Auto merge of #29850 - Kimundi:attributes_that_make_a_statement, r=pnkfelix
See https://github.com/rust-lang/rfcs/pull/16 and https://github.com/rust-lang/rust/issues/15701 - Added syntax support for attributes on expressions and all syntax nodes in statement position. - Extended `#[cfg]` folder to allow removal of statements, and of expressions in optional positions like expression lists and trailing block expressions. - Extended lint checker to recognize lint levels on expressions and locals. - As per RFC, attributes are not yet accepted on `if` expressions. Examples: ```rust let x = y; { ... } assert_eq!((1, #[cfg(unset)] 2, 3), (1, 3)); let FOO = 0; ``` Implementation wise, there are a few rough corners and open questions: - The parser work ended up a bit ugly. - The pretty printer change was based mostly on guessing. - Similar to the `if` case, there are some places in the grammar where a new `Expr` node starts, but where it seemed weird to accept attributes and hence the parser doesn't. This includes: - const expressions in patterns - in the middle of an postfix operator chain (that is, after `.`, before indexing, before calls) - on range expressions, since `#[attr] x .. y` parses as `(#[attr] x) .. y`, which is inconsistent with `#[attr] .. y` which would parse as `#[attr] (.. y)` - Attributes are added as additional `Option<Box<Vec<Attribute>>>` fields in expressions and locals. - Memory impact has not been measured yet. - A cfg-away trailing expression in a block does not currently promote the previous `StmtExpr` in a block to a new trailing expr. That is to say, this won't work: ```rust let x = { #[cfg(foo)] Foo { data: x } #[cfg(not(foo))] Foo { data: y } }; ``` - One-element tuples can have their inner expression removed to become Unit, but just Parenthesis can't. Eg, `(#[cfg(unset)] x,) == ()` but `(#[cfg(unset)] x) == error`. This seemed reasonable to me since tuples and unit are type constructors, but could probably be argued either way. - Attributes on macro nodes are currently unconditionally dropped during macro expansion, which seemed fine since macro disappear at that point? - Attributes on `ast::ExprParens` will be prepend-ed to the inner expression in the hir folder. - The work on pretty printer tests for this did trigger, but not fix errors regarding macros: - expression `foo![]` prints as `foo!()` - expression `foo!{}` prints as `foo!()` - statement `foo![];` prints as `foo!();` - statement `foo!{};` prints as `foo!();` - statement `foo!{}` triggers a `None` unwrap ICE.
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