feat: implement correct naming convention for Ferris

Author: nik-rev

compiler/rustc_interface/src/errors.rs

@@ -24,8 +24,9 @@ pub(crate) struct CrateNameInvalid<'a> {
 pub struct FerrisIdentifier {
     #[primary_span]
     pub spans: Vec<Span>,
-    #[suggestion(code = "ferris", applicability = "maybe-incorrect")]
+    #[suggestion(code = "{ferris_fix}", applicability = "maybe-incorrect")]
     pub first_span: Span,
+    pub ferris_fix: &'static str,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_interface/src/passes.rs

@@ -302,7 +302,52 @@ fn early_lint_checks(tcx: TyCtxt<'_>, (): ()) {
             spans.sort();
             if ident == sym::ferris {
                 let first_span = spans[0];
-                sess.dcx().emit_err(errors::FerrisIdentifier { spans, first_span });
+
+                enum FerrisFix {
+                    SnakeCase,
+                    ScreamingSnakeCase,
+                    PascalCase,
+                }
+
+                impl FerrisFix {
+                    const fn as_str(self) -> &'static str {
+                        match self {
+                            FerrisFix::SnakeCase => "ferris",
+                            FerrisFix::ScreamingSnakeCase => "FERRIS",
+                            FerrisFix::PascalCase => "Ferris",
+                        }
+                    }
+                }
+
+                // Obtain the symbol that is before Ferris.
+                let symbols = &sess.psess.symbol_gallery.symbols.lock();
+                let mut symbols: Vec<_> = symbols.iter().collect();
+
+                // sort by spans
+                symbols.sort_unstable_by_key(|k| k.1);
+                let ferris_fix = match symbols.binary_search_by(|(_, span)| span.cmp(&&first_span)) {
+                    Ok(index) | Err(index) if index > 0 => {
+                        let keyword = symbols[index - 1].0.as_str();
+                        // if we have e.g. `static mut`
+                        if keyword == "mut" {
+                            let keyword_before = symbols.get(index - 2).map(|s| s.0.as_str()).unwrap_or_default();
+                            if keyword_before == "static" {
+                                FerrisFix::ScreamingSnakeCase
+                            } else {
+                                FerrisFix::SnakeCase
+                            }
+                        } else {
+                            match keyword {
+                                "const" | "static" => FerrisFix::ScreamingSnakeCase,
+                                "trait" | "mod" | "union" | "type" | "enum" => FerrisFix::PascalCase,
+                                _ => FerrisFix::SnakeCase,
+                            }
+                        }
+                    },
+                    _ => FerrisFix::SnakeCase,
+                }.as_str();
+
+                sess.dcx().emit_err(errors::FerrisIdentifier { spans, first_span, ferris_fix });
             } else {
                 sess.dcx().emit_err(errors::EmojiIdentifier { spans, ident });
             }

flake.nix

@@ -0,0 +1 @@
+src/tools/nix-dev-shell/flake.nix