Auto merge of rust-lang#13523 - lowr:fix/adjust-expectation-for-if, r=lnicola

fix: disregard type variable expectation for if expressions

Fixes rust-lang#13522

As [the comment](https://github.com/rust-lang/rust-analyzer/blob/8142d1f606dc2e52b1d2b8992671e2bd73379f28/crates/hir-ty/src/infer.rs#L1087-L1090) on `Expectation::adjust_for_branches` explains:

> If the expected type is just a type variable, then don't use an expected type. Otherwise, we might write parts of the type when checking the 'then' block which are incompatible with the 'else' branch.

Note that we already use it in match expressions. I've added tests for them too nevertheless.

Author: bors

crates/hir-ty/src/infer/expr.rs

@@ -85,6 +85,7 @@ impl<'a> InferenceContext<'a> {
         let ty = match &self.body[tgt_expr] {
             Expr::Missing => self.err_ty(),
             &Expr::If { condition, then_branch, else_branch } => {
+                let expected = &expected.adjust_for_branches(&mut self.table);
                 self.infer_expr(
                     condition,
                     &Expectation::has_type(TyKind::Scalar(Scalar::Bool).intern(Interner)),

crates/hir-ty/src/tests/coercion.rs

@@ -122,6 +122,23 @@ fn test() {
     )
 }
 
+#[test]
+fn if_else_adjust_for_branches_discard_type_var() {
+    check_no_mismatches(
+        r#"
+fn test() {
+    let f = || {
+        if true {
+            &""
+        } else {
+            ""
+        }
+    };
+}
+"#,
+    );
+}
+
 #[test]
 fn match_first_coerce() {
     check_no_mismatches(
@@ -182,6 +199,22 @@ fn test() {
     );
 }
 
+#[test]
+fn match_adjust_for_branches_discard_type_var() {
+    check_no_mismatches(
+        r#"
+fn test() {
+    let f = || {
+        match 0i32 {
+            0i32 => &"",
+            _ => "",
+        }
+    };
+}
+"#,
+    );
+}
+
 #[test]
 fn return_coerce_unknown() {
     check_types(