Lint single-use-lifetimes on the AST.

Author: cjgillot

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);
     }
 

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