Auto merge of #54802 - davidtwco:issue-53040, r=pnkfelix

[nll] better error message when returning refs to upvars

Fixes #53040.

r? @nikomatsakis

9 files changed, 63 insertions, 44 deletions

diff --git a/src/test/ui/borrowck/borrowck-describe-lvalue.ast.nll.stderr b/src/test/ui/borrowck/borrowck-describe-lvalue.ast.nll.stderr
index ac6cfac2a16..5721c52ba21 100644
--- a/src/test/ui/borrowck/borrowck-describe-lvalue.ast.nll.stderr
+++ b/src/test/ui/borrowck/borrowck-describe-lvalue.ast.nll.stderr
@@ -20,24 +20,22 @@ LL |                    //[mir]~^ ERROR cannot borrow `x` as mutable more than o
 LL |                    *y = 1;
    |                    ------ first borrow later used here
 
-error: unsatisfied lifetime constraints
+error: captured variable cannot escape `FnMut` closure body
   --> $DIR/borrowck-describe-lvalue.rs:305:16
    |
 LL |              || {
-   |              --
-   |              ||
-   |              |return type of closure is [closure@$DIR/borrowck-describe-lvalue.rs:305:16: 311:18 x:&'2 mut i32]
-   |              lifetime `'1` represents this closure's body
-LL | /                || { //[mir]~ ERROR unsatisfied lifetime constraints
+   |               - inferred to be a `FnMut` closure
+LL | /                || { //[mir]~ ERROR captured variable cannot escape `FnMut` closure body
 LL | |                    let y = &mut x;
 LL | |                    &mut x; //[ast]~ ERROR cannot borrow `**x` as mutable more than once at a time
 LL | |                    //[mir]~^ ERROR cannot borrow `x` as mutable more than once at a time
 LL | |                    *y = 1;
 LL | |                    drop(y);
 LL | |                 }
-   | |_________________^ returning this value requires that `'1` must outlive `'2`
+   | |_________________^ returns a closure that contains a reference to a captured variable, which then escapes the closure body
    |
-   = note: closure implements `FnMut`, so references to captured variables can't escape the closure
+   = note: `FnMut` closures only have access to their captured variables while they are executing...
+   = note: ...therefore, they cannot allow references to captured variables to escape
 
 error[E0503]: cannot use `f.x` because it was mutably borrowed
   --> $DIR/borrowck-describe-lvalue.rs:53:9
diff --git a/src/test/ui/borrowck/borrowck-describe-lvalue.mir.stderr b/src/test/ui/borrowck/borrowck-describe-lvalue.mir.stderr
index ac6cfac2a16..5721c52ba21 100644
--- a/src/test/ui/borrowck/borrowck-describe-lvalue.mir.stderr
+++ b/src/test/ui/borrowck/borrowck-describe-lvalue.mir.stderr
@@ -20,24 +20,22 @@ LL |                    //[mir]~^ ERROR cannot borrow `x` as mutable more than o
 LL |                    *y = 1;
    |                    ------ first borrow later used here
 
-error: unsatisfied lifetime constraints
+error: captured variable cannot escape `FnMut` closure body
   --> $DIR/borrowck-describe-lvalue.rs:305:16
    |
 LL |              || {
-   |              --
-   |              ||
-   |              |return type of closure is [closure@$DIR/borrowck-describe-lvalue.rs:305:16: 311:18 x:&'2 mut i32]
-   |              lifetime `'1` represents this closure's body
-LL | /                || { //[mir]~ ERROR unsatisfied lifetime constraints
+   |               - inferred to be a `FnMut` closure
+LL | /                || { //[mir]~ ERROR captured variable cannot escape `FnMut` closure body
 LL | |                    let y = &mut x;
 LL | |                    &mut x; //[ast]~ ERROR cannot borrow `**x` as mutable more than once at a time
 LL | |                    //[mir]~^ ERROR cannot borrow `x` as mutable more than once at a time
 LL | |                    *y = 1;
 LL | |                    drop(y);
 LL | |                 }
-   | |_________________^ returning this value requires that `'1` must outlive `'2`
+   | |_________________^ returns a closure that contains a reference to a captured variable, which then escapes the closure body
    |
-   = note: closure implements `FnMut`, so references to captured variables can't escape the closure
+   = note: `FnMut` closures only have access to their captured variables while they are executing...
+   = note: ...therefore, they cannot allow references to captured variables to escape
 
 error[E0503]: cannot use `f.x` because it was mutably borrowed
   --> $DIR/borrowck-describe-lvalue.rs:53:9
diff --git a/src/test/ui/borrowck/borrowck-describe-lvalue.rs b/src/test/ui/borrowck/borrowck-describe-lvalue.rs
index 2ef08e75cfd..649de888ab0 100644
--- a/src/test/ui/borrowck/borrowck-describe-lvalue.rs
+++ b/src/test/ui/borrowck/borrowck-describe-lvalue.rs
@@ -302,7 +302,7 @@ fn main() {
         // FIXME(#49824) -- the free region error below should probably not be there
         let mut x = 0;
            || {
-               || { //[mir]~ ERROR unsatisfied lifetime constraints
+               || { //[mir]~ ERROR captured variable cannot escape `FnMut` closure body
                    let y = &mut x;
                    &mut x; //[ast]~ ERROR cannot borrow `**x` as mutable more than once at a time
                    //[mir]~^ ERROR cannot borrow `x` as mutable more than once at a time
diff --git a/src/test/ui/issues/issue-40510-1.nll.stderr b/src/test/ui/issues/issue-40510-1.nll.stderr
index 3a579c04de1..1aeb1a89ead 100644
--- a/src/test/ui/issues/issue-40510-1.nll.stderr
+++ b/src/test/ui/issues/issue-40510-1.nll.stderr
@@ -1,15 +1,13 @@
-error: unsatisfied lifetime constraints
+error: captured variable cannot escape `FnMut` closure body
   --> $DIR/issue-40510-1.rs:18:9
    |
 LL |     || {
-   |     --
-   |     ||
-   |     |return type of closure is &'2 mut std::boxed::Box<()>
-   |     lifetime `'1` represents this closure's body
+   |      - inferred to be a `FnMut` closure
 LL |         &mut x
-   |         ^^^^^^ returning this value requires that `'1` must outlive `'2`
+   |         ^^^^^^ returns a reference to a captured variable which escapes the closure body
    |
-   = note: closure implements `FnMut`, so references to captured variables can't escape the closure
+   = note: `FnMut` closures only have access to their captured variables while they are executing...
+   = note: ...therefore, they cannot allow references to captured variables to escape
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/issues/issue-40510-3.nll.stderr b/src/test/ui/issues/issue-40510-3.nll.stderr
index 84ab2a8216d..c334e592fbc 100644
--- a/src/test/ui/issues/issue-40510-3.nll.stderr
+++ b/src/test/ui/issues/issue-40510-3.nll.stderr
@@ -1,17 +1,15 @@
-error: unsatisfied lifetime constraints
+error: captured variable cannot escape `FnMut` closure body
   --> $DIR/issue-40510-3.rs:18:9
    |
 LL |       || {
-   |       --
-   |       ||
-   |       |return type of closure is [closure@$DIR/issue-40510-3.rs:18:9: 20:10 x:&'2 mut std::vec::Vec<()>]
-   |       lifetime `'1` represents this closure's body
+   |        - inferred to be a `FnMut` closure
 LL | /         || {
 LL | |             x.push(())
 LL | |         }
-   | |_________^ returning this value requires that `'1` must outlive `'2`
+   | |_________^ returns a closure that contains a reference to a captured variable, which then escapes the closure body
    |
-   = note: closure implements `FnMut`, so references to captured variables can't escape the closure
+   = note: `FnMut` closures only have access to their captured variables while they are executing...
+   = note: ...therefore, they cannot allow references to captured variables to escape
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/issues/issue-49824.nll.stderr b/src/test/ui/issues/issue-49824.nll.stderr
index df43158ec9c..2e0463fdd1d 100644
--- a/src/test/ui/issues/issue-49824.nll.stderr
+++ b/src/test/ui/issues/issue-49824.nll.stderr
@@ -1,17 +1,15 @@
-error: unsatisfied lifetime constraints
+error: captured variable cannot escape `FnMut` closure body
   --> $DIR/issue-49824.rs:22:9
    |
 LL |       || {
-   |       --
-   |       ||
-   |       |return type of closure is [closure@$DIR/issue-49824.rs:22:9: 24:10 x:&'2 mut i32]
-   |       lifetime `'1` represents this closure's body
+   |        - inferred to be a `FnMut` closure
 LL | /         || {
 LL | |             let _y = &mut x;
 LL | |         }
-   | |_________^ returning this value requires that `'1` must outlive `'2`
+   | |_________^ returns a closure that contains a reference to a captured variable, which then escapes the closure body
    |
-   = note: closure implements `FnMut`, so references to captured variables can't escape the closure
+   = note: `FnMut` closures only have access to their captured variables while they are executing...
+   = note: ...therefore, they cannot allow references to captured variables to escape
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/nll/issue-53040.rs b/src/test/ui/nll/issue-53040.rs
new file mode 100644
index 00000000000..2b6e67be6d9
--- /dev/null
+++ b/src/test/ui/nll/issue-53040.rs
@@ -0,0 +1,16 @@
+// Copyright 2017 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.
+
+#![feature(nll)]
+
+fn main() {
+    let mut v: Vec<()> = Vec::new();
+    || &mut v;
+}
diff --git a/src/test/ui/nll/issue-53040.stderr b/src/test/ui/nll/issue-53040.stderr
new file mode 100644
index 00000000000..fac9969f193
--- /dev/null
+++ b/src/test/ui/nll/issue-53040.stderr
@@ -0,0 +1,13 @@
+error: captured variable cannot escape `FnMut` closure body
+  --> $DIR/issue-53040.rs:15:8
+   |
+LL |     || &mut v;
+   |      - ^^^^^^ returns a reference to a captured variable which escapes the closure body
+   |      |
+   |      inferred to be a `FnMut` closure
+   |
+   = note: `FnMut` closures only have access to their captured variables while they are executing...
+   = note: ...therefore, they cannot allow references to captured variables to escape
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/regions/regions-return-ref-to-upvar-issue-17403.nll.stderr b/src/test/ui/regions/regions-return-ref-to-upvar-issue-17403.nll.stderr
index 35c1da61ae2..300a5639822 100644
--- a/src/test/ui/regions/regions-return-ref-to-upvar-issue-17403.nll.stderr
+++ b/src/test/ui/regions/regions-return-ref-to-upvar-issue-17403.nll.stderr
@@ -1,13 +1,13 @@
-error: unsatisfied lifetime constraints
+error: captured variable cannot escape `FnMut` closure body
   --> $DIR/regions-return-ref-to-upvar-issue-17403.rs:17:24
    |
 LL |         let mut f = || &mut x; //~ ERROR cannot infer
-   |                     -- ^^^^^^ returning this value requires that `'1` must outlive `'2`
-   |                     ||
-   |                     |return type of closure is &'2 mut i32
-   |                     lifetime `'1` represents this closure's body
+   |                      - ^^^^^^ returns a reference to a captured variable which escapes the closure body
+   |                      |
+   |                      inferred to be a `FnMut` closure
    |
-   = note: closure implements `FnMut`, so references to captured variables can't escape the closure
+   = note: `FnMut` closures only have access to their captured variables while they are executing...
+   = note: ...therefore, they cannot allow references to captured variables to escape
 
 error: aborting due to previous error