Auto merge of #52681 - pnkfelix:z-borrowck-migrate, r=nikomatsakis

Add `-Z borrowck=migrate`

This adds `-Z borrowck=migrate`, which represents the way we want to migrate to NLL under Rust versions to come. It also hooks this new mode into `--edition 2018`, which means we're officially turning NLL on in the 2018 edition.

The basic idea of `-Z borrowck=migrate` that there are cases where NLL is fixing old soundness bugs in the borrow-checker, but in order to avoid just breaking code by immediately rejecting the programs that hit those soundness bugs, we instead use the following strategy:

If your code is accepted by NLL, then we accept it.
If your code is rejected by both NLL and the old AST-borrowck, then we reject it.
If your code is rejected by NLL but accepted by the old AST-borrowck, then we emit the new NLL errors as **warnings**.

These warnings will be turned into hard errors in the future, and they say so in these diagnostics.

Fix #46908

2 files changed, 19 insertions, 19 deletions

diff --git a/src/librustc_errors/diagnostic.rs b/src/librustc_errors/diagnostic.rs
index b1578b697bb..825e31539c8 100644
--- a/src/librustc_errors/diagnostic.rs
+++ b/src/librustc_errors/diagnostic.rs
@@ -99,6 +99,25 @@ impl Diagnostic {
         }
     }
 
+    pub fn is_error(&self) -> bool {
+        match self.level {
+            Level::Bug |
+            Level::Fatal |
+            Level::PhaseFatal |
+            Level::Error |
+            Level::FailureNote => {
+                true
+            }
+
+            Level::Warning |
+            Level::Note |
+            Level::Help |
+            Level::Cancelled => {
+                false
+            }
+        }
+    }
+
     /// Cancel the diagnostic (a structured diagnostic must either be emitted or
     /// canceled or it will panic when dropped).
     pub fn cancel(&mut self) {
diff --git a/src/librustc_errors/diagnostic_builder.rs b/src/librustc_errors/diagnostic_builder.rs
index 8f99ad87cb8..a0f3abda077 100644
--- a/src/librustc_errors/diagnostic_builder.rs
+++ b/src/librustc_errors/diagnostic_builder.rs
@@ -100,25 +100,6 @@ impl<'a> DiagnosticBuilder<'a> {
         buffered_diagnostics.push(diagnostic);
     }
 
-    pub fn is_error(&self) -> bool {
-        match self.level {
-            Level::Bug |
-            Level::Fatal |
-            Level::PhaseFatal |
-            Level::Error |
-            Level::FailureNote => {
-                true
-            }
-
-            Level::Warning |
-            Level::Note |
-            Level::Help |
-            Level::Cancelled => {
-                false
-            }
-        }
-    }
-
     /// Convenience function for internal use, clients should use one of the
     /// span_* methods instead.
     pub fn sub<S: Into<MultiSpan>>(