Auto merge of #68583 - estebank:hrlt, r=oli-obk

Account for HR lifetimes when suggesting introduction of named lifetime

```
error[E0106]: missing lifetime specifier
 --> src/test/ui/suggestions/fn-missing-lifetime-in-item.rs:2:32
  |
2 | struct S2<F: Fn(&i32, &i32) -> &i32>(F);
  |                 ----  ----     ^ expected named lifetime parameter
  |
  = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from argument 1 or argument 2
  = note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
help: consider making the bound lifetime-generic with a new `'a` lifetime
  |
2 | struct S2<F: for<'a> Fn(&'a i32, &'a i32) -> &'a i32>(F);
  |              ^^^^^^^    ^^^^^^^  ^^^^^^^     ^^^
help: consider introducing a named lifetime parameter
  |
2 | struct S2<'a, F: Fn(&'a i32, &'a i32) -> &'a i32>(F);=
  |           ^^^       ^^^^^^^  ^^^^^^^     ^^^
```

Follow up to #68267. Addresses the diagnostics part of #49287.

src/librustc_hir/hir.rs

@@ -2260,10 +2260,10 @@ impl TraitRef<'_> {
 
 #[derive(RustcEncodable, RustcDecodable, Debug, HashStable_Generic)]
 pub struct PolyTraitRef<'hir> {
-    /// The `'a` in `<'a> Foo<&'a T>`.
+    /// The `'a` in `for<'a> Foo<&'a T>`.
     pub bound_generic_params: &'hir [GenericParam<'hir>],
 
-    /// The `Foo<&'a T>` in `<'a> Foo<&'a T>`.
+    /// The `Foo<&'a T>` in `for<'a> Foo<&'a T>`.
     pub trait_ref: TraitRef<'hir>,
 
     pub span: Span,

src/librustc_resolve/diagnostics.rs

@@ -20,6 +20,7 @@ use syntax::ast::{self, Ident, Path};
 use syntax::util::lev_distance::find_best_match_for_name;
 
 use crate::imports::{ImportDirective, ImportDirectiveSubclass, ImportResolver};
+use crate::lifetimes::{ElisionFailureInfo, LifetimeContext};
 use crate::path_names_to_string;
 use crate::{AmbiguityError, AmbiguityErrorMisc, AmbiguityKind};
 use crate::{BindingError, CrateLint, HasGenericParams, LegacyScope, Module, ModuleOrUniformRoot};
@@ -48,6 +49,40 @@ crate struct ImportSuggestion {
     pub path: Path,
 }
 
+crate enum MissingLifetimeSpot<'tcx> {
+    Generics(&'tcx hir::Generics<'tcx>),
+    HigherRanked { span: Span, span_type: ForLifetimeSpanType },
+}
+
+crate enum ForLifetimeSpanType {
+    BoundEmpty,
+    BoundTail,
+    TypeEmpty,
+    TypeTail,
+}
+
+impl ForLifetimeSpanType {
+    crate fn descr(&self) -> &'static str {
+        match self {
+            Self::BoundEmpty | Self::BoundTail => "bound",
+            Self::TypeEmpty | Self::TypeTail => "type",
+        }
+    }
+
+    crate fn suggestion(&self, sugg: &str) -> String {
+        match self {
+            Self::BoundEmpty | Self::TypeEmpty => format!("for<{}> ", sugg),
+            Self::BoundTail | Self::TypeTail => format!(", {}", sugg),
+        }
+    }
+}
+
+impl<'tcx> Into<MissingLifetimeSpot<'tcx>> for &'tcx hir::Generics<'tcx> {
+    fn into(self) -> MissingLifetimeSpot<'tcx> {
+        MissingLifetimeSpot::Generics(self)
+    }
+}
+
 /// Adjust the impl span so that just the `impl` keyword is taken by removing
 /// everything after `<` (`"impl<T> Iterator for A<T> {}" -> "impl"`) and
 /// everything after the first whitespace (`"impl Iterator for A" -> "impl"`).
@@ -1457,72 +1492,206 @@ crate fn show_candidates(
     }
 }
 
-crate fn report_missing_lifetime_specifiers(
-    sess: &Session,
-    span: Span,
-    count: usize,
-) -> DiagnosticBuilder<'_> {
-    struct_span_err!(sess, span, E0106, "missing lifetime specifier{}", pluralize!(count))
-}
+impl<'tcx> LifetimeContext<'_, 'tcx> {
+    crate fn report_missing_lifetime_specifiers(
+        &self,
+        span: Span,
+        count: usize,
+    ) -> DiagnosticBuilder<'tcx> {
+        struct_span_err!(
+            self.tcx.sess,
+            span,
+            E0106,
+            "missing lifetime specifier{}",
+            pluralize!(count)
+        )
+    }
 
-crate fn add_missing_lifetime_specifiers_label(
-    err: &mut DiagnosticBuilder<'_>,
-    span: Span,
-    count: usize,
-    lifetime_names: &FxHashSet<ast::Ident>,
-    snippet: Option<&str>,
-    missing_named_lifetime_spots: &[&hir::Generics<'_>],
-) {
-    if count > 1 {
-        err.span_label(span, format!("expected {} lifetime parameters", count));
-    } else {
-        let suggest_existing = |err: &mut DiagnosticBuilder<'_>, sugg| {
-            err.span_suggestion(
-                span,
-                "consider using the named lifetime",
-                sugg,
-                Applicability::MaybeIncorrect,
-            );
-        };
-        let suggest_new = |err: &mut DiagnosticBuilder<'_>, sugg| {
-            err.span_label(span, "expected named lifetime parameter");
-
-            if let Some(generics) = missing_named_lifetime_spots.iter().last() {
-                let mut introduce_suggestion = vec![];
-                introduce_suggestion.push(match &generics.params {
-                    [] => (generics.span, "<'lifetime>".to_string()),
-                    [param, ..] => (param.span.shrink_to_lo(), "'lifetime, ".to_string()),
-                });
-                introduce_suggestion.push((span, sugg));
-                err.multipart_suggestion(
-                    "consider introducing a named lifetime parameter",
-                    introduce_suggestion,
-                    Applicability::MaybeIncorrect,
-                );
+    crate fn emit_undeclared_lifetime_error(&self, lifetime_ref: &hir::Lifetime) {
+        let mut err = struct_span_err!(
+            self.tcx.sess,
+            lifetime_ref.span,
+            E0261,
+            "use of undeclared lifetime name `{}`",
+            lifetime_ref
+        );
+        err.span_label(lifetime_ref.span, "undeclared lifetime");
+        for missing in &self.missing_named_lifetime_spots {
+            match missing {
+                MissingLifetimeSpot::Generics(generics) => {
+                    let (span, sugg) = if let Some(param) = generics
+                        .params
+                        .iter()
+                        .filter(|p| match p.kind {
+                            hir::GenericParamKind::Type {
+                                synthetic: Some(hir::SyntheticTyParamKind::ImplTrait),
+                                ..
+                            } => false,
+                            _ => true,
+                        })
+                        .next()
+                    {
+                        (param.span.shrink_to_lo(), format!("{}, ", lifetime_ref))
+                    } else {
+                        (generics.span, format!("<{}>", lifetime_ref))
+                    };
+                    err.span_suggestion(
+                        span,
+                        &format!("consider introducing lifetime `{}` here", lifetime_ref),
+                        sugg,
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+                MissingLifetimeSpot::HigherRanked { span, span_type } => {
+                    err.span_suggestion(
+                        *span,
+                        &format!(
+                            "consider making the {} lifetime-generic with a new `{}` lifetime",
+                            span_type.descr(),
+                            lifetime_ref
+                        ),
+                        span_type.suggestion(&lifetime_ref.to_string()),
+                        Applicability::MaybeIncorrect,
+                    );
+                    err.note(
+                        "for more information on higher-ranked polymorphism, visit \
+                            https://doc.rust-lang.org/nomicon/hrtb.html",
+                    );
+                }
             }
-        };
+        }
+        err.emit();
+    }
 
-        match (lifetime_names.len(), lifetime_names.iter().next(), snippet) {
-            (1, Some(name), Some("&")) => {
-                suggest_existing(err, format!("&{} ", name));
-            }
-            (1, Some(name), Some("'_")) => {
-                suggest_existing(err, name.to_string());
-            }
-            (1, Some(name), Some(snippet)) if !snippet.ends_with(">") => {
-                suggest_existing(err, format!("{}<{}>", snippet, name));
-            }
-            (0, _, Some("&")) => {
-                suggest_new(err, "&'lifetime ".to_string());
-            }
-            (0, _, Some("'_")) => {
-                suggest_new(err, "'lifetime".to_string());
-            }
-            (0, _, Some(snippet)) if !snippet.ends_with(">") => {
-                suggest_new(err, format!("{}<'lifetime>", snippet));
+    crate fn is_trait_ref_fn_scope(&mut self, trait_ref: &'tcx hir::PolyTraitRef<'tcx>) -> bool {
+        if let def::Res::Def(_, did) = trait_ref.trait_ref.path.res {
+            if [
+                self.tcx.lang_items().fn_once_trait(),
+                self.tcx.lang_items().fn_trait(),
+                self.tcx.lang_items().fn_mut_trait(),
+            ]
+            .contains(&Some(did))
+            {
+                let (span, span_type) = match &trait_ref.bound_generic_params {
+                    [] => (trait_ref.span.shrink_to_lo(), ForLifetimeSpanType::BoundEmpty),
+                    [.., bound] => (bound.span.shrink_to_hi(), ForLifetimeSpanType::BoundTail),
+                };
+                self.missing_named_lifetime_spots
+                    .push(MissingLifetimeSpot::HigherRanked { span, span_type });
+                return true;
             }
-            _ => {
-                err.span_label(span, "expected lifetime parameter");
+        };
+        false
+    }
+
+    crate fn add_missing_lifetime_specifiers_label(
+        &self,
+        err: &mut DiagnosticBuilder<'_>,
+        span: Span,
+        count: usize,
+        lifetime_names: &FxHashSet<ast::Ident>,
+        params: &[ElisionFailureInfo],
+    ) {
+        if count > 1 {
+            err.span_label(span, format!("expected {} lifetime parameters", count));
+        } else {
+            let snippet = self.tcx.sess.source_map().span_to_snippet(span).ok();
+            let suggest_existing = |err: &mut DiagnosticBuilder<'_>, sugg| {
+                err.span_suggestion(
+                    span,
+                    "consider using the named lifetime",
+                    sugg,
+                    Applicability::MaybeIncorrect,
+                );
+            };
+            let suggest_new =
+                |err: &mut DiagnosticBuilder<'_>, sugg: &str| {
+                    err.span_label(span, "expected named lifetime parameter");
+
+                    for missing in self.missing_named_lifetime_spots.iter().rev() {
+                        let mut introduce_suggestion = vec![];
+                        let msg;
+                        let should_break;
+                        introduce_suggestion.push(match missing {
+                        MissingLifetimeSpot::Generics(generics) => {
+                            msg = "consider introducing a named lifetime parameter".to_string();
+                            should_break = true;
+                            if let Some(param) = generics.params.iter().filter(|p| match p.kind {
+                                hir::GenericParamKind::Type {
+                                    synthetic: Some(hir::SyntheticTyParamKind::ImplTrait),
+                                    ..
+                                } => false,
+                                _ => true,
+                            }).next() {
+                                (param.span.shrink_to_lo(), "'a, ".to_string())
+                            } else {
+                                (generics.span, "<'a>".to_string())
+                            }
+                        }
+                        MissingLifetimeSpot::HigherRanked { span, span_type } => {
+                            msg = format!(
+                                "consider making the {} lifetime-generic with a new `'a` lifetime",
+                                span_type.descr(),
+                            );
+                            should_break = false;
+                            err.note(
+                                "for more information on higher-ranked polymorphism, visit \
+                             https://doc.rust-lang.org/nomicon/hrtb.html",
+                            );
+                            (*span, span_type.suggestion("'a"))
+                        }
+                    });
+                        for param in params {
+                            if let Ok(snippet) =
+                                self.tcx.sess.source_map().span_to_snippet(param.span)
+                            {
+                                if snippet.starts_with("&") && !snippet.starts_with("&'") {
+                                    introduce_suggestion
+                                        .push((param.span, format!("&'a {}", &snippet[1..])));
+                                } else if snippet.starts_with("&'_ ") {
+                                    introduce_suggestion
+                                        .push((param.span, format!("&'a {}", &snippet[4..])));
+                                }
+                            }
+                        }
+                        introduce_suggestion.push((span, sugg.to_string()));
+                        err.multipart_suggestion(
+                            &msg,
+                            introduce_suggestion,
+                            Applicability::MaybeIncorrect,
+                        );
+                        if should_break {
+                            break;
+                        }
+                    }
+                };
+
+            match (
+                lifetime_names.len(),
+                lifetime_names.iter().next(),
+                snippet.as_ref().map(|s| s.as_str()),
+            ) {
+                (1, Some(name), Some("&")) => {
+                    suggest_existing(err, format!("&{} ", name));
+                }
+                (1, Some(name), Some("'_")) => {
+                    suggest_existing(err, name.to_string());
+                }
+                (1, Some(name), Some(snippet)) if !snippet.ends_with(">") => {
+                    suggest_existing(err, format!("{}<{}>", snippet, name));
+                }
+                (0, _, Some("&")) => {
+                    suggest_new(err, "&'a ");
+                }
+                (0, _, Some("'_")) => {
+                    suggest_new(err, "'a");
+                }
+                (0, _, Some(snippet)) if !snippet.ends_with(">") => {
+                    suggest_new(err, &format!("{}<'a>", snippet));
+                }
+                _ => {
+                    err.span_label(span, "expected lifetime parameter");
+                }
             }
         }
     }