Fix "consider borrowing" for else-if

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -2713,6 +2713,31 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         ));
                     }
 
+                    // Don't try to suggest ref/deref on an `if` expression, because:
+                    // - The `if` could be part of a desugared `if else` statement,
+                    //   which would create impossible suggestions such as `if ... { ... } else &if { ... } else { ... }`.
+                    // - In general the suggestions it creates such as `&if ... { ... } else { ... }` are not very helpful.
+                    // We try to generate a suggestion such as `if ... { &... } else { &... }` instead.
+                    if let hir::ExprKind::If(_c, then, els) = expr.kind {
+                        // The `then` of a `Expr::If` always contains a block, and that block may have a final expression that we can borrow
+                        // If the block does not have a final expression, it will return () and we do not make a suggestion to borrow that.
+                        let ExprKind::Block(then, _) = then.kind else { return None };
+                        let Some(then) = then.expr else { return None };
+                        let (mut suggs, help, app, verbose, mutref) =
+                            self.suggest_deref_or_ref(then, checked_ty, expected)?;
+
+                        // If there is no `else`, the return type of this `if` will be (), so suggesting to change the `then` block is useless
+                        let els_expr = match els?.kind {
+                            ExprKind::Block(block, _) => block.expr?,
+                            _ => els?,
+                        };
+                        let (else_suggs, ..) =
+                            self.suggest_deref_or_ref(els_expr, checked_ty, expected)?;
+                        suggs.extend(else_suggs);
+
+                        return Some((suggs, help, app, verbose, mutref));
+                    }
+
                     if let Some((sugg, msg)) = self.can_use_as_ref(expr) {
                         return Some((
                             sugg,

tests/ui/typeck/consider-borrowing-141810-1.rs

@@ -0,0 +1,9 @@
+fn main() {
+    let x = if true {
+        &true
+    } else if false { //~ ERROR `if` and `else` have incompatible types [E0308]
+        true //~ HELP consider borrowing here
+    } else {
+        true
+    };
+}

tests/ui/typeck/consider-borrowing-141810-1.stderr

@@ -0,0 +1,28 @@
+error[E0308]: `if` and `else` have incompatible types
+  --> $DIR/consider-borrowing-141810-1.rs:4:12
+   |
+LL |        let x = if true {
+   |  ______________-
+LL | |          &true
+   | |          ----- expected because of this
+LL | |      } else if false {
+   | | ____________^
+LL | ||         true
+LL | ||     } else {
+LL | ||         true
+LL | ||     };
+   | ||     ^
+   | ||_____|
+   |  |_____`if` and `else` have incompatible types
+   |        expected `&bool`, found `bool`
+   |
+help: consider borrowing here
+   |
+LL ~         &true
+LL |     } else {
+LL ~         &true
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/typeck/consider-borrowing-141810-2.rs

@@ -0,0 +1,8 @@
+fn main() {
+    let x = if true {
+        &()
+    } else if false { //~ ERROR `if` and `else` have incompatible types [E0308]
+    } else {
+    };
+
+}

tests/ui/typeck/consider-borrowing-141810-2.stderr

@@ -0,0 +1,19 @@
+error[E0308]: `if` and `else` have incompatible types
+  --> $DIR/consider-borrowing-141810-2.rs:4:12
+   |
+LL |        let x = if true {
+   |  ______________-
+LL | |          &()
+   | |          --- expected because of this
+LL | |      } else if false {
+   | | ____________^
+LL | ||     } else {
+LL | ||     };
+   | ||     ^
+   | ||_____|
+   |  |_____`if` and `else` have incompatible types
+   |        expected `&()`, found `()`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/typeck/consider-borrowing-141810-3.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let x = if true {
+        &()
+    } else if false { //~ ERROR `if` and `else` have incompatible types [E0308]
+
+    };
+}

tests/ui/typeck/consider-borrowing-141810-3.stderr

@@ -0,0 +1,22 @@
+error[E0308]: `if` and `else` have incompatible types
+  --> $DIR/consider-borrowing-141810-3.rs:4:12
+   |
+LL |        let x = if true {
+   |  ______________-
+LL | |          &()
+   | |          --- expected because of this
+LL | |      } else if false {
+   | | ____________^
+LL | ||
+LL | ||     };
+   | ||     ^
+   | ||_____|
+   |  |_____`if` and `else` have incompatible types
+   |        expected `&()`, found `()`
+   |
+   = note: `if` expressions without `else` evaluate to `()`
+   = note: consider adding an `else` block that evaluates to the expected type
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/typeck/consider-borrowing-141810-4.rs

@@ -0,0 +1,11 @@
+fn baz(x: &String) {}
+
+fn bar() {
+    baz({
+        String::from("hi") //~ ERROR mismatched types
+    });
+}
+
+fn main() {
+    bar();
+}

tests/ui/typeck/consider-borrowing-141810-4.stderr

@@ -0,0 +1,14 @@
+error[E0308]: mismatched types
+  --> $DIR/consider-borrowing-141810-4.rs:5:9
+   |
+LL |         String::from("hi")
+   |         ^^^^^^^^^^^^^^^^^^ expected `&String`, found `String`
+   |
+help: consider borrowing here
+   |
+LL |         &String::from("hi")
+   |         +
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.