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Merged
swernli merged 2 commits into from
Feb 2, 2026
+72 −16
Merged

Better feedback on incorrect syntax for qubit allocation #2897

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16855e1
Better feedback on incorrect syntax for qubit allocation
swernli Jan 27, 2026
93e1659
Merge remote-tracking branch 'origin/main' into swernli/qubit-use-gui…
swernli Jan 29, 2026
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9 changes: 9 additions & 0 deletions source/compiler/qsc_frontend/src/resolve.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -213,6 +213,13 @@ pub(super) enum Error {
#[diagnostic(code("Qsc.Resolve.NotFound"))]
NotAvailable(String, String, #[label] Span),

#[error("`Qubit` not found")]
#[diagnostic(help(
"to allocate qubits, use syntax like `use q = Qubit();` or `use qs = Qubit[N];` or `use (q1, q2) = (Qubit(), Qubit());`"
))]
#[diagnostic(code("Qsc.Resolve.NotFoundQubit"))]
NotFoundQubit(#[label] Span),

#[error("use of unimplemented item `{0}`")]
#[diagnostic(help("this item is not implemented and cannot be used"))]
#[diagnostic(code("Qsc.Resolve.Unimplemented"))]
Expand Down Expand Up @@ -772,6 +779,8 @@ impl Resolver {
format!("{}.{}", dropped_name.namespace, dropped_name.name),
span,
))
} else if name == "Qubit" {
Err(Error::NotFoundQubit(span))
} else {
Err(Error::NotFound(name, span))
}
Expand Down
44 changes: 44 additions & 0 deletions source/compiler/qsc_frontend/src/resolve/tests.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2341,6 +2341,50 @@ fn resolve_local_generic() {
);
}

#[test]
fn incorrect_single_qubit_allocation_syntax_gets_extra_help() {
check(
indoc! {"
namespace A {
operation B() : Unit {
let q = Qubit();
}
}
"},
&expect![[r#"
namespace namespace3 {
operation package2_item1() : Unit {
let local13 = Qubit();
}
}

// NotFoundQubit(Span { lo: 57, hi: 62 })
"#]],
);
}

#[test]
fn incorrect_qubit_array_allocation_syntax_gets_extra_help() {
check(
indoc! {"
namespace A {
operation B() : Unit {
let q = Qubit[5];
}
}
"},
&expect![[r#"
namespace namespace3 {
operation package2_item1() : Unit {
let local13 = Qubit[5];
}
}

// NotFoundQubit(Span { lo: 57, hi: 62 })
"#]],
);
}

#[test]
fn dropped_base_callable_from_unrestricted() {
check_with_capabilities(
Expand Down
32 changes: 16 additions & 16 deletions source/compiler/qsc_parse/src/stmt.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -154,6 +154,7 @@ fn parse_qubit(s: &mut ParserContext) -> Result<Box<StmtKind>> {
}

fn parse_qubit_init(s: &mut ParserContext) -> Result<Box<QubitInit>> {
let help_text = "to allocate qubits, use syntax like `use q = Qubit();` or `use qs = Qubit[N];` or `use (q1, q2) = (Qubit(), Qubit());`";
let lo = s.peek().span.lo;
s.expect(WordKinds::Qubit);
let kind = if let Ok(name) = ident(s) {
Expand All @@ -162,33 +163,32 @@ fn parse_qubit_init(s: &mut ParserContext) -> Result<Box<QubitInit>> {
"qubit initializer",
"identifier",
name.span,
)));
))
.with_help(help_text));
} else if token(s, TokenKind::Open(Delim::Paren)).is_ok() {
token(s, TokenKind::Close(Delim::Paren))?;
token(s, TokenKind::Close(Delim::Paren)).map_err(|e| e.with_help(help_text))?;
QubitInitKind::Single
} else if token(s, TokenKind::Open(Delim::Bracket)).is_ok() {
let size = expr(s)?;
token(s, TokenKind::Close(Delim::Bracket))?;
let size = expr(s).map_err(|e| e.with_help(help_text))?;
token(s, TokenKind::Close(Delim::Bracket)).map_err(|e| e.with_help(help_text))?;
QubitInitKind::Array(size)
} else {
let token = s.peek();
return Err(Error::new(ErrorKind::Rule(
"qubit suffix",
token.kind,
token.span,
)));
return Err(
Error::new(ErrorKind::Rule("qubit suffix", token.kind, token.span))
.with_help(help_text),
);
}
} else if token(s, TokenKind::Open(Delim::Paren)).is_ok() {
let (inits, final_sep) = seq(s, parse_qubit_init)?;
token(s, TokenKind::Close(Delim::Paren))?;
let (inits, final_sep) = seq(s, parse_qubit_init).map_err(|e| e.with_help(help_text))?;
token(s, TokenKind::Close(Delim::Paren)).map_err(|e| e.with_help(help_text))?;
final_sep.reify(inits, QubitInitKind::Paren, QubitInitKind::Tuple)
} else {
let token = s.peek();
return Err(Error::new(ErrorKind::Rule(
"qubit initializer",
token.kind,
token.span,
)));
return Err(
Error::new(ErrorKind::Rule("qubit initializer", token.kind, token.span))
.with_help(help_text),
);
};

Ok(Box::new(QubitInit {
Expand Down
3 changes: 3 additions & 0 deletions source/compiler/qsc_parse/src/stmt/tests.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -144,6 +144,9 @@ fn use_invalid_init() {
hi: 14,
},
),
Some(
"to allocate qubits, use syntax like `use q = Qubit();` or `use qs = Qubit[N];` or `use (q1, q2) = (Qubit(), Qubit());`",
),
)
"#]],
);
Expand Down
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