Show substitution where hovering over generic things

There are few things to note in the implementation:

First, this is a best-effort implementation. Mainly, type aliases may not be shown (due to their eager nature it's harder) and partial pathes (aka. hovering over `Struct` in `Struct::method`) are not supported at all.

Second, we only need to show substitutions in expression and pattern position, because in type position all generic arguments always have to be written explicitly.

crates/hir/src/lib.rs

@@ -3574,6 +3574,61 @@ impl GenericDef {
     }
 }
 
+// We cannot call this `Substitution` unfortunately...
+#[derive(Debug)]
+pub struct GenericSubstitution {
+    def: GenericDefId,
+    subst: Substitution,
+    env: Arc<TraitEnvironment>,
+}
+
+impl GenericSubstitution {
+    fn new(def: GenericDefId, subst: Substitution, env: Arc<TraitEnvironment>) -> Self {
+        Self { def, subst, env }
+    }
+
+    pub fn types(&self, db: &dyn HirDatabase) -> Vec<(Symbol, Type)> {
+        let container = match self.def {
+            GenericDefId::ConstId(id) => Some(id.lookup(db.upcast()).container),
+            GenericDefId::FunctionId(id) => Some(id.lookup(db.upcast()).container),
+            GenericDefId::TypeAliasId(id) => Some(id.lookup(db.upcast()).container),
+            _ => None,
+        };
+        let container_type_params = container
+            .and_then(|container| match container {
+                ItemContainerId::ImplId(container) => Some(container.into()),
+                ItemContainerId::TraitId(container) => Some(container.into()),
+                _ => None,
+            })
+            .map(|container| {
+                db.generic_params(container)
+                    .iter_type_or_consts()
+                    .filter_map(|param| match param.1 {
+                        TypeOrConstParamData::TypeParamData(param) => Some(param.name.clone()),
+                        TypeOrConstParamData::ConstParamData(_) => None,
+                    })
+                    .collect::<Vec<_>>()
+            });
+        let generics = db.generic_params(self.def);
+        let type_params = generics.iter_type_or_consts().filter_map(|param| match param.1 {
+            TypeOrConstParamData::TypeParamData(param) => Some(param.name.clone()),
+            TypeOrConstParamData::ConstParamData(_) => None,
+        });
+        // The `Substitution` is first self then container, we want the reverse order.
+        let self_params = self.subst.type_parameters(Interner).zip(type_params);
+        let container_params = self.subst.as_slice(Interner)[generics.len()..]
+            .iter()
+            .filter_map(|param| param.ty(Interner).cloned())
+            .zip(container_type_params.into_iter().flatten());
+        container_params
+            .chain(self_params)
+            .filter_map(|(ty, name)| {
+                Some((name?.symbol().clone(), Type { ty, env: self.env.clone() }))
+            })
+            .collect()
+    }
+}
+
 /// A single local definition.
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub struct Local {

crates/hir/src/semantics.rs

@@ -49,10 +49,10 @@ use crate::{
     semantics::source_to_def::{ChildContainer, SourceToDefCache, SourceToDefCtx},
     source_analyzer::{name_hygiene, resolve_hir_path, SourceAnalyzer},
     Access, Adjust, Adjustment, Adt, AutoBorrow, BindingMode, BuiltinAttr, Callable, Const,
-    ConstParam, Crate, DeriveHelper, Enum, Field, Function, HasSource, HirFileId, Impl, InFile,
-    InlineAsmOperand, ItemInNs, Label, LifetimeParam, Local, Macro, Module, ModuleDef, Name,
-    OverloadedDeref, Path, ScopeDef, Static, Struct, ToolModule, Trait, TraitAlias, TupleField,
-    Type, TypeAlias, TypeParam, Union, Variant, VariantDef,
+    ConstParam, Crate, DeriveHelper, Enum, Field, Function, GenericSubstitution, HasSource,
+    HirFileId, Impl, InFile, InlineAsmOperand, ItemInNs, Label, LifetimeParam, Local, Macro,
+    Module, ModuleDef, Name, OverloadedDeref, Path, ScopeDef, Static, Struct, ToolModule, Trait,
+    TraitAlias, TupleField, Type, TypeAlias, TypeParam, Union, Variant, VariantDef,
 };
 
 const CONTINUE_NO_BREAKS: ControlFlow<Infallible, ()> = ControlFlow::Continue(());
@@ -1415,7 +1415,7 @@ impl<'db> SemanticsImpl<'db> {
     pub fn resolve_method_call_fallback(
         &self,
         call: &ast::MethodCallExpr,
-    ) -> Option<Either<Function, Field>> {
+    ) -> Option<(Either<Function, Field>, Option<GenericSubstitution>)> {
         self.analyze(call.syntax())?.resolve_method_call_fallback(self.db, call)
     }
 
@@ -1458,18 +1458,33 @@ impl<'db> SemanticsImpl<'db> {
     pub fn resolve_field_fallback(
         &self,
         field: &ast::FieldExpr,
-    ) -> Option<Either<Either<Field, TupleField>, Function>> {
+    ) -> Option<(Either<Either<Field, TupleField>, Function>, Option<GenericSubstitution>)> {
         self.analyze(field.syntax())?.resolve_field_fallback(self.db, field)
     }
 
     pub fn resolve_record_field(
         &self,
         field: &ast::RecordExprField,
     ) -> Option<(Field, Option<Local>, Type)> {
+        self.resolve_record_field_with_substitution(field)
+            .map(|(field, local, ty, _)| (field, local, ty))
+    }
+
+    pub fn resolve_record_field_with_substitution(
+        &self,
+        field: &ast::RecordExprField,
+    ) -> Option<(Field, Option<Local>, Type, GenericSubstitution)> {
         self.analyze(field.syntax())?.resolve_record_field(self.db, field)
     }
 
     pub fn resolve_record_pat_field(&self, field: &ast::RecordPatField) -> Option<(Field, Type)> {
+        self.resolve_record_pat_field_with_subst(field).map(|(field, ty, _)| (field, ty))
+    }
+
+    pub fn resolve_record_pat_field_with_subst(
+        &self,
+        field: &ast::RecordPatField,
+    ) -> Option<(Field, Type, GenericSubstitution)> {
         self.analyze(field.syntax())?.resolve_record_pat_field(self.db, field)
     }
 
@@ -1525,6 +1540,13 @@ impl<'db> SemanticsImpl<'db> {
     }
 
     pub fn resolve_path(&self, path: &ast::Path) -> Option<PathResolution> {
+        self.resolve_path_with_subst(path).map(|(it, _)| it)
+    }
+
+    pub fn resolve_path_with_subst(
+        &self,
+        path: &ast::Path,
+    ) -> Option<(PathResolution, Option<GenericSubstitution>)> {
         self.analyze(path.syntax())?.resolve_path(self.db, path)
     }