Merge #6734

6734: Emit additional diagnostics for hints/help/etc r=lnicola a=jonas-schievink

This makes rust-analyzer diagnostics match native rustc diagnostics in the terminal more closely. Unfortunately all of this is a bodge, since we already provide this information to the client in the form of `DiagnosticRelatedInformation`, but at least VS Code chooses such a poor UI for these that they don't help much, as evidenced [here](https://twitter.com/yaahc_/status/1335297260444250112) and in https://github.com/rust-lang/rust/issues/79741.

This PR papers over these client UI problems by taking the `DiagnosticRelatedInformation` and turning each one into its own hint-level diagnostic, which makes it show up in the source code. Quick fixes are attached to all resulting diagnostics, which makes them more discoverable.

### Example: "Consider removing this semicolon"

![screenshot-2020-12-06-01:27:29](https://user-images.githubusercontent.com/1786438/101268366-46423980-3762-11eb-9a69-1ff0b1806c2f.png)
![screenshot-2020-12-06-01:27:39](https://user-images.githubusercontent.com/1786438/101268367-46dad000-3762-11eb-81fa-afd234d44f17.png)
![screenshot-2020-12-06-01:27:46](https://user-images.githubusercontent.com/1786438/101268368-46dad000-3762-11eb-9205-4b9bd9f4406d.png)

### Example: "Value used after move"

![screenshot-2020-12-06-01:33:00](https://user-images.githubusercontent.com/1786438/101268447-22332800-3763-11eb-85ce-8c742927a2c8.png)
![screenshot-2020-12-06-01:33:07](https://user-images.githubusercontent.com/1786438/101268448-22cbbe80-3763-11eb-8f16-0590895d8bc6.png)


Co-authored-by: Jonas Schievink <[email protected]>

8 files changed, 1093 insertions, 119 deletions

diff --git a/crates/rust-analyzer/src/diagnostics/test_data/clippy_pass_by_ref.txt b/crates/rust-analyzer/src/diagnostics/test_data/clippy_pass_by_ref.txt
index 72f6c5725..7576097b3 100644
--- a/crates/rust-analyzer/src/diagnostics/test_data/clippy_pass_by_ref.txt
+++ b/crates/rust-analyzer/src/diagnostics/test_data/clippy_pass_by_ref.txt
@@ -104,4 +104,168 @@
         },
         fixes: [],
     },
+    MappedRustDiagnostic {
+        url: Url {
+            scheme: "file",
+            host: None,
+            port: None,
+            path: "/test/compiler/lib.rs",
+            query: None,
+            fragment: None,
+        },
+        diagnostic: Diagnostic {
+            range: Range {
+                start: Position {
+                    line: 0,
+                    character: 8,
+                },
+                end: Position {
+                    line: 0,
+                    character: 19,
+                },
+            },
+            severity: Some(
+                Hint,
+            ),
+            code: Some(
+                String(
+                    "trivially_copy_pass_by_ref",
+                ),
+            ),
+            code_description: Some(
+                CodeDescription {
+                    href: Url {
+                        scheme: "https",
+                        host: Some(
+                            Domain(
+                                "rust-lang.github.io",
+                            ),
+                        ),
+                        port: None,
+                        path: "/rust-clippy/master/index.html",
+                        query: None,
+                        fragment: Some(
+                            "trivially_copy_pass_by_ref",
+                        ),
+                    },
+                },
+            ),
+            source: Some(
+                "clippy",
+            ),
+            message: "lint level defined here",
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/compiler/mir/tagset.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 41,
+                                    character: 23,
+                                },
+                                end: Position {
+                                    line: 41,
+                                    character: 28,
+                                },
+                            },
+                        },
+                        message: "original diagnostic",
+                    },
+                ],
+            ),
+            tags: None,
+            data: None,
+        },
+        fixes: [],
+    },
+    MappedRustDiagnostic {
+        url: Url {
+            scheme: "file",
+            host: None,
+            port: None,
+            path: "/test/compiler/mir/tagset.rs",
+            query: None,
+            fragment: None,
+        },
+        diagnostic: Diagnostic {
+            range: Range {
+                start: Position {
+                    line: 41,
+                    character: 23,
+                },
+                end: Position {
+                    line: 41,
+                    character: 28,
+                },
+            },
+            severity: Some(
+                Hint,
+            ),
+            code: Some(
+                String(
+                    "trivially_copy_pass_by_ref",
+                ),
+            ),
+            code_description: Some(
+                CodeDescription {
+                    href: Url {
+                        scheme: "https",
+                        host: Some(
+                            Domain(
+                                "rust-lang.github.io",
+                            ),
+                        ),
+                        port: None,
+                        path: "/rust-clippy/master/index.html",
+                        query: None,
+                        fragment: Some(
+                            "trivially_copy_pass_by_ref",
+                        ),
+                    },
+                },
+            ),
+            source: Some(
+                "clippy",
+            ),
+            message: "consider passing by value instead",
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/compiler/mir/tagset.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 41,
+                                    character: 23,
+                                },
+                                end: Position {
+                                    line: 41,
+                                    character: 28,
+                                },
+                            },
+                        },
+                        message: "original diagnostic",
+                    },
+                ],
+            ),
+            tags: None,
+            data: None,
+        },
+        fixes: [],
+    },
 ]
diff --git a/crates/rust-analyzer/src/diagnostics/test_data/macro_compiler_error.txt b/crates/rust-analyzer/src/diagnostics/test_data/macro_compiler_error.txt
index bbec6a796..bdcf2a38f 100644
--- a/crates/rust-analyzer/src/diagnostics/test_data/macro_compiler_error.txt
+++ b/crates/rust-analyzer/src/diagnostics/test_data/macro_compiler_error.txt
@@ -4,19 +4,19 @@
             scheme: "file",
             host: None,
             port: None,
-            path: "/test/crates/hir_def/src/data.rs",
+            path: "/test/crates/hir_def/src/path.rs",
             query: None,
             fragment: None,
         },
         diagnostic: Diagnostic {
             range: Range {
                 start: Position {
-                    line: 79,
-                    character: 15,
+                    line: 264,
+                    character: 8,
                 },
                 end: Position {
-                    line: 79,
-                    character: 41,
+                    line: 264,
+                    character: 76,
                 },
             },
             severity: Some(
@@ -36,22 +36,22 @@
                                 scheme: "file",
                                 host: None,
                                 port: None,
-                                path: "/test/crates/hir_def/src/path.rs",
+                                path: "/test/crates/hir_def/src/data.rs",
                                 query: None,
                                 fragment: None,
                             },
                             range: Range {
                                 start: Position {
-                                    line: 264,
-                                    character: 8,
+                                    line: 79,
+                                    character: 15,
                                 },
                                 end: Position {
-                                    line: 264,
-                                    character: 76,
+                                    line: 79,
+                                    character: 41,
                                 },
                             },
                         },
-                        message: "Error originated from macro here",
+                        message: "Exact error occurred here",
                     },
                 ],
             ),
@@ -65,19 +65,19 @@
             scheme: "file",
             host: None,
             port: None,
-            path: "/test/crates/hir_def/src/path.rs",
+            path: "/test/crates/hir_def/src/data.rs",
             query: None,
             fragment: None,
         },
         diagnostic: Diagnostic {
             range: Range {
                 start: Position {
-                    line: 264,
-                    character: 8,
+                    line: 79,
+                    character: 15,
                 },
                 end: Position {
-                    line: 264,
-                    character: 76,
+                    line: 79,
+                    character: 41,
                 },
             },
             severity: Some(
@@ -89,33 +89,7 @@
                 "rustc",
             ),
             message: "Please register your known path in the path module",
-            related_information: Some(
-                [
-                    DiagnosticRelatedInformation {
-                        location: Location {
-                            uri: Url {
-                                scheme: "file",
-                                host: None,
-                                port: None,
-                                path: "/test/crates/hir_def/src/data.rs",
-                                query: None,
-                                fragment: None,
-                            },
-                            range: Range {
-                                start: Position {
-                                    line: 79,
-                                    character: 15,
-                                },
-                                end: Position {
-                                    line: 79,
-                                    character: 41,
-                                },
-                            },
-                        },
-                        message: "Exact error occured here",
-                    },
-                ],
-            ),
+            related_information: None,
             tags: None,
             data: None,
         },
diff --git a/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable.txt b/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable.txt
index c709de95f..23d42b4d0 100644
--- a/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable.txt
+++ b/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable.txt
@@ -32,7 +32,33 @@
                 "rustc",
             ),
             message: "unused variable: `foo`\n#[warn(unused_variables)] on by default",
-            related_information: None,
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/driver/subcommand/repl.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 290,
+                                    character: 8,
+                                },
+                                end: Position {
+                                    line: 290,
+                                    character: 11,
+                                },
+                            },
+                        },
+                        message: "consider prefixing with an underscore",
+                    },
+                ],
+            ),
             tags: Some(
                 [
                     Unnecessary,
@@ -87,4 +113,114 @@
             },
         ],
     },
+    MappedRustDiagnostic {
+        url: Url {
+            scheme: "file",
+            host: None,
+            port: None,
+            path: "/test/driver/subcommand/repl.rs",
+            query: None,
+            fragment: None,
+        },
+        diagnostic: Diagnostic {
+            range: Range {
+                start: Position {
+                    line: 290,
+                    character: 8,
+                },
+                end: Position {
+                    line: 290,
+                    character: 11,
+                },
+            },
+            severity: Some(
+                Hint,
+            ),
+            code: Some(
+                String(
+                    "unused_variables",
+                ),
+            ),
+            code_description: None,
+            source: Some(
+                "rustc",
+            ),
+            message: "consider prefixing with an underscore",
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/driver/subcommand/repl.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 290,
+                                    character: 8,
+                                },
+                                end: Position {
+                                    line: 290,
+                                    character: 11,
+                                },
+                            },
+                        },
+                        message: "original diagnostic",
+                    },
+                ],
+            ),
+            tags: None,
+            data: None,
+        },
+        fixes: [
+            CodeAction {
+                title: "consider prefixing with an underscore",
+                group: None,
+                kind: Some(
+                    CodeActionKind(
+                        "quickfix",
+                    ),
+                ),
+                edit: Some(
+                    SnippetWorkspaceEdit {
+                        changes: Some(
+                            {
+                                Url {
+                                    scheme: "file",
+                                    host: None,
+                                    port: None,
+                                    path: "/test/driver/subcommand/repl.rs",
+                                    query: None,
+                                    fragment: None,
+                                }: [
+                                    TextEdit {
+                                        range: Range {
+                                            start: Position {
+                                                line: 290,
+                                                character: 8,
+                                            },
+                                            end: Position {
+                                                line: 290,
+                                                character: 11,
+                                            },
+                                        },
+                                        new_text: "_foo",
+                                    },
+                                ],
+                            },
+                        ),
+                        document_changes: None,
+                    },
+                ),
+                is_preferred: Some(
+                    true,
+                ),
+                data: None,
+            },
+        ],
+    },
 ]
diff --git a/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable_as_hint.txt b/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable_as_hint.txt
index 632f438d7..4e428bedc 100644
--- a/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable_as_hint.txt
+++ b/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable_as_hint.txt
@@ -32,7 +32,33 @@
                 "rustc",
             ),
             message: "unused variable: `foo`\n#[warn(unused_variables)] on by default",
-            related_information: None,
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/driver/subcommand/repl.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 290,
+                                    character: 8,
+                                },
+                                end: Position {
+                                    line: 290,
+                                    character: 11,
+                                },
+                            },
+                        },
+                        message: "consider prefixing with an underscore",
+                    },
+                ],
+            ),
             tags: Some(
                 [
                     Unnecessary,
@@ -87,4 +113,114 @@
             },
         ],
     },
+    MappedRustDiagnostic {
+        url: Url {
+            scheme: "file",
+            host: None,
+            port: None,
+            path: "/test/driver/subcommand/repl.rs",
+            query: None,
+            fragment: None,
+        },
+        diagnostic: Diagnostic {
+            range: Range {
+                start: Position {
+                    line: 290,
+                    character: 8,
+                },
+                end: Position {
+                    line: 290,
+                    character: 11,
+                },
+            },
+            severity: Some(
+                Hint,
+            ),
+            code: Some(
+                String(
+                    "unused_variables",
+                ),
+            ),
+            code_description: None,
+            source: Some(
+                "rustc",
+            ),
+            message: "consider prefixing with an underscore",
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/driver/subcommand/repl.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 290,
+                                    character: 8,
+                                },
+                                end: Position {
+                                    line: 290,
+                                    character: 11,
+                                },
+                            },
+                        },
+                        message: "original diagnostic",
+                    },
+                ],
+            ),
+            tags: None,
+            data: None,
+        },
+        fixes: [
+            CodeAction {
+                title: "consider prefixing with an underscore",
+                group: None,
+                kind: Some(
+                    CodeActionKind(
+                        "quickfix",
+                    ),
+                ),
+                edit: Some(
+                    SnippetWorkspaceEdit {
+                        changes: Some(
+                            {
+                                Url {
+                                    scheme: "file",
+                                    host: None,
+                                    port: None,
+                                    path: "/test/driver/subcommand/repl.rs",
+                                    query: None,
+                                    fragment: None,
+                                }: [
+                                    TextEdit {
+                                        range: Range {
+                                            start: Position {
+                                                line: 290,
+                                                character: 8,
+                                            },
+                                            end: Position {
+                                                line: 290,
+                                                character: 11,
+                                            },
+                                        },
+                                        new_text: "_foo",
+                                    },
+                                ],
+                            },
+                        ),
+                        document_changes: None,
+                    },
+                ),
+                is_preferred: Some(
+                    true,
+                ),
+                data: None,
+            },
+        ],
+    },
 ]
diff --git a/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable_as_info.txt b/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable_as_info.txt
index c0b79428d..4ddd7efae 100644
--- a/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable_as_info.txt
+++ b/crates/rust-analyzer/src/diagnostics/test_data/rustc_unused_variable_as_info.txt
@@ -32,7 +32,33 @@
                 "rustc",
             ),
             message: "unused variable: `foo`\n#[warn(unused_variables)] on by default",
-            related_information: None,
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/driver/subcommand/repl.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 290,
+                                    character: 8,
+                                },
+                                end: Position {
+                                    line: 290,
+                                    character: 11,
+                                },
+                            },
+                        },
+                        message: "consider prefixing with an underscore",
+                    },
+                ],
+            ),
             tags: Some(
                 [
                     Unnecessary,
@@ -87,4 +113,114 @@
             },
         ],
     },
+    MappedRustDiagnostic {
+        url: Url {
+            scheme: "file",
+            host: None,
+            port: None,
+            path: "/test/driver/subcommand/repl.rs",
+            query: None,
+            fragment: None,
+        },
+        diagnostic: Diagnostic {
+            range: Range {
+                start: Position {
+                    line: 290,
+                    character: 8,
+                },
+                end: Position {
+                    line: 290,
+                    character: 11,
+                },
+            },
+            severity: Some(
+                Hint,
+            ),
+            code: Some(
+                String(
+                    "unused_variables",
+                ),
+            ),
+            code_description: None,
+            source: Some(
+                "rustc",
+            ),
+            message: "consider prefixing with an underscore",
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/driver/subcommand/repl.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 290,
+                                    character: 8,
+                                },
+                                end: Position {
+                                    line: 290,
+                                    character: 11,
+                                },
+                            },
+                        },
+                        message: "original diagnostic",
+                    },
+                ],
+            ),
+            tags: None,
+            data: None,
+        },
+        fixes: [
+            CodeAction {
+                title: "consider prefixing with an underscore",
+                group: None,
+                kind: Some(
+                    CodeActionKind(
+                        "quickfix",
+                    ),
+                ),
+                edit: Some(
+                    SnippetWorkspaceEdit {
+                        changes: Some(
+                            {
+                                Url {
+                                    scheme: "file",
+                                    host: None,
+                                    port: None,
+                                    path: "/test/driver/subcommand/repl.rs",
+                                    query: None,
+                                    fragment: None,
+                                }: [
+                                    TextEdit {
+                                        range: Range {
+                                            start: Position {
+                                                line: 290,
+                                                character: 8,
+                                            },
+                                            end: Position {
+                                                line: 290,
+                                                character: 11,
+                                            },
+                                        },
+                                        new_text: "_foo",
+                                    },
+                                ],
+                            },
+                        ),
+                        document_changes: None,
+                    },
+                ),
+                is_preferred: Some(
+                    true,
+                ),
+                data: None,
+            },
+        ],
+    },
 ]
diff --git a/crates/rust-analyzer/src/diagnostics/test_data/rustc_wrong_number_of_parameters.txt b/crates/rust-analyzer/src/diagnostics/test_data/rustc_wrong_number_of_parameters.txt
index b9650f3e4..f455cf25e 100644
--- a/crates/rust-analyzer/src/diagnostics/test_data/rustc_wrong_number_of_parameters.txt
+++ b/crates/rust-analyzer/src/diagnostics/test_data/rustc_wrong_number_of_parameters.txt
@@ -81,4 +81,86 @@
         },
         fixes: [],
     },
+    MappedRustDiagnostic {
+        url: Url {
+            scheme: "file",
+            host: None,
+            port: None,
+            path: "/test/compiler/ty/select.rs",
+            query: None,
+            fragment: None,
+        },
+        diagnostic: Diagnostic {
+            range: Range {
+                start: Position {
+                    line: 218,
+                    character: 4,
+                },
+                end: Position {
+                    line: 230,
+                    character: 5,
+                },
+            },
+            severity: Some(
+                Hint,
+            ),
+            code: Some(
+                String(
+                    "E0061",
+                ),
+            ),
+            code_description: Some(
+                CodeDescription {
+                    href: Url {
+                        scheme: "https",
+                        host: Some(
+                            Domain(
+                                "doc.rust-lang.org",
+                            ),
+                        ),
+                        port: None,
+                        path: "/error-index.html",
+                        query: None,
+                        fragment: Some(
+                            "E0061",
+                        ),
+                    },
+                },
+            ),
+            source: Some(
+                "rustc",
+            ),
+            message: "defined here",
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/compiler/ty/select.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 103,
+                                    character: 17,
+                                },
+                                end: Position {
+                                    line: 103,
+                                    character: 29,
+                                },
+                            },
+                        },
+                        message: "original diagnostic",
+                    },
+                ],
+            ),
+            tags: None,
+            data: None,
+        },
+        fixes: [],
+    },
 ]
diff --git a/crates/rust-analyzer/src/diagnostics/test_data/snap_multi_line_fix.txt b/crates/rust-analyzer/src/diagnostics/test_data/snap_multi_line_fix.txt
index c45f68a91..4cbdb3b92 100644
--- a/crates/rust-analyzer/src/diagnostics/test_data/snap_multi_line_fix.txt
+++ b/crates/rust-analyzer/src/diagnostics/test_data/snap_multi_line_fix.txt
@@ -74,6 +74,309 @@
                         },
                         message: "unnecessary let binding",
                     },
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/src/main.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 2,
+                                    character: 4,
+                                },
+                                end: Position {
+                                    line: 2,
+                                    character: 30,
+                                },
+                            },
+                        },
+                        message: "return the expression directly",
+                    },
+                ],
+            ),
+            tags: None,
+            data: None,
+        },
+        fixes: [
+            CodeAction {
+                title: "return the expression directly",
+                group: None,
+                kind: Some(
+                    CodeActionKind(
+                        "quickfix",
+                    ),
+                ),
+                edit: Some(
+                    SnippetWorkspaceEdit {
+                        changes: Some(
+                            {
+                                Url {
+                                    scheme: "file",
+                                    host: None,
+                                    port: None,
+                                    path: "/test/src/main.rs",
+                                    query: None,
+                                    fragment: None,
+                                }: [
+                                    TextEdit {
+                                        range: Range {
+                                            start: Position {
+                                                line: 2,
+                                                character: 4,
+                                            },
+                                            end: Position {
+                                                line: 2,
+                                                character: 30,
+                                            },
+                                        },
+                                        new_text: "",
+                                    },
+                                    TextEdit {
+                                        range: Range {
+                                            start: Position {
+                                                line: 3,
+                                                character: 4,
+                                            },
+                                            end: Position {
+                                                line: 3,
+                                                character: 5,
+                                            },
+                                        },
+                                        new_text: "(0..10).collect()",
+                                    },
+                                ],
+                            },
+                        ),
+                        document_changes: None,
+                    },
+                ),
+                is_preferred: Some(
+                    true,
+                ),
+                data: None,
+            },
+        ],
+    },
+    MappedRustDiagnostic {
+        url: Url {
+            scheme: "file",
+            host: None,
+            port: None,
+            path: "/test/src/main.rs",
+            query: None,
+            fragment: None,
+        },
+        diagnostic: Diagnostic {
+            range: Range {
+                start: Position {
+                    line: 2,
+                    character: 4,
+                },
+                end: Position {
+                    line: 2,
+                    character: 30,
+                },
+            },
+            severity: Some(
+                Hint,
+            ),
+            code: Some(
+                String(
+                    "let_and_return",
+                ),
+            ),
+            code_description: Some(
+                CodeDescription {
+                    href: Url {
+                        scheme: "https",
+                        host: Some(
+                            Domain(
+                                "rust-lang.github.io",
+                            ),
+                        ),
+                        port: None,
+                        path: "/rust-clippy/master/index.html",
+                        query: None,
+                        fragment: Some(
+                            "let_and_return",
+                        ),
+                    },
+                },
+            ),
+            source: Some(
+                "clippy",
+            ),
+            message: "unnecessary let binding",
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/src/main.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 3,
+                                    character: 4,
+                                },
+                                end: Position {
+                                    line: 3,
+                                    character: 5,
+                                },
+                            },
+                        },
+                        message: "original diagnostic",
+                    },
+                ],
+            ),
+            tags: None,
+            data: None,
+        },
+        fixes: [
+            CodeAction {
+                title: "return the expression directly",
+                group: None,
+                kind: Some(
+                    CodeActionKind(
+                        "quickfix",
+                    ),
+                ),
+                edit: Some(
+                    SnippetWorkspaceEdit {
+                        changes: Some(
+                            {
+                                Url {
+                                    scheme: "file",
+                                    host: None,
+                                    port: None,
+                                    path: "/test/src/main.rs",
+                                    query: None,
+                                    fragment: None,
+                                }: [
+                                    TextEdit {
+                                        range: Range {
+                                            start: Position {
+                                                line: 2,
+                                                character: 4,
+                                            },
+                                            end: Position {
+                                                line: 2,
+                                                character: 30,
+                                            },
+                                        },
+                                        new_text: "",
+                                    },
+                                    TextEdit {
+                                        range: Range {
+                                            start: Position {
+                                                line: 3,
+                                                character: 4,
+                                            },
+                                            end: Position {
+                                                line: 3,
+                                                character: 5,
+                                            },
+                                        },
+                                        new_text: "(0..10).collect()",
+                                    },
+                                ],
+                            },
+                        ),
+                        document_changes: None,
+                    },
+                ),
+                is_preferred: Some(
+                    true,
+                ),
+                data: None,
+            },
+        ],
+    },
+    MappedRustDiagnostic {
+        url: Url {
+            scheme: "file",
+            host: None,
+            port: None,
+            path: "/test/src/main.rs",
+            query: None,
+            fragment: None,
+        },
+        diagnostic: Diagnostic {
+            range: Range {
+                start: Position {
+                    line: 2,
+                    character: 4,
+                },
+                end: Position {
+                    line: 2,
+                    character: 30,
+                },
+            },
+            severity: Some(
+                Hint,
+            ),
+            code: Some(
+                String(
+                    "let_and_return",
+                ),
+            ),
+            code_description: Some(
+                CodeDescription {
+                    href: Url {
+                        scheme: "https",
+                        host: Some(
+                            Domain(
+                                "rust-lang.github.io",
+                            ),
+                        ),
+                        port: None,
+                        path: "/rust-clippy/master/index.html",
+                        query: None,
+                        fragment: Some(
+                            "let_and_return",
+                        ),
+                    },
+                },
+            ),
+            source: Some(
+                "clippy",
+            ),
+            message: "return the expression directly",
+            related_information: Some(
+                [
+                    DiagnosticRelatedInformation {
+                        location: Location {
+                            uri: Url {
+                                scheme: "file",
+                                host: None,
+                                port: None,
+                                path: "/test/src/main.rs",
+                                query: None,
+                                fragment: None,
+                            },
+                            range: Range {
+                                start: Position {
+                                    line: 3,
+                                    character: 4,
+                                },
+                                end: Position {
+                                    line: 3,
+                                    character: 5,
+                                },
+                            },
+                        },
+                        message: "original diagnostic",
+                    },
                 ],
             ),
             tags: None,
diff --git a/crates/rust-analyzer/src/diagnostics/to_proto.rs b/crates/rust-analyzer/src/diagnostics/to_proto.rs
index 324019614..f16f97131 100644
--- a/crates/rust-analyzer/src/diagnostics/to_proto.rs
+++ b/crates/rust-analyzer/src/diagnostics/to_proto.rs
@@ -75,8 +75,10 @@ fn diagnostic_related_information(
 }
 
 enum MappedRustChildDiagnostic {
-    Related(lsp_types::DiagnosticRelatedInformation),
-    SuggestedFix(lsp_ext::CodeAction),
+    Related {
+        related: lsp_types::DiagnosticRelatedInformation,
+        suggested_fix: Option<lsp_ext::CodeAction>,
+    },
     MessageLine(String),
 }
 
@@ -103,23 +105,32 @@ fn map_rust_child_diagnostic(
     }
 
     if edit_map.is_empty() {
-        MappedRustChildDiagnostic::Related(lsp_types::DiagnosticRelatedInformation {
-            location: location(workspace_root, spans[0]),
-            message: rd.message.clone(),
-        })
+        MappedRustChildDiagnostic::Related {
+            related: lsp_types::DiagnosticRelatedInformation {
+                location: location(workspace_root, spans[0]),
+                message: rd.message.clone(),
+            },
+            suggested_fix: None,
+        }
     } else {
-        MappedRustChildDiagnostic::SuggestedFix(lsp_ext::CodeAction {
-            title: rd.message.clone(),
-            group: None,
-            kind: Some(lsp_types::CodeActionKind::QUICKFIX),
-            edit: Some(lsp_ext::SnippetWorkspaceEdit {
-                // FIXME: there's no good reason to use edit_map here....
-                changes: Some(edit_map),
-                document_changes: None,
+        MappedRustChildDiagnostic::Related {
+            related: lsp_types::DiagnosticRelatedInformation {
+                location: location(workspace_root, spans[0]),
+                message: rd.message.clone(),
+            },
+            suggested_fix: Some(lsp_ext::CodeAction {
+                title: rd.message.clone(),
+                group: None,
+                kind: Some(lsp_types::CodeActionKind::QUICKFIX),
+                edit: Some(lsp_ext::SnippetWorkspaceEdit {
+                    // FIXME: there's no good reason to use edit_map here....
+                    changes: Some(edit_map),
+                    document_changes: None,
+                }),
+                is_preferred: Some(true),
+                data: None,
             }),
-            is_preferred: Some(true),
-            data: None,
-        })
+        }
     }
 }
 
@@ -179,8 +190,12 @@ pub(crate) fn map_rust_diagnostic_to_lsp(
     for child in &rd.children {
         let child = map_rust_child_diagnostic(workspace_root, &child);
         match child {
-            MappedRustChildDiagnostic::Related(related) => related_information.push(related),
-            MappedRustChildDiagnostic::SuggestedFix(code_action) => fixes.push(code_action),
+            MappedRustChildDiagnostic::Related { related, suggested_fix } => {
+                related_information.push(related);
+                if let Some(code_action) = suggested_fix {
+                    fixes.push(code_action);
+                }
+            }
             MappedRustChildDiagnostic::MessageLine(message_line) => {
                 format_to!(message, "\n{}", message_line);
 
@@ -219,7 +234,7 @@ pub(crate) fn map_rust_diagnostic_to_lsp(
 
     primary_spans
         .iter()
-        .map(|primary_span| {
+        .flat_map(|primary_span| {
             let location = location(workspace_root, &primary_span);
 
             let mut message = message.clone();
@@ -229,72 +244,100 @@ pub(crate) fn map_rust_diagnostic_to_lsp(
                 }
             }
 
+            // Each primary diagnostic span may result in multiple LSP diagnostics.
+            let mut diagnostics = Vec::new();
+
+            let mut related_macro_info = None;
+
             // If error occurs from macro expansion, add related info pointing to
             // where the error originated
             // Also, we would generate an additional diagnostic, so that exact place of macro
             // will be highlighted in the error origin place.
-            let additional_diagnostic =
-                if !is_from_macro(&primary_span.file_name) && primary_span.expansion.is_some() {
-                    let in_macro_location = location_naive(workspace_root, &primary_span);
+            if !is_from_macro(&primary_span.file_name) && primary_span.expansion.is_some() {
+                let in_macro_location = location_naive(workspace_root, &primary_span);
 
-                    // Add related information for the main disagnostic.
-                    related_information.push(lsp_types::DiagnosticRelatedInformation {
-                        location: in_macro_location.clone(),
-                        message: "Error originated from macro here".to_string(),
-                    });
+                // Add related information for the main disagnostic.
+                related_macro_info = Some(lsp_types::DiagnosticRelatedInformation {
+                    location: in_macro_location.clone(),
+                    message: "Error originated from macro here".to_string(),
+                });
 
-                    // For the additional in-macro diagnostic we add the inverse message pointing to the error location in code.
-                    let information_for_additional_diagnostic =
-                        vec![lsp_types::DiagnosticRelatedInformation {
-                            location: location.clone(),
-                            message: "Exact error occured here".to_string(),
-                        }];
+                // For the additional in-macro diagnostic we add the inverse message pointing to the error location in code.
+                let information_for_additional_diagnostic =
+                    vec![lsp_types::DiagnosticRelatedInformation {
+                        location: location.clone(),
+                        message: "Exact error occurred here".to_string(),
+                    }];
 
-                    let diagnostic = lsp_types::Diagnostic {
-                        range: in_macro_location.range,
-                        severity,
-                        code: code.clone().map(lsp_types::NumberOrString::String),
-                        code_description: code_description.clone(),
-                        source: Some(source.clone()),
-                        message: message.clone(),
-                        related_information: Some(information_for_additional_diagnostic),
-                        tags: if tags.is_empty() { None } else { Some(tags.clone()) },
-                        data: None,
-                    };
-
-                    Some(MappedRustDiagnostic {
-                        url: in_macro_location.uri,
-                        diagnostic,
-                        fixes: fixes.clone(),
-                    })
-                } else {
-                    None
+                let diagnostic = lsp_types::Diagnostic {
+                    range: in_macro_location.range,
+                    severity,
+                    code: code.clone().map(lsp_types::NumberOrString::String),
+                    code_description: code_description.clone(),
+                    source: Some(source.clone()),
+                    message: message.clone(),
+                    related_information: Some(information_for_additional_diagnostic),
+                    tags: if tags.is_empty() { None } else { Some(tags.clone()) },
+                    data: None,
                 };
 
-            let diagnostic = lsp_types::Diagnostic {
-                range: location.range,
-                severity,
-                code: code.clone().map(lsp_types::NumberOrString::String),
-                code_description: code_description.clone(),
-                source: Some(source.clone()),
-                message,
-                related_information: if related_information.is_empty() {
-                    None
-                } else {
-                    Some(related_information.clone())
+                diagnostics.push(MappedRustDiagnostic {
+                    url: in_macro_location.uri,
+                    diagnostic,
+                    fixes: fixes.clone(),
+                });
+            }
+
+            // Emit the primary diagnostic.
+            diagnostics.push(MappedRustDiagnostic {
+                url: location.uri.clone(),
+                diagnostic: lsp_types::Diagnostic {
+                    range: location.range,
+                    severity,
+                    code: code.clone().map(lsp_types::NumberOrString::String),
+                    code_description: code_description.clone(),
+                    source: Some(source.clone()),
+                    message,
+                    related_information: if related_information.is_empty() {
+                        None
+                    } else {
+                        let mut related = related_information.clone();
+                        related.extend(related_macro_info);
+                        Some(related)
+                    },
+                    tags: if tags.is_empty() { None } else { Some(tags.clone()) },
+                    data: None,
                 },
-                tags: if tags.is_empty() { None } else { Some(tags.clone()) },
-                data: None,
-            };
+                fixes: fixes.clone(),
+            });
 
-            let main_diagnostic =
-                MappedRustDiagnostic { url: location.uri, diagnostic, fixes: fixes.clone() };
-            match additional_diagnostic {
-                None => vec![main_diagnostic],
-                Some(additional_diagnostic) => vec![main_diagnostic, additional_diagnostic],
+            // Emit hint-level diagnostics for all `related_information` entries such as "help"s.
+            // This is useful because they will show up in the user's editor, unlike
+            // `related_information`, which just produces hard-to-read links, at least in VS Code.
+            let back_ref = lsp_types::DiagnosticRelatedInformation {
+                location,
+                message: "original diagnostic".to_string(),
+            };
+            for info in &related_information {
+                diagnostics.push(MappedRustDiagnostic {
+                    url: info.location.uri.clone(),
+                    fixes: fixes.clone(), // share fixes to make them easier to apply
+                    diagnostic: lsp_types::Diagnostic {
+                        range: info.location.range,
+                        severity: Some(lsp_types::DiagnosticSeverity::Hint),
+                        code: code.clone().map(lsp_types::NumberOrString::String),
+                        code_description: code_description.clone(),
+                        source: Some(source.clone()),
+                        message: info.message.clone(),
+                        related_information: Some(vec![back_ref.clone()]),
+                        tags: None, // don't apply modifiers again
+                        data: None,
+                    },
+                });
             }
+
+            diagnostics
         })
-        .flatten()
         .collect()
 }