Point to definition when modifying field of immutable variable

[estebank]

Given a file

struct S {
    x: i32,
}
fn foo() {
    let s = S { x: 42 };
    s.x += 1;
}
fn bar(s: S) {
    s.x += 1;
}

Provide the following output:

error: cannot assign to immutable field `s.x`
 --> $DIR/issue-35937.rs:16:5
  |
5 |     let s = S { x: 42 };
  |         - consider changing this to `mut s`
6 |     s.x += 1;
  |     ^^^^^^^^ cannot mutably borrow immutable field

error: cannot assign to immutable field `s.x`
 --> $DIR/issue-35937.rs:20:5
  |
8 | fn bar(s: S) {
  |        - consider changing this to `mut s`
9 |     s.x += 1;
  |     ^^^^^^^^ cannot mutably borrow immutable field

Follow up to #40445. Fix #35937.

[rust-highfive]

r? @pnkfelix

(rust_highfive has picked a reviewer for you, use r? to override)

[Mark-Simulacrum]

It'd be nice to avoid the phrasing "immutable field" since it's not really the field that's immutable, but more the binding of the struct. I'm not sure I can come up with anything better than "immutable binding," though, which isn't great either.

The help message does alleviate most of the potential pain from this, so while nice to improve, certainly not necessary.

[pnkfelix]

@bors r+

[bors]

📌 Commit 289772f has been approved by pnkfelix

[bors]

⌛ Testing commit 289772f with merge 83d832d...

[bors]

💔 Test failed - status-travis

[estebank]

@bors retry

[bors]

⌛ Testing commit 289772f with merge d1023ae...

[bors]

💔 Test failed - status-travis

[alexcrichton]

@bors: retry * network error

…

On Sun, Apr 2, 2017 at 12:06 PM, bors ***@***.***> wrote: 💔 Test failed - status-travis <https://travis-ci.org/rust-lang/rust/builds/217835654> — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#40767 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAD95IYapgdutfh1OLNNGqyDfVQyFaSCks5rr-OPgaJpZM4MnLU7> .

[bors]

⌛ Testing commit 289772f with merge 7f2ec26...

[bors]

💔 Test failed - status-travis

[estebank]

@bors retry

[bors]

⌛ Testing commit 289772f with merge cd0a4f3...

[bors]

💔 Test failed - status-travis

[alexcrichton]

@bors: retry * network failure w/ centos #40474

…

On Mon, Apr 3, 2017 at 12:52 PM, bors ***@***.***> wrote: 💔 Test failed - status-travis <https://travis-ci.org/rust-lang/rust/builds/218183568> — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#40767 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAD95MV5yw_Q3VTl2iQ_uY3N9xvjHpvWks5rsT_agaJpZM4MnLU7> .

[bors]

⌛ Testing commit 289772f with merge ceaaebc...

[bors]

💔 Test failed - status-travis

[estebank]

@bors retry

[alexcrichton]

@bors: r-

I think that's a legitimate error unfortunately:

[00:12:10]    Compiling rustc_borrowck v0.0.0 (file:///checkout/src/librustc_borrowck)
[00:12:10] error[E0425]: cannot find value `local_def` in this scope
[00:12:10]    --> /checkout/src/librustc_borrowck/borrowck/mod.rs:690:25
[00:12:10]     |
[00:12:10] 690 |                         local_def = err.cmt.get_def().map(|nid| self.tcx.hir.span(nid));
[00:12:10]     |                         ^^^^^^^^^ not found in this scope
[00:12:10] 
[00:12:11] error: no method named `get_def` found for type `std::rc::Rc<rustc::middle::mem_categorization::cmt_<'tcx>>` in the current scope
[00:12:11]    --> /checkout/src/librustc_borrowck/borrowck/mod.rs:690:45
[00:12:11]     |
[00:12:11] 690 |                         local_def = err.cmt.get_def().map(|nid| self.tcx.hir.span(nid));
[00:12:11]     |                                             ^^^^^^^
[00:12:11] 
[00:12:11] error[E0277]: the trait bound `str: core::marker::Sized` is not satisfied
[00:12:11]    --> /checkout/src/librustc_borrowck/borrowck/mod.rs:666:13
[00:12:11]     |
[00:12:11] 666 |         let msg = match err.code {
[00:12:11]     |             ^^^ the trait `core::marker::Sized` is not implemented for `str`
[00:12:11]     |
[00:12:11]     = note: `str` does not have a constant size known at compile-time
[00:12:11]     = note: all local variables must have a statically known size
[00:12:11] 
[00:12:13] error: aborting due to 2 previous errors
[00:12:13] 
[00:12:13] error: Could not compile `rustc_borrowck`.
[00:12:13] Build failed, waiting for other jobs to finish...
[00:13:06] error: build failed
[00:13:06] 
[00:13:06] 
[00:13:06] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "-j" "4" "--target" "x86_64-unknown-linux-gnu" "--release" "--locked" "--features" " jemalloc" "--manifest-path" "/checkout/src/rustc/Cargo.toml"
[00:13:06] expected success, got: exit code: 101
[00:13:06] 
[00:13:06] 
[00:13:06] Build completed unsuccessfully in 0:10:40
[00:13:06] Makefile:24: recipe for target 'all' failed
[00:13:06] make: *** [all] Error 1

[estebank]

@alexcrichton it seems like #40841 supersedes this PR.