Add help for potentially missing crate in Cargo.toml

On resolve errors where there might be a missing crate, mention `cargo add foo`:

```
error[E0433]: failed to resolve: use of unresolved module or unlinked crate `nope`
  --> $DIR/conflicting-impl-with-err.rs:4:11
   |
LL | impl From<nope::Thing> for Error {
   |           ^^^^ use of unresolved module or unlinked crate `nope`
   |
   = help: if you wanted to use a crate named `nope`, use `cargo add nope` to add it to your `Cargo.toml`
```

Author: estebank

compiler/rustc_resolve/src/diagnostics.rs

@@ -809,7 +809,6 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                     }
                     err.multipart_suggestion(msg, suggestions, applicability);
                 }
-
                 if let Some(ModuleOrUniformRoot::Module(module)) = module
                     && let Some(module) = module.opt_def_id()
                     && let Some(segment) = segment
@@ -2046,7 +2045,10 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                             self.current_crate_outer_attr_insert_span,
                             format!("extern crate {ident};\n"),
                         )],
-                        format!("consider importing the `{ident}` crate"),
+                        format!(
+                            "if you wanted to use a crate named `{ident}`, use `cargo add {ident}` \
+                            to add it to your `Cargo.toml` and import it in your code",
+                        ),
                         Applicability::MaybeIncorrect,
                     )),
                 )
@@ -2225,6 +2227,14 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                 let descr = binding.res().descr();
                 (format!("{descr} `{ident}` is not a crate or module"), suggestion)
             } else {
+                let suggestion = suggestion.or(Some((
+                    vec![],
+                    format!(
+                        "if you wanted to use a crate named `{ident}`, use `cargo add {ident}` to \
+                         add it to your `Cargo.toml`",
+                    ),
+                    Applicability::MaybeIncorrect,
+                )));
                 (format!("use of unresolved module or unlinked crate `{ident}`"), suggestion)
             }
         }

tests/rustdoc-ui/ice-unresolved-import-100241.stderr

@@ -4,7 +4,7 @@ error[E0432]: unresolved import `inner`
 LL |     pub use inner::S;
    |             ^^^^^ use of unresolved module or unlinked crate `inner`
    |
-help: consider importing the `inner` crate
+help: if you wanted to use a crate named `inner`, use `cargo add inner` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate inner;
    |

tests/rustdoc-ui/intra-doc/unresolved-import-recovery.stderr

@@ -4,7 +4,7 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `unr
 LL | use unresolved_crate::module::Name;
    |     ^^^^^^^^^^^^^^^^ use of unresolved module or unlinked crate `unresolved_crate`
    |
-help: consider importing the `unresolved_crate` crate
+help: if you wanted to use a crate named `unresolved_crate`, use `cargo add unresolved_crate` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate unresolved_crate;
    |

tests/rustdoc-ui/issues/issue-61732.stderr

@@ -4,7 +4,7 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `r#m
 LL | pub(in crate::r#mod) fn main() {}
    |               ^^^^^ use of unresolved module or unlinked crate `r#mod`
    |
-help: consider importing the `r#mod` crate
+help: if you wanted to use a crate named `r#mod`, use `cargo add r#mod` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate r#mod;
    |

tests/ui/attributes/field-attributes-vis-unresolved.stderr

@@ -4,7 +4,7 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `non
 LL |     pub(in nonexistent) field: u8
    |            ^^^^^^^^^^^ use of unresolved module or unlinked crate `nonexistent`
    |
-help: consider importing the `nonexistent` crate
+help: if you wanted to use a crate named `nonexistent`, use `cargo add nonexistent` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate nonexistent;
    |
@@ -15,7 +15,7 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `non
 LL |     pub(in nonexistent) u8
    |            ^^^^^^^^^^^ use of unresolved module or unlinked crate `nonexistent`
    |
-help: consider importing the `nonexistent` crate
+help: if you wanted to use a crate named `nonexistent`, use `cargo add nonexistent` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate nonexistent;
    |

tests/ui/coherence/conflicting-impl-with-err.stderr

@@ -3,12 +3,16 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `nop
    |
 LL | impl From<nope::Thing> for Error {
    |           ^^^^ use of unresolved module or unlinked crate `nope`
+   |
+   = help: if you wanted to use a crate named `nope`, use `cargo add nope` to add it to your `Cargo.toml`
 
 error[E0433]: failed to resolve: use of unresolved module or unlinked crate `nope`
   --> $DIR/conflicting-impl-with-err.rs:5:16
    |
 LL |     fn from(_: nope::Thing) -> Self {
    |                ^^^^ use of unresolved module or unlinked crate `nope`
+   |
+   = help: if you wanted to use a crate named `nope`, use `cargo add nope` to add it to your `Cargo.toml`
 
 error: aborting due to 2 previous errors
 

tests/ui/delegation/bad-resolve.stderr

@@ -86,6 +86,8 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `unr
    |
 LL | reuse unresolved_prefix::{a, b, c};
    |       ^^^^^^^^^^^^^^^^^ use of unresolved module or unlinked crate `unresolved_prefix`
+   |
+   = help: if you wanted to use a crate named `unresolved_prefix`, use `cargo add unresolved_prefix` to add it to your `Cargo.toml`
 
 error[E0433]: failed to resolve: `crate` in paths can only be used in start position
   --> $DIR/bad-resolve.rs:44:29

tests/ui/error-codes/E0432.stderr

@@ -4,7 +4,7 @@ error[E0432]: unresolved import `something`
 LL | use something::Foo;
    |     ^^^^^^^^^ use of unresolved module or unlinked crate `something`
    |
-help: consider importing the `something` crate
+help: if you wanted to use a crate named `something`, use `cargo add something` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate something;
    |

tests/ui/extern-flag/multiple-opts.stderr

@@ -3,6 +3,8 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `som
    |
 LL |     somedep::somefun();
    |     ^^^^^^^ use of unresolved module or unlinked crate `somedep`
+   |
+   = help: if you wanted to use a crate named `somedep`, use `cargo add somedep` to add it to your `Cargo.toml`
 
 error: aborting due to 1 previous error
 

tests/ui/extern-flag/noprelude.stderr

@@ -3,6 +3,8 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `som
    |
 LL |     somedep::somefun();
    |     ^^^^^^^ use of unresolved module or unlinked crate `somedep`
+   |
+   = help: if you wanted to use a crate named `somedep`, use `cargo add somedep` to add it to your `Cargo.toml`
 
 error: aborting due to 1 previous error
 

tests/ui/foreign/stashed-issue-121451.stderr

@@ -3,6 +3,8 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `lib
    |
 LL | extern "C" fn _f() -> libc::uintptr_t {}
    |                       ^^^^ use of unresolved module or unlinked crate `libc`
+   |
+   = help: if you wanted to use a crate named `libc`, use `cargo add libc` to add it to your `Cargo.toml`
 
 error: aborting due to 1 previous error
 

tests/ui/hygiene/extern-prelude-from-opaque-fail-2018.stderr

@@ -24,6 +24,7 @@ LL |         fn f() { my_core::mem::drop(0); }
 LL | a!();
    | ---- in this macro invocation
    |
+   = help: if you wanted to use a crate named `my_core`, use `cargo add my_core` to add it to your `Cargo.toml`
    = help: consider importing this module:
            std::mem
    = note: this error originates in the macro `a` (in Nightly builds, run with -Z macro-backtrace for more info)
@@ -34,6 +35,7 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `my_
 LL |     fn f() { my_core::mem::drop(0); }
    |              ^^^^^^^ use of unresolved module or unlinked crate `my_core`
    |
+   = help: if you wanted to use a crate named `my_core`, use `cargo add my_core` to add it to your `Cargo.toml`
 help: consider importing this module
    |
 LL +     use std::mem;

tests/ui/hygiene/extern-prelude-from-opaque-fail.stderr

@@ -24,6 +24,7 @@ LL |         fn f() { my_core::mem::drop(0); }
 LL | a!();
    | ---- in this macro invocation
    |
+   = help: if you wanted to use a crate named `my_core`, use `cargo add my_core` to add it to your `Cargo.toml`
    = help: consider importing this module:
            my_core::mem
    = note: this error originates in the macro `a` (in Nightly builds, run with -Z macro-backtrace for more info)
@@ -34,6 +35,7 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `my_
 LL |     fn f() { my_core::mem::drop(0); }
    |              ^^^^^^^ use of unresolved module or unlinked crate `my_core`
    |
+   = help: if you wanted to use a crate named `my_core`, use `cargo add my_core` to add it to your `Cargo.toml`
 help: consider importing this module
    |
 LL +     use my_core::mem;

tests/ui/impl-trait/issue-72911.stderr

@@ -3,12 +3,16 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `foo
    |
 LL | fn gather_from_file(dir_entry: &foo::MissingItem) -> impl Iterator<Item = Lint> {
    |                                 ^^^ use of unresolved module or unlinked crate `foo`
+   |
+   = help: if you wanted to use a crate named `foo`, use `cargo add foo` to add it to your `Cargo.toml`
 
 error[E0433]: failed to resolve: use of unresolved module or unlinked crate `foo`
   --> $DIR/issue-72911.rs:16:41
    |
 LL | fn lint_files() -> impl Iterator<Item = foo::MissingItem> {
    |                                         ^^^ use of unresolved module or unlinked crate `foo`
+   |
+   = help: if you wanted to use a crate named `foo`, use `cargo add foo` to add it to your `Cargo.toml`
 
 error: aborting due to 2 previous errors
 

tests/ui/impl-trait/stashed-diag-issue-121504.stderr

@@ -3,6 +3,8 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `xyz
    |
 LL | impl MyTrait for xyz::T {
    |                  ^^^ use of unresolved module or unlinked crate `xyz`
+   |
+   = help: if you wanted to use a crate named `xyz`, use `cargo add xyz` to add it to your `Cargo.toml`
 
 error: aborting due to 1 previous error
 

tests/ui/imports/import-from-missing-star-2.stderr

@@ -4,7 +4,7 @@ error[E0432]: unresolved import `spam`
 LL |     use spam::*;
    |         ^^^^ use of unresolved module or unlinked crate `spam`
    |
-help: consider importing the `spam` crate
+help: if you wanted to use a crate named `spam`, use `cargo add spam` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate spam;
    |

tests/ui/imports/import-from-missing-star-3.stderr

@@ -4,7 +4,7 @@ error[E0432]: unresolved import `spam`
 LL |     use spam::*;
    |         ^^^^ use of unresolved module or unlinked crate `spam`
    |
-help: consider importing the `spam` crate
+help: if you wanted to use a crate named `spam`, use `cargo add spam` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate spam;
    |
@@ -15,7 +15,7 @@ error[E0432]: unresolved import `spam`
 LL |         use spam::*;
    |             ^^^^ use of unresolved module or unlinked crate `spam`
    |
-help: consider importing the `spam` crate
+help: if you wanted to use a crate named `spam`, use `cargo add spam` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate spam;
    |

tests/ui/imports/import-from-missing-star.stderr

@@ -4,7 +4,7 @@ error[E0432]: unresolved import `spam`
 LL | use spam::*;
    |     ^^^^ use of unresolved module or unlinked crate `spam`
    |
-help: consider importing the `spam` crate
+help: if you wanted to use a crate named `spam`, use `cargo add spam` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate spam;
    |

tests/ui/imports/import3.stderr

@@ -4,7 +4,7 @@ error[E0432]: unresolved import `main`
 LL | use main::bar;
    |     ^^^^ use of unresolved module or unlinked crate `main`
    |
-help: consider importing the `main` crate
+help: if you wanted to use a crate named `main`, use `cargo add main` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate main;
    |

tests/ui/imports/issue-109343.stderr

@@ -4,7 +4,7 @@ error[E0432]: unresolved import `unresolved`
 LL | pub use unresolved::f;
    |         ^^^^^^^^^^ use of unresolved module or unlinked crate `unresolved`
    |
-help: consider importing the `unresolved` crate
+help: if you wanted to use a crate named `unresolved`, use `cargo add unresolved` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate unresolved;
    |

tests/ui/imports/issue-1697.rs

@@ -3,6 +3,6 @@
 use unresolved::*;
 //~^ ERROR unresolved import `unresolved` [E0432]
 //~| NOTE use of unresolved module or unlinked crate `unresolved`
-//~| HELP consider importing the `unresolved` crate
+//~| HELP if you wanted to use a crate named `unresolved`, use `cargo add unresolved` to add it to your `Cargo.toml`
 
 fn main() {}

tests/ui/imports/issue-1697.stderr

@@ -4,7 +4,7 @@ error[E0432]: unresolved import `unresolved`
 LL | use unresolved::*;
    |     ^^^^^^^^^^ use of unresolved module or unlinked crate `unresolved`
    |
-help: consider importing the `unresolved` crate
+help: if you wanted to use a crate named `unresolved`, use `cargo add unresolved` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate unresolved;
    |

tests/ui/imports/issue-33464.stderr

@@ -4,7 +4,7 @@ error[E0432]: unresolved import `abc`
 LL | use abc::one_el;
    |     ^^^ use of unresolved module or unlinked crate `abc`
    |
-help: consider importing the `abc` crate
+help: if you wanted to use a crate named `abc`, use `cargo add abc` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate abc;
    |
@@ -15,7 +15,7 @@ error[E0432]: unresolved import `abc`
 LL | use abc::{a, bbb, cccccc};
    |     ^^^ use of unresolved module or unlinked crate `abc`
    |
-help: consider importing the `abc` crate
+help: if you wanted to use a crate named `abc`, use `cargo add abc` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate abc;
    |
@@ -26,7 +26,7 @@ error[E0432]: unresolved import `a_very_long_name`
 LL | use a_very_long_name::{el, el2};
    |     ^^^^^^^^^^^^^^^^ use of unresolved module or unlinked crate `a_very_long_name`
    |
-help: consider importing the `a_very_long_name` crate
+help: if you wanted to use a crate named `a_very_long_name`, use `cargo add a_very_long_name` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate a_very_long_name;
    |

tests/ui/imports/issue-36881.stderr

@@ -4,7 +4,7 @@ error[E0432]: unresolved import `issue_36881_aux`
 LL |     use issue_36881_aux::Foo;
    |         ^^^^^^^^^^^^^^^ use of unresolved module or unlinked crate `issue_36881_aux`
    |
-help: consider importing the `issue_36881_aux` crate
+help: if you wanted to use a crate named `issue_36881_aux`, use `cargo add issue_36881_aux` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate issue_36881_aux;
    |

tests/ui/imports/issue-37887.stderr

@@ -4,7 +4,7 @@ error[E0432]: unresolved import `test`
 LL |     use test::*;
    |         ^^^^ use of unresolved module or unlinked crate `test`
    |
-help: consider importing the `test` crate
+help: if you wanted to use a crate named `test`, use `cargo add test` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate test;
    |

tests/ui/imports/issue-53269.stderr

@@ -4,7 +4,7 @@ error[E0432]: unresolved import `nonexistent_module`
 LL |     use nonexistent_module::mac;
    |         ^^^^^^^^^^^^^^^^^^ use of unresolved module or unlinked crate `nonexistent_module`
    |
-help: consider importing the `nonexistent_module` crate
+help: if you wanted to use a crate named `nonexistent_module`, use `cargo add nonexistent_module` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate nonexistent_module;
    |

tests/ui/imports/issue-55457.stderr

@@ -13,7 +13,7 @@ error[E0432]: unresolved import `non_existent`
 LL | use non_existent::non_existent;
    |     ^^^^^^^^^^^^ use of unresolved module or unlinked crate `non_existent`
    |
-help: consider importing the `non_existent` crate
+help: if you wanted to use a crate named `non_existent`, use `cargo add non_existent` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate non_existent;
    |

tests/ui/imports/issue-81413.stderr

@@ -4,7 +4,7 @@ error[E0432]: unresolved import `doesnt_exist`
 LL | pub use doesnt_exist::*;
    |         ^^^^^^^^^^^^ use of unresolved module or unlinked crate `doesnt_exist`
    |
-help: consider importing the `doesnt_exist` crate
+help: if you wanted to use a crate named `doesnt_exist`, use `cargo add doesnt_exist` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate doesnt_exist;
    |

tests/ui/imports/tool-mod-child.stderr

@@ -4,7 +4,7 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `cli
 LL | use clippy::a::b;
    |     ^^^^^^ use of unresolved module or unlinked crate `clippy`
    |
-help: consider importing the `clippy` crate
+help: if you wanted to use a crate named `clippy`, use `cargo add clippy` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate clippy;
    |
@@ -15,7 +15,7 @@ error[E0432]: unresolved import `clippy`
 LL | use clippy::a;
    |     ^^^^^^ use of unresolved module or unlinked crate `clippy`
    |
-help: consider importing the `clippy` crate
+help: if you wanted to use a crate named `clippy`, use `cargo add clippy` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate clippy;
    |
@@ -26,7 +26,7 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `rus
 LL | use rustdoc::a::b;
    |     ^^^^^^^ use of unresolved module or unlinked crate `rustdoc`
    |
-help: consider importing the `rustdoc` crate
+help: if you wanted to use a crate named `rustdoc`, use `cargo add rustdoc` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate rustdoc;
    |
@@ -37,7 +37,7 @@ error[E0432]: unresolved import `rustdoc`
 LL | use rustdoc::a;
    |     ^^^^^^^ use of unresolved module or unlinked crate `rustdoc`
    |
-help: consider importing the `rustdoc` crate
+help: if you wanted to use a crate named `rustdoc`, use `cargo add rustdoc` to add it to your `Cargo.toml` and import it in your code
    |
 LL + extern crate rustdoc;
    |