Rollup merge of #98183 - dtolnay:emptybound, r=lcnr

Fix pretty printing of empty bound lists in where-clause

Repro:

```rust
macro_rules! assert_item_stringify {
    ($item:item $expected:literal) => {
        assert_eq!(stringify!($item), $expected);
    };
}

fn main() {
    assert_item_stringify! {
        fn f<'a, T>() where 'a:, T: {}
        "fn f<'a, T>() where 'a:, T: {}"
    }
}
```

Previously this assertion would fail because rustc renders the where-clause as `where 'a, T` which is invalid syntax.

This PR makes the above assertion pass.

This bug also affects `-Zunpretty=expanded`. The intention is for that to emit syntactically valid code, but the buggy output is not valid Rust syntax.

```console
$ rustc <(echo "fn f<'a, T>() where 'a:, T: {}") -Zunpretty=expanded
#![feature(prelude_import)]
#![no_std]
#[prelude_import]
use ::std::prelude::rust_2015::*;
#[macro_use]
extern crate std;
fn f<'a, T>() where 'a, T {}
```

```console
$ rustc <(echo "fn f<'a, T>() where 'a:, T: {}") -Zunpretty=expanded | rustc -
error: expected `:`, found `,`
 --> <anon>:7:23
  |
7 | fn f<'a, T>() where 'a, T {}
  |                       ^ expected `:`
```

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -814,7 +814,7 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
     }
 
     fn bounds_to_string(&self, bounds: &[ast::GenericBound]) -> String {
-        Self::to_string(|s| s.print_type_bounds("", bounds))
+        Self::to_string(|s| s.print_type_bounds(bounds))
     }
 
     fn pat_to_string(&self, pat: &ast::Pat) -> String {
@@ -991,7 +991,12 @@ impl<'a> State<'a> {
                     Term::Const(c) => self.print_expr_anon_const(c, &[]),
                 }
             }
-            ast::AssocConstraintKind::Bound { bounds } => self.print_type_bounds(":", &*bounds),
+            ast::AssocConstraintKind::Bound { bounds } => {
+                if !bounds.is_empty() {
+                    self.word_nbsp(":");
+                    self.print_type_bounds(&bounds);
+                }
+            }
         }
     }
 
@@ -1045,11 +1050,14 @@ impl<'a> State<'a> {
             }
             ast::TyKind::Path(Some(ref qself), ref path) => self.print_qpath(path, qself, false),
             ast::TyKind::TraitObject(ref bounds, syntax) => {
-                let prefix = if syntax == ast::TraitObjectSyntax::Dyn { "dyn" } else { "" };
+                if syntax == ast::TraitObjectSyntax::Dyn {
-                self.print_type_bounds(prefix, &bounds);
+                    self.word_nbsp("dyn");
+                }
+                self.print_type_bounds(bounds);
             }
             ast::TyKind::ImplTrait(_, ref bounds) => {
-                self.print_type_bounds("impl", &bounds);
+                self.word_nbsp("impl");
+                self.print_type_bounds(bounds);
             }
             ast::TyKind::Array(ref ty, ref length) => {
                 self.word("[");
@@ -1549,17 +1557,13 @@ impl<'a> State<'a> {
         }
     }
 
-    pub fn print_type_bounds(&mut self, prefix: &'static str, bounds: &[ast::GenericBound]) {
+    pub fn print_type_bounds(&mut self, bounds: &[ast::GenericBound]) {
-        if !bounds.is_empty() {
-            self.word(prefix);
         let mut first = true;
         for bound in bounds {
-                if !(first && prefix.is_empty()) {
-                    self.nbsp();
-                }
             if first {
                 first = false;
             } else {
+                self.nbsp();
                 self.word_space("+");
             }
 
@@ -1574,20 +1578,12 @@ impl<'a> State<'a> {
             }
         }
     }
-    }
 
     pub(crate) fn print_lifetime(&mut self, lifetime: ast::Lifetime) {
         self.print_name(lifetime.ident.name)
     }
 
-    pub(crate) fn print_lifetime_bounds(
+    pub(crate) fn print_lifetime_bounds(&mut self, bounds: &ast::GenericBounds) {
-        &mut self,
-        lifetime: ast::Lifetime,
-        bounds: &ast::GenericBounds,
-    ) {
-        self.print_lifetime(lifetime);
-        if !bounds.is_empty() {
-            self.word(": ");
         for (i, bound) in bounds.iter().enumerate() {
             if i != 0 {
                 self.word(" + ");
@@ -1598,7 +1594,6 @@ impl<'a> State<'a> {
             }
         }
     }
-    }
 
     pub(crate) fn print_generic_params(&mut self, generic_params: &[ast::GenericParam]) {
         if generic_params.is_empty() {
@@ -1613,11 +1608,18 @@ impl<'a> State<'a> {
             match param.kind {
                 ast::GenericParamKind::Lifetime => {
                     let lt = ast::Lifetime { id: param.id, ident: param.ident };
-                    s.print_lifetime_bounds(lt, &param.bounds)
+                    s.print_lifetime(lt);
+                    if !param.bounds.is_empty() {
+                        s.word_nbsp(":");
+                        s.print_lifetime_bounds(&param.bounds)
+                    }
                 }
                 ast::GenericParamKind::Type { ref default } => {
                     s.print_ident(param.ident);
-                    s.print_type_bounds(":", &param.bounds);
+                    if !param.bounds.is_empty() {
+                        s.word_nbsp(":");
+                        s.print_type_bounds(&param.bounds);
+                    }
                     if let Some(ref default) = default {
                         s.space();
                         s.word_space("=");
@@ -1630,7 +1632,10 @@ impl<'a> State<'a> {
                     s.space();
                     s.word_space(":");
                     s.print_type(ty);
-                    s.print_type_bounds(":", &param.bounds);
+                    if !param.bounds.is_empty() {
+                        s.word_nbsp(":");
+                        s.print_type_bounds(&param.bounds);
+                    }
                     if let Some(ref default) = default {
                         s.space();
                         s.word_space("=");

compiler/rustc_ast_pretty/src/pprust/state/item.rs

@@ -114,7 +114,10 @@ impl<'a> State<'a> {
         self.word_space("type");
         self.print_ident(ident);
         self.print_generic_params(&generics.params);
-        self.print_type_bounds(":", bounds);
+        if !bounds.is_empty() {
+            self.word_nbsp(":");
+            self.print_type_bounds(bounds);
+        }
         self.print_where_clause_parts(where_clauses.0.0, before_predicates);
         if let Some(ty) = ty {
             self.space();
@@ -320,7 +323,10 @@ impl<'a> State<'a> {
                         real_bounds.push(b.clone());
                     }
                 }
-                self.print_type_bounds(":", &real_bounds);
+                if !real_bounds.is_empty() {
+                    self.word_nbsp(":");
+                    self.print_type_bounds(&real_bounds);
+                }
                 self.print_where_clause(&generics.where_clause);
                 self.word(" ");
                 self.bopen();
@@ -347,7 +353,10 @@ impl<'a> State<'a> {
                     }
                 }
                 self.nbsp();
-                self.print_type_bounds("=", &real_bounds);
+                if !real_bounds.is_empty() {
+                    self.word_nbsp("=");
+                    self.print_type_bounds(&real_bounds);
+                }
                 self.print_where_clause(&generics.where_clause);
                 self.word(";");
                 self.end(); // end inner head-block
@@ -618,14 +627,23 @@ impl<'a> State<'a> {
             }) => {
                 self.print_formal_generic_params(bound_generic_params);
                 self.print_type(bounded_ty);
-                self.print_type_bounds(":", bounds);
+                self.word(":");
+                if !bounds.is_empty() {
+                    self.nbsp();
+                    self.print_type_bounds(bounds);
+                }
             }
             ast::WherePredicate::RegionPredicate(ast::WhereRegionPredicate {
                 lifetime,
                 bounds,
                 ..
             }) => {
-                self.print_lifetime_bounds(*lifetime, bounds);
+                self.print_lifetime(*lifetime);
+                self.word(":");
+                if !bounds.is_empty() {
+                    self.nbsp();
+                    self.print_lifetime_bounds(bounds);
+                }
             }
             ast::WherePredicate::EqPredicate(ast::WhereEqPredicate { lhs_ty, rhs_ty, .. }) => {
                 self.print_type(lhs_ty);

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -1355,7 +1355,10 @@ impl<'a> Parser<'a> {
                     s.print_mutability(mut_ty.mutbl, false);
                     s.popen();
                     s.print_type(&mut_ty.ty);
-                    s.print_type_bounds(" +", &bounds);
+                    if !bounds.is_empty() {
+                        s.word(" + ");
+                        s.print_type_bounds(&bounds);
+                    }
                     s.pclose()
                 });
 

src/test/pretty/where-clauses.rs

@@ -2,4 +2,7 @@
 
 fn f<'a, 'b, T>(t: T) -> isize where T: 'a, 'a: 'b, T: Eq { 0 }
 
+// This is legal syntax, sometimes generated by macros. `where T: $($bound+)*`
+fn zero_bounds<'a, T>() where 'a:, T: {}
+
 fn main() {}