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Document never type fallback in !'s docs #124419

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merged 5 commits into from
May 2, 2024
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Document never type fallback in !'s docs #124419

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Document never type fallback in `!`'s docs
WaffleLapkin Apr 26, 2024
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WaffleLapkin Apr 26, 2024
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WaffleLapkin May 2, 2024
e2eb053
Slightly reformat !'s docs after applying github suggestions
WaffleLapkin May 2, 2024
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fixup links in never type docs
WaffleLapkin May 2, 2024
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Apply suggestions from code review 
Co-authored-by: Kevin Reid <kpreid@switchb.org>
Co-authored-by: Herman Skogseth <herman.skogseth@me.com>
Co-authored-by: Jubilee <46493976+workingjubilee@users.noreply.github.com>
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@WaffleLapkin @kpreid
@skogseth @workingjubilee
4 people authored May 2, 2024
commit 3a40e838bfc22c6415e32fcd94fd48e83e1844b1
15 changes: 7 additions & 8 deletions library/core/src/primitive_docs.rs
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Expand Up @@ -270,8 +270,8 @@ mod prim_bool {}
///
/// # Never type fallback
///
/// When the compiler sees a value of type `!` it implicitly inserts a coercion (if possible)
/// to allow type checker to infer any type:
/// When the compiler sees a value of type `!` in a [coercion site](https://doc.rust-lang.org/reference/type-coercions.html#coercion-sites), it implicitly inserts a coercion
/// to allow the type checker to infer any type:
///
/// ```rust,ignore (illustrative-and-has-placeholders)
/// // this
Expand All @@ -286,8 +286,7 @@ mod prim_bool {}
// FIXME: use `core::convert::absurd` here instead, once it's merged
/// ```
///
/// While it's convenient to be able to use non-diverging code in one of the branches (like
/// `if a { b } else { return }`) this could lead to compilation errors:
/// This can lead to compilation errors if the type cannot be inferred:
///
/// ```compile_fail
/// // this
Expand All @@ -298,17 +297,17 @@ mod prim_bool {}
/// ```
///
/// To prevent such errors, the compiler remembers where it inserted `absurd` calls, and
/// if it can't infer their type, it sets the type to the fallback type:
/// if it can't infer the type, it uses the fallback type instead:
/// ```rust, ignore
/// type Fallback = /* An arbitrarily selected type! */;
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"Arbitrarily selected" here sounds like "the compiler picks from some set", not "we made a choice and taught it to the compiler". I'm not sure what would be better, though.

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There's nothing stopping us from having arbitrarily complicated fallback logic, aside from a reluctance to implement anything more complex, which is why I suggested it that way.

We do pick from a set.

The set has one member. :^)

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Hoping to grow the set to two :p

I'm also not a fan of "arbitrary" here because we did purposefully choose the fallback type...

But I'm not sure how to phrase this properly :/

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@workingjubilee workingjubilee Apr 27, 2024 •
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Whenever I use arbitrary I strongly mean the "the reason for why is because I said so", because often there were multiple reasonable choices we could have made, but we could only make one (one set, with any number of members).

/// { absurd::<Fallback>(panic!()) }
/// ```
///
/// This is what is known as "never type fallback".
///
/// Historically fallback was [`()`], causing confusing behavior where `!` spontaneously coerced
/// to `()`, even though `()` was never mentioned (because of the fallback). There are plans to
/// change it in 2024 edition (and possibly in all editions on a later date), see
/// Historically, the fallback type was [`()`], causing confusing behavior where `!` spontaneously coerced
/// to `()`, even when it would not infer `()` without the fallback. There are plans to
/// change it in the [2024 edition](https://doc.rust-lang.org/nightly/edition-guide/rust-2024/index.html) (and possibly in all editions on a later date); see
/// [Tracking Issue for making `!` fall back to `!`][fallback-ti].
///
/// [`()`]: prim@unit
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