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Do not trigger unused_braces for while let #75083

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merged 1 commit into from
Aug 4, 2020
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Do not trigger unused_braces for while let #75083

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5 changes: 4 additions & 1 deletion src/librustc_lint/unused.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -520,7 +520,10 @@ trait UnusedDelimLint {
(cond, UnusedDelimsCtx::IfCond, true, Some(left), Some(right))
}

While(ref cond, ref block, ..) => {
// Do not lint `unused_braces` in `while let` expressions.
While(ref cond, ref block, ..)
if !matches!(cond.kind, Let(_, _)) || Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX =>
{
let left = e.span.lo() + rustc_span::BytePos(5);
let right = block.span.lo();
(cond, UnusedDelimsCtx::WhileCond, true, Some(left), Some(right))
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11 changes: 2 additions & 9 deletions src/test/ui/lint/issue-74883-unused-paren-baren-yield.rs
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Expand Up @@ -15,15 +15,8 @@ fn main() {
while let Some(_) = ((yield)) {} //~ ERROR: unnecessary parentheses
{{yield}}; //~ ERROR: unnecessary braces
{( yield )}; //~ ERROR: unnecessary parentheses

// FIXME: Reduce duplicate warnings.
// Perhaps we should tweak checks in `BlockRetValue`?
while let Some(_) = {(yield)} {}
//~^ ERROR: unnecessary braces
//~| ERROR: unnecessary parentheses
while let Some(_) = {{yield}} {}
//~^ ERROR: unnecessary braces
//~| ERROR: unnecessary braces
while let Some(_) = {(yield)} {} //~ ERROR: unnecessary parentheses
while let Some(_) = {{yield}} {} //~ ERROR: unnecessary braces

// FIXME: It'd be great if we could also warn them.
((yield));
Expand Down
18 changes: 3 additions & 15 deletions src/test/ui/lint/issue-74883-unused-paren-baren-yield.stderr
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Expand Up @@ -34,29 +34,17 @@ error: unnecessary parentheses around block return value
LL | {( yield )};
| ^^^^^^^^^ help: remove these parentheses

error: unnecessary braces around `let` scrutinee expression
--> $DIR/issue-74883-unused-paren-baren-yield.rs:21:29
|
LL | while let Some(_) = {(yield)} {}
| ^^^^^^^^^ help: remove these braces

error: unnecessary parentheses around block return value
--> $DIR/issue-74883-unused-paren-baren-yield.rs:21:30
--> $DIR/issue-74883-unused-paren-baren-yield.rs:18:30
|
LL | while let Some(_) = {(yield)} {}
| ^^^^^^^ help: remove these parentheses

error: unnecessary braces around `let` scrutinee expression
--> $DIR/issue-74883-unused-paren-baren-yield.rs:24:29
|
LL | while let Some(_) = {{yield}} {}
| ^^^^^^^^^ help: remove these braces

error: unnecessary braces around block return value
--> $DIR/issue-74883-unused-paren-baren-yield.rs:24:30
--> $DIR/issue-74883-unused-paren-baren-yield.rs:19:30
|
LL | while let Some(_) = {{yield}} {}
| ^^^^^^^ help: remove these braces

error: aborting due to 8 previous errors
error: aborting due to 6 previous errors

12 changes: 12 additions & 0 deletions src/test/ui/lint/unused-braces-while-let-with-mutable-value.rs
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@@ -0,0 +1,12 @@
// check-pass

#![deny(unused_braces)]

fn main() {
let mut a = Some(3);
// Shouldn't warn below `a`.
while let Some(ref mut v) = {a} {
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So it is not linted for unnamed struct literal in the future?

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not quite sure what you mean here, we can't warn for { a } if it's used in patterns because it can
actually change the behavior there. From the docs

    /// Due to `ref` pattern, there can be a difference between using
    /// `{ expr }` and `expr` in pattern-matching contexts. This means
    /// that we should only lint `unused_parens` and not `unused_braces`
    /// in this case.
    ///
    /// ```rust
    /// let mut a = 7;
    /// let ref b = { a }; // We actually borrow a copy of `a` here.
    /// a += 1; // By mutating `a` we invalidate any borrows of `a`.
    /// assert_eq!(b + 1, a); // `b` does not borrow `a`, so we can still use it here.
    /// ```

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@tesuji tesuji Aug 3, 2020 •
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Playground: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=d28588845e2466227a28572bb6929b30

What I trying to say is that currently we don't allow struct literals in if, while-let.

struct Foo {
     a: u32
}

fn foo() {
     while let Foo { a } = Foo { a: 42} {
          ()
     }
}

By enabling around { a }, unnamed structs in the future might be hard to be used/migrate.
I am not against this changes, also am not authorative.
What I want is improving the precision of the lint around {( yield )}.

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So you want forbid while let pat = { a }?

I still can't completely follow you here.

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Yeah, I think it kind of ambiguous in parsing. But I'm not authoritative here.
so let's cc parser devs like @petrochenkov @estebank

I'm sorry for causing this noise.

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We already allow while let pat = { a } { /**/ } so I don't really think we can change that now

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But with the unused braces warning people will feel discouraged to use it.

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while let pat = { a } also doesn't cause ambiguity here as while let _ = expects expr { so the { of { a } must be part of expr here

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I'm kinda get it your point. Does it mean parsing unnamed structs at expr is unambiguous and allowed?
Even though struct literals are forbidden.

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@lcnr lcnr Aug 3, 2020 •
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But with the unused braces warning people will feel discouraged to use it.

The unused braces warning can break code here and imo rustc should try quite hard to not emit potentially incorrect warnings

a.as_mut().map(|a| std::mem::swap(a, v));
break;
}
}