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Rollup merge of #117988 - estebank:issue-106020, r=cjgillot

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Handle attempts to have multiple `cfg`d tail expressions

When encountering code that seems like it might be trying to have multiple tail expressions depending on `cfg` information, suggest alternatives that will success to parse.

```rust
fn foo() -> String {
    #[cfg(feature = "validation")]
    [1, 2, 3].iter().map(|c| c.to_string()).collect::<String>()
    #[cfg(not(feature = "validation"))]
    String::new()
}
```

```
error: expected `;`, found `#`
  --> $DIR/multiple-tail-expr-behind-cfg.rs:5:64
   |
LL |     #[cfg(feature = "validation")]
   |     ------------------------------ only `;` terminated statements or tail expressions are allowed after this attribute
LL |     [1, 2, 3].iter().map(|c| c.to_string()).collect::<String>()
   |                                                                ^ expected `;` here
LL |     #[cfg(not(feature = "validation"))]
   |     - unexpected token
   |
help: add `;` here
   |
LL |     [1, 2, 3].iter().map(|c| c.to_string()).collect::<String>();
   |                                                                +
help: alternatively, consider surrounding the expression with a block
   |
LL |     { [1, 2, 3].iter().map(|c| c.to_string()).collect::<String>() }
   |     +                                                             +
help: it seems like you are trying to provide different expressions depending on `cfg`, consider using `if cfg!(..)`
   |
LL ~     if cfg!(feature = "validation") {
LL ~         [1, 2, 3].iter().map(|c| c.to_string()).collect::<String>()
LL ~     } else if cfg!(not(feature = "validation")) {
LL ~         String::new()
LL +     }
   |
```

Fix #106020.

r? `@oli-obk`
This commit is contained in:
Michael Goulet 2023-11-19 19:14:34 -08:00 • committed by GitHub
commit e6a3ca0c65
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14
compiler/rustc_parse/src/parser/stmt.rs
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@ -617,6 +617,20 @@ impl<'a> Parser<'a> {
let mut add_semi_to_stmt = false;
match &mut stmt.kind {
// Expression without semicolon.
StmtKind::Expr(expr)
if classify::expr_requires_semi_to_be_stmt(expr)
&& !expr.attrs.is_empty()
&& ![token::Eof, token::Semi, token::CloseDelim(Delimiter::Brace)]
.contains(&self.token.kind) =>
{
// The user has written `#[attr] expr` which is unsupported. (#106020)
self.attr_on_non_tail_expr(&expr);
// We already emitted an error, so don't emit another type error
let sp = expr.span.to(self.prev_token.span);
*expr = self.mk_expr_err(sp);
}
// Expression without semicolon.
StmtKind::Expr(expr)
if self.token != token::Eof && classify::expr_requires_semi_to_be_stmt(expr) =>