Auto merge of #125051 - dtolnay:printletelse, r=compiler-errors

Pretty-print let-else with added parenthesization when needed

Rustc used to produce invalid syntax for the following code, which is problematic because it means we cannot apply rustfmt to the output of `-Zunpretty=expanded`.

```rust
macro_rules! expr {
    ($e:expr) => { $e };
}

fn main() {
    let _ = expr!(loop {}) else { return; };
}
```

```console
$ rustc repro.rs -Zunpretty=expanded | rustfmt
error: `loop...else` loops are not supported
 --> <stdin>:9:29
  |
9 | fn main() { let _ = loop {} else { return; }; }
  |                     ----    ^^^^^^^^^^^^^^^^
  |                     |
  |                     `else` is attached to this loop
  |
  = note: consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
```

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1238,7 +1238,11 @@ fn print_stmt(&mut self, st: &ast::Stmt) {
                 if let Some((init, els)) = loc.kind.init_else_opt() {
                     self.nbsp();
                     self.word_space("=");
-                    self.print_expr(init, FixupContext::default());
+                    self.print_expr_cond_paren(
+                        init,
+                        els.is_some() && classify::expr_trailing_brace(init).is_some(),
+                        FixupContext::default(),
+                    );
                     if let Some(els) = els {
                         self.cbox(INDENT_UNIT);
                         self.ibox(INDENT_UNIT);

tests/ui/macros/stringify.rs

@@ -675,6 +675,11 @@ fn test_stmt() {
         "let (a, b): (u32, u32) = (1, 2);",
         "let (a, b): (u32, u32) = (1, 2)"
     );
+    c2!(stmt,
+        [ let _ = f() else { return; } ],
+        "let _ = f() else { return; };",
+        "let _ = f() else { return; }",
+    );
     macro_rules! c2_let_expr_minus_one {
         ([ $expr:expr ], $stmt_expected:expr, $tokens_expected:expr $(,)?) => {
             c2!(stmt, [ let _ = $expr - 1 ], $stmt_expected, $tokens_expected);
@@ -685,6 +690,16 @@ macro_rules! c2_let_expr_minus_one {
         "let _ = match void {} - 1;",
         "let _ = match void {} - 1",
     );
+    macro_rules! c2_let_expr_else_return {
+        ([ $expr:expr ], $stmt_expected:expr, $tokens_expected:expr $(,)?) => {
+            c2!(stmt, [ let _ = $expr else { return; } ], $stmt_expected, $tokens_expected);
+        };
+    }
+    c2_let_expr_else_return!(
+        [ f() ],
+        "let _ = f() else { return; };",
+        "let _ = f() else { return; }",
+    );
 
     // StmtKind::Item
     c1!(stmt, [ struct S; ], "struct S;");

tests/ui/unpretty/let-else.rs

@@ -0,0 +1,11 @@
+//@ compile-flags: -Zunpretty=expanded
+//@ check-pass
+
+macro_rules! expr {
+    ($e:expr) => { $e };
+}
+
+fn main() {
+    let _ = expr!(1 + 1) else { return; };
+    let _ = expr!(loop {}) else { return; };
+}

tests/ui/unpretty/let-else.stdout

@@ -0,0 +1,15 @@
+#![feature(prelude_import)]
+#![no_std]
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
+#[macro_use]
+extern crate std;
+//@ compile-flags: -Zunpretty=expanded
+//@ check-pass
+
+macro_rules! expr { ($e:expr) => { $e }; }
+
+fn main() {
+    let _ = 1 + 1 else { return; };
+    let _ = (loop {}) else { return; };
+}