Rollup merge of rust-lang#64404 - GuillaumeGomez:err-E0495, r=cramertj

Add long error explanation for E0495

Part of rust-lang#61137.

Author: tmandry

src/librustc/error_codes.rs

@@ -1580,6 +1580,47 @@ where
 ```
 "##,
 
+E0495: r##"
+A lifetime cannot be determined in the given situation.
+
+Erroneous code example:
+
+```compile_fail,E0495
+fn transmute_lifetime<'a, 'b, T>(t: &'a (T,)) -> &'b T {
+    match (&t,) { // error!
+        ((u,),) => u,
+    }
+}
+
+let y = Box::new((42,));
+let x = transmute_lifetime(&y);
+```
+
+In this code, you have two ways to solve this issue:
+ 1. Enforce that `'a` lives at least as long as `'b`.
+ 2. Use the same lifetime requirement for both input and output values.
+
+So for the first solution, you can do it by replacing `'a` with `'a: 'b`:
+
+```
+fn transmute_lifetime<'a: 'b, 'b, T>(t: &'a (T,)) -> &'b T {
+    match (&t,) { // ok!
+        ((u,),) => u,
+    }
+}
+```
+
+In the second you can do it by simply removing `'b` so they both use `'a`:
+
+```
+fn transmute_lifetime<'a, T>(t: &'a (T,)) -> &'a T {
+    match (&t,) { // ok!
+        ((u,),) => u,
+    }
+}
+```
+"##,
+
 E0496: r##"
 A lifetime name is shadowing another lifetime name. Erroneous code example:
 
@@ -2275,8 +2316,6 @@ rejected in your own crates.
     E0488, // lifetime of variable does not enclose its declaration
     E0489, // type/lifetime parameter not in scope here
     E0490, // a value of type `..` is borrowed for too long
-    E0495, // cannot infer an appropriate lifetime due to conflicting
-           // requirements
     E0566, // conflicting representation hints
     E0623, // lifetime mismatch where both parameters are anonymous regions
     E0628, // generators cannot have explicit parameters

src/test/ui/associated-types/cache/project-fn-ret-contravariant.transmute.stderr

@@ -23,3 +23,4 @@ LL |    bar(foo, x)
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0495`.

src/test/ui/associated-types/cache/project-fn-ret-invariant.transmute.stderr

@@ -19,3 +19,4 @@ LL | fn baz<'a,'b>(x: Type<'a>) -> Type<'static> {
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0495`.

src/test/ui/c-variadic/variadic-ffi-4.stderr

@@ -209,4 +209,5 @@ LL | | }
 
 error: aborting due to 8 previous errors
 
-For more information about this error, try `rustc --explain E0308`.
+Some errors have detailed explanations: E0308, E0495.
+For more information about an error, try `rustc --explain E0308`.

src/test/ui/error-codes/E0621-does-not-trigger-for-closures.stderr

@@ -27,3 +27,4 @@ LL |     invoke(&x, |a, b| if a > b { a } else { b });
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0495`.

src/test/ui/impl-header-lifetime-elision/dyn-trait.stderr

@@ -19,3 +19,4 @@ LL | fn with_dyn_debug_static<'a>(x: Box<dyn Debug + 'a>) {
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0495`.

src/test/ui/in-band-lifetimes/mismatched_trait_impl-2.stderr

@@ -18,3 +18,4 @@ LL | |     }
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0495`.

src/test/ui/in-band-lifetimes/mismatched_trait_impl.stderr

@@ -32,3 +32,4 @@ LL |         x
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0495`.

src/test/ui/issues/issue-16683.stderr

@@ -27,3 +27,4 @@ LL | trait T<'a> {
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0495`.

src/test/ui/issues/issue-17758.stderr

@@ -28,3 +28,4 @@ LL | trait Foo<'a> {
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0495`.