Add help suggesting matches to let_chains lint

Author: estebank

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -639,7 +639,11 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
         };
     }
     gate_all!(if_let_guard, "`if let` guards are experimental");
-    gate_all!(let_chains, "`let` expressions in this position are experimental");
+    gate_all!(
+        let_chains,
+        "`let` expressions in this position are experimental",
+        "you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`"
+    );
     gate_all!(
         async_closure,
         "async closures are unstable",

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

@@ -33,6 +33,7 @@ LL |         () if (let 0 = 1) => {}
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`
 
 error[E0658]: `let` expressions in this position are experimental
   --> $DIR/feature-gate.rs:14:18
@@ -42,6 +43,7 @@ LL |         () if (((let 0 = 1))) => {}
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`
 
 error[E0658]: `let` expressions in this position are experimental
   --> $DIR/feature-gate.rs:18:23
@@ -51,6 +53,7 @@ LL |         () if true && let 0 = 1 => {}
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`
 
 error[E0658]: `let` expressions in this position are experimental
   --> $DIR/feature-gate.rs:22:15
@@ -60,6 +63,7 @@ LL |         () if let 0 = 1 && true => {}
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`
 
 error[E0658]: `let` expressions in this position are experimental
   --> $DIR/feature-gate.rs:26:16
@@ -69,6 +73,7 @@ LL |         () if (let 0 = 1) && true => {}
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`
 
 error[E0658]: `let` expressions in this position are experimental
   --> $DIR/feature-gate.rs:30:24
@@ -78,6 +83,7 @@ LL |         () if true && (let 0 = 1) => {}
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`
 
 error[E0658]: `let` expressions in this position are experimental
   --> $DIR/feature-gate.rs:34:16
@@ -87,6 +93,7 @@ LL |         () if (let 0 = 1) && (let 0 = 1) => {}
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`
 
 error[E0658]: `let` expressions in this position are experimental
   --> $DIR/feature-gate.rs:34:31
@@ -96,6 +103,7 @@ LL |         () if (let 0 = 1) && (let 0 = 1) => {}
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`
 
 error[E0658]: `let` expressions in this position are experimental
   --> $DIR/feature-gate.rs:40:15
@@ -105,6 +113,7 @@ LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 =
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`
 
 error[E0658]: `let` expressions in this position are experimental
   --> $DIR/feature-gate.rs:40:28
@@ -114,6 +123,7 @@ LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 =
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`
 
 error[E0658]: `let` expressions in this position are experimental
   --> $DIR/feature-gate.rs:40:42
@@ -123,6 +133,7 @@ LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 =
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`
 
 error[E0658]: `let` expressions in this position are experimental
   --> $DIR/feature-gate.rs:40:55
@@ -132,6 +143,7 @@ LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 =
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`
 
 error[E0658]: `let` expressions in this position are experimental
   --> $DIR/feature-gate.rs:40:68
@@ -141,6 +153,7 @@ LL |         () if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 =
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`
 
 error[E0658]: `let` expressions in this position are experimental
   --> $DIR/feature-gate.rs:52:15
@@ -150,6 +163,7 @@ LL |         () if let Range { start: _, end: _ } = (true..true) && false => {}
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`
 
 error[E0658]: `let` expressions in this position are experimental
   --> $DIR/feature-gate.rs:68:16
@@ -159,6 +173,7 @@ LL |     use_expr!((let 0 = 1 && 0 == 0));
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`
 
 error[E0658]: `let` expressions in this position are experimental
   --> $DIR/feature-gate.rs:71:16
@@ -168,6 +183,7 @@ LL |     use_expr!((let 0 = 1));
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
    = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`
 
 error: `let` expressions are not supported here
   --> $DIR/feature-gate.rs:10:16