Introduce the mismatched_lifetime_syntaxes lint

Author: shepmaster

compiler/rustc_hir/src/hir.rs

@@ -89,7 +89,7 @@ impl ParamName {
     }
 }
 
-#[derive(Debug, Copy, Clone, PartialEq, Eq, HashStable_Generic)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, HashStable_Generic)]
 pub enum LifetimeName {
     /// User-given names or fresh (synthetic) names.
     Param(LocalDefId),
@@ -158,11 +158,23 @@ impl Lifetime {
     }
 
     pub fn is_anonymous(&self) -> bool {
-        self.ident.name == kw::Empty || self.ident.name == kw::UnderscoreLifetime
+        self.is_syntactically_hidden() || self.is_syntactically_anonymous()
+    }
+
+    pub fn is_syntactically_hidden(&self) -> bool {
+        self.ident.name == kw::Empty
+    }
+
+    pub fn is_syntactically_anonymous(&self) -> bool {
+        self.ident.name == kw::UnderscoreLifetime
+    }
+
+    pub fn is_static(&self) -> bool {
+        self.res == LifetimeName::Static
     }
 
     pub fn suggestion_position(&self, is_ref: bool) -> (LifetimeSuggestionPosition, Span) {
-        if self.ident.name == kw::Empty {
+        if self.is_syntactically_hidden() {
             if is_ref {
                 (LifetimeSuggestionPosition::Ampersand, self.ident.span)
             } else if self.ident.span.is_empty() {

compiler/rustc_lint/messages.ftl

@@ -504,6 +504,25 @@ 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
+    .label_mismatched_lifetime_syntaxes_inputs =
+        {$n_inputs ->
+            [one] this lifetime flows
+            *[other] these lifetimes flow
+        } to the output
+    .label_mismatched_lifetime_syntaxes_outputs =
+        the elided {$n_outputs ->
+            [one] lifetime gets
+            *[other] lifetimes get
+        } resolved as `{$lifetime_name}`
+
+lint_mismatched_lifetime_syntaxes_suggestion_hidden =
+    one option is to consistently hide the lifetime
+
+lint_mismatched_lifetime_syntaxes_suggestion_named =
+    one option is to consistently use `{$lifetime_name}`
+
 lint_missing_fragment_specifier = missing fragment specifier
 
 lint_missing_unsafe_on_extern = extern blocks should be unsafe

compiler/rustc_lint/src/lib.rs

@@ -57,6 +57,7 @@ mod invalid_from_utf8;
 mod late;
 mod let_underscore;
 mod levels;
+mod lifetime_style;
 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_style::*;
 use macro_expr_fragment_specifier_2024_migration::*;
 use map_unit_fn::*;
 use multiple_supertrait_upcastable::*;
@@ -244,6 +246,7 @@ late_lint_methods!(
             IfLetRescope: IfLetRescope::default(),
             StaticMutRefs: StaticMutRefs,
             UnqualifiedLocalImports: UnqualifiedLocalImports,
+            LifetimeStyle: LifetimeStyle,
         ]
     ]
 );