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Andrew V. Teylu
574d8774b9
The AST pretty printer produces invalid Rust when a block expression is
the base of an index operation inside a macro expansion. This is a gap
in the existing `FixupContext` parenthesization machinery — the approach
handles statement position but not the case where a block-index is
nested inside another expression.
The following is a correct program:
```rust
macro_rules! block_arr {
() => {{ [0u8; 4] }};
}
macro_rules! as_slice {
() => {{ &block_arr!()[..] }};
}
fn main() { let _: &[u8] = as_slice!(); }
```
But `rustc -Zunpretty=expanded` produces output that is not valid Rust,
because the closing brace of `{ [0u8; 4] }` creates a statement
boundary, causing the parser to treat `[..]` as a separate expression:
```rust
fn main() { let _: &[u8] = { &{ [0u8; 4] }[..] }; }
```
```
error: expected one of `.`, `;`, `?`, `}`, or an operator, found `[`
```
Fixed output after this change:
```rust
fn main() { let _: &[u8] = { &({ [0u8; 4] })[..] }; }
```
Since `{ ... }[...]` never parses as indexing a block regardless of
context, the fix unconditionally parenthesizes "complete" expressions
(block, match, if, loop, etc.) when they appear as the base of an index
operation.