feat(add): Stabilize MSRV-aware version req selection

This is part of rust-lang#9930 for rust-lang/rfcs#3537

This will make it easier to maintain an MSRV-compliant `Cargo.toml` but
leaves validation up to the user as the resolver will pick newer
versions.
This helps the MSRV-aware workflows enumerated in
rust-lang/rfcs#3537 though it could be confusing to the workflow with an
MSRV-compatible lockfile.
PR rust-lang#13561 at least raises awareness of that discrepancy.

There is an unresolved question on differences in the resolver vs
`cargo add` for dealing with an unset `rust-version`.
However, we are only stabilizing the `cargo add` side which is a very
light two-way door as this is a UX-focused command and not a
programmatic command.

This hasn't gotten much end-user testing but, as its UX focused, that
seems fine.

As such, this seems like it is ready to stabilize.

src/bin/cargo/commands/add.rs

@@ -87,7 +87,7 @@ Example uses:
 - Depend on crates with the same name from different registries"),
             flag(
                 "ignore-rust-version",
-                "Ignore `rust-version` specification in packages (unstable)"
+                "Ignore `rust-version` specification in packages"
             ),
         ])
         .arg_manifest_path_without_unsupported_path_tip()
@@ -206,14 +206,6 @@ pub fn exec(gctx: &mut GlobalContext, args: &ArgMatches) -> CliResult {
     let dependencies = parse_dependencies(gctx, args)?;
 
     let ignore_rust_version = args.flag("ignore-rust-version");
-    if ignore_rust_version && !gctx.cli_unstable().msrv_policy {
-        return Err(CliError::new(
-            anyhow::format_err!(
-                "`--ignore-rust-version` is unstable; pass `-Zmsrv-policy` to enable support for it"
-            ),
-            101,
-        ));
-    }
     let honor_rust_version = !ignore_rust_version;
 
     let options = AddOptions {

src/cargo/ops/cargo_add/mod.rs

@@ -615,7 +615,7 @@ fn get_latest_dependency(
                 )
             })?;
 
-            if gctx.cli_unstable().msrv_policy && honor_rust_version {
+            if honor_rust_version {
                 let (req_msrv, is_msrv) = spec
                     .rust_version()
                     .cloned()

src/doc/man/cargo-add.md

@@ -141,11 +141,6 @@ which enables all specified features.
 
 {{#option "`--ignore-rust-version`" }}
 Ignore `rust-version` specification in packages.
-
-This option is unstable and available only on the
-[nightly channel](https://doc.rust-lang.org/book/appendix-07-nightly-rust.html)
-and requires the `-Z unstable-options` flag to enable.
-See <https://github.com/rust-lang/cargo/issues/5579> for more information.
 {{/option}}
 
 {{/options}}

src/doc/man/generated_txt/cargo-add.txt

@@ -131,12 +131,6 @@ OPTIONS
        --ignore-rust-version
            Ignore rust-version specification in packages.
 
-           This option is unstable and available only on the nightly channel
-           <https://doc.rust-lang.org/book/appendix-07-nightly-rust.html> and
-           requires the -Z unstable-options flag to enable. See
-           <https://github.com/rust-lang/cargo/issues/5579> for more
-           information.
-
    Display Options
        -v, --verbose
            Use verbose output. May be specified twice for “very verbose”

src/doc/src/commands/cargo-add.md

@@ -137,11 +137,7 @@ which enables all specified features.</dd>
 
 
 <dt class="option-term" id="option-cargo-add---ignore-rust-version"><a class="option-anchor" href="#option-cargo-add---ignore-rust-version"></a><code>--ignore-rust-version</code></dt>
-<dd class="option-desc">Ignore <code>rust-version</code> specification in packages.</p>
-<p>This option is unstable and available only on the
-<a href="https://doc.rust-lang.org/book/appendix-07-nightly-rust.html">nightly channel</a>
-and requires the <code>-Z unstable-options</code> flag to enable.
-See <a href="https://github.com/rust-lang/cargo/issues/5579">https://github.com/rust-lang/cargo/issues/5579</a> for more information.</dd>
+<dd class="option-desc">Ignore <code>rust-version</code> specification in packages.</dd>
 
 
 </dl>

src/doc/src/reference/unstable.md

@@ -326,8 +326,6 @@ Documentation updates:
 
 ## msrv-policy
 - [#9930](https://github.com/rust-lang/cargo/issues/9930) (MSRV-aware resolver)
-- [#10653](https://github.com/rust-lang/cargo/issues/10653) (MSRV-aware cargo-add)
-- [#10903](https://github.com/rust-lang/cargo/issues/10903) (MSRV-aware cargo-install)
 
 The `msrv-policy` feature enables experiments in MSRV-aware policy for cargo in
 preparation for an upcoming RFC.

src/etc/man/cargo-add.1

@@ -162,11 +162,6 @@ which enables all specified features.
 \fB\-\-ignore\-rust\-version\fR
 .RS 4
 Ignore \fBrust\-version\fR specification in packages.
-.sp
-This option is unstable and available only on the
-\fInightly channel\fR <https://doc.rust\-lang.org/book/appendix\-07\-nightly\-rust.html>
-and requires the \fB\-Z unstable\-options\fR flag to enable.
-See <https://github.com/rust\-lang/cargo/issues/5579> for more information.
 .RE
 .SS "Display Options"
 .sp