Introduce the mismatched_lifetime_syntaxes lint

Author: shepmaster

compiler/rustc_hir/src/hir.rs

@@ -206,7 +206,7 @@ impl ParamName {
     }
 }
 
-#[derive(Debug, Copy, Clone, PartialEq, Eq, HashStable_Generic)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, HashStable_Generic)]
 pub enum LifetimeKind {
     /// User-given names or fresh (synthetic) names.
     Param(LocalDefId),

compiler/rustc_lint/messages.ftl

@@ -518,6 +518,28 @@ lint_metavariable_still_repeating = variable `{$name}` is still repeating at thi
 
 lint_metavariable_wrong_operator = meta-variable repeats with different Kleene operator
 
+lint_mismatched_lifetime_syntaxes =
+    lifetime flowing from input to output with different syntax can be confusing
+    .label_mismatched_lifetime_syntaxes_inputs =
+        {$n_inputs ->
+            [one] this lifetime flows
+            *[other] these lifetimes flow
+        } to the output
+    .label_mismatched_lifetime_syntaxes_outputs =
+        the {$n_outputs ->
+            [one] lifetime gets
+            *[other] lifetimes get
+        } resolved as `{$lifetime_name}`
+
+lint_mismatched_lifetime_syntaxes_suggestion_explicit =
+    one option is to consistently use `{$lifetime_name}`
+
+lint_mismatched_lifetime_syntaxes_suggestion_implicit =
+    one option is to consistently remove the lifetime
+
+lint_mismatched_lifetime_syntaxes_suggestion_mixed =
+    one option is to remove the lifetime for references and use the anonymous lifetime for paths
+
 lint_missing_fragment_specifier = missing fragment specifier
 
 lint_missing_unsafe_on_extern = extern blocks should be unsafe

compiler/rustc_lint/src/lib.rs

@@ -55,6 +55,7 @@ mod invalid_from_utf8;
 mod late;
 mod let_underscore;
 mod levels;
+mod lifetime_syntax;
 mod lints;
 mod macro_expr_fragment_specifier_2024_migration;
 mod map_unit_fn;
@@ -96,6 +97,7 @@ use impl_trait_overcaptures::ImplTraitOvercaptures;
 use internal::*;
 use invalid_from_utf8::*;
 use let_underscore::*;
+use lifetime_syntax::*;
 use macro_expr_fragment_specifier_2024_migration::*;
 use map_unit_fn::*;
 use multiple_supertrait_upcastable::*;
@@ -246,6 +248,7 @@ late_lint_methods!(
             StaticMutRefs: StaticMutRefs,
             UnqualifiedLocalImports: UnqualifiedLocalImports,
             CheckTransmutes: CheckTransmutes,
+            LifetimeSyntax: LifetimeSyntax,
         ]
     ]
 );