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Suggest using slice when encountering let x = ""[..];
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@ -244,6 +244,23 @@ impl Path {
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pub fn is_global(&self) -> bool {
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!self.segments.is_empty() && self.segments[0].name == keywords::CrateRoot.name()
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}
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/// Wether this path is any of `::std::ops::{Range, RangeTo, RangeFrom}`.
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pub fn is_range(&self) -> bool {
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let mut base = ["{{root}}", "std", "ops"].iter().map(|p| p.to_string()).collect::<Vec<_>>();
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let range_paths = ["Range", "RangeTo", "RangeFrom"];
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let segments = self.segments.iter()
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.map(|segment| format!("{}", segment.name))
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.collect::<Vec<String>>();
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for path in &range_paths {
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base.push(path.to_string());
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if base == segments {
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return true;
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}
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base.pop();
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}
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false
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}
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}
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impl fmt::Debug for Path {
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@ -581,6 +581,8 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
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trait_ref.self_ty()));
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}
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self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
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// Try to report a help message
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if !trait_ref.has_infer_types() &&
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self.predicate_can_apply(obligation.param_env, trait_ref) {
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@ -821,6 +823,33 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
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err.emit();
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}
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/// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a
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/// suggestion to borrow the initializer in order to use have a slice instead.
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fn suggest_borrow_on_unsized_slice(&self,
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code: &ObligationCauseCode<'tcx>,
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err: &mut DiagnosticBuilder<'tcx>) {
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if let &ObligationCauseCode::VariableType(node_id) = code {
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let parent_node = self.tcx.hir.get_parent_node(node_id);
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if let Some(hir::map::NodeLocal(ref local)) = self.tcx.hir.find(parent_node) {
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if let Some(ref expr) = local.init {
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if let hir::ExprIndex(_, ref index) = expr.node {
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if let hir::ExprStruct(hir::QPath::Resolved(None, ref path),
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..) = index.node {
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if let (Ok(snippet), true) = (
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self.tcx.sess.codemap().span_to_snippet(expr.span),
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path.is_range()
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) {
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err.span_suggestion(expr.span,
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"consider a slice instead",
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format!("&{}", snippet));
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}
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}
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}
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}
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}
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}
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}
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fn report_arg_count_mismatch(
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&self,
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span: Span,
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17
src/test/ui/suggestions/str-array-assignment.rs
Normal file
17
src/test/ui/suggestions/str-array-assignment.rs
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@ -0,0 +1,17 @@
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// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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fn main() {
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let s = "abc";
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let t = if true { s[..2] } else { s };
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let u: &str = if true { s[..2] } else { s };
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let v = s[..2];
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let w: &str = s[..2];
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}
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44
src/test/ui/suggestions/str-array-assignment.stderr
Normal file
44
src/test/ui/suggestions/str-array-assignment.stderr
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@ -0,0 +1,44 @@
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error[E0308]: if and else have incompatible types
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--> $DIR/str-array-assignment.rs:13:11
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13 | let t = if true { s[..2] } else { s };
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected str, found &str
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= note: expected type `str`
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found type `&str`
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error[E0308]: mismatched types
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--> $DIR/str-array-assignment.rs:14:27
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11 | fn main() {
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| - expected `()` because of default return type
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...
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14 | let u: &str = if true { s[..2] } else { s };
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| ^^^^^^ expected &str, found str
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= note: expected type `&str`
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found type `str`
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error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
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--> $DIR/str-array-assignment.rs:15:7
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15 | let v = s[..2];
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| ^ ------ help: consider a slice instead: `&s[..2]`
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| |
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| `str` does not have a constant size known at compile-time
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= help: the trait `std::marker::Sized` is not implemented for `str`
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= note: all local variables must have a statically known size
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error[E0308]: mismatched types
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--> $DIR/str-array-assignment.rs:16:17
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16 | let w: &str = s[..2];
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| ^^^^^^ expected &str, found str
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= note: expected type `&str`
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found type `str`
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= help: try with `&s[..2]`
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error: aborting due to 4 previous errors
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