Add explanations and suggestions to `irrefutable_let_patterns` lint

compiler/rustc_lint_defs/src/builtin.rs

@@ -1814,14 +1814,12 @@ declare_lint! {
 }
 
 declare_lint! {
-    /// The `irrefutable_let_patterns` lint detects detects [irrefutable
-    /// patterns] in [`if let`] and [`while let`] statements.
-    ///
-    ///
+    /// The `irrefutable_let_patterns` lint detects [irrefutable patterns]
+    /// in [`if let`]s, [`while let`]s, and `if let` guards.
     ///
     /// ### Example
     ///
-    /// ```rust
+    /// ```
     /// if let _ = 123 {
     ///     println!("always runs!");
     /// }

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -366,14 +366,31 @@ fn unreachable_pattern(tcx: TyCtxt<'_>, span: Span, id: HirId, catchall: Option<
 }
 
 fn irrefutable_let_pattern(tcx: TyCtxt<'_>, span: Span, id: HirId, source: hir::MatchSource) {
-    tcx.struct_span_lint_hir(IRREFUTABLE_LET_PATTERNS, id, span, |lint| {
-        let msg = match source {
-            hir::MatchSource::IfLetDesugar { .. } => "irrefutable `if let` pattern",
-            hir::MatchSource::WhileLetDesugar => "irrefutable `while let` pattern",
-            hir::MatchSource::IfLetGuardDesugar => "irrefutable `if let` guard",
-            _ => bug!(),
-        };
-        lint.build(msg).emit()
+    tcx.struct_span_lint_hir(IRREFUTABLE_LET_PATTERNS, id, span, |lint| match source {
+        hir::MatchSource::IfLetDesugar { .. } => {
+            let mut diag = lint.build("irrefutable `if let` pattern");
+            diag.note("this pattern will always match, so the `if let` is useless");
+            diag.help("consider replacing the `if let` with a `let`");
+            diag.emit()
+        }
+        hir::MatchSource::WhileLetDesugar => {
+            let mut diag = lint.build("irrefutable `while let` pattern");
+            diag.note("this pattern will always match, so the loop will never exit");
+            diag.help("consider instead using a `loop { ... }` with a `let` inside it");
+            diag.emit()
+        }
+        hir::MatchSource::IfLetGuardDesugar => {
+            let mut diag = lint.build("irrefutable `if let` guard pattern");
+            diag.note("this pattern will always match, so the guard is useless");
+            diag.help("consider removing the guard and adding a `let` inside the match arm");
+            diag.emit()
+        }
+        _ => {
+            bug!(
+                "expected `if let`, `while let`, or `if let` guard HIR match source, found {:?}",
+                source,
+            )
+        }
     });
 }
 
@@ -387,7 +404,7 @@ fn check_if_let_guard<'p, 'tcx>(
     report_arm_reachability(&cx, &report, hir::MatchSource::IfLetGuardDesugar);
 
     if report.non_exhaustiveness_witnesses.is_empty() {
-        // The match is exhaustive, i.e. the if let pattern is irrefutable.
+        // The match is exhaustive, i.e. the `if let` pattern is irrefutable.
         irrefutable_let_pattern(cx.tcx, pat.span, pat_id, hir::MatchSource::IfLetGuardDesugar)
     }
 }

src/test/ui/closures/2229_closure_analysis/diagnostics/closure-origin-single-variant-diagnostics.stderr

@@ -17,6 +17,8 @@ LL | |         }
    | |_________^
    |
    = note: `#[warn(irrefutable_let_patterns)]` on by default
+   = note: this pattern will always match, so the `if let` is useless
+   = help: consider replacing the `if let` with a `let`
 
 error[E0382]: use of moved value: `c`
   --> $DIR/closure-origin-single-variant-diagnostics.rs:25:13

src/test/ui/expr/if/if-let.stderr

@@ -10,6 +10,8 @@ LL | |     });
    | |_______- in this macro invocation
    |
    = note: `#[warn(irrefutable_let_patterns)]` on by default
+   = note: this pattern will always match, so the `if let` is useless
+   = help: consider replacing the `if let` with a `let`
    = note: this warning originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 warning: irrefutable `if let` pattern
@@ -23,6 +25,8 @@ LL | |         println!("irrefutable pattern");
 LL | |     });
    | |_______- in this macro invocation
    |
+   = note: this pattern will always match, so the `if let` is useless
+   = help: consider replacing the `if let` with a `let`
    = note: this warning originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 warning: irrefutable `if let` pattern
@@ -32,6 +36,9 @@ LL | /     if let a = 1 {
 LL | |         println!("irrefutable pattern");
 LL | |     }
    | |_____^
+   |
+   = note: this pattern will always match, so the `if let` is useless
+   = help: consider replacing the `if let` with a `let`
 
 warning: irrefutable `if let` pattern
   --> $DIR/if-let.rs:30:5
@@ -44,6 +51,9 @@ LL | |     } else {
 LL | |         println!("else in irrefutable `if let`");
 LL | |     }
    | |_____^
+   |
+   = note: this pattern will always match, so the `if let` is useless
+   = help: consider replacing the `if let` with a `let`
 
 warning: irrefutable `if let` pattern
   --> $DIR/if-let.rs:40:12
@@ -53,6 +63,9 @@ LL |       } else if let a = 1 {
 LL | |         println!("irrefutable pattern");
 LL | |     }
    | |_____^
+   |
+   = note: this pattern will always match, so the `if let` is useless
+   = help: consider replacing the `if let` with a `let`
 
 warning: irrefutable `if let` pattern
   --> $DIR/if-let.rs:46:12
@@ -62,6 +75,9 @@ LL |       } else if let a = 1 {
 LL | |         println!("irrefutable pattern");
 LL | |     }
    | |_____^
+   |
+   = note: this pattern will always match, so the `if let` is useless
+   = help: consider replacing the `if let` with a `let`
 
 warning: 6 warnings emitted
 

src/test/ui/pattern/usefulness/deny-irrefutable-let-patterns.rs

@@ -1,3 +1,6 @@
+#![feature(if_let_guard)]
+#![allow(incomplete_features)]
+
 #![deny(irrefutable_let_patterns)]
 
 fn main() {
@@ -6,4 +9,9 @@ fn main() {
     while let _ = 5 { //~ ERROR irrefutable `while let` pattern
         break;
     }
+
+    match 5 {
+        _ if let _ = 2 => {} //~ ERROR irrefutable `if let` guard pattern
+        _ => {}
+    }
 }

src/test/ui/pattern/usefulness/deny-irrefutable-let-patterns.stderr

@@ -1,22 +1,36 @@
 error: irrefutable `if let` pattern
-  --> $DIR/deny-irrefutable-let-patterns.rs:4:5
+  --> $DIR/deny-irrefutable-let-patterns.rs:7:5
    |
 LL |     if let _ = 5 {}
    |     ^^^^^^^^^^^^^^^
    |
 note: the lint level is defined here
-  --> $DIR/deny-irrefutable-let-patterns.rs:1:9
+  --> $DIR/deny-irrefutable-let-patterns.rs:4:9
    |
 LL | #![deny(irrefutable_let_patterns)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: this pattern will always match, so the `if let` is useless
+   = help: consider replacing the `if let` with a `let`
 
 error: irrefutable `while let` pattern
-  --> $DIR/deny-irrefutable-let-patterns.rs:6:5
+  --> $DIR/deny-irrefutable-let-patterns.rs:9:5
    |
 LL | /     while let _ = 5 {
 LL | |         break;
 LL | |     }
    | |_____^
+   |
+   = note: this pattern will always match, so the loop will never exit
+   = help: consider instead using a `loop { ... }` with a `let` inside it
 
-error: aborting due to 2 previous errors
+error: irrefutable `if let` guard pattern
+  --> $DIR/deny-irrefutable-let-patterns.rs:14:18
+   |
+LL |         _ if let _ = 2 => {}
+   |                  ^
+   |
+   = note: this pattern will always match, so the guard is useless
+   = help: consider removing the guard and adding a `let` inside the match arm
+
+error: aborting due to 3 previous errors
 

src/test/ui/rfc-2294-if-let-guard/warns.stderr

@@ -1,4 +1,4 @@
-error: irrefutable `if let` guard
+error: irrefutable `if let` guard pattern
   --> $DIR/warns.rs:7:24
    |
 LL |         Some(x) if let () = x => {}
@@ -9,6 +9,8 @@ note: the lint level is defined here
    |
 LL | #[deny(irrefutable_let_patterns)]
    |        ^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: this pattern will always match, so the guard is useless
+   = help: consider removing the guard and adding a `let` inside the match arm
 
 error: unreachable pattern
   --> $DIR/warns.rs:16:25

src/test/ui/while-let.stderr

@@ -10,6 +10,8 @@ LL | |     });
    | |_______- in this macro invocation
    |
    = note: `#[warn(irrefutable_let_patterns)]` on by default
+   = note: this pattern will always match, so the loop will never exit
+   = help: consider instead using a `loop { ... }` with a `let` inside it
    = note: this warning originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 warning: irrefutable `while let` pattern
@@ -23,6 +25,8 @@ LL | |         println!("irrefutable pattern");
 LL | |     });
    | |_______- in this macro invocation
    |
+   = note: this pattern will always match, so the loop will never exit
+   = help: consider instead using a `loop { ... }` with a `let` inside it
    = note: this warning originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
 warning: irrefutable `while let` pattern
@@ -33,6 +37,9 @@ LL | |         println!("irrefutable pattern");
 LL | |         break;
 LL | |     }
    | |_____^
+   |
+   = note: this pattern will always match, so the loop will never exit
+   = help: consider instead using a `loop { ... }` with a `let` inside it
 
 warning: 3 warnings emitted