introducing lint reason annotations (RFC 2383)

This is just for the `reason =` name-value meta-item; the
`#[expect(lint)]` attribute also described in the RFC is a problem for
another day.

The place where we were directly calling `emit()` on a match block
(whose arms returned a mutable reference to a diagnostic-builder) was
admittedly cute, but no longer plausibly natural after adding the
if-let to the end of the `LintSource::Node` arm.

This regards #54503.

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

@@ -0,0 +1,16 @@
+#![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)]
+//~^ WARN unknown lint
+
+fn main() {}

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

@@ -0,0 +1,45 @@
+error[E0452]: malformed lint attribute
+  --> $DIR/reasons-erroneous.rs:1:58
+   |
+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:4:40
+   |
+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:7:29
+   |
+LL | #![warn(bare_trait_objects, reasons = "leaders to no sure land, guides their bearings lost")]
+   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0452]: malformed lint attribute
+  --> $DIR/reasons-erroneous.rs:9:23
+   |
+LL | #![warn(box_pointers, blerp = "or in league with robbers have reversed the signposts")]
+   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0452]: malformed lint attribute
+  --> $DIR/reasons-erroneous.rs:11:36
+   |
+LL | #![warn(elided_lifetimes_in_paths, reason("disrespectful to ancestors", "irresponsible to heirs"))]
+   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: unknown lint: `reason`
+  --> $DIR/reasons-erroneous.rs:13:44
+   |
+LL | #![warn(ellipsis_inclusive_range_patterns, reason)]
+   |                                            ^^^^^^
+   |
+   = note: #[warn(unknown_lints)] on by default
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0452`.

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

@@ -0,0 +1,19 @@
+#![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:12:13
+   |
+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 +1,31 @@
+// compile-pass
+
+#![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:19:29
+   |
+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:3:9
+   |
+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:28:9
+   |
+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:7:5
+   |
+LL |     nonstandard_style,
+   |     ^^^^^^^^^^^^^^^^^
+   = note: #[warn(non_snake_case)] implied by #[warn(nonstandard_style)]
+