Introduce -Zprecise-pre-release unstable flag

This change introduces the feature but does not yet attempt an implementation.
The actual implementation will happen in future PRs

r? @epage

src/cargo/core/features.rs

@@ -779,6 +779,7 @@ unstable_cli_options!(
     next_lockfile_bump: bool,
     no_index_update: bool = ("Do not update the registry index even if the cache is outdated"),
     panic_abort_tests: bool = ("Enable support to run tests with -Cpanic=abort"),
+    precise_pre_release: bool = ("Enable pre-release versions to be selected with `update --precise`"),
     profile_rustflags: bool = ("Enable the `rustflags` option in profiles in .cargo/config.toml file"),
     publish_timeout: bool = ("Enable the `publish.timeout` key in .cargo/config.toml file"),
     rustdoc_map: bool = ("Allow passing external documentation mappings to rustdoc"),
@@ -1125,6 +1126,7 @@ impl CliUnstable {
             "no-index-update" => self.no_index_update = parse_empty(k, v)?,
             "panic-abort-tests" => self.panic_abort_tests = parse_empty(k, v)?,
             "profile-rustflags" => self.profile_rustflags = parse_empty(k, v)?,
+            "precise-pre-release" => self.precise_pre_release = parse_empty(k, v)?,
             "trim-paths" => self.trim_paths = parse_empty(k, v)?,
             "publish-timeout" => self.publish_timeout = parse_empty(k, v)?,
             "rustdoc-map" => self.rustdoc_map = parse_empty(k, v)?,

src/doc/src/reference/unstable.md

@@ -72,6 +72,7 @@ For the latest nightly, see the [nightly version] of this page.
     * [direct-minimal-versions](#direct-minimal-versions) — Forces the resolver to use the lowest compatible version instead of the highest.
     * [public-dependency](#public-dependency) --- Allows dependencies to be classified as either public or private.
     * [msrv-policy](#msrv-policy) --- MSRV-aware resolver and version selection
+    * [precise-pre-release](#precise-pre-release) --- Allows pre-release versions to be selected with `update --precise`
 * Output behavior
     * [out-dir](#out-dir) --- Adds a directory where artifacts are copied to.
     * [Different binary name](#different-binary-name) --- Assign a name to the built binary that is separate from the crate name.
@@ -315,6 +316,25 @@ Documentation updates:
 The `msrv-policy` feature enables experiments in MSRV-aware policy for cargo in
 preparation for an upcoming RFC.
 
+## precise-pre-release
+
+* Tracking Issue: [#13290](https://github.com/rust-lang/rust/issues/13290)
+* RFC: [#3493](https://github.com/rust-lang/rfcs/pull/3493)
+
+The `precise-pre-release` feature allows pre-release versions to be selected with `update --precise`
+even when a pre-release is not specified by a projects `Cargo.toml`.
+
+Take for example this `Cargo.toml`.
+
+```toml
+[dependencies]
+my-dependency = "0.1.1"
+```
+
+It's possible to update `my-dependancy` to a pre-release with `update -Zprecise-pre-release -p my-dependency --precise 0.1.2-pre.0`.
+This is because `0.1.2-pre.0` is considered compatible with `0.1.1`.
+It would not be possible to upgrade to `0.2.0-pre.0` from `0.1.1` in the same way.
+
 ## build-std
 * Tracking Repository: <https://github.com/rust-lang/wg-cargo-std-aware>
 

tests/testsuite/cargo/z_help/stdout.log

@@ -22,6 +22,7 @@ Available unstable (nightly-only) flags:
     -Z mtime-on-use             Configure Cargo to update the mtime of used files
     -Z no-index-update          Do not update the registry index even if the cache is outdated
     -Z panic-abort-tests        Enable support to run tests with -Cpanic=abort
+    -Z precise-pre-release      Enable pre-release versions to be selected with `update --precise`
     -Z profile-rustflags        Enable the `rustflags` option in profiles in .cargo/config.toml file
     -Z publish-timeout          Enable the `publish.timeout` key in .cargo/config.toml file
     -Z rustdoc-map              Allow passing external documentation mappings to rustdoc

tests/testsuite/main.rs

@@ -134,6 +134,7 @@ mod patch;
 mod path;
 mod paths;
 mod pkgid;
+mod precise_pre_release;
 mod proc_macro;
 mod profile_config;
 mod profile_custom;

tests/testsuite/precise_pre_release.rs

@@ -0,0 +1,60 @@
+//! Tests for selecting pre-release versions with `update --precise`.
+
+use cargo_test_support::project;
+
+#[cargo_test]
+fn requires_nightly_cargo() {
+    cargo_test_support::registry::init();
+
+    for version in ["0.1.1", "0.1.2-pre.0"] {
+        cargo_test_support::registry::Package::new("my-dependency", version).publish();
+    }
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "package"
+            [dependencies]
+            my-dependency = "0.1.1"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("update -p my-dependency --precise 0.1.2-pre.0")
+        .with_status(101)
+        // This error is suffering from #12579 but still demonstrates that updating to
+        // a pre-release does not work on stable
+        .with_stderr(
+            r#"[UPDATING] `dummy-registry` index
+[ERROR] failed to select a version for the requirement `my-dependency = "^0.1.1"`
+candidate versions found which didn't match: 0.1.2-pre.0
+location searched: `dummy-registry` index (which is replacing registry `crates-io`)
+required by package `package v0.0.0 ([ROOT]/foo)`
+if you are looking for the prerelease package it needs to be specified explicitly
+    my-dependency = { version = "0.1.2-pre.0" }
+perhaps a crate was updated and forgotten to be re-vendored?"#,
+        )
+        .run()
+}
+
+#[cargo_test]
+fn feature_exists() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "package"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("-Zprecise-pre-release update")
+        .masquerade_as_nightly_cargo(&["precise-pre-release"])
+        .with_stderr("")
+        .run()
+}