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Improve compiler error message for wrong generic parameter order #72271

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Jun 23, 2020
Merged

Improve compiler error message for wrong generic parameter order #72271

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73 changes: 67 additions & 6 deletions src/librustc_typeck/astconv.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,8 +8,7 @@
use crate::collect::PlaceholderHirTyCollector;
use crate::middle::resolve_lifetime as rl;
use crate::require_c_abi_if_c_variadic;
use rustc_ast::ast::ParamKindOrd;
use rustc_ast::util::lev_distance::find_best_match_for_name;
use rustc_ast::{ast::ParamKindOrd, util::lev_distance::find_best_match_for_name};
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_errors::ErrorReported;
use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticId, FatalError};
Expand All @@ -27,7 +26,7 @@ use rustc_middle::ty::{GenericParamDef, GenericParamDefKind};
use rustc_session::lint::builtin::{AMBIGUOUS_ASSOCIATED_ITEMS, LATE_BOUND_LIFETIME_ARGUMENTS};
use rustc_session::parse::feature_err;
use rustc_session::Session;
use rustc_span::symbol::{sym, Ident, Symbol};
use rustc_span::symbol::{kw, sym, Ident, Symbol};
use rustc_span::{MultiSpan, Span, DUMMY_SP};
use rustc_target::spec::abi;
use rustc_trait_selection::traits;
Expand Down Expand Up @@ -475,7 +474,12 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {

/// Report an error that a generic argument did not match the generic parameter that was
/// expected.
fn generic_arg_mismatch_err(sess: &Session, arg: &GenericArg<'_>, kind: &'static str) {
fn generic_arg_mismatch_err(
sess: &Session,
arg: &GenericArg<'_>,
kind: &'static str,
help: Option<&str>,
) {
let mut err = struct_span_err!(
sess,
arg.span(),
Expand Down Expand Up @@ -503,6 +507,10 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
let (first, last) =
if kind_ord < arg_ord { (kind, arg.descr()) } else { (arg.descr(), kind) };
err.note(&format!("{} arguments must be provided before {} arguments", first, last));

if let Some(help) = help {
err.help(help);
}
err.emit();
}

Expand Down Expand Up @@ -648,7 +656,60 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
if arg_count.correct.is_ok()
&& arg_count.explicit_late_bound == ExplicitLateBound::No
{
Self::generic_arg_mismatch_err(tcx.sess, arg, kind.descr());
// We're going to iterate over the parameters to sort them out, and
// show that order to the user as a possible order for the parameters
let mut param_types_present = defs
.params
.clone()
.into_iter()
.map(|param| {
(
match param.kind {
GenericParamDefKind::Lifetime => {
ParamKindOrd::Lifetime
}
GenericParamDefKind::Type { .. } => {
ParamKindOrd::Type
}
GenericParamDefKind::Const => {
ParamKindOrd::Const
}
},
param,
)
})
.collect::<Vec<(ParamKindOrd, GenericParamDef)>>();
param_types_present.sort_by_key(|(ord, _)| *ord);
let (mut param_types_present, ordered_params): (
Vec<ParamKindOrd>,
Vec<GenericParamDef>,
) = param_types_present.into_iter().unzip();
param_types_present.dedup();

Self::generic_arg_mismatch_err(
tcx.sess,
arg,
kind.descr(),
Some(&format!(
"reorder the arguments: {}: `<{}>`",
param_types_present
.into_iter()
.map(|ord| format!("{}s", ord.to_string()))
.collect::<Vec<String>>()
.join(", then "),
ordered_params
.into_iter()
.filter_map(|param| {
if param.name == kw::SelfUpper {
None
} else {
Some(param.name.to_string())
}
})
.collect::<Vec<String>>()
.join(", ")
)),
);
}

// We've reported the error, but we want to make sure that this
Expand Down Expand Up @@ -680,7 +741,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
assert_eq!(kind, "lifetime");
let provided =
force_infer_lt.expect("lifetimes ought to have been inferred");
Self::generic_arg_mismatch_err(tcx.sess, provided, kind);
Self::generic_arg_mismatch_err(tcx.sess, provided, kind, None);
rakshith-ravi marked this conversation as resolved.
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}

break;
Expand Down
1 change: 1 addition & 0 deletions src/test/ui/const-generics/const-arg-type-arg-misordered.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,7 @@ LL | fn foo<const N: usize>() -> Array<N, ()> {
| ^
|
= note: type arguments must be provided before constant arguments
= help: reorder the arguments: types, then consts: `<T, N>`

error: aborting due to previous error; 1 warning emitted

Expand Down
2 changes: 2 additions & 0 deletions src/test/ui/suggestions/suggest-move-types.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -125,6 +125,7 @@ LL | struct Cl<'a, 'b, 'c, T, U, V, M: ThreeWithLifetime<T, 'a, A=(), B=(), C=()
| ^^
|
= note: lifetime arguments must be provided before type arguments
= help: reorder the arguments: lifetimes, then types: `<'a, 'b, 'c, T, U, V>`

error[E0747]: lifetime provided when a type was expected
--> $DIR/suggest-move-types.rs:82:56
Expand All @@ -133,6 +134,7 @@ LL | struct Dl<'a, 'b, 'c, T, U, V, M: ThreeWithLifetime<T, 'a, A=(), B=(), U, '
| ^^
|
= note: lifetime arguments must be provided before type arguments
= help: reorder the arguments: lifetimes, then types: `<'a, 'b, 'c, T, U, V>`

error: aborting due to 12 previous errors

Expand Down