Auto merge of rust-lang#6696 - dtolnay-contrib:regex, r=Manishearth

Downgrade trivial_regex to nursery

See rust-lang#6690. I think there is still value in a trivial_regex lint, but only if clippy can tell that the regex is only ever constructed and applied to a single input.

```rust
let regex = Regex::new("trivial_regex")?;
println!("{}", regex.is_match(s));
// `regex` never used again
```

---

changelog: remove `trivial_regex` from default set of enabled lints

clippy_lints/src/lib.rs

@@ -1628,7 +1628,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&reference::DEREF_ADDROF),
         LintId::of(&reference::REF_IN_DEREF),
         LintId::of(&regex::INVALID_REGEX),
-        LintId::of(&regex::TRIVIAL_REGEX),
         LintId::of(&repeat_once::REPEAT_ONCE),
         LintId::of(&returns::LET_AND_RETURN),
         LintId::of(&returns::NEEDLESS_RETURN),
@@ -1791,7 +1790,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&ranges::MANUAL_RANGE_CONTAINS),
         LintId::of(&redundant_field_names::REDUNDANT_FIELD_NAMES),
         LintId::of(&redundant_static_lifetimes::REDUNDANT_STATIC_LIFETIMES),
-        LintId::of(&regex::TRIVIAL_REGEX),
         LintId::of(&returns::LET_AND_RETURN),
         LintId::of(&returns::NEEDLESS_RETURN),
         LintId::of(&single_component_path_imports::SINGLE_COMPONENT_PATH_IMPORTS),
@@ -2021,6 +2019,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&needless_borrow::NEEDLESS_BORROW),
         LintId::of(&path_buf_push_overwrite::PATH_BUF_PUSH_OVERWRITE),
         LintId::of(&redundant_pub_crate::REDUNDANT_PUB_CRATE),
+        LintId::of(&regex::TRIVIAL_REGEX),
         LintId::of(&strings::STRING_LIT_AS_BYTES),
         LintId::of(&transmute::USELESS_TRANSMUTE),
         LintId::of(&use_self::USE_SELF),

clippy_lints/src/regex.rs

@@ -35,14 +35,16 @@ declare_clippy_lint! {
     /// `str::starts_with`, `str::ends_with` or `std::contains` or other `str`
     /// methods.
     ///
-    /// **Known problems:** None.
+    /// **Known problems:** If the same regex is going to be applied to multiple
+    /// inputs, the precomputations done by `Regex` construction can give
+    /// significantly better performance than any of the `str`-based methods.
     ///
     /// **Example:**
     /// ```ignore
     /// Regex::new("^foobar")
     /// ```
     pub TRIVIAL_REGEX,
-    style,
+    nursery,
     "trivial regular expressions"
 }