Rollup merge of #86671 - m-ou-se:non-fmt-panic-future-incompatible, r=nikomatsakis

Turn non_fmt_panic into a future_incompatible edition lint.

This turns the `non_fmt_panic` lint into a future_incompatible edition lint, so it becomes part of the `rust_2021_compatibility` group. See https://github.com/rust-lang/rust/issues/85894.

This lint produces both warnings about semantical changes (e.g. `panic!("{{")`) and things that will become hard errors (e.g. `panic!("{")`). So I added a `explain_reason: false` that supresses the default "this will become a hard error" or "the semantics will change" message, and instead added a note depending on the situation. (cc `@rylev)`

r? `@nikomatsakis`

1 files changed, 8 insertions, 3 deletions

diff --git a/compiler/rustc_middle/src/lint.rs b/compiler/rustc_middle/src/lint.rs
index 8180d853f60..8d0fdf9128a 100644
--- a/compiler/rustc_middle/src/lint.rs
+++ b/compiler/rustc_middle/src/lint.rs
@@ -398,9 +398,14 @@ pub fn struct_lint_level<'s, 'd>(
                  it will become a hard error in a future release!"
                     .to_owned()
             };
-            let citation = format!("for more information, see {}", future_incompatible.reference);
-            err.warn(&explanation);
-            err.note(&citation);
+            if future_incompatible.explain_reason {
+                err.warn(&explanation);
+            }
+            if !future_incompatible.reference.is_empty() {
+                let citation =
+                    format!("for more information, see {}", future_incompatible.reference);
+                err.note(&citation);
+            }
         }
 
         // Finally, run `decorate`. This function is also responsible for emitting the diagnostic.