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Don't recommend const fns on a stable build without a note about nightlies #28507

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Sep 20, 2015
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Don't recommend const fns on a stable build without a note about nightlies #28507

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12 changes: 10 additions & 2 deletions src/librustc/diagnostics.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -298,10 +298,18 @@ const FOO: i32 = { 0 }; // but brackets are useless here
```
"##,

// FIXME(#24111) Change the language here when const fn stabilizes
E0015: r##"
The only functions that can be called in static or constant expressions are
`const` functions. Rust currently does not support more general compile-time
function execution.
`const` functions, and struct/enum constructors. `const` functions are only
available on a nightly compiler. Rust currently does not support more general
compile-time function execution.

```
const FOO: Option<u8> = Some(1); // enum constructor
struct Bar {x: u8}
const BAR: Bar = Bar {x: 1}; // struct constructor
```

See [RFC 911] for more details on the design of `const fn`s.

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20 changes: 16 additions & 4 deletions src/librustc/middle/check_const.rs
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Expand Up @@ -39,6 +39,7 @@ use util::nodemap::NodeMap;
use rustc_front::hir;
use syntax::ast;
use syntax::codemap::Span;
use syntax::feature_gate::UnstableFeatures;
use rustc_front::visit::{self, FnKind, Visitor};

use std::collections::hash_map::Entry;
Expand Down Expand Up @@ -709,10 +710,21 @@ fn check_expr<'a, 'tcx>(v: &mut CheckCrateVisitor<'a, 'tcx>,
if !is_const {
v.add_qualif(ConstQualif::NOT_CONST);
if v.mode != Mode::Var {
span_err!(v.tcx.sess, e.span, E0015,
"function calls in {}s are limited to \
constant functions, \
struct and enum constructors", v.msg());
// FIXME(#24111) Remove this check when const fn stabilizes
if let UnstableFeatures::Disallow = v.tcx.sess.opts.unstable_features {
span_err!(v.tcx.sess, e.span, E0015,
"function calls in {}s are limited to \
struct and enum constructors", v.msg());
v.tcx.sess.span_note(e.span,
"a limited form of compile-time function \
evaluation is available on a nightly \
compiler via `const fn`");
} else {
span_err!(v.tcx.sess, e.span, E0015,
"function calls in {}s are limited to \
constant functions, \
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Did you intend to mention "constant functions" on stable Rust?

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Yes, I did. That's why I mention that "it is available on nightlies".

I'm okay with suggesting unstable features on a stable compiler, as long as it is mentioned that they are only available on nightlies (so that the user can evaluate switching to a nightly if constfn was indeed what they wanted). I want to avoid the bad UX of "compiler suggests thing, try thing, thing doesn't work because stable".

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Oh, I see what you mean. Fixed.

struct and enum constructors", v.msg());
}
}
}
}
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