Properly suggest deref in else block

Author: mibac138

compiler/rustc_typeck/src/check/demand.rs

@@ -360,6 +360,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         false
     }
 
+    crate fn hir_id_sole_block_element(
+        &self,
+        hir_id: hir::HirId,
+    ) -> Option<&'tcx rustc_hir::Expr<'tcx>> {
+        let node: Option<Node<'_>> = self.tcx.hir().find(hir_id);
+        match node {
+            Some(Node::Expr(rustc_hir::Expr {
+                kind: rustc_hir::ExprKind::Block(block, ..),
+                ..
+            })) if block.stmts.len() == 0 => block.expr,
+            _ => None,
+        }
+    }
+
     /// This function is used to determine potential "simple" improvements or users' errors and
     /// provide them useful help. For example:
     ///
@@ -626,6 +640,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 };
                                 let suggestion = if is_struct_pat_shorthand_field {
                                     format!("{}: *{}", code, code)
+                                } else if let Some(expr) =
+                                    self.hir_id_sole_block_element(expr.hir_id)
+                                {
+                                    if let Ok(inner_code) = sm.span_to_snippet(expr.span) {
+                                        format!("*{}", inner_code)
+                                    } else {
+                                        format!("*{}", code)
+                                    }
                                 } else {
                                     format!("*{}", code)
                                 };

src/test/ui/deref-suggestion.rs

@@ -45,4 +45,14 @@ fn main() {
     //~^ ERROR mismatched types
     let r = R { i: i };
     //~^ ERROR mismatched types
+
+
+    let a = &1;
+    let b = &2;
+    let val: i32 = if true {
+        a + 1
+    } else {
+        b
+        //~^ ERROR mismatched types
+    };
 }

src/test/ui/deref-suggestion.stderr

@@ -89,6 +89,15 @@ LL |     let r = R { i: i };
    |                    expected `u32`, found `&{integer}`
    |                    help: consider dereferencing the borrow: `*i`
 
-error: aborting due to 10 previous errors
+error[E0308]: mismatched types
+  --> $DIR/deref-suggestion.rs:55:9
+   |
+LL |         b
+   |         ^
+   |         |
+   |         expected `i32`, found `&{integer}`
+   |         help: consider dereferencing the borrow: `*b`
+
+error: aborting due to 11 previous errors
 
 For more information about this error, try `rustc --explain E0308`.