Add clearer error message using &str + &str
This is the first part of #39018. One of the common things for new users coming from more dynamic languages like JavaScript, Python or Ruby is to use `+` to concatenate strings. However, this doesn't work that way in Rust unless the first type is a `String`. This commit adds a check for this use case and outputs a new error as well as a suggestion to guide the user towards the desired behavior. It also adds a new test case to test the output of the error.
This commit is contained in:
Michael Gattozzi
parent
0f8a296475
commit
b54f593cff
@ -13,7 +13,9 @@
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use super::FnCtxt;
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use hir::def_id::DefId;
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use rustc::ty::{Ty, TypeFoldable, PreferMutLvalue, TypeVariants};
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use rustc::ty::TypeVariants::{TyStr, TyRef};
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use rustc::infer::type_variable::TypeVariableOrigin;
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use errors;
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use syntax::ast;
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use syntax::symbol::Symbol;
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use rustc::hir;
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@ -237,9 +239,17 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
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};
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if let Some(missing_trait) = missing_trait {
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span_note!(&mut err, lhs_expr.span,
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"an implementation of `{}` might be missing for `{}`",
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missing_trait, lhs_ty);
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if missing_trait == "std::ops::Add" &&
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self.check_str_addition(expr, lhs_expr, lhs_ty,
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rhs_expr, rhs_ty_var, &mut err) {
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// This has nothing here because it means we did string
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// concatenation (e.g. "Hello " + "World!"). This means
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// we don't want the span in the else clause to be emmitted
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} else {
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span_note!(&mut err, lhs_expr.span,
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"an implementation of `{}` might be missing for `{}`",
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missing_trait, lhs_ty);
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}
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}
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err.emit();
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}
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@ -254,6 +264,47 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
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(rhs_ty_var, return_ty)
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}
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fn check_str_addition(&self,
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expr: &'gcx hir::Expr,
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lhs_expr: &'gcx hir::Expr,
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lhs_ty: Ty<'tcx>,
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rhs_expr: &'gcx hir::Expr,
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rhs_ty_var: Ty<'tcx>,
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mut err: &mut errors::DiagnosticBuilder) -> bool {
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// If this function returns false it means we use it to make sure we print
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// out the an "implementation of span_note!" above where this function is
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// called and if true we don't.
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let mut is_string_addition = false;
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let rhs_ty = self.check_expr_coercable_to_type(rhs_expr, rhs_ty_var);
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if let TyRef(_, l_ty) = lhs_ty.sty {
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if let TyRef(_, r_ty) = rhs_ty.sty {
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if l_ty.ty.sty == TyStr && r_ty.ty.sty == TyStr {
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span_note!(&mut err, lhs_expr.span,
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"`+` can't be used to concatenate two `&str` strings");
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let codemap = self.tcx.sess.codemap();
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let suggestion =
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match (codemap.span_to_snippet(lhs_expr.span),
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codemap.span_to_snippet(rhs_expr.span)) {
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(Ok(lstring), Ok(rstring)) =>
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format!("{}.to_owned() + {}", lstring, rstring),
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_ => format!("<expression>")
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};
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err.span_suggestion(expr.span,
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&format!("to_owned() can be used to create an owned `String` \
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from a string reference. String concatenation \
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appends the string on the right to the string \
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on the left and may require reallocation. This \
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requires ownership of the string on the left."), suggestion);
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is_string_addition = true;
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}
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}
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}
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is_string_addition
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}
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pub fn check_user_unop(&self,
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op_str: &str,
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mname: &str,
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28
src/test/parse-fail/issue-39018.stderr
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28
src/test/parse-fail/issue-39018.stderr
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@ -0,0 +1,28 @@
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error[E0369]: binary operation `+` cannot be applied to type `&'static str`
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--> src/test/ui/span/issue-39018.rs:2:13
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2 | let x = "Hello " + "World!";
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| ^^^^^^^^
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note: `+` can't be used to concatenate two `&str` strings
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--> src/test/ui/span/issue-39018.rs:2:13
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2 | let x = "Hello " + "World!";
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| ^^^^^^^^
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help: to_owned() can be used to create an owned `String` from a string reference. This allows concatenation since the `String` is owned.
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| let x = "Hello ".to_owned() + "World!";
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error[E0369]: binary operation `+` cannot be applied to type `World`
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--> src/test/ui/span/issue-39018.rs:7:13
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7 | let y = World::Hello + World::Goodbye;
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| ^^^^^^^^^^^^
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note: an implementation of `std::ops::Add` might be missing for `World`
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--> src/test/ui/span/issue-39018.rs:7:13
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7 | let y = World::Hello + World::Goodbye;
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| ^^^^^^^^^^^^
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error: aborting due to 2 previous errors
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23
src/test/ui/span/issue-39018.rs
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23
src/test/ui/span/issue-39018.rs
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@ -0,0 +1,23 @@
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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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pub fn main() {
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let x = "Hello " + "World!";
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// Make sure that the span outputs a warning
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// for not having an implementation for std::ops::Add
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// that won't output for the above string concatenation
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let y = World::Hello + World::Goodbye;
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}
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enum World {
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Hello,
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Goodbye,
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}
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28
src/test/ui/span/issue-39018.stderr
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28
src/test/ui/span/issue-39018.stderr
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@ -0,0 +1,28 @@
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error[E0369]: binary operation `+` cannot be applied to type `&'static str`
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--> $DIR/issue-39018.rs:12:13
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12 | let x = "Hello " + "World!";
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| ^^^^^^^^
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note: `+` can't be used to concatenate two `&str` strings
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--> $DIR/issue-39018.rs:12:13
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12 | let x = "Hello " + "World!";
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| ^^^^^^^^
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help: to_owned() can be used to create an owned `String` from a string reference. String concatenation appends the string on the right to the string on the left and may require reallocation. This requires ownership of the string on the left.
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| let x = "Hello ".to_owned() + "World!";
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error[E0369]: binary operation `+` cannot be applied to type `World`
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--> $DIR/issue-39018.rs:17:13
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17 | let y = World::Hello + World::Goodbye;
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| ^^^^^^^^^^^^
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note: an implementation of `std::ops::Add` might be missing for `World`
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--> $DIR/issue-39018.rs:17:13
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17 | let y = World::Hello + World::Goodbye;
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| ^^^^^^^^^^^^
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error: aborting due to 2 previous errors
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