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[WIP] Suggest use mut when mutating a non mutable local #51260

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[WIP] Suggest use mut when mutating a non mutable local #51260

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9 changes: 5 additions & 4 deletions src/librustc_mir/borrow_check/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1716,10 +1716,11 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
.cannot_borrow_path_as_mutable(span, &item_msg, Origin::Mir);
err.span_label(span, "cannot borrow as mutable");

if place != place_err {
if let Some(name) = self.describe_place(place_err) {
err.note(&format!("the value which is causing this path not to be \
mutable is...: `{}`", name));
if let &Place::Local(local) = place_err {
let local_decl = &self.mir.local_decls[local];
if local_decl.is_user_variable && local_decl.mutability == Mutability::Not {
err.span_label(local_decl.source_info.span,
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Here we should use the span_suggestion method. Something like this (I took the prompt etc from AST borrowck):

let local_name = local_decl.name.unwrap();
err.span_suggestion(
    local_decl.source_info.span,
    &format!("consider making `{}` mutable", local_name),
    format!("mut {}", local_name),
);

This will encode the suggestion into the JSON etc so that tools like rustfix or IDEs can apply it automatically.

format!("consider changing this to `mut {}`", local_decl.name.unwrap()));
}
}
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@nikomatsakis nikomatsakis Jun 1, 2018 •
edited
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Also, I was noting on Zulip at 4:40pm (ET) yesterday, I think we want to move this suggestion code so that it comes after the match. That will require restructuring this method somewhat. The problem with the current setup is that your suggestion will only fire when you are trying to borrow something, but not — I think — in tests like let x = 3; x = 4; (try it!). I would combine it with some of the other suggestion-like code in the method too, yielding something like this:

let mut err = match kind {
  Reservation(...) | Write(...) => {
    if no error { return } else { construct_err_object() }
  }
  Reservation(...) | Write(...) => /* some variant of the preivous arm */,
  Reservation(...) | Write(...) => ...,
};

// If we get here, then there *is* an error
match place_err {
    Place::Local(l) => {
        // suggest changing to `mut` variable
    }
    Place::Projection(...) => {
        // suggest changing to `&mut` borrow (see [link 1])
    }
}

err.emit()

link 1 is

rust/src/librustc_mir/borrow_check/mod.rs

Lines 1723 to 1728 in 61f35e5

let err_info = if let Place::Projection(
box Projection {
base: Place::Local(local),
elem: ProjectionElem::Deref
}
) = *place_err {

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We are also going to want to watch out for "upvars" — those are cases where the place_err will not be a Place::Local but we want to make a similar suggestion. We can discuss on Zulip though, I'm just going to edit your PR description to have a check-list so things don't get forgotten.


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3 changes: 3 additions & 0 deletions src/test/ui/augmented-assignments.nll.stderr
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Expand Up @@ -17,6 +17,9 @@ LL | | x; //~ value moved here
error[E0596]: cannot borrow immutable item `y` as mutable
--> $DIR/augmented-assignments.rs:30:5
|
LL | let y = Int(2);
| - consider changing this to `mut y`
LL | //~^ consider changing this to `mut y`
LL | y //~ error: cannot borrow immutable local variable `y` as mutable
| ^ cannot borrow as mutable

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2 changes: 2 additions & 0 deletions src/test/ui/borrowck/issue-45983.nll.stderr
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Expand Up @@ -21,6 +21,8 @@ LL | give_any(|y| x = Some(y));
error[E0596]: cannot borrow immutable item `x` as mutable
--> $DIR/issue-45983.rs:17:14
|
LL | let x = None;
| - consider changing this to `mut x`
LL | give_any(|y| x = Some(y));
| ^^^^^^^^^^^^^^^ cannot borrow as mutable

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3 changes: 3 additions & 0 deletions src/test/ui/codemap_tests/huge_multispan_highlight.nll.stderr
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@@ -1,6 +1,9 @@
error[E0596]: cannot borrow immutable item `x` as mutable
--> $DIR/huge_multispan_highlight.rs:100:13
|
LL | let x = "foo";
| - consider changing this to `mut x`
...
LL | let y = &mut x; //~ ERROR cannot borrow
| ^^^^^^ cannot borrow as mutable

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2 changes: 2 additions & 0 deletions src/test/ui/did_you_mean/issue-34337.nll.stderr
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@@ -1,6 +1,8 @@
error[E0596]: cannot borrow immutable item `key` as mutable
--> $DIR/issue-34337.rs:16:9
|
LL | let ref mut key = v[0];
| ----------- consider changing this to `mut key`
LL | get(&mut key); //~ ERROR cannot borrow
| ^^^^^^^^ cannot borrow as mutable

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4 changes: 2 additions & 2 deletions src/test/ui/did_you_mean/issue-35937.nll.stderr
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@@ -1,10 +1,10 @@
error[E0596]: cannot borrow immutable item `f.v` as mutable
--> $DIR/issue-35937.rs:17:5
|
LL | let f = Foo { v: Vec::new() };
| - consider changing this to `mut f`
LL | f.v.push("cat".to_string()); //~ ERROR cannot borrow
| ^^^ cannot borrow as mutable
|
= note: the value which is causing this path not to be mutable is...: `f`

error[E0384]: cannot assign twice to immutable variable `s.x`
--> $DIR/issue-35937.rs:26:5
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2 changes: 2 additions & 0 deletions src/test/ui/did_you_mean/issue-37139.nll.stderr
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@@ -1,6 +1,8 @@
error[E0596]: cannot borrow immutable item `x` as mutable
--> $DIR/issue-37139.rs:22:18
|
LL | TestEnum::Item(ref mut x) => {
| --------- consider changing this to `mut x`
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@pnkfelix pnkfelix May 31, 2018 •
edited
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yeah we definitely don't want to suggest this...

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Yes, need to look at this more carefully.

LL | test(&mut x); //~ ERROR cannot borrow immutable
| ^^^^^^ cannot borrow as mutable

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2 changes: 0 additions & 2 deletions src/test/ui/did_you_mean/issue-38147-1.nll.stderr
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Expand Up @@ -3,8 +3,6 @@ error[E0596]: cannot borrow immutable item `*self.s` as mutable
|
LL | self.s.push('x'); //~ ERROR cannot borrow data mutably
| ^^^^^^ cannot borrow as mutable
|
= note: the value which is causing this path not to be mutable is...: `*self`

error: aborting due to previous error

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2 changes: 0 additions & 2 deletions src/test/ui/did_you_mean/issue-38147-4.nll.stderr
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Expand Up @@ -3,8 +3,6 @@ error[E0596]: cannot borrow immutable item `*f.s` as mutable
|
LL | f.s.push('x'); //~ ERROR cannot borrow data mutably
| ^^^ cannot borrow as mutable
|
= note: the value which is causing this path not to be mutable is...: `*f`

error: aborting due to previous error

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24 changes: 2 additions & 22 deletions src/test/ui/did_you_mean/issue-39544.nll.stderr
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@@ -1,90 +1,70 @@
error[E0596]: cannot borrow immutable item `z.x` as mutable
--> $DIR/issue-39544.rs:21:13
|
LL | let z = Z { x: X::Y };
| - consider changing this to `mut z`
LL | let _ = &mut z.x; //~ ERROR cannot borrow
| ^^^^^^^^ cannot borrow as mutable
|
= note: the value which is causing this path not to be mutable is...: `z`

error[E0596]: cannot borrow immutable item `self.x` as mutable
--> $DIR/issue-39544.rs:26:17
|
LL | let _ = &mut self.x; //~ ERROR cannot borrow
| ^^^^^^^^^^^ cannot borrow as mutable
|
= note: the value which is causing this path not to be mutable is...: `*self`

error[E0596]: cannot borrow immutable item `self.x` as mutable
--> $DIR/issue-39544.rs:30:17
|
LL | let _ = &mut self.x; //~ ERROR cannot borrow
| ^^^^^^^^^^^ cannot borrow as mutable
|
= note: the value which is causing this path not to be mutable is...: `*self`

error[E0596]: cannot borrow immutable item `other.x` as mutable
--> $DIR/issue-39544.rs:31:17
|
LL | let _ = &mut other.x; //~ ERROR cannot borrow
| ^^^^^^^^^^^^ cannot borrow as mutable
|
= note: the value which is causing this path not to be mutable is...: `*other`

error[E0596]: cannot borrow immutable item `self.x` as mutable
--> $DIR/issue-39544.rs:35:17
|
LL | let _ = &mut self.x; //~ ERROR cannot borrow
| ^^^^^^^^^^^ cannot borrow as mutable
|
= note: the value which is causing this path not to be mutable is...: `*self`

error[E0596]: cannot borrow immutable item `other.x` as mutable
--> $DIR/issue-39544.rs:36:17
|
LL | let _ = &mut other.x; //~ ERROR cannot borrow
| ^^^^^^^^^^^^ cannot borrow as mutable
|
= note: the value which is causing this path not to be mutable is...: `*other`

error[E0596]: cannot borrow immutable item `self.x` as mutable
--> $DIR/issue-39544.rs:40:17
|
LL | let _ = &mut self.x; //~ ERROR cannot borrow
| ^^^^^^^^^^^ cannot borrow as mutable
|
= note: the value which is causing this path not to be mutable is...: `*self`

error[E0596]: cannot borrow immutable item `other.x` as mutable
--> $DIR/issue-39544.rs:41:17
|
LL | let _ = &mut other.x; //~ ERROR cannot borrow
| ^^^^^^^^^^^^ cannot borrow as mutable
|
= note: the value which is causing this path not to be mutable is...: `*other`

error[E0596]: cannot borrow immutable item `other.x` as mutable
--> $DIR/issue-39544.rs:45:17
|
LL | let _ = &mut other.x; //~ ERROR cannot borrow
| ^^^^^^^^^^^^ cannot borrow as mutable
|
= note: the value which is causing this path not to be mutable is...: `*other`

error[E0596]: cannot borrow immutable item `z.x` as mutable
--> $DIR/issue-39544.rs:51:13
|
LL | let _ = &mut z.x; //~ ERROR cannot borrow
| ^^^^^^^^ cannot borrow as mutable
|
= note: the value which is causing this path not to be mutable is...: `z`

error[E0596]: cannot borrow immutable item `w.x` as mutable
--> $DIR/issue-39544.rs:52:13
|
LL | let _ = &mut w.x; //~ ERROR cannot borrow
| ^^^^^^^^ cannot borrow as mutable
|
= note: the value which is causing this path not to be mutable is...: `*w`

error[E0594]: cannot assign to immutable item `*x.0`
--> $DIR/issue-39544.rs:58:5
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4 changes: 2 additions & 2 deletions src/test/ui/issue-36400.nll.stderr
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@@ -1,10 +1,10 @@
error[E0596]: cannot borrow immutable item `*x` as mutable
--> $DIR/issue-36400.rs:15:7
|
LL | let x = Box::new(3);
| - consider changing this to `mut x`
LL | f(&mut *x); //~ ERROR cannot borrow immutable
| ^^^^^^^ cannot borrow as mutable
|
= note: the value which is causing this path not to be mutable is...: `x`

error: aborting due to previous error

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2 changes: 2 additions & 0 deletions src/test/ui/macros/span-covering-argument-1.nll.stderr
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@@ -1,6 +1,8 @@
error[E0596]: cannot borrow immutable item `foo` as mutable
--> $DIR/span-covering-argument-1.rs:15:14
|
LL | let $s = 0;
| -- consider changing this to `mut foo`
LL | *&mut $s = 0;
| ^^^^^^^ cannot borrow as mutable
...
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2 changes: 0 additions & 2 deletions src/test/ui/span/borrowck-call-is-borrow-issue-12224.nll.stderr
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Expand Up @@ -23,8 +23,6 @@ error[E0596]: cannot borrow immutable item `*f.f` as mutable
|
LL | f.f.call_mut(())
| ^^^ cannot borrow as mutable
|
= note: the value which is causing this path not to be mutable is...: `*f`

error[E0507]: cannot move out of borrowed content
--> $DIR/borrowck-call-is-borrow-issue-12224.rs:66:13
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2 changes: 0 additions & 2 deletions src/test/ui/span/borrowck-object-mutability.nll.stderr
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Expand Up @@ -9,8 +9,6 @@ error[E0596]: cannot borrow immutable item `*x` as mutable
|
LL | x.borrowed_mut(); //~ ERROR cannot borrow
| ^ cannot borrow as mutable
|
= note: the value which is causing this path not to be mutable is...: `x`

error: aborting due to 2 previous errors

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