Auto merge of #105805 - yanchen4791:issue-105227-fix, r=estebank

Suggest adding named lifetime when the return contains value borrowed from more than one lifetimes of function inputs

fix for #105227.

The problem: The suggestion of adding an explicit `'_` lifetime bound is **incorrect** when the function's return type contains a value which could be borrowed from more than one lifetimes of the function's inputs. Instead, a named lifetime parameter can be introduced in such a case.

The solution: Checking the number of elided lifetimes in the function signature. If more than one lifetimes found in the function inputs when the suggestion of adding explicit `'_` lifetime, change it to using named lifetime parameter `'a` instead.

src/test/ui/impl-trait/static-return-lifetime-infered.rs

@@ -6,12 +6,10 @@ impl A {
     fn iter_values_anon(&self) -> impl Iterator<Item=u32> {
         self.x.iter().map(|a| a.0)
         //~^ ERROR: captures lifetime that does not appear in bounds
-        //~| ERROR: captures lifetime that does not appear in bounds
     }
     fn iter_values<'a>(&'a self) -> impl Iterator<Item=u32> {
         self.x.iter().map(|a| a.0)
         //~^ ERROR: captures lifetime that does not appear in bounds
-        //~| ERROR: captures lifetime that does not appear in bounds
     }
 }
 

src/test/ui/impl-trait/static-return-lifetime-infered.stderr

@@ -12,23 +12,10 @@ LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + '_ {
    |                                                           ++++
 
 error[E0700]: hidden type for `impl Iterator<Item = u32>` captures lifetime that does not appear in bounds
-  --> $DIR/static-return-lifetime-infered.rs:7:9
-   |
-LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> {
-   |                         ----- hidden type `Map<std::slice::Iter<'_, (u32, u32)>, [closure@$DIR/static-return-lifetime-infered.rs:7:27: 7:30]>` captures the anonymous lifetime defined here
-LL |         self.x.iter().map(|a| a.0)
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-help: to declare that `impl Iterator<Item = u32>` captures `'_`, you can add an explicit `'_` lifetime bound
-   |
-LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> + '_ {
-   |                                                           ++++
-
-error[E0700]: hidden type for `impl Iterator<Item = u32>` captures lifetime that does not appear in bounds
-  --> $DIR/static-return-lifetime-infered.rs:12:9
+  --> $DIR/static-return-lifetime-infered.rs:11:9
    |
 LL |     fn iter_values<'a>(&'a self) -> impl Iterator<Item=u32> {
-   |                    -- hidden type `Map<std::slice::Iter<'a, (u32, u32)>, [closure@$DIR/static-return-lifetime-infered.rs:12:27: 12:30]>` captures the lifetime `'a` as defined here
+   |                    -- hidden type `Map<std::slice::Iter<'a, (u32, u32)>, [closure@$DIR/static-return-lifetime-infered.rs:11:27: 11:30]>` captures the lifetime `'a` as defined here
 LL |         self.x.iter().map(|a| a.0)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
@@ -37,19 +24,6 @@ help: to declare that `impl Iterator<Item = u32>` captures `'a`, you can add an
 LL |     fn iter_values<'a>(&'a self) -> impl Iterator<Item=u32> + 'a {
    |                                                             ++++
 
-error[E0700]: hidden type for `impl Iterator<Item = u32>` captures lifetime that does not appear in bounds
-  --> $DIR/static-return-lifetime-infered.rs:12:9
-   |
-LL |     fn iter_values<'a>(&'a self) -> impl Iterator<Item=u32> {
-   |                    -- hidden type `Map<std::slice::Iter<'a, (u32, u32)>, [closure@$DIR/static-return-lifetime-infered.rs:12:27: 12:30]>` captures the lifetime `'a` as defined here
-LL |         self.x.iter().map(|a| a.0)
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-help: to declare that `impl Iterator<Item = u32>` captures `'a`, you can add an explicit `'a` lifetime bound
-   |
-LL |     fn iter_values<'a>(&'a self) -> impl Iterator<Item=u32> + 'a {
-   |                                                             ++++
-
-error: aborting due to 4 previous errors
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0700`.

src/test/ui/lifetimes/issue-105227.fixed

@@ -0,0 +1,26 @@
+// Regression test for issue #105227.
+
+// run-rustfix
+#![allow(warnings)]
+fn chars0<'a>(v :(&'a  str, &'a str)) -> impl Iterator<Item = char> + 'a  {
+//~^ HELP to declare that `impl Iterator<Item = char>` captures `'_`, you can introduce a named lifetime parameter `'a`
+    v.0.chars().chain(v.1.chars())
+    //~^ ERROR hidden type for `impl Iterator<Item = char>` captures lifetime that does not appear in bounds
+}
+
+fn chars1<'a>(v0 : &'a  str, v1 : &'a str) -> impl Iterator<Item = char> + 'a  {
+//~^ HELP to declare that `impl Iterator<Item = char>` captures `'_`, you can introduce a named lifetime parameter `'a`
+    v0.chars().chain(v1.chars())
+    //~^ ERROR hidden type for `impl Iterator<Item = char>` captures lifetime that does not appear in bound
+}
+
+fn chars2<'b>(v0 : &'b str, v1 : &'b str, v2 : &'b str) ->
+//~^ HELP to declare that `impl Iterator<Item = char>` captures `'_`, you can use the named lifetime parameter `'b`
+    (impl Iterator<Item = char> + 'b , &'b str)
+{
+    (v0.chars().chain(v1.chars()), v2)
+    //~^ ERROR hidden type for `impl Iterator<Item = char>` captures lifetime that does not appear in bound
+}
+
+fn main() {
+}

src/test/ui/lifetimes/issue-105227.rs

@@ -0,0 +1,26 @@
+// Regression test for issue #105227.
+
+// run-rustfix
+#![allow(warnings)]
+fn chars0(v :(& str, &str)) -> impl Iterator<Item = char> {
+//~^ HELP to declare that `impl Iterator<Item = char>` captures `'_`, you can introduce a named lifetime parameter `'a`
+    v.0.chars().chain(v.1.chars())
+    //~^ ERROR hidden type for `impl Iterator<Item = char>` captures lifetime that does not appear in bounds
+}
+
+fn chars1(v0 : & str, v1 : &str) -> impl Iterator<Item = char> {
+//~^ HELP to declare that `impl Iterator<Item = char>` captures `'_`, you can introduce a named lifetime parameter `'a`
+    v0.chars().chain(v1.chars())
+    //~^ ERROR hidden type for `impl Iterator<Item = char>` captures lifetime that does not appear in bound
+}
+
+fn chars2<'b>(v0 : &str, v1 : &'_ str, v2 : &'b str) ->
+//~^ HELP to declare that `impl Iterator<Item = char>` captures `'_`, you can use the named lifetime parameter `'b`
+    (impl Iterator<Item = char>, &'b str)
+{
+    (v0.chars().chain(v1.chars()), v2)
+    //~^ ERROR hidden type for `impl Iterator<Item = char>` captures lifetime that does not appear in bound
+}
+
+fn main() {
+}

src/test/ui/lifetimes/issue-105227.stderr

@@ -0,0 +1,47 @@
+error[E0700]: hidden type for `impl Iterator<Item = char>` captures lifetime that does not appear in bounds
+  --> $DIR/issue-105227.rs:7:5
+   |
+LL | fn chars0(v :(& str, &str)) -> impl Iterator<Item = char> {
+   |               ----- hidden type `std::iter::Chain<Chars<'_>, Chars<'_>>` captures the anonymous lifetime defined here
+LL |
+LL |     v.0.chars().chain(v.1.chars())
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: to declare that `impl Iterator<Item = char>` captures `'_`, you can introduce a named lifetime parameter `'a`
+   |
+LL | fn chars0<'a>(v :(&'a  str, &'a str)) -> impl Iterator<Item = char> + 'a  {
+   |          ++++      ++        ++                                     ++++
+
+error[E0700]: hidden type for `impl Iterator<Item = char>` captures lifetime that does not appear in bounds
+  --> $DIR/issue-105227.rs:13:5
+   |
+LL | fn chars1(v0 : & str, v1 : &str) -> impl Iterator<Item = char> {
+   |                ----- hidden type `std::iter::Chain<Chars<'_>, Chars<'_>>` captures the anonymous lifetime defined here
+LL |
+LL |     v0.chars().chain(v1.chars())
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: to declare that `impl Iterator<Item = char>` captures `'_`, you can introduce a named lifetime parameter `'a`
+   |
+LL | fn chars1<'a>(v0 : &'a  str, v1 : &'a str) -> impl Iterator<Item = char> + 'a  {
+   |          ++++       ++             ++                                    ++++
+
+error[E0700]: hidden type for `impl Iterator<Item = char>` captures lifetime that does not appear in bounds
+  --> $DIR/issue-105227.rs:21:5
+   |
+LL | fn chars2<'b>(v0 : &str, v1 : &'_ str, v2 : &'b str) ->
+   |                    ---- hidden type `std::iter::Chain<Chars<'_>, Chars<'_>>` captures the anonymous lifetime defined here
+...
+LL |     (v0.chars().chain(v1.chars()), v2)
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: to declare that `impl Iterator<Item = char>` captures `'_`, you can use the named lifetime parameter `'b`
+   |
+LL ~ fn chars2<'b>(v0 : &'b str, v1 : &'b str, v2 : &'b str) ->
+LL |
+LL ~     (impl Iterator<Item = char> + 'b , &'b str)
+   |
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0700`.