Add long error explanation for E0312

src/librustc/error_codes.rs

@@ -1347,6 +1347,39 @@ struct Foo<T: 'static> {
 ```
 "##,
 
+E0312: r##"
+Reference's lifetime of borrowed content doesn't match the expected lifetime.
+
+Erroneous code example:
+
+```compile_fail,E0312
+pub fn opt_str<'a>(maybestr: &'a Option<String>) -> &'static str {
+    if maybestr.is_none() {
+        "(none)"
+    } else {
+        let s: &'a str = maybestr.as_ref().unwrap();
+        s  // Invalid lifetime!
+    }
+}
+```
+
+To fix this error, either lessen the expected lifetime or find a way to not have
+to use this reference outside of its current scope (by running the code directly
+in the same block for example?):
+
+```
+// In this case, we can fix the issue by switching from "static" lifetime to 'a
+pub fn opt_str<'a>(maybestr: &'a Option<String>) -> &'a str {
+    if maybestr.is_none() {
+        "(none)"
+    } else {
+        let s: &'a str = maybestr.as_ref().unwrap();
+        s  // Ok!
+    }
+}
+```
+"##,
+
 E0317: r##"
 This error occurs when an `if` expression without an `else` block is used in a
 context where a type other than `()` is expected, for example a `let`
@@ -2202,7 +2235,6 @@ static X: u32 = 42;
 //  E0304, // expected signed integer constant
 //  E0305, // expected constant
     E0311, // thing may not live long enough
-    E0312, // lifetime of reference outlives lifetime of borrowed content
     E0313, // lifetime of borrowed pointer outlives lifetime of captured
            // variable
     E0314, // closure outlives stack frame

src/test/ui/issues/issue-10291.stderr

@@ -20,3 +20,4 @@ LL | fn test<'x>(x: &'x isize) {
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0312`.

src/test/ui/issues/issue-52533.stderr

@@ -17,3 +17,4 @@ LL |     foo(|a, b| b)
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0312`.

src/test/ui/lub-if.stderr

@@ -26,3 +26,4 @@ LL | pub fn opt_str3<'a>(maybestr: &'a Option<String>) -> &'static str {
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0312`.

src/test/ui/lub-match.stderr

@@ -26,3 +26,4 @@ LL | pub fn opt_str3<'a>(maybestr: &'a Option<String>) -> &'static str {
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0312`.

src/test/ui/nll/issue-52742.stderr

@@ -20,3 +20,4 @@ LL | |     }
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0312`.

src/test/ui/nll/issue-55401.stderr

@@ -13,3 +13,4 @@ LL | fn static_to_a_to_static_through_ref_in_tuple<'a>(x: &'a u32) -> &'static u
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0312`.

src/test/ui/nll/user-annotations/constant-in-expr-normalize.stderr

@@ -13,3 +13,4 @@ LL | fn foo<'a>(_: &'a u32) -> &'static u32 {
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0312`.

src/test/ui/nll/user-annotations/constant-in-expr-trait-item-1.stderr

@@ -13,3 +13,4 @@ LL | fn foo<'a>(_: &'a u32) -> &'static u32 {
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0312`.

src/test/ui/nll/user-annotations/constant-in-expr-trait-item-2.stderr

@@ -13,3 +13,4 @@ LL | fn foo<'a, T: Foo<'a>>() -> &'static u32 {
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0312`.

src/test/ui/regions/regions-early-bound-error-method.stderr

@@ -17,3 +17,4 @@ LL |     fn or<'b,G:GetRef<'b>>(&self, g2: G) -> &'a isize {
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0312`.

src/test/ui/regions/regions-early-bound-error.stderr

@@ -17,3 +17,4 @@ LL | fn get<'a,'b,G:GetRef<'a, isize>>(g1: G, b: &'b isize) -> &'b isize {
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0312`.

src/test/ui/regions/regions-nested-fns.stderr

@@ -57,3 +57,4 @@ LL | fn nested<'x>(x: &'x isize) {
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0312`.

src/test/ui/regions/regions-static-bound.migrate.stderr

@@ -30,4 +30,5 @@ LL |     static_id_indirect(&v);
 
 error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0621`.
+Some errors have detailed explanations: E0312, E0621.
+For more information about an error, try `rustc --explain E0312`.

src/test/ui/unboxed-closures/unboxed-closures-infer-argument-types-two-region-pointers.stderr

@@ -23,3 +23,4 @@ LL | |     });
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0312`.

src/test/ui/wf/wf-static-method.stderr

@@ -105,4 +105,5 @@ LL |     <IndirectEvil>::static_evil(b)
 
 error: aborting due to 5 previous errors
 
-For more information about this error, try `rustc --explain E0478`.
+Some errors have detailed explanations: E0312, E0478.
+For more information about an error, try `rustc --explain E0312`.