From ac3290e8d949448193014c85096f8772db331d46 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Esteban=20K=C3=BCber?= <esteban@kuber.com.ar>
Date: Sun, 17 Mar 2019 22:42:51 -0700
Subject: [PATCH] Add suggestion to use `&*var` when `&str: From<String>` is
 expected

---
 .../src/language-features/on-unimplemented.md   | 12 ++++++++++++
 src/libcore/convert.rs                          |  6 ++++++
 src/test/ui/suggestions/into-str.rs             |  6 ++++++
 src/test/ui/suggestions/into-str.stderr         | 17 +++++++++++++++++
 4 files changed, 41 insertions(+)
 create mode 100644 src/test/ui/suggestions/into-str.rs
 create mode 100644 src/test/ui/suggestions/into-str.stderr

diff --git a/src/doc/unstable-book/src/language-features/on-unimplemented.md b/src/doc/unstable-book/src/language-features/on-unimplemented.md
index f787f629756f..b156038fba06 100644
--- a/src/doc/unstable-book/src/language-features/on-unimplemented.md
+++ b/src/doc/unstable-book/src/language-features/on-unimplemented.md
@@ -138,3 +138,15 @@ error[E0277]: `&str` is not an iterator
   = help: the trait `std::iter::Iterator` is not implemented for `&str`
   = note: required by `std::iter::IntoIterator::into_iter`
 ```
+
+If you need to filter on multiple attributes, you can use `all` in the following way:
+
+```rust,compile_fail
+#[rustc_on_unimplemented(
+    on(
+        all(_Self="&str", T="std::string::String"),
+        note="you can coerce a `{T}` into a `{Self}` by writing `&*variable`"
+    )
+)]
+pub trait From<T>: Sized { /* ... */ }
+```
\ No newline at end of file
diff --git a/src/libcore/convert.rs b/src/libcore/convert.rs
index 774d648558b4..cb574a53dbcc 100644
--- a/src/libcore/convert.rs
+++ b/src/libcore/convert.rs
@@ -352,6 +352,12 @@ pub trait Into<T>: Sized {
 /// [`from`]: trait.From.html#tymethod.from
 /// [book]: ../../book/ch09-00-error-handling.html
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_on_unimplemented(
+    on(
+        all(_Self="&str", T="std::string::String"),
+        note="you can coerce a `{T}` into a `{Self}` by writing `&*variable`"
+    )
+)]
 pub trait From<T>: Sized {
     /// Performs the conversion.
     #[stable(feature = "rust1", since = "1.0.0")]
diff --git a/src/test/ui/suggestions/into-str.rs b/src/test/ui/suggestions/into-str.rs
new file mode 100644
index 000000000000..9793ee801d18
--- /dev/null
+++ b/src/test/ui/suggestions/into-str.rs
@@ -0,0 +1,6 @@
+fn foo<'a, T>(_t: T) where T: Into<&'a str> {}
+
+fn main() {
+    foo(String::new());
+    //~^ ERROR the trait bound `&str: std::convert::From<std::string::String>` is not satisfied
+}
diff --git a/src/test/ui/suggestions/into-str.stderr b/src/test/ui/suggestions/into-str.stderr
new file mode 100644
index 000000000000..f341d3637021
--- /dev/null
+++ b/src/test/ui/suggestions/into-str.stderr
@@ -0,0 +1,17 @@
+error[E0277]: the trait bound `&str: std::convert::From<std::string::String>` is not satisfied
+  --> $DIR/into-str.rs:4:5
+   |
+LL |     foo(String::new());
+   |     ^^^ the trait `std::convert::From<std::string::String>` is not implemented for `&str`
+   |
+   = note: you can coerce a `std::string::String` into a `&str` by writing `&*variable`
+   = note: required because of the requirements on the impl of `std::convert::Into<&str>` for `std::string::String`
+note: required by `foo`
+  --> $DIR/into-str.rs:1:1
+   |
+LL | fn foo<'a, T>(_t: T) where T: Into<&'a str> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.