improve diagnosts for GATs

compiler/rustc_middle/src/middle/resolve_lifetime.rs

@@ -49,6 +49,20 @@ pub enum Region {
     Free(DefId, /* lifetime decl */ DefId),
 }
 
+/// This is used in diagnostics to improve suggestions for missing generic arguments.
+/// It gives information on the type of lifetimes that are in scope for a particular `PathSegment`,
+/// so that we can e.g. suggest elided-lifetimes-in-paths of the form <'_, '_> e.g.
+#[derive(Clone, PartialEq, Eq, Hash, TyEncodable, TyDecodable, Debug, HashStable)]
+pub enum LifetimeScopeForPath {
+    // Contains all lifetime names that are in scope and could possibly be used in generics
+    // arguments of path.
+    NonElided(Vec<String>),
+
+    // Information that allows us to suggest args of the form `<'_>` in case
+    // no generic arguments were provided for a path.
+    Elided,
+}
+
 /// A set containing, at most, one known element.
 /// If two distinct values are inserted into a set, then it
 /// becomes `Many`, which can be used to detect ambiguities.

compiler/rustc_middle/src/query/mod.rs

@@ -1301,6 +1301,10 @@ rustc_queries! {
         desc { "looking up late bound vars" }
     }
 
+    query lifetime_scope_map(_: LocalDefId) -> Option<FxHashMap<ItemLocalId, LifetimeScopeForPath>> {
+        desc { "finds the lifetime scope for an HirId of a PathSegment" }
+    }
+
     query visibility(def_id: DefId) -> ty::Visibility {
         eval_always
         desc { |tcx| "computing visibility of `{}`", tcx.def_path_str(def_id) }

compiler/rustc_middle/src/ty/context.rs

@@ -9,7 +9,7 @@ use crate::infer::canonical::{Canonical, CanonicalVarInfo, CanonicalVarInfos};
 use crate::lint::{struct_lint_level, LintDiagnosticBuilder, LintLevelSource};
 use crate::middle;
 use crate::middle::cstore::{CrateStoreDyn, EncodedMetadata};
-use crate::middle::resolve_lifetime::{self, ObjectLifetimeDefault};
+use crate::middle::resolve_lifetime::{self, LifetimeScopeForPath, ObjectLifetimeDefault};
 use crate::middle::stability;
 use crate::mir::interpret::{self, Allocation, ConstValue, Scalar};
 use crate::mir::{Body, Field, Local, Place, PlaceElem, ProjectionKind, Promoted};
@@ -2686,6 +2686,10 @@ impl<'tcx> TyCtxt<'tcx> {
                 .iter(),
         )
     }
+
+    pub fn lifetime_scope(self, id: HirId) -> Option<LifetimeScopeForPath> {
+        self.lifetime_scope_map(id.owner).and_then(|mut map| map.remove(&id.local_id))
+    }
 }
 
 impl TyCtxtAt<'tcx> {

compiler/rustc_middle/src/ty/query/mod.rs

@@ -9,7 +9,9 @@ use crate::middle::exported_symbols::{ExportedSymbol, SymbolExportLevel};
 use crate::middle::lib_features::LibFeatures;
 use crate::middle::privacy::AccessLevels;
 use crate::middle::region;
-use crate::middle::resolve_lifetime::{ObjectLifetimeDefault, Region, ResolveLifetimes};
+use crate::middle::resolve_lifetime::{
+    LifetimeScopeForPath, ObjectLifetimeDefault, Region, ResolveLifetimes,
+};
 use crate::middle::stability::{self, DeprecationEntry};
 use crate::mir;
 use crate::mir::interpret::GlobalId;