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Document restrictions of the main function #345

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Merged
merged 3 commits into from
May 24, 2018
Merged

Document restrictions of the main function #345

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7a4edcd
Document multiple return types in main
Havvy May 22, 2018
f15b529
More restrictions on main
Havvy May 24, 2018
3348d21
Address feedback for main fn changes
Havvy May 24, 2018
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32 changes: 23 additions & 9 deletions src/crates-and-source-files.md
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Expand Up @@ -11,11 +11,11 @@
> UTF8BOM : `\uFEFF`
> SHEBANG : `#!` ~[`[` `\n`] ~`\n`<sup>*</sup>

Although Rust, like any other language, can be implemented by an interpreter as
well as a compiler, the only existing implementation is a compiler,
and the language has
always been designed to be compiled. For these reasons, this section assumes a
compiler.

> Note: Although Rust, like any other language, can be implemented by an
> interpreter as well as a compiler, the only existing implementation is a
> compiler,and the language has always been designed to be compiled. For these
> reasons, this section assumes a compiler.

Rust's semantics obey a *phase distinction* between compile-time and
run-time.[^phase-distinction] Semantic rules that have a *static
Expand Down Expand Up @@ -66,9 +66,19 @@ apply to the crate as a whole.
#![warn(non_camel_case_types)]
```

A crate that contains a `main` function can be compiled to an executable. If a
`main` function is present, its return type must be `()`
("[unit]") and it must take no arguments.
A crate that contains a `main` [function] can be compiled to an executable. If a
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Not from this PR, but binary crates are determined by the crate type, not the presence of main.

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There's a lot on that page I don't like, but I wanted to keep this change self-contained.

`main` function is present, it must take no arguments, must not declare any
[trait or lifetime bounds], must not have any [where clauses], and its return
type must be one of the following:

* `()`
* `bool`
* `i32`
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Bool and i32 don't implement Termination at the moment.

<!-- `!` -->
* `Result<T, E> where T: on this list, E: Error`

> Note: The implementation of which return types is allowed is the unstable
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Are allowed.
'Is determined by'?

> [`Termination`] trait.

The optional [_UTF8 byte order mark_] (UTF8BOM production) indicates that the
file is encoded in UTF8. It can only occur at the beginning of the file and
Expand Down Expand Up @@ -98,9 +108,13 @@ fn main() {

[module]: items/modules.html
[module path]: paths.html
[attributes]: items-and-attributes.html
[attributes]: attributes.html
[unit]: types.html#tuple-types
[_InnerAttribute_]: attributes.html
[_Item_]: items.html
[_shebang_]: https://en.wikipedia.org/wiki/Shebang_(Unix)
[_utf8 byte order mark_]: https://en.wikipedia.org/wiki/Byte_order_mark#UTF-8
[function]: items/functions.html
[`Termination`]: ../std/process/trait.Termination.html
[where clause]: items/where-clauses.html
[trait or lifetime bounds]: trait-bounds.html