Allow storing format_args!() in variable

Fixes rust-lang#92698

Tracking issue for super let: rust-lang#139076

This change allows:

```rust
let name = "world";
let f = format_args!("hello {name}!");

println!("{f}");
```

This will need an FCP.

This implementation makes use of `super let`, which is unstable and might not exist in the future in its current form. However, it is entirely reasonable to assume future Rust will always have _a_ way of expressing temporary lifetimes like this, since the (stable) `pin!()` macro needs this. (This was also the motivation for merging rust-lang#139114.)

> [!NOTE]
> This PR causes many subtle changes in diagnostics output. Most of those are good. Some of those are bad. I've collected all the bad ones in the last commit. Those still need fixing. Marking this PR as draft.

(This is a second version of rust-lang#139135)

 Allow storing `format_args!()` in variable

Fixes #92698

Tracking issue for super let: #139076

Tracking issue for format_args: #99012

This change allows:

```rust
let name = "world";
let f = format_args!("hello {name}!"); // New: Store format_args!() for later!

println!("{f}");
```

This will need an FCP.

This implementation makes use of `super let`, which is unstable and might not exist in the future in its current form. However, it is entirely reasonable to assume future Rust will always have _a_ way of expressing temporary lifetimes like this, since the (stable) `pin!()` macro needs this too. (This was also the motivation for merging #139114.)

(This is a second version of #139135)

 Allow storing `format_args!()` in variable

Fixes rust-lang/rust#92698

Tracking issue for super let: rust-lang/rust#139076

Tracking issue for format_args: rust-lang/rust#99012

This change allows:

```rust
let name = "world";
let f = format_args!("hello {name}!"); // New: Store format_args!() for later!

println!("{f}");
```

This will need an FCP.

This implementation makes use of `super let`, which is unstable and might not exist in the future in its current form. However, it is entirely reasonable to assume future Rust will always have _a_ way of expressing temporary lifetimes like this, since the (stable) `pin!()` macro needs this too. (This was also the motivation for merging rust-lang/rust#139114.)

(This is a second version of rust-lang/rust#139135)

 Allow storing `format_args!()` in variable

Fixes rust-lang/rust#92698

Tracking issue for super let: rust-lang/rust#139076

Tracking issue for format_args: rust-lang/rust#99012

This change allows:

```rust
let name = "world";
let f = format_args!("hello {name}!"); // New: Store format_args!() for later!

println!("{f}");
```

This will need an FCP.

This implementation makes use of `super let`, which is unstable and might not exist in the future in its current form. However, it is entirely reasonable to assume future Rust will always have _a_ way of expressing temporary lifetimes like this, since the (stable) `pin!()` macro needs this too. (This was also the motivation for merging rust-lang/rust#139114.)

(This is a second version of rust-lang/rust#139135)

 Allow storing `format_args!()` in variable

Fixes rust-lang/rust#92698

Tracking issue for super let: rust-lang/rust#139076

Tracking issue for format_args: rust-lang/rust#99012

This change allows:

```rust
let name = "world";
let f = format_args!("hello {name}!"); // New: Store format_args!() for later!

println!("{f}");
```

This will need an FCP.

This implementation makes use of `super let`, which is unstable and might not exist in the future in its current form. However, it is entirely reasonable to assume future Rust will always have _a_ way of expressing temporary lifetimes like this, since the (stable) `pin!()` macro needs this too. (This was also the motivation for merging rust-lang/rust#139114.)

(This is a second version of rust-lang/rust#139135)

…elmann

 Allow storing `format_args!()` in variable

Fixes rust-lang#92698

Tracking issue for super let: rust-lang#139076

Tracking issue for format_args: rust-lang#99012

This change allows:

```rust
let name = "world";
let f = format_args!("hello {name}!"); // New: Store format_args!() for later!

println!("{f}");
```

This will need an FCP.

This implementation makes use of `super let`, which is unstable and might not exist in the future in its current form. However, it is entirely reasonable to assume future Rust will always have _a_ way of expressing temporary lifetimes like this, since the (stable) `pin!()` macro needs this too. (This was also the motivation for merging rust-lang#139114.)

(This is a second version of rust-lang#139135)