Fix whitespace after fragment specifiers in macro pretty printing

[aytey]

When a macro-generating-macro captures fragment specifier tokens (like $x:ident) as tt metavariables and replays them before a keyword (like where), the pretty printer concatenates them into an invalid fragment specifier (e.g. $x:identwhere instead of $x:ident where).

This happens because tt captures preserve the original token spacing. When the fragment specifier name (e.g. ident) was originally the last token before a closing delimiter, it retains JointHidden spacing. The print_tts function only checks space_between for Spacing::Alone tokens, so JointHidden tokens skip the space check entirely, causing adjacent identifier-like tokens to merge.

The fix adds a check in print_tts to insert a space between adjacent identifier-like tokens (identifiers and keywords) regardless of the original spacing, preventing them from being concatenated into invalid tokens.

This is similar to the existing space_between mechanism that prevents token merging for Spacing::Alone tokens, extended to also handle Joint/JointHidden cases where two identifier-like tokens would merge.

Example

before (rustc 1.96.0-nightly (3b1b0ef4d 2026-03-11)):

#![feature(prelude_import)]
#![no_std]
extern crate std;
#[prelude_import]
use ::std::prelude::rust_2015::*;
//@ pretty-mode:expanded
//@ pp-exact:macro-fragment-specifier-whitespace.pp

// Test that fragment specifier names in macro definitions are properly
// separated from the following keyword/identifier token when pretty-printed.
// This is a regression test for a bug where `$x:ident` followed by `where`
// was pretty-printed as `$x:identwhere` (an invalid fragment specifier).

macro_rules! outer {
    ($d:tt $($params:tt)*) =>
    {
        #[macro_export] macro_rules! inner
        { ($($params)* where $d($rest:tt)*) => {}; }
    };
}
#[macro_export]
macro_rules! inner { ($x:identwhere $ ($rest : tt)*) => {}; }

fn main() {}

after (this branch):

#![feature(prelude_import)]
#![no_std]
extern crate std;
#[prelude_import]
use ::std::prelude::rust_2015::*;
//@ pretty-mode:expanded
//@ pp-exact:macro-fragment-specifier-whitespace.pp

// Test that fragment specifier names in macro definitions are properly
// separated from the following keyword/identifier token when pretty-printed.
// This is a regression test for a bug where `$x:ident` followed by `where`
// was pretty-printed as `$x:identwhere` (an invalid fragment specifier).

macro_rules! outer {
    ($d:tt $($params:tt)*) =>
    {
        #[macro_export] macro_rules! inner
        { ($($params)* where $d($rest:tt)*) => {}; }
    };
}
#[macro_export]
macro_rules! inner { ($x:ident where $ ($rest : tt)*) => {}; }

fn main() {}

Notice the $x:identwhere in the before — an invalid fragment specifier that causes a hard parse error. The after correctly separates it as $x:ident where.

[rustbot]

r? @mati865

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[Kivooeo]

can you show before and after please :)

[aytey]

can you show before and after please :)

before (rustc 1.95.0-nightly (873d4682c 2026-01-25)):

#![feature(prelude_import)]
#![no_std]
extern crate std;
#[prelude_import]
use ::std::prelude::rust_2015::*;
macro_rules! outer {
    ($d:tt $($params:tt)*) =>
    {
        #[macro_export] macro_rules! inner
        { ($($params)* where $d($rest:tt)*) => {}; }
    };
}
#[macro_export]
macro_rules! inner { ($x:identwhere $ ($rest : tt)*) => {}; }

fn main() {}

after (this branch):

#![feature(prelude_import)]
#![no_std]
extern crate std;
#[prelude_import]
use ::std::prelude::rust_2015::*;
macro_rules! outer {
    ($d:tt $($params:tt)*) =>
    {
        #[macro_export] macro_rules! inner
        { ($($params)* where $d($rest:tt)*) => {}; }
    };
}
#[macro_export]
macro_rules! inner { ($x:ident where $ ($rest : tt)*) => {}; }

fn main() {}

Notice the $x:identwhere in the before.

[Kivooeo]

yeah, new behaviour looks correct! thanks

r? me @bors r+ rollup

I also noticed that you have opened a few PRs on pretty printer today, if it's not too much trouble, could you update the description of each one to show the differences before and after? this would make the review process easier, as it would be possible to see the visual differences. I'll probably take a look at the others at a later time

[rust-bors]

📌 Commit c777685 has been approved by Kivooeo

It is now in the queue for this repository.

[aytey]

I also noticed that you have opened a few PRs on pretty printer today, if it's not too much trouble, could you update the description of each one to show the differences before and after?

@Kivooeo: all done!

I'll probably take a look at the others at a later time

Thanks 🫶