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Do not desugar if-let-else to match arm guards #41316

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Merged
merged 5 commits into from Apr 15, 2017
Merged

Do not desugar if-let-else to match arm guards #41316

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65fafd9
[41272] - code for desugaring iflet changed
Apr 15, 2017
4ac11be
[41272] - Fixed to conform to rust project style
su225 Apr 15, 2017
65b04fa
Fixed aesthetics and test
su225 Apr 15, 2017
6fc7d45
Fixed aesthetics of if let block
su225 Apr 15, 2017
5649b37
Aesthetic issue. Corrected indentation
su225 Apr 15, 2017
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96 changes: 9 additions & 87 deletions src/librustc/hir/lowering.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2033,7 +2033,6 @@ impl<'a> LoweringContext<'a> {
//
// match <sub_expr> {
// <pat> => <body>,
// [_ if <else_opt_if_cond> => <else_opt_if_body>,]
// _ => [<else_opt> | ()]
// }

Expand All @@ -2047,93 +2046,16 @@ impl<'a> LoweringContext<'a> {
arms.push(self.arm(hir_vec![pat], body_expr));
}

// `[_ if <else_opt_if_cond> => <else_opt_if_body>,]`
// `_ => [<else_opt> | ()]`
// _ => [<else_opt>|()]
{
let mut current: Option<&Expr> = else_opt.as_ref().map(|p| &**p);
let mut else_exprs: Vec<Option<&Expr>> = vec![current];

// First, we traverse the AST and recursively collect all
// `else` branches into else_exprs, e.g.:
//
// if let Some(_) = x {
// ...
// } else if ... { // Expr1
// ...
// } else if ... { // Expr2
// ...
// } else { // Expr3
// ...
// }
//
// ... results in else_exprs = [Some(&Expr1),
// Some(&Expr2),
// Some(&Expr3)]
//
// Because there also the case there is no `else`, these
// entries can also be `None`, as in:
//
// if let Some(_) = x {
// ...
// } else if ... { // Expr1
// ...
// } else if ... { // Expr2
// ...
// }
//
// ... results in else_exprs = [Some(&Expr1),
// Some(&Expr2),
// None]
//
// The last entry in this list is always translated into
// the final "unguard" wildcard arm of the `match`. In the
// case of a `None`, it becomes `_ => ()`.
loop {
if let Some(e) = current {
// There is an else branch at this level
if let ExprKind::If(_, _, ref else_opt) = e.node {
// The else branch is again an if-expr
current = else_opt.as_ref().map(|p| &**p);
else_exprs.push(current);
} else {
// The last item in the list is not an if-expr,
// stop here
break
}
} else {
// We have no more else branch
break
}
}

// Now translate the list of nested else-branches into the
// arms of the match statement.
for else_expr in else_exprs {
if let Some(else_expr) = else_expr {
let (guard, body) = if let ExprKind::If(ref cond,
ref then,
_) = else_expr.node {
let then = self.lower_block(then, false);
(Some(cond),
self.expr_block(then, ThinVec::new()))
} else {
(None,
self.lower_expr(else_expr))
};

arms.push(hir::Arm {
attrs: hir_vec![],
pats: hir_vec![self.pat_wild(e.span)],
guard: guard.map(|e| P(self.lower_expr(e))),
body: P(body),
});
} else {
// There was no else-branch, push a noop
let pat_under = self.pat_wild(e.span);
let unit = self.expr_tuple(e.span, hir_vec![]);
arms.push(self.arm(hir_vec![pat_under], unit));
}
}
let wildcard_arm: Option<&Expr> = else_opt.as_ref().map(|p| &**p);
let wildcard_pattern = self.pat_wild(e.span);
let body = if let Some(else_expr) = wildcard_arm {
P(self.lower_expr(else_expr))
} else {
self.expr_tuple(e.span, hir_vec![])
};
arms.push(self.arm(hir_vec![wildcard_pattern], body));
}

let contains_else_clause = else_opt.is_some();
Expand Down
29 changes: 29 additions & 0 deletions src/test/run-pass/issue-41272.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,29 @@
// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

struct Foo;

impl Foo {
fn bar(&mut self) -> bool { true }
}

fn error(foo: &mut Foo) {
if let Some(_) = Some(true) {
} else if foo.bar() {}
}

fn ok(foo: &mut Foo) {
if let Some(_) = Some(true) {
} else {
if foo.bar() {}
}
}

fn main() {}