Auto merge of #54683 - zackmdavis:critique_of_pure_lints, r=petrochenkov

lint reasons (RFC 2883, part 1)

This implements the `reason =` functionality described in [the RFC](https://github.com/rust-lang/rfcs/blob/master/text/2383-lint-reasons.md) under a `lint_reasons` feature gate.

![lint_reasons_pt_1](https://user-images.githubusercontent.com/1076988/46252097-eed51000-c418-11e8-8212-939d3f02f95d.png)

src/test/ui/feature-gates/feature-gate-lint-reasons.rs

@@ -0,0 +1,4 @@
+#![warn(nonstandard_style, reason = "the standard should be respected")]
+//~^ ERROR lint reasons are experimental
+
+fn main() {}

src/test/ui/feature-gates/feature-gate-lint-reasons.stderr

@@ -0,0 +1,11 @@
+error[E0658]: lint reasons are experimental (see issue #54503)
+  --> $DIR/feature-gate-lint-reasons.rs:1:28
+   |
+LL | #![warn(nonstandard_style, reason = "the standard should be respected")]
+   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(lint_reasons)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/lint/empty-lint-attributes.rs

@@ -0,0 +1,17 @@
+#![feature(lint_reasons)]
+
+// run-pass
+
+// Empty (and reason-only) lint attributes are legal—although we may want to
+// lint them in the future (Issue #55112).
+
+#![allow()]
+#![warn(reason = "observationalism")]
+
+#[forbid()]
+fn devoir() {}
+
+#[deny(reason = "ultion")]
+fn waldgrave() {}
+
+fn main() {}

src/test/ui/lint/reasons-erroneous.rs

@@ -0,0 +1,24 @@
+#![feature(lint_reasons)]
+
+#![warn(absolute_paths_not_starting_with_crate, reason = 0)]
+//~^ ERROR malformed lint attribute
+//~| HELP reason must be a string literal
+#![warn(anonymous_parameters, reason = b"consider these, for we have condemned them")]
+//~^ ERROR malformed lint attribute
+//~| HELP reason must be a string literal
+#![warn(bare_trait_objects, reasons = "leaders to no sure land, guides their bearings lost")]
+//~^ ERROR malformed lint attribute
+#![warn(box_pointers, blerp = "or in league with robbers have reversed the signposts")]
+//~^ ERROR malformed lint attribute
+#![warn(elided_lifetimes_in_paths, reason("disrespectful to ancestors", "irresponsible to heirs"))]
+//~^ ERROR malformed lint attribute
+#![warn(ellipsis_inclusive_range_patterns, reason = "born barren", reason = "a freak growth")]
+//~^ ERROR malformed lint attribute
+//~| HELP reason in lint attribute must come last
+#![warn(keyword_idents, reason = "root in rubble", macro_use_extern_crate)]
+//~^ ERROR malformed lint attribute
+//~| HELP reason in lint attribute must come last
+#![warn(missing_copy_implementations, reason)]
+//~^ WARN unknown lint
+
+fn main() {}

src/test/ui/lint/reasons-erroneous.stderr

@@ -0,0 +1,61 @@
+error[E0452]: malformed lint attribute
+  --> $DIR/reasons-erroneous.rs:3:58
+   |
+LL | #![warn(absolute_paths_not_starting_with_crate, reason = 0)]
+   |                                                          ^
+   |
+   = help: reason must be a string literal
+
+error[E0452]: malformed lint attribute
+  --> $DIR/reasons-erroneous.rs:6:40
+   |
+LL | #![warn(anonymous_parameters, reason = b"consider these, for we have condemned them")]
+   |                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: reason must be a string literal
+
+error[E0452]: malformed lint attribute
+  --> $DIR/reasons-erroneous.rs:9:29
+   |
+LL | #![warn(bare_trait_objects, reasons = "leaders to no sure land, guides their bearings lost")]
+   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0452]: malformed lint attribute
+  --> $DIR/reasons-erroneous.rs:11:23
+   |
+LL | #![warn(box_pointers, blerp = "or in league with robbers have reversed the signposts")]
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0452]: malformed lint attribute
+  --> $DIR/reasons-erroneous.rs:13:36
+   |
+LL | #![warn(elided_lifetimes_in_paths, reason("disrespectful to ancestors", "irresponsible to heirs"))]
+   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0452]: malformed lint attribute
+  --> $DIR/reasons-erroneous.rs:15:44
+   |
+LL | #![warn(ellipsis_inclusive_range_patterns, reason = "born barren", reason = "a freak growth")]
+   |                                            ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: reason in lint attribute must come last
+
+error[E0452]: malformed lint attribute
+  --> $DIR/reasons-erroneous.rs:18:25
+   |
+LL | #![warn(keyword_idents, reason = "root in rubble", macro_use_extern_crate)]
+   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: reason in lint attribute must come last
+
+warning: unknown lint: `reason`
+  --> $DIR/reasons-erroneous.rs:21:39
+   |
+LL | #![warn(missing_copy_implementations, reason)]
+   |                                       ^^^^^^
+   |
+   = note: #[warn(unknown_lints)] on by default
+
+error: aborting due to 7 previous errors
+
+For more information about this error, try `rustc --explain E0452`.

src/test/ui/lint/reasons-forbidden.rs

@@ -0,0 +1,21 @@
+#![feature(lint_reasons)]
+
+#![forbid(
+    unsafe_code,
+    //~^ NOTE `forbid` level set here
+    reason = "our errors & omissions insurance policy doesn't cover unsafe Rust"
+)]
+
+use std::ptr;
+
+fn main() {
+    let a_billion_dollar_mistake = ptr::null();
+
+    #[allow(unsafe_code)]
+    //~^ ERROR allow(unsafe_code) overruled by outer forbid(unsafe_code)
+    //~| NOTE overruled by previous forbid
+    //~| NOTE our errors & omissions insurance policy doesn't cover unsafe Rust
+    unsafe {
+        *a_billion_dollar_mistake
+    }
+}

src/test/ui/lint/reasons-forbidden.stderr

@@ -0,0 +1,14 @@
+error[E0453]: allow(unsafe_code) overruled by outer forbid(unsafe_code)
+  --> $DIR/reasons-forbidden.rs:14:13
+   |
+LL |     unsafe_code,
+   |     ----------- `forbid` level set here
+...
+LL |     #[allow(unsafe_code)]
+   |             ^^^^^^^^^^^ overruled by previous forbid
+   |
+   = note: our errors & omissions insurance policy doesn't cover unsafe Rust
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0453`.

src/test/ui/lint/reasons.rs

@@ -0,0 +1,33 @@
+// compile-pass
+
+#![feature(lint_reasons)]
+
+#![warn(elided_lifetimes_in_paths,
+        //~^ NOTE lint level defined here
+        reason = "explicit anonymous lifetimes aid reasoning about ownership")]
+#![warn(
+    nonstandard_style,
+    //~^ NOTE lint level defined here
+    reason = r#"people shouldn't have to change their usual style habits
+to contribute to our project"#
+)]
+#![allow(unused, reason = "unused code has never killed anypony")]
+
+use std::fmt;
+
+pub struct CheaterDetectionMechanism {}
+
+impl fmt::Debug for CheaterDetectionMechanism {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        //~^ WARN hidden lifetime parameters in types are deprecated
+        //~| NOTE explicit anonymous lifetimes aid
+        //~| HELP indicate the anonymous lifetime
+        fmt.debug_struct("CheaterDetectionMechanism").finish()
+    }
+}
+
+fn main() {
+    let Social_exchange_psychology = CheaterDetectionMechanism {};
+    //~^ WARN should have a snake case name such as
+    //~| NOTE people shouldn't have to change their usual style habits
+}

src/test/ui/lint/reasons.stderr

@@ -0,0 +1,28 @@
+warning: hidden lifetime parameters in types are deprecated
+  --> $DIR/reasons.rs:21:29
+   |
+LL |     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+   |                             ^^^^^^^^^^^^^^- help: indicate the anonymous lifetime: `<'_>`
+   |
+   = note: explicit anonymous lifetimes aid reasoning about ownership
+note: lint level defined here
+  --> $DIR/reasons.rs:5:9
+   |
+LL | #![warn(elided_lifetimes_in_paths,
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: variable `Social_exchange_psychology` should have a snake case name such as `social_exchange_psychology`
+  --> $DIR/reasons.rs:30:9
+   |
+LL |     let Social_exchange_psychology = CheaterDetectionMechanism {};
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: people shouldn't have to change their usual style habits
+           to contribute to our project
+note: lint level defined here
+  --> $DIR/reasons.rs:9:5
+   |
+LL |     nonstandard_style,
+   |     ^^^^^^^^^^^^^^^^^
+   = note: #[warn(non_snake_case)] implied by #[warn(nonstandard_style)]
+