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Improve error message for non-exhaustive patterns #31020

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Jan 27, 2016
Merged

Improve error message for non-exhaustive patterns #31020

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6100743
Improve error message for non-exhaustive patterns
regexident Jan 19, 2016
48e8326
Refined error message.
regexident Jan 19, 2016
70692ce
Refined error message to truncate at 3 and hint at number of hidden p…
regexident Jan 21, 2016
2c7a19a
Adjusted PR to better match project's coding style.
regexident Jan 22, 2016
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40 changes: 22 additions & 18 deletions src/librustc/middle/check_match.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -368,31 +368,34 @@ fn raw_pat<'a>(p: &'a Pat) -> &'a Pat {
fn check_exhaustive(cx: &MatchCheckCtxt, sp: Span, matrix: &Matrix, source: hir::MatchSource) {
match is_useful(cx, matrix, &[DUMMY_WILD_PAT], ConstructWitness) {
UsefulWithWitness(pats) => {
let witness = match &pats[..] {
[ref witness] => &**witness,
[] => DUMMY_WILD_PAT,
_ => unreachable!()
let witnesses = match &pats[..] {
[] => vec![DUMMY_WILD_PAT],
[p..] => {
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This may be a good chance to get rid of the slice pattern here? (they're quite buggy)

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So simply do if pats.is_empty() { … } else { … } instead?

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Yes.

p.iter().map(|w| &**w ).collect()
}
};
match source {
hir::MatchSource::ForLoopDesugar => {
// `witness` has the form `Some(<head>)`, peel off the `Some`
let witness = match witness.node {
// `witnesses[0]` has the form `Some(<head>)`, peel off the `Some`
let witness = match witnesses[0].node {
hir::PatEnum(_, Some(ref pats)) => match &pats[..] {
[ref pat] => &**pat,
_ => unreachable!(),
},
_ => unreachable!(),
};

span_err!(cx.tcx.sess, sp, E0297,
"refutable pattern in `for` loop binding: \
`{}` not covered",
pat_to_string(witness));
},
_ => {
let pattern_strings: Vec<_> = witnesses.iter().map(|w| {
pat_to_string(w)
}).take(10).collect();
span_err!(cx.tcx.sess, sp, E0004,
"non-exhaustive patterns: `{}` not covered",
pat_to_string(witness)
pattern_strings.join("`, `")
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Maybe delimit the last element with "and"? I think this reads a bit weird otherwise.

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Good point. Was wondering about this, too.

Would something like this look good to you?

let joined_string = match pattern_strings {
    [pattern] => pattern,
    [head.., tail] => head.join("`, `") + "` and `" + tail
}

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This also uses slice patterns, and apparently requires the "advanced_slice_pattern" feature gate.

I'm not sure about the optimal solution, maybe something like this?

let joined_string = match pattern_strings.split_last() {
    Some((tail, head)) => {
        if head.is_empty() {
            tail.clone()
        } else {
            head.join("`, `") + "` and `" + tail
        }
    },
    None => unreachable!()
};

);
},
}
Expand Down Expand Up @@ -594,14 +597,14 @@ impl<'tcx, 'container> ty::AdtDefData<'tcx, 'container> {
}
}

fn missing_constructor(cx: &MatchCheckCtxt, &Matrix(ref rows): &Matrix,
left_ty: Ty, max_slice_length: usize) -> Option<Constructor> {
fn missing_constructors(cx: &MatchCheckCtxt, &Matrix(ref rows): &Matrix,
left_ty: Ty, max_slice_length: usize) -> Vec<Constructor> {
let used_constructors: Vec<Constructor> = rows.iter()
.flat_map(|row| pat_constructors(cx, row[0], left_ty, max_slice_length))
.collect();
all_constructors(cx, left_ty, max_slice_length)
.into_iter()
.find(|c| !used_constructors.contains(c))
.filter(|c| !used_constructors.contains(c)).collect()
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Nit: can you put the .collect() on the next line, to match the formatting above?

}

/// This determines the set of all possible constructors of a pattern matching
Expand Down Expand Up @@ -680,8 +683,8 @@ fn is_useful(cx: &MatchCheckCtxt,

let constructors = pat_constructors(cx, v[0], left_ty, max_slice_length);
if constructors.is_empty() {
match missing_constructor(cx, matrix, left_ty, max_slice_length) {
None => {
match &missing_constructors(cx, matrix, left_ty, max_slice_length)[..] {
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Nit: I think I'd prefer:

let constructors = missing_costructors(...);
if constructors.is_empty() {
    ...
} else {
    ...
}

Slice patterns are kind of unreliable and, besides, it has less rightward drift.

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Well, not unreliable per se, just likely to change.

[] => {
all_constructors(cx, left_ty, max_slice_length).into_iter().map(|c| {
match is_useful_specialized(cx, matrix, v, c.clone(), left_ty, witness) {
UsefulWithWitness(pats) => UsefulWithWitness({
Expand All @@ -701,7 +704,7 @@ fn is_useful(cx: &MatchCheckCtxt,
}).find(|result| result != &NotUseful).unwrap_or(NotUseful)
},

Some(constructor) => {
[constructors..] => {
let matrix = rows.iter().filter_map(|r| {
if pat_is_binding_or_wild(&cx.tcx.def_map.borrow(), raw_pat(r[0])) {
Some(r[1..].to_vec())
Expand All @@ -711,10 +714,11 @@ fn is_useful(cx: &MatchCheckCtxt,
}).collect();
match is_useful(cx, &matrix, &v[1..], witness) {
UsefulWithWitness(pats) => {
let arity = constructor_arity(cx, &constructor, left_ty);
let wild_pats = vec![DUMMY_WILD_PAT; arity];
let enum_pat = construct_witness(cx, &constructor, wild_pats, left_ty);
let mut new_pats = vec![enum_pat];
let mut new_pats: Vec<_> = constructors.into_iter().map(|constructor| {
let arity = constructor_arity(cx, &constructor, left_ty);
let wild_pats = vec![DUMMY_WILD_PAT; arity];
construct_witness(cx, &constructor, wild_pats, left_ty)
}).collect();
new_pats.extend(pats);
UsefulWithWitness(new_pats)
},
Expand Down
2 changes: 1 addition & 1 deletion src/test/compile-fail/non-exhaustive-pattern-witness.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -34,7 +34,7 @@ fn struct_with_a_nested_enum_and_vector() {

fn enum_with_multiple_missing_variants() {
match Color::Red {
//~^ ERROR non-exhaustive patterns: `Red` not covered
//~^ ERROR non-exhaustive patterns: `Red`, `Green` not covered
Color::CustomRGBA { .. } => ()
}
}
Expand Down