Suggest using slice when encountering let x = ""[..];

This commit is contained in:
Esteban Küber 2017-11-24 14:47:40 -08:00
parent 71da1c21eb
commit fa6ae4c828
4 changed files with 107 additions and 0 deletions

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@ -244,6 +244,23 @@ impl Path {
pub fn is_global(&self) -> bool {
!self.segments.is_empty() && self.segments[0].name == keywords::CrateRoot.name()
}
/// Wether this path is any of `::std::ops::{Range, RangeTo, RangeFrom}`.
pub fn is_range(&self) -> bool {
let mut base = ["{{root}}", "std", "ops"].iter().map(|p| p.to_string()).collect::<Vec<_>>();
let range_paths = ["Range", "RangeTo", "RangeFrom"];
let segments = self.segments.iter()
.map(|segment| format!("{}", segment.name))
.collect::<Vec<String>>();
for path in &range_paths {
base.push(path.to_string());
if base == segments {
return true;
}
base.pop();
}
false
}
}
impl fmt::Debug for Path {

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@ -581,6 +581,8 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
trait_ref.self_ty()));
}
self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
// Try to report a help message
if !trait_ref.has_infer_types() &&
self.predicate_can_apply(obligation.param_env, trait_ref) {
@ -821,6 +823,33 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
err.emit();
}
/// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a
/// suggestion to borrow the initializer in order to use have a slice instead.
fn suggest_borrow_on_unsized_slice(&self,
code: &ObligationCauseCode<'tcx>,
err: &mut DiagnosticBuilder<'tcx>) {
if let &ObligationCauseCode::VariableType(node_id) = code {
let parent_node = self.tcx.hir.get_parent_node(node_id);
if let Some(hir::map::NodeLocal(ref local)) = self.tcx.hir.find(parent_node) {
if let Some(ref expr) = local.init {
if let hir::ExprIndex(_, ref index) = expr.node {
if let hir::ExprStruct(hir::QPath::Resolved(None, ref path),
..) = index.node {
if let (Ok(snippet), true) = (
self.tcx.sess.codemap().span_to_snippet(expr.span),
path.is_range()
) {
err.span_suggestion(expr.span,
"consider a slice instead",
format!("&{}", snippet));
}
}
}
}
}
}
}
fn report_arg_count_mismatch(
&self,
span: Span,

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@ -0,0 +1,17 @@
// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
fn main() {
let s = "abc";
let t = if true { s[..2] } else { s };
let u: &str = if true { s[..2] } else { s };
let v = s[..2];
let w: &str = s[..2];
}

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@ -0,0 +1,44 @@
error[E0308]: if and else have incompatible types
--> $DIR/str-array-assignment.rs:13:11
|
13 | let t = if true { s[..2] } else { s };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected str, found &str
|
= note: expected type `str`
found type `&str`
error[E0308]: mismatched types
--> $DIR/str-array-assignment.rs:14:27
|
11 | fn main() {
| - expected `()` because of default return type
...
14 | let u: &str = if true { s[..2] } else { s };
| ^^^^^^ expected &str, found str
|
= note: expected type `&str`
found type `str`
error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
--> $DIR/str-array-assignment.rs:15:7
|
15 | let v = s[..2];
| ^ ------ help: consider a slice instead: `&s[..2]`
| |
| `str` does not have a constant size known at compile-time
|
= help: the trait `std::marker::Sized` is not implemented for `str`
= note: all local variables must have a statically known size
error[E0308]: mismatched types
--> $DIR/str-array-assignment.rs:16:17
|
16 | let w: &str = s[..2];
| ^^^^^^ expected &str, found str
|
= note: expected type `&str`
found type `str`
= help: try with `&s[..2]`
error: aborting due to 4 previous errors