add test for &format!(...) in an if expression's block

This is the real-world way that the unintentional stabilization of
lifetime extension for format arguments was used.

Author: dianne

tests/ui/borrowck/format-args-temporary-scopes.e2021.stderr

@@ -0,0 +1,16 @@
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/format-args-temporary-scopes.rs:25:41
+   |
+LL |     println!("{:?}{:?}", (), if true { &format!("") } else { "" });
+   |                              -----------^^^^^^^^^^^--------------
+   |                              |          |           |
+   |                              |          |           temporary value is freed at the end of this statement
+   |                              |          creates a temporary value which is freed while still in use
+   |                              borrow later used here
+   |
+   = note: consider using a `let` binding to create a longer lived value
+   = note: this error originates in the macro `format` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0716`.

tests/ui/borrowck/format-args-temporary-scopes.e2024.stderr

@@ -1,5 +1,5 @@
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/format-args-temporary-scopes.rs:13:25
+  --> $DIR/format-args-temporary-scopes.rs:12:25
    |
 LL |     println!("{:?}", { &temp() });
    |                      ---^^^^^---
@@ -11,7 +11,7 @@ LL |     println!("{:?}", { &temp() });
    = note: consider using a `let` binding to create a longer lived value
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/format-args-temporary-scopes.rs:19:29
+  --> $DIR/format-args-temporary-scopes.rs:18:29
    |
 LL |     println!("{:?}{:?}", { &temp() }, ());
    |                          ---^^^^^---
@@ -22,6 +22,19 @@ LL |     println!("{:?}{:?}", { &temp() }, ());
    |
    = note: consider using a `let` binding to create a longer lived value
 
-error: aborting due to 2 previous errors
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/format-args-temporary-scopes.rs:25:41
+   |
+LL |     println!("{:?}{:?}", (), if true { &format!("") } else { "" });
+   |                                        -^^^^^^^^^^-
+   |                                        ||         |
+   |                                        ||         temporary value is freed at the end of this statement
+   |                                        |creates a temporary value which is freed while still in use
+   |                                        borrow later used here
+   |
+   = note: consider using a `let` binding to create a longer lived value
+   = note: this error originates in the macro `format` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0716`.

tests/ui/borrowck/format-args-temporary-scopes.rs

@@ -1,7 +1,6 @@
 //! Test for #145784 as it relates to format arguments: arguments to macros such as `println!`
 //! should obey normal temporary scoping rules.
 //@ revisions: e2021 e2024
-//@ [e2021] check-pass
 //@ [e2021] edition: 2021
 //@ [e2024] edition: 2024
 
@@ -18,4 +17,11 @@ fn main() {
     // outlive the result of the block, making this compile.
     println!("{:?}{:?}", { &temp() }, ());
     //[e2024]~^ ERROR: temporary value dropped while borrowed [E0716]
+
+    // In real-world projects, this typically appeared in `if` expressions with a `&str` in one
+    // branch and a reference to a `String` temporary in the other. Since the consequent and `else`
+    // blocks of `if` expressions are temporary scopes in all editions, this affects Rust 2021 and
+    // earlier as well.
+    println!("{:?}{:?}", (), if true { &format!("") } else { "" });
+    //~^ ERROR: temporary value dropped while borrowed [E0716]
 }