Rollup merge of #92843 - camelid:str-concat-sugg, r=davidtwco

Improve string concatenation suggestion

Before:

    error[E0369]: cannot add `&str` to `&str`
     --> file.rs:2:22
      |
    2 |     let _x = "hello" + " world";
      |              ------- ^ -------- &str
      |              |       |
      |              |       `+` cannot be used to concatenate two `&str` strings
      |              &str
      |
    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
      |
    2 |     let _x = "hello".to_owned() + " world";
      |              ~~~~~~~~~~~~~~~~~~

After:

    error[E0369]: cannot add `&str` to `&str`
     --> file.rs:2:22
      |
    2 |     let _x = "hello" + " world";
      |              ------- ^ -------- &str
      |              |       |
      |              |       `+` cannot be used to concatenate two `&str` strings
      |              &str
      |
      = note: string concatenation requires an owned `String` on the left
    help: create an owned `String` from a string reference
      |
    2 |     let _x = "hello".to_owned() + " world";
      |                     +++++++++++

6 files changed, 81 insertions, 85 deletions

diff --git a/compiler/rustc_typeck/src/check/op.rs b/compiler/rustc_typeck/src/check/op.rs
index c20c457de85..74516acbfcf 100644
--- a/compiler/rustc_typeck/src/check/op.rs
+++ b/compiler/rustc_typeck/src/check/op.rs
@@ -549,16 +549,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         is_assign: IsAssign,
         op: hir::BinOp,
     ) -> bool {
-        let source_map = self.tcx.sess.source_map();
-        let remove_borrow_msg = "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 msg = "`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 str_concat_note = "string concatenation requires an owned `String` on the left";
+        let rm_borrow_msg = "remove the borrow to obtain an owned `String`";
+        let to_owned_msg = "create an owned `String` from a string reference";
 
         let string_type = self.tcx.get_diagnostic_item(sym::String);
         let is_std_string = |ty: Ty<'tcx>| match ty.ty_adt_def() {
@@ -574,31 +567,23 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     ) =>
             {
                 if let IsAssign::No = is_assign { // Do not supply this message if `&str += &str`
-                    err.span_label(
-                        op.span,
-                        "`+` cannot be used to concatenate two `&str` strings",
-                    );
-                    match source_map.span_to_snippet(lhs_expr.span) {
-                        Ok(lstring) => {
-                            err.span_suggestion(
-                                lhs_expr.span,
-                                if lstring.starts_with('&') {
-                                    remove_borrow_msg
-                                } else {
-                                    msg
-                                },
-                                if let Some(stripped) = lstring.strip_prefix('&') {
-                                    // let a = String::new();
-                                    // let _ = &a + "bar";
-                                    stripped.to_string()
-                                } else {
-                                    format!("{}.to_owned()", lstring)
-                                },
-                                Applicability::MachineApplicable,
-                            )
-                        }
-                        _ => err.help(msg),
-                    };
+                    err.span_label(op.span, "`+` cannot be used to concatenate two `&str` strings");
+                    err.note(str_concat_note);
+                    if let hir::ExprKind::AddrOf(_, _, lhs_inner_expr) = lhs_expr.kind {
+                        err.span_suggestion_verbose(
+                            lhs_expr.span.until(lhs_inner_expr.span),
+                            rm_borrow_msg,
+                            "".to_owned(),
+                            Applicability::MachineApplicable
+                        );
+                    } else {
+                        err.span_suggestion_verbose(
+                            lhs_expr.span.shrink_to_hi(),
+                            to_owned_msg,
+                            ".to_owned()".to_owned(),
+                            Applicability::MachineApplicable
+                        );
+                    }
                 }
                 true
             }
@@ -609,32 +594,30 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     op.span,
                     "`+` cannot be used to concatenate a `&str` with a `String`",
                 );
-                match (
-                    source_map.span_to_snippet(lhs_expr.span),
-                    source_map.span_to_snippet(rhs_expr.span),
-                    is_assign,
-                ) {
-                    (Ok(l), Ok(r), IsAssign::No) => {
-                        let to_string = if let Some(stripped) = l.strip_prefix('&') {
-                            // let a = String::new(); let b = String::new();
-                            // let _ = &a + b;
-                            stripped.to_string()
+                match is_assign {
+                    IsAssign::No => {
+                        let sugg_msg;
+                        let lhs_sugg = if let hir::ExprKind::AddrOf(_, _, lhs_inner_expr) = lhs_expr.kind {
+                            sugg_msg = "remove the borrow on the left and add one on the right";
+                            (lhs_expr.span.until(lhs_inner_expr.span), "".to_owned())
                         } else {
-                            format!("{}.to_owned()", l)
+                            sugg_msg = "create an owned `String` on the left and add a borrow on the right";
+                            (lhs_expr.span.shrink_to_hi(), ".to_owned()".to_owned())
                         };
-                        err.multipart_suggestion(
-                            msg,
-                            vec![
-                                (lhs_expr.span, to_string),
-                                (rhs_expr.span, format!("&{}", r)),
-                            ],
+                        let suggestions = vec![
+                            lhs_sugg,
+                            (rhs_expr.span.shrink_to_lo(), "&".to_owned()),
+                        ];
+                        err.multipart_suggestion_verbose(
+                            sugg_msg,
+                            suggestions,
                             Applicability::MachineApplicable,
                         );
                     }
-                    _ => {
-                        err.help(msg);
+                    IsAssign::Yes => {
+                        err.note(str_concat_note);
                     }
-                };
+                }
                 true
             }
             _ => false,
diff --git a/src/test/ui/issues/issue-47377.stderr b/src/test/ui/issues/issue-47377.stderr
index a7b8b1ca861..4f0fd948e76 100644
--- a/src/test/ui/issues/issue-47377.stderr
+++ b/src/test/ui/issues/issue-47377.stderr
@@ -7,10 +7,11 @@ LL |      let _a = b + ", World!";
    |               | `+` cannot be used to concatenate two `&str` strings
    |               &str
    |
-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
+   = note: string concatenation requires an owned `String` on the left
+help: create an owned `String` from a string reference
    |
 LL |      let _a = b.to_owned() + ", World!";
-   |               ~~~~~~~~~~~~
+   |                +++++++++++
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/issues/issue-47380.stderr b/src/test/ui/issues/issue-47380.stderr
index f6222c77e2e..b04ac5536c4 100644
--- a/src/test/ui/issues/issue-47380.stderr
+++ b/src/test/ui/issues/issue-47380.stderr
@@ -7,10 +7,11 @@ LL |     println!("🦀🦀🦀🦀🦀"); let _a = b + ", World!";
    |                                      | `+` cannot be used to concatenate two `&str` strings
    |                                      &str
    |
-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
+   = note: string concatenation requires an owned `String` on the left
+help: create an owned `String` from a string reference
    |
 LL |     println!("🦀🦀🦀🦀🦀"); let _a = b.to_owned() + ", World!";
-   |                                      ~~~~~~~~~~~~
+   |                                       +++++++++++
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/span/issue-39018.stderr b/src/test/ui/span/issue-39018.stderr
index 92e86bf5d6c..e2e7ce1ed18 100644
--- a/src/test/ui/span/issue-39018.stderr
+++ b/src/test/ui/span/issue-39018.stderr
@@ -7,10 +7,11 @@ LL |     let x = "Hello " + "World!";
    |             |        `+` cannot be used to concatenate two `&str` strings
    |             &str
    |
-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
+   = note: string concatenation requires an owned `String` on the left
+help: create an owned `String` from a string reference
    |
 LL |     let x = "Hello ".to_owned() + "World!";
-   |             ~~~~~~~~~~~~~~~~~~~
+   |                     +++++++++++
 
 error[E0369]: cannot add `World` to `World`
   --> $DIR/issue-39018.rs:8:26
@@ -46,10 +47,10 @@ LL |     let x = "Hello " + "World!".to_owned();
    |             |        `+` cannot be used to concatenate a `&str` with a `String`
    |             &str
    |
-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
+help: create an owned `String` on the left and add a borrow on the right
    |
 LL |     let x = "Hello ".to_owned() + &"World!".to_owned();
-   |             ~~~~~~~~~~~~~~~~~~~   ~~~~~~~~~~~~~~~~~~~~
+   |                     +++++++++++   +
 
 error[E0369]: cannot add `&String` to `&String`
   --> $DIR/issue-39018.rs:26:16
@@ -60,10 +61,12 @@ LL |     let _ = &a + &b;
    |             |  `+` cannot be used to concatenate two `&str` strings
    |             &String
    |
-help: 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
+   = note: string concatenation requires an owned `String` on the left
+help: remove the borrow to obtain an owned `String`
    |
-LL |     let _ = a + &b;
-   |             ~
+LL -     let _ = &a + &b;
+LL +     let _ = a + &b;
+   | 
 
 error[E0369]: cannot add `String` to `&String`
   --> $DIR/issue-39018.rs:27:16
@@ -74,10 +77,11 @@ LL |     let _ = &a + b;
    |             |  `+` cannot be used to concatenate a `&str` with a `String`
    |             &String
    |
-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
+help: remove the borrow on the left and add one on the right
    |
-LL |     let _ = a + &b;
-   |             ~   ~~
+LL -     let _ = &a + b;
+LL +     let _ = a + &b;
+   | 
 
 error[E0308]: mismatched types
   --> $DIR/issue-39018.rs:29:17
@@ -97,10 +101,10 @@ LL |     let _ = e + b;
    |             | `+` cannot be used to concatenate a `&str` with a `String`
    |             &String
    |
-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
+help: create an owned `String` on the left and add a borrow on the right
    |
 LL |     let _ = e.to_owned() + &b;
-   |             ~~~~~~~~~~~~   ~~
+   |              +++++++++++   +
 
 error[E0369]: cannot add `&String` to `&String`
   --> $DIR/issue-39018.rs:31:15
@@ -111,10 +115,11 @@ LL |     let _ = e + &b;
    |             | `+` cannot be used to concatenate two `&str` strings
    |             &String
    |
-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
+   = note: string concatenation requires an owned `String` on the left
+help: create an owned `String` from a string reference
    |
 LL |     let _ = e.to_owned() + &b;
-   |             ~~~~~~~~~~~~
+   |              +++++++++++
 
 error[E0369]: cannot add `&str` to `&String`
   --> $DIR/issue-39018.rs:32:15
@@ -125,10 +130,11 @@ LL |     let _ = e + d;
    |             | `+` cannot be used to concatenate two `&str` strings
    |             &String
    |
-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
+   = note: string concatenation requires an owned `String` on the left
+help: create an owned `String` from a string reference
    |
 LL |     let _ = e.to_owned() + d;
-   |             ~~~~~~~~~~~~
+   |              +++++++++++
 
 error[E0369]: cannot add `&&str` to `&String`
   --> $DIR/issue-39018.rs:33:15
@@ -139,10 +145,11 @@ LL |     let _ = e + &d;
    |             | `+` cannot be used to concatenate two `&str` strings
    |             &String
    |
-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
+   = note: string concatenation requires an owned `String` on the left
+help: create an owned `String` from a string reference
    |
 LL |     let _ = e.to_owned() + &d;
-   |             ~~~~~~~~~~~~
+   |              +++++++++++
 
 error[E0369]: cannot add `&&str` to `&&str`
   --> $DIR/issue-39018.rs:34:16
@@ -169,10 +176,11 @@ LL |     let _ = c + &d;
    |             | `+` cannot be used to concatenate two `&str` strings
    |             &str
    |
-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
+   = note: string concatenation requires an owned `String` on the left
+help: create an owned `String` from a string reference
    |
 LL |     let _ = c.to_owned() + &d;
-   |             ~~~~~~~~~~~~
+   |              +++++++++++
 
 error[E0369]: cannot add `&str` to `&str`
   --> $DIR/issue-39018.rs:37:15
@@ -183,10 +191,11 @@ LL |     let _ = c + d;
    |             | `+` cannot be used to concatenate two `&str` strings
    |             &str
    |
-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
+   = note: string concatenation requires an owned `String` on the left
+help: create an owned `String` from a string reference
    |
 LL |     let _ = c.to_owned() + d;
-   |             ~~~~~~~~~~~~
+   |              +++++++++++
 
 error: aborting due to 14 previous errors
 
diff --git a/src/test/ui/str/str-concat-on-double-ref.stderr b/src/test/ui/str/str-concat-on-double-ref.stderr
index dee28897f4d..bd354679f78 100644
--- a/src/test/ui/str/str-concat-on-double-ref.stderr
+++ b/src/test/ui/str/str-concat-on-double-ref.stderr
@@ -7,10 +7,11 @@ LL |     let c = a + b;
    |             | `+` cannot be used to concatenate two `&str` strings
    |             &String
    |
-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
+   = note: string concatenation requires an owned `String` on the left
+help: create an owned `String` from a string reference
    |
 LL |     let c = a.to_owned() + b;
-   |             ~~~~~~~~~~~~
+   |              +++++++++++
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/terminal-width/non-1-width-unicode-multiline-label.stderr b/src/test/ui/terminal-width/non-1-width-unicode-multiline-label.stderr
index 480f442fedd..bf277362dba 100644
--- a/src/test/ui/terminal-width/non-1-width-unicode-multiline-label.stderr
+++ b/src/test/ui/terminal-width/non-1-width-unicode-multiline-label.stderr
@@ -7,10 +7,11 @@ LL | ...ཽཾཿ྄ཱྀྀྂྃ྅྆྇ྈྉྊྋྌྍྎྏྐྑྒྒྷྔ
    |                                                  |              `+` cannot be used to concatenate two `&str` strings
    |                                                  &str
    |
-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
+   = note: string concatenation requires an owned `String` on the left
+help: create an owned `String` from a string reference
    |
 LL |     let _ = "ༀ༁༂༃༄༅༆༇༈༉༊་༌།༎༏༐༑༒༓༔༕༖༗༘༙༚༛༜༝༞༟༠༡༢༣༤༥༦༧༨༩༪༫༬༭༮༯༰༱༲༳༴༵༶༷༸༹༺༻༼༽༾༿ཀཁགགྷངཅཆཇ཈ཉཊཋཌཌྷཎཏཐདདྷནཔཕབབྷམཙཚཛཛྷཝཞཟའཡརལཤཥསཧཨཀྵཪཫཬ཭཮཯཰ཱཱཱིིུུྲྀཷླྀཹེཻོཽཾཿ྄ཱྀྀྂྃ྅྆྇ྈྉྊྋྌྍྎྏྐྑྒྒྷྔྕྖྗ྘ྙྚྛྜྜྷྞྟྠྡྡྷྣྤྥྦྦྷྨྩྪྫྫྷྭྮྯྰྱྲླྴྵྶྷྸྐྵྺྻྼ྽྾྿࿀࿁࿂࿃࿄࿅࿆࿇࿈࿉࿊࿋࿌࿍࿎࿏࿐࿑࿒࿓࿔࿕࿖࿗࿘࿙࿚"; let _a = unicode_is_fun.to_owned() + " really fun!";
-   |                                                                                                                                                                           ~~~~~~~~~~~~~~~~~~~~~~~~~
+   |                                                                                                                                                                                         +++++++++++
 
 error: aborting due to previous error