Merge TraitItem & ImplItem into AssocItem` by Centril · Pull Request #67131 · rust-lang/rust (original) (raw)

Dylan-DPC-zz pushed a commit to Dylan-DPC-zz/rust that referenced this pull request

…enkov

parse: merge fn syntax + cleanup item parsing

Here we continue the work in rust-lang#67131 in particular to merge the grammars of fn items in various positions.

A list of language level changes (as sanctioned by the language team in rust-lang#65041 (comment) and rust-lang#67131):

• self parameters are now syntactically allowed as the first parameter irrespective of item context (and in function pointers). Instead, semantic validation (ast_validation) is used.

• Syntactically, fn items in extern { ... } blocks can now have bodies (fn foo() { ... } as opposed to fn foo();). As above, we use semantic restrictions instead.

• Syntactically, fn items in free contexts (directly in a file or a module) can now be without bodies (fn foo(); as opposed to fn foo() { ... }. As above, we use semantic restrictions instead, including for non-ident parameter patterns.

• const extern fn feature gating is now done post-expansion such that we do not have conditional compatibilities of function qualifiers in parsing.

• The FnFrontMatter grammar becomes:

  Extern = "extern" StringLit ;
  FnQual = "const"? "async"? "unsafe"? Extern? ;
  FnFrontMatter = FnQual "fn" ;

  That is, all item contexts now syntactically allow const async unsafe extern "C" fn and use semantic restrictions to rule out combinations previously prevented syntactically. The semantic restrictions include in particular:

  • fns in extern { ... } can have no qualifiers.
  • const and async cannot be combined.

• To fuse the list-of-items parsing in the 4 contexts that items are allowed, we now must permit inner attributes (#![attr]) inside trait Foo { ... } definitions. That is, we now allow e.g. trait Foo { #![attr] }. This was probably an oversight due to not using a uniform parsing mechanism, which we now do have (fn parse_item_list). The semantic support (including e.g. for linting) falls out directly from the attributes infrastructure. To ensure this, we include a test for lints.

Put together, these grammar changes allow us to substantially reduce the complexity of item parsing and its grammar. There are however some other non-language improvements that allow the compression to take place.

A list of compiler-internal changes (in particular noting the parser-external data-structure changes):

• We use enum AllowPlus/RecoverQPath/AllowCVariadic { Yes, No } in parser/ty.rs instead of passing around 3 different bools. I felt this was necessary as it was becoming mentally taxing to track which-is-which.

• fn visit_trait_item and fn visit_impl_item are merged into fn visit_assoc_item which now is passed an AssocCtxt to check which one it is.

• We change FnKind to:

  pub enum FnKind<'a> {
      Fn(FnCtxt, Ident, &'a FnSig, &'a Visibility, Option<&'a Block>),
      Closure(&'a FnDecl, &'a Expr),
  }

  with:

  pub enum FnCtxt {
      Free,
      Foreign,
      Assoc(AssocCtxt),
  }

  This is then taken advantage of in tweaking the various semantic restrictions as well as in pretty printing.

• In ItemKind::Fn, we change P<Block> to Option<P<Block>>.

• In ForeignItemKind::Fn, we change P<FnDecl> to FnSig and P<Block> to Option<P<Block>>.

• We change ast::{Unsafety, Spanned<Constness>}> into enum ast::{Unsafe, Const} { Yes(Span), No } respectively. This change in formulation allow us to exclude Span in the case of No, which facilitates parsing. Moreover, we also add a Span to IsAsync which is renamed to Async. The new Spans in Unsafety and Async are then taken advantage of for better diagnostics. A reason this change was made is to have a more uniform and clear naming scheme.

  The HIR keeps the structures in AST (with those definitions moved into HIR) for now to avoid regressing perf.

• Various cleanups, bug fixes, and diagnostics improvements are made along the way. It is probably best to understand those via the diffs.

I would recommend reviewing this commit-by-commit with whitespace changes hidden.

r? @estebank @petrochenkov