Fix whitespace after fragment specifiers in macro pretty printing

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -329,6 +329,19 @@ fn print_crate_inner<'a>(
 /// - #63896: `#[allow(unused,` must be printed rather than `#[allow(unused ,`
 /// - #73345: `#[allow(unused)]` must be printed rather than `# [allow(unused)]`
 ///
+/// Returns `true` if both token trees are identifier-like tokens that would
+/// merge into a single token if printed without a space between them.
+/// E.g. `ident` + `where` would merge into `identwhere`.
+fn idents_would_merge(tt1: &TokenTree, tt2: &TokenTree) -> bool {
+    fn is_ident_like(tt: &TokenTree) -> bool {
+        matches!(
+            tt,
+            TokenTree::Token(Token { kind: token::Ident(..) | token::NtIdent(..), .. }, _,)
+        )
+    }
+    is_ident_like(tt1) && is_ident_like(tt2)
+}
+
 fn space_between(tt1: &TokenTree, tt2: &TokenTree) -> bool {
     use Delimiter::*;
     use TokenTree::{Delimited as Del, Token as Tok};
@@ -794,6 +807,13 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
             if let Some(next) = iter.peek() {
                 if spacing == Spacing::Alone && space_between(tt, next) {
                     self.space();
+                } else if spacing != Spacing::Alone && idents_would_merge(tt, next) {
+                    // When tokens from macro `tt` captures preserve their
+                    // original `Joint`/`JointHidden` spacing, adjacent
+                    // identifier-like tokens can be concatenated without a
+                    // space (e.g. `$x:identwhere`). Insert a space to
+                    // prevent this.
+                    self.space();
                 }
             }
         }

tests/pretty/macro-fragment-specifier-whitespace.pp

@@ -0,0 +1,24 @@
+#![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() {}

tests/pretty/macro-fragment-specifier-whitespace.rs

@@ -0,0 +1,19 @@
+//@ 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)*) => {};
+        }
+    };
+}
+outer!($ $x:ident);
+
+fn main() {}