Unrolled build for #150130

Rollup merge of #150130 - aerooneqq:delegation-one-line-trait-impl, r=petrochenkov

Support syntax for one-line trait reuse

This PR adds support for reusing the whole trait with a one-line reuse syntax and is part of the delegation feature #118212:
```rust
trait T {
  fn foo(&self);
}

struct S;
impl T for S { ... }

struct Wrapper(S);
reuse impl T for Wrapper { self.0 }
```

The core idea is that we already have support for glob reuse, so in this scenario we want to transform one-line reuse into a trait impl block with a glob reuse in the following way:
```rust
//Before
reuse impl T for Wrapper { self.0 }

//After
impl T for Wrapper {
  reuse T::* { self.0 }
}
```

It seems like this task can be solved during parsing stage, when we encountered a one-line trait reuse, we can expand into this impl block right away, and the code which was already written to expand glob delegations will take care about the rest. We will copy trait path into glob reuse path.

The implementation of the transformation reuses already existing methods for `impl` parsing, however, we do not parse inner `impl` items, instead we parse "inner items" as delegation body. Thus, we do not have to deal with generics, consts, unsafe and other `impl` related features.

Other syntax possibility is trying to shorten one-line reuse by replacing `impl` keyword with `reuse` keyword:
```rust
reuse T for Wrapper { self.0 }
```
In this case implementation may become more complicated, and the syntax more confusing, as keywords such as `const` or `unsafe` will precede `reuse`, and there are also generics:
```rust
unsafe reuse<T1, T2> T for Wrapper { self.0 }
```

In the first (currently implemented) version reuse is placed in the beginning of the item, and it is clear that we will reuse trait implementation, while in the second, shorter version, the `reuse` keyword may be lost in generics and keywords that may precede `impl`.

r? ``@petrochenkov``

Author: rust-timer

compiler/rustc_parse/messages.ftl

@@ -162,6 +162,8 @@ parse_default_not_followed_by_item = `default` is not followed by an item
     .label = the `default` qualifier
     .note = only `fn`, `const`, `type`, or `impl` items may be prefixed by `default`
 
+parse_delegation_non_trait_impl_reuse = only trait impls can be reused
+
 parse_do_catch_syntax_removed = found removed `do catch` syntax
     .note = following RFC #2388, the new non-placeholder syntax is `try`
     .suggestion = replace with the new syntax

compiler/rustc_parse/src/errors.rs

@@ -3671,3 +3671,10 @@ pub(crate) struct VarargsWithoutPattern {
     #[suggestion(code = "_: ...", applicability = "machine-applicable")]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(parse_delegation_non_trait_impl_reuse)]
+pub(crate) struct ImplReuseInherentImpl {
+    #[primary_span]
+    pub span: Span,
+}

compiler/rustc_parse/src/parser/item.rs

@@ -117,6 +117,11 @@ impl<'a> Parser<'a> {
     }
 }
 
+enum ReuseKind {
+    Path,
+    Impl,
+}
+
 impl<'a> Parser<'a> {
     pub fn parse_item(&mut self, force_collect: ForceCollect) -> PResult<'a, Option<Box<Item>>> {
         let fn_parse_mode =
@@ -249,9 +254,9 @@ impl<'a> Parser<'a> {
         } else if self.check_keyword_case(exp!(Trait), case) || self.check_trait_front_matter() {
             // TRAIT ITEM
             self.parse_item_trait(attrs, lo)?
-        } else if self.check_impl_frontmatter() {
+        } else if self.check_impl_frontmatter(0) {
             // IMPL ITEM
-            self.parse_item_impl(attrs, def_())?
+            self.parse_item_impl(attrs, def_(), false)?
         } else if let Const::Yes(const_span) = self.parse_constness(case) {
             // CONST ITEM
             self.recover_const_mut(const_span);
@@ -265,8 +270,8 @@ impl<'a> Parser<'a> {
                 rhs,
                 define_opaque: None,
             }))
-        } else if self.is_reuse_path_item() {
-            self.parse_item_delegation()?
+        } else if let Some(kind) = self.is_reuse_item() {
+            self.parse_item_delegation(attrs, def_(), kind)?
         } else if self.check_keyword_case(exp!(Mod), case)
             || self.check_keyword_case(exp!(Unsafe), case) && self.is_keyword_ahead(1, &[kw::Mod])
         {
@@ -367,16 +372,25 @@ impl<'a> Parser<'a> {
     /// When parsing a statement, would the start of a path be an item?
     pub(super) fn is_path_start_item(&mut self) -> bool {
         self.is_kw_followed_by_ident(kw::Union) // no: `union::b`, yes: `union U { .. }`
-        || self.is_reuse_path_item()
+        || self.is_reuse_item().is_some() // yes: `reuse impl Trait for Struct { self.0 }`, yes: `reuse some_path::foo;`
         || self.check_trait_front_matter() // no: `auto::b`, yes: `auto trait X { .. }`
         || self.is_async_fn() // no(2015): `async::b`, yes: `async fn`
         || matches!(self.is_macro_rules_item(), IsMacroRulesItem::Yes{..}) // no: `macro_rules::b`, yes: `macro_rules! mac`
     }
 
-    fn is_reuse_path_item(&mut self) -> bool {
+    fn is_reuse_item(&mut self) -> Option<ReuseKind> {
+        if !self.token.is_keyword(kw::Reuse) {
+            return None;
+        }
+
         // no: `reuse ::path` for compatibility reasons with macro invocations
-        self.token.is_keyword(kw::Reuse)
-            && self.look_ahead(1, |t| t.is_path_start() && *t != token::PathSep)
+        if self.look_ahead(1, |t| t.is_path_start() && *t != token::PathSep) {
+            Some(ReuseKind::Path)
+        } else if self.check_impl_frontmatter(1) {
+            Some(ReuseKind::Impl)
+        } else {
+            None
+        }
     }
 
     /// Are we sure this could not possibly be a macro invocation?
@@ -560,6 +574,7 @@ impl<'a> Parser<'a> {
         &mut self,
         attrs: &mut AttrVec,
         defaultness: Defaultness,
+        is_reuse: bool,
     ) -> PResult<'a, ItemKind> {
         let mut constness = self.parse_constness(Case::Sensitive);
         let safety = self.parse_safety(Case::Sensitive);
@@ -628,7 +643,11 @@ impl<'a> Parser<'a> {
 
         generics.where_clause = self.parse_where_clause()?;
 
-        let impl_items = self.parse_item_list(attrs, |p| p.parse_impl_item(ForceCollect::No))?;
+        let impl_items = if is_reuse {
+            Default::default()
+        } else {
+            self.parse_item_list(attrs, |p| p.parse_impl_item(ForceCollect::No))?
+        };
 
         let (of_trait, self_ty) = match ty_second {
             Some(ty_second) => {
@@ -699,10 +718,76 @@ impl<'a> Parser<'a> {
         Ok(ItemKind::Impl(Impl { generics, of_trait, self_ty, items: impl_items, constness }))
     }
 
-    fn parse_item_delegation(&mut self) -> PResult<'a, ItemKind> {
+    fn parse_item_delegation(
+        &mut self,
+        attrs: &mut AttrVec,
+        defaultness: Defaultness,
+        kind: ReuseKind,
+    ) -> PResult<'a, ItemKind> {
         let span = self.token.span;
         self.expect_keyword(exp!(Reuse))?;
 
+        let item_kind = match kind {
+            ReuseKind::Path => self.parse_path_like_delegation(),
+            ReuseKind::Impl => self.parse_impl_delegation(span, attrs, defaultness),
+        }?;
+
+        self.psess.gated_spans.gate(sym::fn_delegation, span.to(self.prev_token.span));
+
+        Ok(item_kind)
+    }
+
+    fn parse_delegation_body(&mut self) -> PResult<'a, Option<Box<Block>>> {
+        Ok(if self.check(exp!(OpenBrace)) {
+            Some(self.parse_block()?)
+        } else {
+            self.expect(exp!(Semi))?;
+            None
+        })
+    }
+
+    fn parse_impl_delegation(
+        &mut self,
+        span: Span,
+        attrs: &mut AttrVec,
+        defaultness: Defaultness,
+    ) -> PResult<'a, ItemKind> {
+        let mut impl_item = self.parse_item_impl(attrs, defaultness, true)?;
+        let ItemKind::Impl(Impl { items, of_trait, .. }) = &mut impl_item else { unreachable!() };
+
+        let until_expr_span = span.to(self.prev_token.span);
+
+        let Some(of_trait) = of_trait else {
+            return Err(self
+                .dcx()
+                .create_err(errors::ImplReuseInherentImpl { span: until_expr_span }));
+        };
+
+        let body = self.parse_delegation_body()?;
+        let whole_reuse_span = span.to(self.prev_token.span);
+
+        items.push(Box::new(AssocItem {
+            id: DUMMY_NODE_ID,
+            attrs: Default::default(),
+            span: whole_reuse_span,
+            tokens: None,
+            vis: Visibility {
+                kind: VisibilityKind::Inherited,
+                span: whole_reuse_span,
+                tokens: None,
+            },
+            kind: AssocItemKind::DelegationMac(Box::new(DelegationMac {
+                qself: None,
+                prefix: of_trait.trait_ref.path.clone(),
+                suffixes: None,
+                body,
+            })),
+        }));
+
+        Ok(impl_item)
+    }
+
+    fn parse_path_like_delegation(&mut self) -> PResult<'a, ItemKind> {
         let (qself, path) = if self.eat_lt() {
             let (qself, path) = self.parse_qpath(PathStyle::Expr)?;
             (Some(qself), path)
@@ -713,43 +798,35 @@ impl<'a> Parser<'a> {
         let rename = |this: &mut Self| {
             Ok(if this.eat_keyword(exp!(As)) { Some(this.parse_ident()?) } else { None })
         };
-        let body = |this: &mut Self| {
-            Ok(if this.check(exp!(OpenBrace)) {
-                Some(this.parse_block()?)
-            } else {
-                this.expect(exp!(Semi))?;
-                None
-            })
-        };
 
-        let item_kind = if self.eat_path_sep() {
+        Ok(if self.eat_path_sep() {
             let suffixes = if self.eat(exp!(Star)) {
                 None
             } else {
                 let parse_suffix = |p: &mut Self| Ok((p.parse_path_segment_ident()?, rename(p)?));
                 Some(self.parse_delim_comma_seq(exp!(OpenBrace), exp!(CloseBrace), parse_suffix)?.0)
             };
-            let deleg = DelegationMac { qself, prefix: path, suffixes, body: body(self)? };
-            ItemKind::DelegationMac(Box::new(deleg))
+
+            ItemKind::DelegationMac(Box::new(DelegationMac {
+                qself,
+                prefix: path,
+                suffixes,
+                body: self.parse_delegation_body()?,
+            }))
         } else {
             let rename = rename(self)?;
             let ident = rename.unwrap_or_else(|| path.segments.last().unwrap().ident);
-            let deleg = Delegation {
+
+            ItemKind::Delegation(Box::new(Delegation {
                 id: DUMMY_NODE_ID,
                 qself,
                 path,
                 ident,
                 rename,
-                body: body(self)?,
+                body: self.parse_delegation_body()?,
                 from_glob: false,
-            };
-            ItemKind::Delegation(Box::new(deleg))
-        };
-
-        let span = span.to(self.prev_token.span);
-        self.psess.gated_spans.gate(sym::fn_delegation, span);
-
-        Ok(item_kind)
+            }))
+        })
     }
 
     fn parse_item_list<T>(
@@ -2594,7 +2671,7 @@ impl<'a> Parser<'a> {
         Ok(body)
     }
 
-    fn check_impl_frontmatter(&mut self) -> bool {
+    fn check_impl_frontmatter(&mut self, look_ahead: usize) -> bool {
         const ALL_QUALS: &[Symbol] = &[kw::Const, kw::Unsafe];
         // In contrast to the loop below, this call inserts `impl` into the
         // list of expected tokens shown in diagnostics.
@@ -2603,7 +2680,7 @@ impl<'a> Parser<'a> {
         }
         let mut i = 0;
         while i < ALL_QUALS.len() {
-            let action = self.look_ahead(i, |token| {
+            let action = self.look_ahead(i + look_ahead, |token| {
                 if token.is_keyword(kw::Impl) {
                     return Some(true);
                 }
@@ -2618,6 +2695,7 @@ impl<'a> Parser<'a> {
             }
             i += 1;
         }
+
         self.is_keyword_ahead(i, &[kw::Impl])
     }
 

tests/pretty/delegation-impl-reuse.pp

@@ -0,0 +1,46 @@
+#![feature(prelude_import)]
+#![no_std]
+//@ pretty-compare-only
+//@ pretty-mode:expanded
+//@ pp-exact:delegation-impl-reuse.pp
+
+#![allow(incomplete_features)]
+#![feature(fn_delegation)]
+#[macro_use]
+extern crate std;
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
+
+trait Trait<T> {
+    fn foo(&self) {}
+    fn bar(&self) {}
+    fn baz(&self) {}
+}
+
+struct S;
+
+impl Trait<{
+        struct S;
+        0
+    }> for S {
+    reuse Trait<{
+            struct S;
+            0
+        }>::foo {
+        self.0
+    }
+    reuse Trait<{
+            struct S;
+            0
+        }>::bar {
+        self.0
+    }
+    reuse Trait<{
+            struct S;
+            0
+        }>::baz {
+        self.0
+    }
+}
+
+fn main() {}

tests/pretty/delegation-impl-reuse.rs

@@ -0,0 +1,18 @@
+//@ pretty-compare-only
+//@ pretty-mode:expanded
+//@ pp-exact:delegation-impl-reuse.pp
+
+#![allow(incomplete_features)]
+#![feature(fn_delegation)]
+
+trait Trait<T> {
+    fn foo(&self) {}
+    fn bar(&self) {}
+    fn baz(&self) {}
+}
+
+struct S;
+
+reuse impl Trait<{ struct S; 0 }> for S { self.0 }
+
+fn main() {}

tests/ui/delegation/impl-reuse-bad-path.rs

@@ -0,0 +1,31 @@
+#![allow(incomplete_features)]
+#![feature(fn_delegation)]
+
+mod unresolved {
+    struct S;
+    reuse impl unresolved for S { self.0 }
+    //~^ ERROR failed to resolve: use of unresolved module or unlinked crate `unresolved`
+    //~| ERROR cannot find trait `unresolved` in this scope
+
+    trait T {}
+    reuse impl T for unresolved { self.0 }
+    //~^ ERROR empty glob delegation is not supported
+    //~| ERROR cannot find type `unresolved` in this scope
+}
+
+mod wrong_entities {
+    trait T {}
+    struct Trait;
+    struct S;
+
+    reuse impl Trait for S { self.0 }
+    //~^ ERROR expected trait, found struct `Trait`
+    //~| ERROR expected trait, found struct `Trait`
+
+    mod TraitModule {}
+    reuse impl TraitModule for S { self.0 }
+    //~^ ERROR expected trait, found module `TraitModule`
+    //~| ERROR expected trait, found module `TraitModule`
+}
+
+fn main() {}