issue-39544.stderr

@@ -1,99 +1,99 @@
-error[E0596]: cannot borrow immutable field `z.x` as mutable
+error[E0596]: cannot borrow field `z.x` of immutable binding as mutable
   --> $DIR/issue-39544.rs:21:18
    |
 20 |     let z = Z { x: X::Y };
    |         - consider changing this to `mut z`
 21 |     let _ = &mut z.x; //~ ERROR cannot borrow
-   |                  ^^^ cannot mutably borrow immutable field
+   |                  ^^^ cannot mutably borrow field of immutable binding
 
-error[E0596]: cannot borrow immutable field `self.x` as mutable
+error[E0596]: cannot borrow field `self.x` of immutable binding as mutable
   --> $DIR/issue-39544.rs:26:22
    |
 25 |     fn foo<'z>(&'z self) {
    |                -------- use `&'z mut self` here to make mutable
 26 |         let _ = &mut self.x; //~ ERROR cannot borrow
-   |                      ^^^^^^ cannot mutably borrow immutable field
+   |                      ^^^^^^ cannot mutably borrow field of immutable binding
 
-error[E0596]: cannot borrow immutable field `self.x` as mutable
+error[E0596]: cannot borrow field `self.x` of immutable binding as mutable
   --> $DIR/issue-39544.rs:30:22
    |
 29 |     fn foo1(&self, other: &Z) {
    |             ----- use `&mut self` here to make mutable
 30 |         let _ = &mut self.x; //~ ERROR cannot borrow
-   |                      ^^^^^^ cannot mutably borrow immutable field
+   |                      ^^^^^^ cannot mutably borrow field of immutable binding
 
-error[E0596]: cannot borrow immutable field `other.x` as mutable
+error[E0596]: cannot borrow field `other.x` of immutable binding as mutable
   --> $DIR/issue-39544.rs:31:22
    |
 29 |     fn foo1(&self, other: &Z) {
    |                           -- use `&mut Z` here to make mutable
 30 |         let _ = &mut self.x; //~ ERROR cannot borrow
 31 |         let _ = &mut other.x; //~ ERROR cannot borrow
-   |                      ^^^^^^^ cannot mutably borrow immutable field
+   |                      ^^^^^^^ cannot mutably borrow field of immutable binding
 
-error[E0596]: cannot borrow immutable field `self.x` as mutable
+error[E0596]: cannot borrow field `self.x` of immutable binding as mutable
   --> $DIR/issue-39544.rs:35:22
    |
 34 |     fn foo2<'a>(&'a self, other: &Z) {
    |                 -------- use `&'a mut self` here to make mutable
 35 |         let _ = &mut self.x; //~ ERROR cannot borrow
-   |                      ^^^^^^ cannot mutably borrow immutable field
+   |                      ^^^^^^ cannot mutably borrow field of immutable binding
 
-error[E0596]: cannot borrow immutable field `other.x` as mutable
+error[E0596]: cannot borrow field `other.x` of immutable binding as mutable
   --> $DIR/issue-39544.rs:36:22
    |
 34 |     fn foo2<'a>(&'a self, other: &Z) {
    |                                  -- use `&mut Z` here to make mutable
 35 |         let _ = &mut self.x; //~ ERROR cannot borrow
 36 |         let _ = &mut other.x; //~ ERROR cannot borrow
-   |                      ^^^^^^^ cannot mutably borrow immutable field
+   |                      ^^^^^^^ cannot mutably borrow field of immutable binding
 
-error[E0596]: cannot borrow immutable field `self.x` as mutable
+error[E0596]: cannot borrow field `self.x` of immutable binding as mutable
   --> $DIR/issue-39544.rs:40:22
    |
 39 |     fn foo3<'a>(self: &'a Self, other: &Z) {
    |                       -------- use `&'a mut Self` here to make mutable
 40 |         let _ = &mut self.x; //~ ERROR cannot borrow
-   |                      ^^^^^^ cannot mutably borrow immutable field
+   |                      ^^^^^^ cannot mutably borrow field of immutable binding
 
-error[E0596]: cannot borrow immutable field `other.x` as mutable
+error[E0596]: cannot borrow field `other.x` of immutable binding as mutable
   --> $DIR/issue-39544.rs:41:22
    |
 39 |     fn foo3<'a>(self: &'a Self, other: &Z) {
    |                                        -- use `&mut Z` here to make mutable
 40 |         let _ = &mut self.x; //~ ERROR cannot borrow
 41 |         let _ = &mut other.x; //~ ERROR cannot borrow
-   |                      ^^^^^^^ cannot mutably borrow immutable field
+   |                      ^^^^^^^ cannot mutably borrow field of immutable binding
 
-error[E0596]: cannot borrow immutable field `other.x` as mutable
+error[E0596]: cannot borrow field `other.x` of immutable binding as mutable
   --> $DIR/issue-39544.rs:45:22
    |
 44 |     fn foo4(other: &Z) {
    |                    -- use `&mut Z` here to make mutable
 45 |         let _ = &mut other.x; //~ ERROR cannot borrow
-   |                      ^^^^^^^ cannot mutably borrow immutable field
+   |                      ^^^^^^^ cannot mutably borrow field of immutable binding
 
-error[E0596]: cannot borrow immutable field `z.x` as mutable
+error[E0596]: cannot borrow field `z.x` of immutable binding as mutable
   --> $DIR/issue-39544.rs:51:18
    |
 50 | pub fn with_arg(z: Z, w: &Z) {
    |                 - consider changing this to `mut z`
 51 |     let _ = &mut z.x; //~ ERROR cannot borrow
-   |                  ^^^ cannot mutably borrow immutable field
+   |                  ^^^ cannot mutably borrow field of immutable binding
 
-error[E0596]: cannot borrow immutable field `w.x` as mutable
+error[E0596]: cannot borrow field `w.x` of immutable binding as mutable
   --> $DIR/issue-39544.rs:52:18
    |
 50 | pub fn with_arg(z: Z, w: &Z) {
    |                          -- use `&mut Z` here to make mutable
 51 |     let _ = &mut z.x; //~ ERROR cannot borrow
 52 |     let _ = &mut w.x; //~ ERROR cannot borrow
-   |                  ^^^ cannot mutably borrow immutable field
+   |                  ^^^ cannot mutably borrow field of immutable binding
 
-error[E0594]: cannot assign to immutable borrowed content `*x.0`
+error[E0594]: cannot assign to borrowed content `*x.0` of immutable binding
   --> $DIR/issue-39544.rs:58:5
    |
-58 |     *x.0 = 1; //~ ERROR cannot assign to immutable borrowed content
+58 |     *x.0 = 1;
    |     ^^^^^^^^ cannot borrow as mutable
 
 error: aborting due to 12 previous errors

src/test/ui/span/borrowck-call-is-borrow-issue-12224.rs

@@ -42,7 +42,7 @@ fn test3<F>(f: &mut F) where F: FnMut() {
 
 fn test4(f: &Test) {
     f.f.call_mut(())
-    //~^ ERROR: cannot borrow immutable `Box` content `*f.f` as mutable
+    //~^ ERROR: cannot borrow `Box` content `*f.f` of immutable binding as mutable
 }
 
 fn test5(f: &mut Test) {

src/test/ui/span/borrowck-call-is-borrow-issue-12224.stderr

@@ -19,7 +19,7 @@ error[E0596]: cannot borrow immutable borrowed content `*f` as mutable
 35 |     (*f)();
    |     ^^^^ cannot borrow as mutable
 
-error[E0596]: cannot borrow immutable `Box` content `*f.f` as mutable
+error[E0596]: cannot borrow `Box` content `*f.f` of immutable binding as mutable
   --> $DIR/borrowck-call-is-borrow-issue-12224.rs:44:5
    |
 43 | fn test4(f: &Test) {