Auto merge of #55134 - davidtwco:issue-55118, r=pnkfelix

NLL: change compare-mode=nll to use borrowck=migrate

Fixes #55118.

This PR is split into two parts:

The first commit is a minor change that fixes a flaw in the existing `borrowck=migrate` implementation whereby a lint that was promoted to an error in the AST borrow checker would result in the same lint from the NLL borrow checker being downgraded to a warning in migrate mode. This PR fixes this by ensuring lints are exempt from buffering in the NLL borrow checker.

The second commit updates `compiletest` to make the NLL compare mode use `-Z borrowck=migrate` rather than `-Z borrowck=mir`. The third commit shows all the test output changes that result from this.

r? @pnkfelix

src/librustc_mir/borrow_check/mod.rs

@@ -320,20 +320,20 @@ fn do_mir_borrowck<'a, 'gcx, 'tcx>(
                 continue;
             }
 
-            let mut err = tcx.struct_span_lint_node(
+            let mut_span = tcx.sess.source_map().span_until_non_whitespace(span);
+            tcx.struct_span_lint_node(
                 UNUSED_MUT,
                 vsi[local_decl.source_info.scope].lint_root,
                 span,
                 "variable does not need to be mutable",
-            );
-            let mut_span = tcx.sess.source_map().span_until_non_whitespace(span);
-            err.span_suggestion_short_with_applicability(
+            )
+            .span_suggestion_short_with_applicability(
                 mut_span,
                 "remove this `mut`",
                 String::new(),
-                Applicability::MachineApplicable);
-
-            err.buffer(&mut mbcx.errors_buffer);
+                Applicability::MachineApplicable,
+            )
+            .emit();
         }
     }
 

src/test/ui/access-mode-in-closures.nll.stderr

@@ -13,6 +13,18 @@ note: move occurs because `v` has type `std::vec::Vec<isize>`, which does not im
 LL |         match *s { sty(v) => v } //~ ERROR cannot move out
    |                        ^
 
-error: aborting due to previous error
+error[E0507]: cannot move out of `s.0` which is behind a `&` reference
+  --> $DIR/access-mode-in-closures.rs:19:24
+   |
+LL |     let _foo = unpack(|s| {
+   |                        - help: consider changing this to be a mutable reference: `&mut sty`
+LL |         // Test that `s` is moved here.
+LL |         match *s { sty(v) => v } //~ ERROR cannot move out
+   |                        ^
+   |                        |
+   |                        cannot move out of `s.0` which is behind a `&` reference
+   |                        `s` is a `&` reference, so the data it refers to cannot be moved
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0507`.

src/test/ui/binop/binop-move-semantics.nll.stderr

@@ -32,6 +32,18 @@ error[E0507]: cannot move out of borrowed content
 LL |     *n;  //~ ERROR: cannot move out of borrowed content
    |     ^^ cannot move out of borrowed content
 
+error[E0507]: cannot move out of `*n` which is behind a `&` reference
+  --> $DIR/binop-move-semantics.rs:42:5
+   |
+LL |     let n = &y;
+   |             -- help: consider changing this to be a mutable reference: `&mut y`
+...
+LL |     *n;  //~ ERROR: cannot move out of borrowed content
+   |     ^^
+   |     |
+   |     cannot move out of `*n` which is behind a `&` reference
+   |     `n` is a `&` reference, so the data it refers to cannot be moved
+
 error[E0502]: cannot borrow `f` as immutable because it is also borrowed as mutable
   --> $DIR/binop-move-semantics.rs:64:5
    |
@@ -62,7 +74,7 @@ LL | |     &mut f;  //~ ERROR: cannot borrow `f` as mutable because it is also b
    |       |    immutable borrow later used here
    |       mutable borrow occurs here
 
-error: aborting due to 6 previous errors
+error: aborting due to 7 previous errors
 
 Some errors occurred: E0382, E0502, E0507.
 For more information about an error, try `rustc --explain E0382`.

src/test/ui/borrowck/borrowck-closures-unique.nll.stderr

@@ -40,15 +40,19 @@ LL |     let c2 = || set(x); //~ ERROR two closures require unique access to `x`
 LL |     c1;
    |     -- first borrow later used here
 
-error[E0594]: cannot assign to `x`, as it is not declared as mutable
+warning[E0594]: cannot assign to `x`, as it is not declared as mutable
   --> $DIR/borrowck-closures-unique.rs:57:38
    |
 LL | fn e(x: &'static mut isize) {
    |      - help: consider changing this to be mutable: `mut x`
 LL |     let c1 = |y: &'static mut isize| x = y; //~ ERROR closure cannot assign to immutable argument
    |                                      ^^^^^ cannot assign
+   |
+   = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
+           It represents potential unsoundness in your code.
+           This warning will become a hard error in the future.
 
-error: aborting due to 4 previous errors
+error: aborting due to 3 previous errors
 
 Some errors occurred: E0500, E0524, E0594.
 For more information about an error, try `rustc --explain E0500`.

src/test/ui/borrowck/borrowck-describe-lvalue.ast.nll.stderr

@@ -20,7 +20,7 @@ LL |                    //[mir]~^ ERROR cannot borrow `x` as mutable more than o
 LL |                    *y = 1;
    |                    ------ first borrow later used here
 
-error: captured variable cannot escape `FnMut` closure body
+warning: captured variable cannot escape `FnMut` closure body
   --> $DIR/borrowck-describe-lvalue.rs:305:16
    |
 LL |              || {
@@ -36,6 +36,9 @@ LL | |                 }
    |
    = 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
+   = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
+           It represents potential unsoundness in your code.
+           This warning will become a hard error in the future.
 
 error[E0503]: cannot use `f.x` because it was mutably borrowed
   --> $DIR/borrowck-describe-lvalue.rs:53:9
@@ -339,7 +342,7 @@ LL |             &[_, F {x: ref xf, ..}] => println!("{}", xf),
 LL |         drop(x);
    |              - mutable borrow later used here
 
-error[E0502]: cannot borrow `*block.current` as immutable because it is also borrowed as mutable
+warning[E0502]: cannot borrow `*block.current` as immutable because it is also borrowed as mutable
   --> $DIR/borrowck-describe-lvalue.rs:245:29
    |
 LL |             let x = &mut block;
@@ -349,8 +352,12 @@ LL |             let p: &'a u8 = &*block.current;
 ...
 LL |             drop(x);
    |                  - mutable borrow later used here
+   |
+   = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
+           It represents potential unsoundness in your code.
+           This warning will become a hard error in the future.
 
-error[E0502]: cannot borrow `*block.current` as immutable because it is also borrowed as mutable
+warning[E0502]: cannot borrow `*block.current` as immutable because it is also borrowed as mutable
   --> $DIR/borrowck-describe-lvalue.rs:260:33
    |
 LL |             let x = &mut block;
@@ -360,6 +367,10 @@ LL |             let p : *const u8 = &*(*block).current;
 ...
 LL |             drop(x);
    |                  - mutable borrow later used here
+   |
+   = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
+           It represents potential unsoundness in your code.
+           This warning will become a hard error in the future.
 
 error[E0382]: use of moved value: `x`
   --> $DIR/borrowck-describe-lvalue.rs:318:22
@@ -371,7 +382,7 @@ LL |                 drop(x); //[ast]~ ERROR use of moved value: `x`
    |
    = note: move occurs because `x` has type `std::vec::Vec<i32>`, which does not implement the `Copy` trait
 
-error: aborting due to 32 previous errors
+error: aborting due to 29 previous errors
 
 Some errors occurred: E0382, E0499, E0502, E0503.
 For more information about an error, try `rustc --explain E0382`.

src/test/ui/borrowck/borrowck-fn-in-const-a.ast.nll.stderr

@@ -0,0 +1,20 @@
+error[E0507]: cannot move out of borrowed content
+  --> $DIR/borrowck-fn-in-const-a.rs:19:16
+   |
+LL |         return *x //[ast]~ ERROR cannot move out of borrowed content [E0507]
+   |                ^^ cannot move out of borrowed content
+
+error[E0507]: cannot move out of `*x` which is behind a `&` reference
+  --> $DIR/borrowck-fn-in-const-a.rs:19:16
+   |
+LL |     fn broken(x: &String) -> String {
+   |                  ------- help: consider changing this to be a mutable reference: `&mut std::string::String`
+LL |         return *x //[ast]~ ERROR cannot move out of borrowed content [E0507]
+   |                ^^
+   |                |
+   |                cannot move out of `*x` which is behind a `&` reference
+   |                `x` is a `&` reference, so the data it refers to cannot be moved
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0507`.