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 2017-01-14 15:25:33 -05:00
parent 0f8a296475
commit b54f593cff
No known key found for this signature in database
GPG Key ID: 16E2FE8B38672F66
4 changed files with 133 additions and 3 deletions

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@ -13,7 +13,9 @@
use super::FnCtxt; use super::FnCtxt;
use hir::def_id::DefId; use hir::def_id::DefId;
use rustc::ty::{Ty, TypeFoldable, PreferMutLvalue, TypeVariants}; use rustc::ty::{Ty, TypeFoldable, PreferMutLvalue, TypeVariants};
use rustc::ty::TypeVariants::{TyStr, TyRef};
use rustc::infer::type_variable::TypeVariableOrigin; use rustc::infer::type_variable::TypeVariableOrigin;
use errors;
use syntax::ast; use syntax::ast;
use syntax::symbol::Symbol; use syntax::symbol::Symbol;
use rustc::hir; use rustc::hir;
@ -237,10 +239,18 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
}; };
if let Some(missing_trait) = missing_trait { if let Some(missing_trait) = missing_trait {
if missing_trait == "std::ops::Add" &&
self.check_str_addition(expr, lhs_expr, lhs_ty,
rhs_expr, rhs_ty_var, &mut err) {
// This has nothing here because it means we did string
// concatenation (e.g. "Hello " + "World!"). This means
// we don't want the span in the else clause to be emmitted
} else {
span_note!(&mut err, lhs_expr.span, span_note!(&mut err, lhs_expr.span,
"an implementation of `{}` might be missing for `{}`", "an implementation of `{}` might be missing for `{}`",
missing_trait, lhs_ty); missing_trait, lhs_ty);
} }
}
err.emit(); err.emit();
} }
} }
@ -254,6 +264,47 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
(rhs_ty_var, return_ty) (rhs_ty_var, return_ty)
} }
fn check_str_addition(&self,
expr: &'gcx hir::Expr,
lhs_expr: &'gcx hir::Expr,
lhs_ty: Ty<'tcx>,
rhs_expr: &'gcx hir::Expr,
rhs_ty_var: Ty<'tcx>,
mut err: &mut errors::DiagnosticBuilder) -> bool {
// If this function returns false it means we use it to make sure we print
// out the an "implementation of span_note!" above where this function is
// called and if true we don't.
let mut is_string_addition = false;
let rhs_ty = self.check_expr_coercable_to_type(rhs_expr, rhs_ty_var);
if let TyRef(_, l_ty) = lhs_ty.sty {
if let TyRef(_, r_ty) = rhs_ty.sty {
if l_ty.ty.sty == TyStr && r_ty.ty.sty == TyStr {
span_note!(&mut err, lhs_expr.span,
"`+` can't be used to concatenate two `&str` strings");
let codemap = self.tcx.sess.codemap();
let suggestion =
match (codemap.span_to_snippet(lhs_expr.span),
codemap.span_to_snippet(rhs_expr.span)) {
(Ok(lstring), Ok(rstring)) =>
format!("{}.to_owned() + {}", lstring, rstring),
_ => format!("<expression>")
};
err.span_suggestion(expr.span,
&format!("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."), suggestion);
is_string_addition = true;
}
}
}
is_string_addition
}
pub fn check_user_unop(&self, pub fn check_user_unop(&self,
op_str: &str, op_str: &str,
mname: &str, mname: &str,

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@ -0,0 +1,28 @@
error[E0369]: binary operation `+` cannot be applied to type `&'static str`
--> src/test/ui/span/issue-39018.rs:2:13
|
2 | let x = "Hello " + "World!";
| ^^^^^^^^
|
note: `+` can't be used to concatenate two `&str` strings
--> src/test/ui/span/issue-39018.rs:2:13
|
2 | let x = "Hello " + "World!";
| ^^^^^^^^
help: to_owned() can be used to create an owned `String` from a string reference. This allows concatenation since the `String` is owned.
| let x = "Hello ".to_owned() + "World!";
error[E0369]: binary operation `+` cannot be applied to type `World`
--> src/test/ui/span/issue-39018.rs:7:13
|
7 | let y = World::Hello + World::Goodbye;
| ^^^^^^^^^^^^
|
note: an implementation of `std::ops::Add` might be missing for `World`
--> src/test/ui/span/issue-39018.rs:7:13
|
7 | let y = World::Hello + World::Goodbye;
| ^^^^^^^^^^^^
error: aborting due to 2 previous errors

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@ -0,0 +1,23 @@
// 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.
pub fn main() {
let x = "Hello " + "World!";
// Make sure that the span outputs a warning
// for not having an implementation for std::ops::Add
// that won't output for the above string concatenation
let y = World::Hello + World::Goodbye;
}
enum World {
Hello,
Goodbye,
}

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@ -0,0 +1,28 @@
error[E0369]: binary operation `+` cannot be applied to type `&'static str`
--> $DIR/issue-39018.rs:12:13
|
12 | let x = "Hello " + "World!";
| ^^^^^^^^
|
note: `+` can't be used to concatenate two `&str` strings
--> $DIR/issue-39018.rs:12:13
|
12 | let x = "Hello " + "World!";
| ^^^^^^^^
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.
| let x = "Hello ".to_owned() + "World!";
error[E0369]: binary operation `+` cannot be applied to type `World`
--> $DIR/issue-39018.rs:17:13
|
17 | let y = World::Hello + World::Goodbye;
| ^^^^^^^^^^^^
|
note: an implementation of `std::ops::Add` might be missing for `World`
--> $DIR/issue-39018.rs:17:13
|
17 | let y = World::Hello + World::Goodbye;
| ^^^^^^^^^^^^
error: aborting due to 2 previous errors