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

Author: bors

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/associated-types/associated-types-project-from-hrtb-in-fn-body.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`.