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Use a distinct error code for "if may be missing an else clause" #36915

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Oct 18, 2016
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Use a distinct error code for "if may be missing an else clause" #36915

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9f7cc5f
Use a distinct error code for "if may be missing an else clause"
jfirebaugh Oct 2, 2016
71f9e2e
Continue to use struct_span_err! macro
jfirebaugh Oct 3, 2016
080ee04
Revise long diagnostic
jfirebaugh Oct 3, 2016
d07602b
Remove FIXME
jfirebaugh Oct 15, 2016
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17 changes: 17 additions & 0 deletions src/librustc/diagnostics.rs
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Expand Up @@ -1431,6 +1431,23 @@ fn make_child<'elve>(x: &mut &'elve isize, y: &mut &'elve isize) {
```
"##,

E0317: r##"
This error occurs when an `if` expression without an `else` block is used in a
context where a type other than `()` is expected, for example a `let`
expression:

```compile_fail,E0317
fn main() {
let x = 5;
let a = if x == 5 { 1 };
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@nagisa nagisa Oct 3, 2016 •
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This example can be made more relatable if written as such:

if condition {
    some_value
}

Missing semicolon after last expression in an if is likely the most common reason for this error by a large degree.

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That example produces the generic "mismatched types" variant of E0308 though, not "if may be missing an else clause".

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Does it? That’s weird. I would expect else-less if as a last expression in a function body/block to produce the “if may be missing an else clause” error instead.

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It seems to be dependent on the expected type in the expression's context:

fn foo() -> i32 {
    if true {
        5
    }
}

produces:

error[E0308]: if may be missing an else clause
 --> <anon>:2:5
  |
2 |     if true {
  |     ^ expected (), found i32
  |

However, change i32 to () and it's:

error[E0308]: mismatched types
 --> <anon>:3:9
  |
3 |         5
  |         ^ expected (), found integral variable
  |

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This makes sense to me -- even if you added an else, given that the expected type of this expression is (), the body would have to be changed. That is, the body and the fn signature are in conflict. We choose to blame the body. You could make a case the other way, but blaming the if seems wrong, since there is nothing you can change about the if itself (without changing something else) that would make this code compile.

Not that the compiler thinks this hard when selecting what to blame; it's semi-random. Just saying that in this case the span makes sense. It'd be nice if we could give a better error message though, explaining where the () came from.

I think that Elm does some nice tracking here. We should copy them. =)

}
```

An `if` expression without an `else` block has the type `()`, so this is a type
error. To resolve it, add an `else` block having the same type as the `if`
block.
"##,

E0398: r##"
In Rust 1.3, the default object lifetime bounds are expected to change, as
described in RFC #1156 [1]. You are getting a warning because the compiler
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15 changes: 10 additions & 5 deletions src/librustc/infer/error_reporting.rs
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Expand Up @@ -577,11 +577,16 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
terr: &TypeError<'tcx>)
-> DiagnosticBuilder<'tcx>
{
// FIXME: do we want to use a different error code for each origin?
let mut diag = struct_span_err!(
self.tcx.sess, trace.origin.span(), E0308,
"{}", trace.origin.as_failure_str()
);
let span = trace.origin.span();
let failure_str = trace.origin.as_failure_str();
let mut diag = match trace.origin {
TypeOrigin::IfExpressionWithNoElse(_) => {
struct_span_err!(self.tcx.sess, span, E0317, "{}", failure_str)
},
_ => {
struct_span_err!(self.tcx.sess, span, E0308, "{}", failure_str)
},
};
self.note_type_err(&mut diag, trace.origin, None, Some(trace.values), terr);
diag
}
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2 changes: 1 addition & 1 deletion src/test/compile-fail/if-without-else-result.rs
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Expand Up @@ -10,7 +10,7 @@

fn main() {
let a = if true { true };
//~^ ERROR if may be missing an else clause
//~^ ERROR if may be missing an else clause [E0317]
//~| expected type `()`
//~| found type `bool`
//~| expected (), found bool
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