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Add an expression parser that uses match-arm's boundary rules #1593

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Feb 28, 2024
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Add an expression parser that uses match-arm's boundary rules #1593

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179 changes: 117 additions & 62 deletions src/expr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1185,6 +1185,122 @@ pub(crate) mod parsing {
pub fn parse_without_eager_brace(input: ParseStream) -> Result<Expr> {
ambiguous_expr(input, AllowStruct(false))
}

/// An alternative to the primary `Expr::parse` parser (from the
/// [`Parse`] trait) for syntactic positions in which expression
/// boundaries are placed more eagerly than done by the typical
/// expression grammar. This includes expressions at the head of a
/// statement or in the right-hand side of a `match` arm.
///
/// Compare the following cases:
///
/// 1.
/// ```
/// # let result = ();
/// # let guard = false;
/// # let cond = true;
/// # let f = true;
/// # let g = f;
/// #
/// let _ = match result {
/// () if guard => if cond { f } else { g }
/// () => false,
/// };
/// ```
///
/// 2.
/// ```
/// # let cond = true;
/// # let f = ();
/// # let g = f;
/// #
/// let _ = || {
/// if cond { f } else { g }
/// ()
/// };
/// ```
///
/// 3.
/// ```
/// # let cond = true;
/// # let f = || ();
/// # let g = f;
/// #
/// let _ = [if cond { f } else { g } ()];
/// ```
///
/// The same sequence of tokens `if cond { f } else { g } ()` appears in
/// expression position 3 times. The first two syntactic positions use
/// eager placement of expression boundaries, and parse as `Expr::If`,
/// with the adjacent `()` becoming `Pat::Tuple` or `Expr::Tuple`. In
/// contrast, the third case uses standard expression boundaries and
/// parses as `Expr::Call`.
///
/// As with [`parse_without_eager_brace`], this ambiguity in the Rust
/// grammar is independent of precedence.
///
/// [`parse_without_eager_brace`]: Self::parse_without_eager_brace
#[cfg(feature = "full")]
#[cfg_attr(doc_cfg, doc(cfg(all(feature = "full", feature = "parsing"))))]
pub fn parse_with_earlier_boundary_rule(input: ParseStream) -> Result<Expr> {
let mut attrs = input.call(expr_attrs)?;
let mut expr = if input.peek(token::Group) {
let allow_struct = AllowStruct(true);
let atom = expr_group(input, allow_struct)?;
if continue_parsing_early(&atom) {
trailer_helper(input, atom)?
} else {
atom
}
} else if input.peek(Token![if]) {
Expr::If(input.parse()?)
} else if input.peek(Token![while]) {
Expr::While(input.parse()?)
} else if input.peek(Token![for])
&& !(input.peek2(Token![<]) && (input.peek3(Lifetime) || input.peek3(Token![>])))
{
Expr::ForLoop(input.parse()?)
} else if input.peek(Token![loop]) {
Expr::Loop(input.parse()?)
} else if input.peek(Token![match]) {
Expr::Match(input.parse()?)
} else if input.peek(Token![try]) && input.peek2(token::Brace) {
Expr::TryBlock(input.parse()?)
} else if input.peek(Token![unsafe]) {
Expr::Unsafe(input.parse()?)
} else if input.peek(Token![const]) && input.peek2(token::Brace) {
Expr::Const(input.parse()?)
} else if input.peek(token::Brace) {
Expr::Block(input.parse()?)
} else if input.peek(Lifetime) {
atom_labeled(input)?
} else {
let allow_struct = AllowStruct(true);
unary_expr(input, allow_struct)?
};

if continue_parsing_early(&expr) {
attrs.extend(expr.replace_attrs(Vec::new()));
expr.replace_attrs(attrs);

let allow_struct = AllowStruct(true);
return parse_expr(input, expr, allow_struct, Precedence::Any);
}

if input.peek(Token![.]) && !input.peek(Token![..]) || input.peek(Token![?]) {
expr = trailer_helper(input, expr)?;

attrs.extend(expr.replace_attrs(Vec::new()));
expr.replace_attrs(attrs);

let allow_struct = AllowStruct(true);
return parse_expr(input, expr, allow_struct, Precedence::Any);
}

attrs.extend(expr.replace_attrs(Vec::new()));
expr.replace_attrs(attrs);
Ok(expr)
}
}

#[cfg(feature = "full")]
Expand Down Expand Up @@ -2010,67 +2126,6 @@ pub(crate) mod parsing {
}
}

#[cfg(feature = "full")]
pub(crate) fn expr_early(input: ParseStream) -> Result<Expr> {
let mut attrs = input.call(expr_attrs)?;
let mut expr = if input.peek(token::Group) {
let allow_struct = AllowStruct(true);
let atom = expr_group(input, allow_struct)?;
if continue_parsing_early(&atom) {
trailer_helper(input, atom)?
} else {
atom
}
} else if input.peek(Token![if]) {
Expr::If(input.parse()?)
} else if input.peek(Token![while]) {
Expr::While(input.parse()?)
} else if input.peek(Token![for])
&& !(input.peek2(Token![<]) && (input.peek3(Lifetime) || input.peek3(Token![>])))
{
Expr::ForLoop(input.parse()?)
} else if input.peek(Token![loop]) {
Expr::Loop(input.parse()?)
} else if input.peek(Token![match]) {
Expr::Match(input.parse()?)
} else if input.peek(Token![try]) && input.peek2(token::Brace) {
Expr::TryBlock(input.parse()?)
} else if input.peek(Token![unsafe]) {
Expr::Unsafe(input.parse()?)
} else if input.peek(Token![const]) && input.peek2(token::Brace) {
Expr::Const(input.parse()?)
} else if input.peek(token::Brace) {
Expr::Block(input.parse()?)
} else if input.peek(Lifetime) {
atom_labeled(input)?
} else {
let allow_struct = AllowStruct(true);
unary_expr(input, allow_struct)?
};

if continue_parsing_early(&expr) {
attrs.extend(expr.replace_attrs(Vec::new()));
expr.replace_attrs(attrs);

let allow_struct = AllowStruct(true);
return parse_expr(input, expr, allow_struct, Precedence::Any);
}

if input.peek(Token![.]) && !input.peek(Token![..]) || input.peek(Token![?]) {
expr = trailer_helper(input, expr)?;

attrs.extend(expr.replace_attrs(Vec::new()));
expr.replace_attrs(attrs);

let allow_struct = AllowStruct(true);
return parse_expr(input, expr, allow_struct, Precedence::Any);
}

attrs.extend(expr.replace_attrs(Vec::new()));
expr.replace_attrs(attrs);
Ok(expr)
}

#[cfg(feature = "full")]
fn continue_parsing_early(mut expr: &Expr) -> bool {
while let Expr::Group(group) = expr {
Expand Down Expand Up @@ -2900,7 +2955,7 @@ pub(crate) mod parsing {
},
fat_arrow_token: input.parse()?,
body: {
let body = input.call(expr_early)?;
let body = Expr::parse_with_earlier_boundary_rule(input)?;
requires_comma = requires_terminator(&body);
Box::new(body)
},
Expand Down
2 changes: 1 addition & 1 deletion src/stmt.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -334,7 +334,7 @@ pub(crate) mod parsing {
allow_nosemi: AllowNoSemi,
mut attrs: Vec<Attribute>,
) -> Result<Stmt> {
let mut e = expr::parsing::expr_early(input)?;
let mut e = Expr::parse_with_earlier_boundary_rule(input)?;

let mut attr_target = &mut e;
loop {
Expand Down