Merge #11771

11771: feat: Visualize compiler inserted reborrows via inlay hints r=Veykril a=Veykril

Disabled by default.

![image](https://user-images.githubusercontent.com/3757771/159165178-baaf968a-4381-468e-933f-5326ca1b203d.png)

Closes https://github.com/rust-analyzer/rust-analyzer/issues/11275


Co-authored-by: Lukas Wirth <lukastw97@gmail.com>

crates/hir/src/semantics.rs

@@ -8,6 +8,7 @@ use base_db::{FileId, FileRange};
 use hir_def::{
     body, macro_id_to_def_id,
     resolver::{self, HasResolver, Resolver, TypeNs},
+    type_ref::Mutability,
     AsMacroCall, FunctionId, MacroId, TraitId, VariantId,
 };
 use hir_expand::{
@@ -313,6 +314,11 @@ impl<'db, DB: HirDatabase> Semantics<'db, DB> {
         self.imp.resolve_type(ty)
     }
 
+    // FIXME: Figure out a nice interface to inspect adjustments
+    pub fn is_implicit_reborrow(&self, expr: &ast::Expr) -> Option<Mutability> {
+        self.imp.is_implicit_reborrow(expr)
+    }
+
     pub fn type_of_expr(&self, expr: &ast::Expr) -> Option<TypeInfo> {
         self.imp.type_of_expr(expr)
     }
@@ -900,6 +906,10 @@ impl<'db> SemanticsImpl<'db> {
         Type::new_with_resolver(self.db, &scope.resolver, ty)
     }
 
+    fn is_implicit_reborrow(&self, expr: &ast::Expr) -> Option<Mutability> {
+        self.analyze(expr.syntax()).is_implicit_reborrow(self.db, expr)
+    }
+
     fn type_of_expr(&self, expr: &ast::Expr) -> Option<TypeInfo> {
         self.analyze(expr.syntax())
             .type_of_expr(self.db, expr)

crates/hir/src/source_analyzer.rs

@@ -20,12 +20,14 @@ use hir_def::{
     macro_id_to_def_id,
     path::{ModPath, Path, PathKind},
     resolver::{resolver_for_scope, Resolver, TypeNs, ValueNs},
+    type_ref::Mutability,
     AsMacroCall, DefWithBodyId, FieldId, FunctionId, LocalFieldId, ModuleDefId, VariantId,
 };
 use hir_expand::{hygiene::Hygiene, name::AsName, HirFileId, InFile};
 use hir_ty::{
     diagnostics::{record_literal_missing_fields, record_pattern_missing_fields},
-    InferenceResult, Interner, Substitution, TyExt, TyLoweringContext,
+    Adjust, Adjustment, AutoBorrow, InferenceResult, Interner, Substitution, TyExt,
+    TyLoweringContext,
 };
 use syntax::{
     ast::{self, AstNode},
@@ -139,6 +141,23 @@ impl SourceAnalyzer {
         Some(res)
     }
 
+    pub(crate) fn is_implicit_reborrow(
+        &self,
+        db: &dyn HirDatabase,
+        expr: &ast::Expr,
+    ) -> Option<Mutability> {
+        let expr_id = self.expr_id(db, expr)?;
+        let infer = self.infer.as_ref()?;
+        let adjustments = infer.expr_adjustments.get(&expr_id)?;
+        adjustments.windows(2).find_map(|slice| match slice {
+            &[Adjustment {kind: Adjust::Deref(None), ..}, Adjustment {kind: Adjust::Borrow(AutoBorrow::Ref(m)), ..}] => Some(match m {
+                hir_ty::Mutability::Mut => Mutability::Mut,
+                hir_ty::Mutability::Not => Mutability::Shared,
+            }),
+            _ => None,
+        })
+    }
+
     pub(crate) fn type_of_expr(
         &self,
         db: &dyn HirDatabase,