Improve docs for struct_lint_level function.

compiler/rustc_lint/src/context.rs

@@ -574,6 +574,11 @@ pub trait LintContext: Sized {
     fn sess(&self) -> &Session;
     fn lints(&self) -> &LintStore;
 
+    /// Emit a lint at the appropriate level, with an optional associated span and an existing diagnostic.
+    ///
+    /// Return value of the `decorate` closure is ignored, see [`struct_lint_level`] for a detailed explanation.
+    ///
+    /// [`struct_lint_level`]: rustc_middle::lint::struct_lint_level#decorate-signature
     fn lookup_with_diagnostics(
         &self,
         lint: &'static Lint,
@@ -872,6 +877,11 @@ pub trait LintContext: Sized {
 
     // FIXME: These methods should not take an Into<MultiSpan> -- instead, callers should need to
     // set the span in their `decorate` function (preferably using set_span).
+    /// Emit a lint at the appropriate level, with an optional associated span.
+    ///
+    /// Return value of the `decorate` closure is ignored, see [`struct_lint_level`] for a detailed explanation.
+    ///
+    /// [`struct_lint_level`]: rustc_middle::lint::struct_lint_level#decorate-signature
     fn lookup<S: Into<MultiSpan>>(
         &self,
         lint: &'static Lint,
@@ -893,6 +903,11 @@ pub trait LintContext: Sized {
         self.lookup(lint, Some(span), decorator.msg(), |diag| decorator.decorate_lint(diag));
     }
 
+    /// Emit a lint at the appropriate level, with an associated span.
+    ///
+    /// Return value of the `decorate` closure is ignored, see [`struct_lint_level`] for a detailed explanation.
+    ///
+    /// [`struct_lint_level`]: rustc_middle::lint::struct_lint_level#decorate-signature
     fn struct_span_lint<S: Into<MultiSpan>>(
         &self,
         lint: &'static Lint,
@@ -914,6 +929,10 @@ pub trait LintContext: Sized {
     }
 
     /// Emit a lint at the appropriate level, with no associated span.
+    ///
+    /// Return value of the `decorate` closure is ignored, see [`struct_lint_level`] for a detailed explanation.
+    ///
+    /// [`struct_lint_level`]: rustc_middle::lint::struct_lint_level#decorate-signature
     fn lint(
         &self,
         lint: &'static Lint,

compiler/rustc_lint/src/levels.rs

@@ -1069,6 +1069,10 @@ impl<'s, P: LintLevelsProvider> LintLevelsBuilder<'s, P> {
 
     /// Used to emit a lint-related diagnostic based on the current state of
     /// this lint context.
+    ///
+    /// Return value of the `decorate` closure is ignored, see [`struct_lint_level`] for a detailed explanation.
+    ///
+    /// [`struct_lint_level`]: rustc_middle::lint::struct_lint_level#decorate-signature
     pub(crate) fn struct_lint(
         &self,
         lint: &'static Lint,

compiler/rustc_middle/src/lint.rs

@@ -274,6 +274,39 @@ pub fn explain_lint_level_source(
     }
 }
 
+/// The innermost function for emitting lints.
+///
+/// If you are loocking to implement a lint, look for higher level functions,
+/// for example:
+/// - [`TyCtxt::emit_spanned_lint`]
+/// - [`TyCtxt::struct_span_lint_hir`]
+/// - [`TyCtxt::emit_lint`]
+/// - [`TyCtxt::struct_lint_node`]
+/// - `LintContext::lookup`
+///
+/// ## `decorate` signature
+///
+/// The return value of `decorate` is ignored by this function. So what is the
+/// point of returning `&'b mut DiagnosticBuilder<'a, ()>`?
+///
+/// There are 2 reasons for this signature.
+///
+/// First of all, it prevents accidental use of `.emit()` -- it's clear that the
+/// builder will be later used and shouldn't be emitted right away (this is
+/// especially important because the old API expected you to call `.emit()` in
+/// the closure).
+///
+/// Second of all, it makes the most common case of adding just a single label
+/// /suggestion much nicer, since [`DiagnosticBuilder`] methods return
+/// `&mut DiagnosticBuilder`, you can just chain methods, without needed
+/// awkward `{ ...; }`:
+/// ```ignore pseudo-code
+/// struct_lint_level(
+///     ...,
+///     |lint| lint.span_label(sp, "lbl")
+///     //          ^^^^^^^^^^^^^^^^^^^^^ returns `&mut DiagnosticBuilder` by default
+/// )
+/// ```
 pub fn struct_lint_level(
     sess: &Session,
     lint: &'static Lint,

compiler/rustc_middle/src/ty/context.rs

@@ -2826,6 +2826,11 @@ impl<'tcx> TyCtxt<'tcx> {
         })
     }
 
+    /// Emit a lint at the appropriate level for a hir node, with an associated span.
+    ///
+    /// Return value of the `decorate` closure is ignored, see [`struct_lint_level`] for a detailed explanation.
+    ///
+    /// [`struct_lint_level`]: rustc_middle::lint::struct_lint_level#decorate-signature
     pub fn struct_span_lint_hir(
         self,
         lint: &'static Lint,
@@ -2851,6 +2856,11 @@ impl<'tcx> TyCtxt<'tcx> {
         self.struct_lint_node(lint, id, decorator.msg(), |diag| decorator.decorate_lint(diag))
     }
 
+    /// Emit a lint at the appropriate level for a hir node.
+    ///
+    /// Return value of the `decorate` closure is ignored, see [`struct_lint_level`] for a detailed explanation.
+    ///
+    /// [`struct_lint_level`]: rustc_middle::lint::struct_lint_level#decorate-signature
     pub fn struct_lint_node(
         self,
         lint: &'static Lint,