rough solution

Author: karolzwolak

compiler/rustc_lint/src/context.rs

@@ -62,7 +62,11 @@ pub struct LintStore {
     lint_groups: FxIndexMap<&'static str, LintGroup>,
 }
 
-impl LintStoreMarker for LintStore {}
+impl LintStoreMarker for LintStore {
+    fn lint_groups(&self) -> Box<dyn Iterator<Item = (&'static str, Vec<LintId>, bool)> + '_> {
+        Box::new(self.get_lint_groups())
+    }
+}
 
 /// The target of the `by_name` map, which accounts for renaming/deprecation.
 #[derive(Debug)]

compiler/rustc_middle/src/lint.rs

@@ -211,11 +211,22 @@ impl LintExpectation {
 }
 
 fn explain_lint_level_source(
+    sess: &Session,
     lint: &'static Lint,
     level: Level,
     src: LintLevelSource,
     err: &mut Diag<'_, ()>,
 ) {
+    fn lint_group_name(lint: &'static Lint, sess: &Session) -> Option<&'static str> {
+        let mut lint_groups_iter = sess.lint_groups();
+        let lint_id = LintId::of(lint);
+        lint_groups_iter
+            .find(|lint_group| {
+                let lints = &lint_group.1;
+                lints.iter().find(|lint_group_lint| **lint_group_lint == lint_id).is_some()
+            })
+            .map(|lint_group| lint_group.0)
+    }
     let name = lint.name_lower();
     if let Level::Allow = level {
         // Do not point at `#[allow(compat_lint)]` as the reason for a compatibility lint
@@ -224,7 +235,13 @@ fn explain_lint_level_source(
     }
     match src {
         LintLevelSource::Default => {
-            err.note_once(format!("`#[{}({})]` on by default", level.as_str(), name));
+            let level_str = level.as_str();
+            err.note_once(format!("`#[{level_str}({name})]` on by default"));
+            if let Some(group_name) = lint_group_name(lint, sess) {
+                err.note_once(format!(
+                    "`#[{level_str}({name})]` implied by `#[{level_str}({group_name})]`"
+                ));
+            }
         }
         LintLevelSource::CommandLine(lint_flag_val, orig_level) => {
             let flag = orig_level.to_cmd_flag();
@@ -427,7 +444,7 @@ pub fn lint_level(
             decorate(&mut err);
         }
 
-        explain_lint_level_source(lint, level, src, &mut err);
+        explain_lint_level_source(sess, lint, level, src, &mut err);
         err.emit()
     }
     lint_level_impl(sess, lint, level, span, Box::new(decorate))

compiler/rustc_session/src/session.rs

@@ -44,6 +44,7 @@ use crate::config::{
     SwitchWithOptPath,
 };
 use crate::filesearch::FileSearch;
+use crate::lint::LintId;
 use crate::parse::{ParseSess, add_feature_diagnostics};
 use crate::search_paths::SearchPath;
 use crate::{errors, filesearch, lint};
@@ -139,7 +140,9 @@ pub struct CompilerIO {
     pub temps_dir: Option<PathBuf>,
 }
 
-pub trait LintStoreMarker: Any + DynSync + DynSend {}
+pub trait LintStoreMarker: Any + DynSync + DynSend {
+    fn lint_groups(&self) -> Box<dyn Iterator<Item = (&'static str, Vec<LintId>, bool)> + '_>;
+}
 
 /// Represents the data associated with a compilation
 /// session for a single crate.
@@ -603,6 +606,13 @@ impl Session {
             (&*self.target.staticlib_prefix, &*self.target.staticlib_suffix)
         }
     }
+
+    pub fn lint_groups(&self) -> Box<dyn Iterator<Item = (&'static str, Vec<LintId>, bool)> + '_> {
+        match self.lint_store {
+            Some(ref lint_store) => lint_store.lint_groups(),
+            None => Box::new(std::iter::empty()),
+        }
+    }
 }
 
 // JUSTIFICATION: defn of the suggested wrapper fns