fix(msrv): Put MSRV-aware resolver behind a config

This is a part of #13540 which is a party of #9930.

The config is `resolver.something-like-precedence` with values:
- `something-like-maximum` (default)
- `something-like-rust-version`

This is punting on the actual config schema so we can implement
`package.resolver` and `edition = "2024"` support as we want the
MSRV-aware resolver available without `cargo_features`.

src/cargo/core/workspace.rs

@@ -26,7 +26,10 @@ use crate::util::errors::{CargoResult, ManifestError};
 use crate::util::interning::InternedString;
 use crate::util::lints::check_implicit_features;
 use crate::util::toml::{read_manifest, InheritableFields};
-use crate::util::{context::ConfigRelativePath, Filesystem, GlobalContext, IntoUrl};
+use crate::util::{
+    context::CargoResolverConfig, context::CargoResolverPrecedence, context::ConfigRelativePath,
+    Filesystem, GlobalContext, IntoUrl,
+};
 use cargo_util::paths;
 use cargo_util::paths::normalize_path;
 use cargo_util_schemas::manifest;
@@ -100,6 +103,7 @@ pub struct Workspace<'gctx> {
 
     /// The resolver behavior specified with the `resolver` field.
     resolve_behavior: ResolveBehavior,
+    resolve_honors_rust_version: bool,
     honor_rust_version: Option<bool>,
 
     /// Workspace-level custom metadata
@@ -207,7 +211,7 @@ impl<'gctx> Workspace<'gctx> {
             .load_workspace_config()?
             .and_then(|cfg| cfg.custom_metadata);
         ws.find_members()?;
-        ws.set_resolve_behavior();
+        ws.set_resolve_behavior()?;
         ws.validate()?;
         Ok(ws)
     }
@@ -230,6 +234,7 @@ impl<'gctx> Workspace<'gctx> {
             loaded_packages: RefCell::new(HashMap::new()),
             ignore_lock: false,
             resolve_behavior: ResolveBehavior::V1,
+            resolve_honors_rust_version: false,
             honor_rust_version: None,
             custom_metadata: None,
         }
@@ -248,7 +253,7 @@ impl<'gctx> Workspace<'gctx> {
             .packages
             .insert(root_path, MaybePackage::Virtual(manifest));
         ws.find_members()?;
-        ws.set_resolve_behavior();
+        ws.set_resolve_behavior()?;
         // TODO: validation does not work because it walks up the directory
         // tree looking for the root which is a fake file that doesn't exist.
         Ok(ws)
@@ -284,11 +289,11 @@ impl<'gctx> Workspace<'gctx> {
         ws.members.push(ws.current_manifest.clone());
         ws.member_ids.insert(id);
         ws.default_members.push(ws.current_manifest.clone());
-        ws.set_resolve_behavior();
+        ws.set_resolve_behavior()?;
         Ok(ws)
     }
 
-    fn set_resolve_behavior(&mut self) {
+    fn set_resolve_behavior(&mut self) -> CargoResult<()> {
         // - If resolver is specified in the workspace definition, use that.
         // - If the root package specifies the resolver, use that.
         // - If the root package specifies edition 2021, use v2.
@@ -299,7 +304,36 @@ impl<'gctx> Workspace<'gctx> {
                 .resolve_behavior()
                 .unwrap_or_else(|| p.manifest().edition().default_resolve_behavior()),
             MaybePackage::Virtual(vm) => vm.resolve_behavior().unwrap_or(ResolveBehavior::V1),
+        };
+
+        match self.gctx().get::<CargoResolverConfig>("resolver") {
+            Ok(CargoResolverConfig {
+                something_like_precedence: Some(precedence),
+            }) => {
+                if self.gctx().cli_unstable().msrv_policy {
+                    self.resolve_honors_rust_version =
+                        precedence == CargoResolverPrecedence::SomethingLikeRustVersion;
+                } else {
+                    self.gctx()
+                        .shell()
+                        .warn("ignoring `resolver` config table without `-Zmsrv-policy`")?;
+                }
+            }
+            Ok(CargoResolverConfig {
+                something_like_precedence: None,
+            }) => {}
+            Err(err) => {
+                if self.gctx().cli_unstable().msrv_policy {
+                    return Err(err);
+                } else {
+                    self.gctx()
+                        .shell()
+                        .warn("ignoring `resolver` config table without `-Zmsrv-policy`")?;
+                }
+            }
         }
+
+        Ok(())
     }
 
     /// Returns the current package of this workspace.
@@ -616,7 +650,9 @@ impl<'gctx> Workspace<'gctx> {
     }
 
     pub fn resolve_honors_rust_version(&self) -> bool {
-        self.gctx().cli_unstable().msrv_policy && self.honor_rust_version.unwrap_or(true)
+        // Give CLI precedence
+        self.honor_rust_version
+            .unwrap_or(self.resolve_honors_rust_version)
     }
 
     pub fn custom_metadata(&self) -> Option<&toml::Value> {

src/cargo/util/context/mod.rs

@@ -2644,6 +2644,19 @@ impl BuildTargetConfig {
     }
 }
 
+#[derive(Debug, Deserialize)]
+#[serde(rename_all = "kebab-case")]
+pub struct CargoResolverConfig {
+    pub something_like_precedence: Option<CargoResolverPrecedence>,
+}
+
+#[derive(Debug, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "kebab-case")]
+pub enum CargoResolverPrecedence {
+    SomethingLikeMaximum,
+    SomethingLikeRustVersion,
+}
+
 #[derive(Deserialize, Default)]
 #[serde(rename_all = "kebab-case")]
 pub struct TermConfig {

src/doc/src/reference/unstable.md

@@ -336,15 +336,26 @@ This was stabilized in 1.79 in [#13608](https://github.com/rust-lang/cargo/pull/
 
 ### MSRV-aware resolver
 
-By default, `-Zmsrv-policy` enables an MSRV-aware resolver.
+`-Zmsrv-policy` allows access to an MSRV-aware resolver which can be enabled with:
+- `resolver.something-like-precedence` config field
+
 The resolver will prefer dependencies with a `package.rust-version` that is the same or older than your project's MSRV.
 Your project's MSRV is determined by taking the lowest `package.rust-version` set among your workspace members.
 If there is none set, your toolchain version will be used with the intent to pick up the version from rustup's `rust-toolchain.toml`, if present.
 
-MSRV-incompatible dependencies can still be selected by:
+#### `resolver.something-like-precedence`
+* Type: string
+* Default: "something-like-maximum"
+* Environment: `CARGO_RESOLVER_SOMETHING_LIKE_PRECEDENCE`
+
+Select which policy should be used when resolving dependencies.  Values include
+- `something-like-maximum`: prefer highest compatible versions of a package
+- `something-like-rust-version`: prefer versions of packages compatible with your project's Rust version
+
+Can be overridden with
+- `--ignore-rust-version` CLI option
 - Setting the dependency's version requirement too high
 - Specifying the version to `cargo update` with `--precise`
-- Passing `--ignore-rust-version`
 
 ## precise-pre-release
 

tests/testsuite/rust_version.rs

@@ -576,6 +576,7 @@ fn resolve_unstable_config_on_stable() {
         )
         .with_stderr(
             "\
+[WARNING] ignoring `resolver` config table without `-Zmsrv-policy`
 [UPDATING] `dummy-registry` index
 [LOCKING] 3 packages to latest compatible versions
 ",
@@ -595,6 +596,7 @@ foo v0.0.1 ([CWD])
         .env("CARGO_RESOLVER_SOMETHING_LIKE_PRECEDENCE", "non-existent")
         .with_stderr(
             "\
+[WARNING] ignoring `resolver` config table without `-Zmsrv-policy`
 [UPDATING] `dummy-registry` index
 [LOCKING] 3 packages to latest compatible versions
 ",