Lint single-use lifetimes during AST resolution

compiler/rustc_ast/src/ast.rs

@@ -1976,6 +1976,8 @@ pub struct BareFnTy {
     pub ext: Extern,
     pub generic_params: Vec<GenericParam>,
     pub decl: P<FnDecl>,
+    /// Span of the `fn(...) -> ...` part.
+    pub decl_span: Span,
 }
 
 /// The various kinds of type recognized by the compiler.

compiler/rustc_ast/src/mut_visit.rs

@@ -460,10 +460,11 @@ pub fn noop_visit_ty<T: MutVisitor>(ty: &mut P<Ty>, vis: &mut T) {
             vis.visit_mt(mt);
         }
         TyKind::BareFn(bft) => {
-            let BareFnTy { unsafety, ext: _, generic_params, decl } = bft.deref_mut();
+            let BareFnTy { unsafety, ext: _, generic_params, decl, decl_span } = bft.deref_mut();
             visit_unsafety(unsafety, vis);
             generic_params.flat_map_in_place(|param| vis.flat_map_generic_param(param));
             vis.visit_fn_decl(decl);
+            vis.visit_span(decl_span);
         }
         TyKind::Tup(tys) => visit_vec(tys, |ty| vis.visit_ty(ty)),
         TyKind::Paren(ty) => vis.visit_ty(ty),

compiler/rustc_ast/src/visit.rs

@@ -89,6 +89,16 @@ impl<'a> FnKind<'a> {
     }
 }
 
+#[derive(Copy, Clone, Debug)]
+pub enum LifetimeCtxt {
+    /// Appears in a reference type.
+    Rptr,
+    /// Appears as a bound on a type or another lifetime.
+    Bound,
+    /// Appears as a generic argument.
+    GenericArg,
+}
+
 /// Each method of the `Visitor` trait is a hook to be potentially
 /// overridden. Each method's default implementation recursively visits
 /// the substructure of the input via the corresponding `walk` method;
@@ -184,7 +194,7 @@ pub trait Visitor<'ast>: Sized {
     fn visit_label(&mut self, label: &'ast Label) {
         walk_label(self, label)
     }
-    fn visit_lifetime(&mut self, lifetime: &'ast Lifetime) {
+    fn visit_lifetime(&mut self, lifetime: &'ast Lifetime, _: LifetimeCtxt) {
         walk_lifetime(self, lifetime)
     }
     fn visit_mac_call(&mut self, mac: &'ast MacCall) {
@@ -414,7 +424,7 @@ pub fn walk_ty<'a, V: Visitor<'a>>(visitor: &mut V, typ: &'a Ty) {
         TyKind::Slice(ref ty) | TyKind::Paren(ref ty) => visitor.visit_ty(ty),
         TyKind::Ptr(ref mutable_type) => visitor.visit_ty(&mutable_type.ty),
         TyKind::Rptr(ref opt_lifetime, ref mutable_type) => {
-            walk_list!(visitor, visit_lifetime, opt_lifetime);
+            walk_list!(visitor, visit_lifetime, opt_lifetime, LifetimeCtxt::Rptr);
             visitor.visit_ty(&mutable_type.ty)
         }
         TyKind::Tup(ref tuple_element_types) => {
@@ -507,7 +517,7 @@ where
     V: Visitor<'a>,
 {
     match generic_arg {
-        GenericArg::Lifetime(lt) => visitor.visit_lifetime(lt),
+        GenericArg::Lifetime(lt) => visitor.visit_lifetime(lt, LifetimeCtxt::GenericArg),
         GenericArg::Type(ty) => visitor.visit_ty(ty),
         GenericArg::Const(ct) => visitor.visit_anon_const(ct),
     }
@@ -599,7 +609,9 @@ pub fn walk_foreign_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a ForeignI
 pub fn walk_param_bound<'a, V: Visitor<'a>>(visitor: &mut V, bound: &'a GenericBound) {
     match *bound {
         GenericBound::Trait(ref typ, ref modifier) => visitor.visit_poly_trait_ref(typ, modifier),
-        GenericBound::Outlives(ref lifetime) => visitor.visit_lifetime(lifetime),
+        GenericBound::Outlives(ref lifetime) => {
+            visitor.visit_lifetime(lifetime, LifetimeCtxt::Bound)
+        }
     }
 }
 
@@ -639,7 +651,7 @@ pub fn walk_where_predicate<'a, V: Visitor<'a>>(visitor: &mut V, predicate: &'a
         WherePredicate::RegionPredicate(WhereRegionPredicate {
             ref lifetime, ref bounds, ..
         }) => {
-            visitor.visit_lifetime(lifetime);
+            visitor.visit_lifetime(lifetime, LifetimeCtxt::Bound);
             walk_list!(visitor, visit_param_bound, bounds, BoundKind::Bound);
         }
         WherePredicate::EqPredicate(WhereEqPredicate { ref lhs_ty, ref rhs_ty, .. }) => {

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1070,7 +1070,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
         visit::walk_label(self, label);
     }
 
-    fn visit_lifetime(&mut self, lifetime: &'a Lifetime) {
+    fn visit_lifetime(&mut self, lifetime: &'a Lifetime, _: visit::LifetimeCtxt) {
         self.check_lifetime(lifetime.ident);
         visit::walk_lifetime(self, lifetime);
     }

compiler/rustc_ast_passes/src/node_count.rs

@@ -106,7 +106,7 @@ impl<'ast> Visitor<'ast> for NodeCounter {
         self.count += 1;
         walk_variant(self, v)
     }
-    fn visit_lifetime(&mut self, lifetime: &Lifetime) {
+    fn visit_lifetime(&mut self, lifetime: &Lifetime, _: visit::LifetimeCtxt) {
         self.count += 1;
         walk_lifetime(self, lifetime)
     }

compiler/rustc_lint/src/context.rs

@@ -819,6 +819,43 @@ pub trait LintContext: Sized {
                         "see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information",
                     );
                 },
+                BuiltinLintDiagnostics::SingleUseLifetime {
+                    param_span,
+                    use_span: Some((use_span, elide)),
+                    deletion_span,
+                } => {
+                    debug!(?param_span, ?use_span, ?deletion_span);
+                    db.span_label(param_span, "this lifetime...");
+                    db.span_label(use_span, "...is used only here");
+                    let msg = "elide the single-use lifetime";
+                    let (use_span, replace_lt) = if elide {
+                        let use_span = sess.source_map().span_extend_while(
+                            use_span,
+                            char::is_whitespace,
+                        ).unwrap_or(use_span);
+                        (use_span, String::new())
+                    } else {
+                        (use_span, "'_".to_owned())
+                    };
+                    db.multipart_suggestion(
+                        msg,
+                        vec![(deletion_span, String::new()), (use_span, replace_lt)],
+                        Applicability::MachineApplicable,
+                    );
+                },
+                BuiltinLintDiagnostics::SingleUseLifetime {
+                    param_span: _,
+                    use_span: None,
+                    deletion_span,
+                } => {
+                    debug!(?deletion_span);
+                    db.span_suggestion(
+                        deletion_span,
+                        "elide the unused lifetime",
+                        String::new(),
+                        Applicability::MachineApplicable,
+                    );
+                },
             }
             // Rewrap `db`, and pass control to the user.
             decorate(LintDiagnosticBuilder::new(db));

compiler/rustc_lint/src/early.rs

@@ -239,6 +239,7 @@ impl<'a, T: EarlyLintPass> ast_visit::Visitor<'a> for EarlyContextAndPass<'a, T>
 
     fn visit_generic_param(&mut self, param: &'a ast::GenericParam) {
         run_early_pass!(self, check_generic_param, param);
+        self.check_id(param.id);
         ast_visit::walk_generic_param(self, param);
     }
 
@@ -272,7 +273,7 @@ impl<'a, T: EarlyLintPass> ast_visit::Visitor<'a> for EarlyContextAndPass<'a, T>
         });
     }
 
-    fn visit_lifetime(&mut self, lt: &'a ast::Lifetime) {
+    fn visit_lifetime(&mut self, lt: &'a ast::Lifetime, _: ast_visit::LifetimeCtxt) {
         run_early_pass!(self, check_lifetime, lt);
         self.check_id(lt.id);
     }

compiler/rustc_lint/src/levels.rs

@@ -14,7 +14,7 @@ use rustc_middle::lint::{
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::{RegisteredTools, TyCtxt};
 use rustc_session::lint::{
-    builtin::{self, FORBIDDEN_LINT_GROUPS, UNFULFILLED_LINT_EXPECTATIONS},
+    builtin::{self, FORBIDDEN_LINT_GROUPS, SINGLE_USE_LIFETIMES, UNFULFILLED_LINT_EXPECTATIONS},
     Level, Lint, LintExpectationId, LintId,
 };
 use rustc_session::parse::{add_feature_diagnostics, feature_err};
@@ -259,6 +259,14 @@ impl<'s> LintLevelsBuilder<'s> {
         let sess = self.sess;
         let bad_attr = |span| struct_span_err!(sess, span, E0452, "malformed lint attribute input");
         for (attr_index, attr) in attrs.iter().enumerate() {
+            if attr.has_name(sym::automatically_derived) {
+                self.current_specs_mut().insert(
+                    LintId::of(SINGLE_USE_LIFETIMES),
+                    (Level::Allow, LintLevelSource::Default),
+                );
+                continue;
+            }
+
             let level = match Level::from_attr(attr) {
                 None => continue,
                 Some(Level::Expect(unstable_id)) if let Some(hir_id) = source_hir_id => {

compiler/rustc_lint_defs/src/lib.rs

@@ -423,7 +423,11 @@ pub enum BuiltinLintDiagnostics {
     DeprecatedMacro(Option<Symbol>, Span),
     MissingAbi(Span, Abi),
     UnusedDocComment(Span),
-    UnusedBuiltinAttribute { attr_name: Symbol, macro_name: String, invoc_span: Span },
+    UnusedBuiltinAttribute {
+        attr_name: Symbol,
+        macro_name: String,
+        invoc_span: Span,
+    },
     PatternsInFnsWithoutBody(Span, Ident),
     LegacyDeriveHelpers(Span),
     ProcMacroBackCompat(String),
@@ -435,6 +439,16 @@ pub enum BuiltinLintDiagnostics {
     UnicodeTextFlow(Span, String),
     UnexpectedCfg((Symbol, Span), Option<(Symbol, Span)>),
     DeprecatedWhereclauseLocation(Span, String),
+    SingleUseLifetime {
+        /// Span of the parameter which declares this lifetime.
+        param_span: Span,
+        /// Span of the code that should be removed when eliding this lifetime.
+        /// This span should include leading or trailing comma.
+        deletion_span: Span,
+        /// Span of the single use, or None if the lifetime is never used.
+        /// If true, the lifetime will be fully elided.
+        use_span: Option<(Span, bool)>,
+    },
 }
 
 /// Lints that are buffered up early on in the `Session` before the

compiler/rustc_parse/src/parser/ty.rs

@@ -518,6 +518,7 @@ impl<'a> Parser<'a> {
             kind: rustc_ast::VisibilityKind::Inherited,
             tokens: None,
         };
+        let span_start = self.token.span;
         let ast::FnHeader { ext, unsafety, constness, asyncness } =
             self.parse_fn_front_matter(&inherited_vis)?;
         let decl = self.parse_fn_decl(|_| false, AllowPlus::No, recover_return_sign)?;
@@ -531,7 +532,8 @@ impl<'a> Parser<'a> {
         if let ast::Async::Yes { span, .. } = asyncness {
             self.error_fn_ptr_bad_qualifier(whole_span, span, "async");
         }
-        Ok(TyKind::BareFn(P(BareFnTy { ext, unsafety, generic_params: params, decl })))
+        let decl_span = span_start.to(self.token.span);
+        Ok(TyKind::BareFn(P(BareFnTy { ext, unsafety, generic_params: params, decl, decl_span })))
     }
 
     /// Emit an error for the given bad function pointer qualifier.

compiler/rustc_passes/src/hir_stats.rs

@@ -318,7 +318,7 @@ impl<'v> ast_visit::Visitor<'v> for StatCollector<'v> {
         ast_visit::walk_variant(self, v)
     }
 
-    fn visit_lifetime(&mut self, lifetime: &'v ast::Lifetime) {
+    fn visit_lifetime(&mut self, lifetime: &'v ast::Lifetime, _: ast_visit::LifetimeCtxt) {
         self.record("Lifetime", Id::None, lifetime);
         ast_visit::walk_lifetime(self, lifetime)
     }