Make the rust-version error recommend `cargo update --precise -p cr…

…ate@ver`

People encountering a dependency with a newer `rust-version` requirement
may not know about `cargo update --precise`, or may consider alternate
approaches that may be harmful (such as pinning with a `=` dependency).

Provide specific guidance in the error message.

src/cargo/ops/cargo_compile.rs

@@ -661,12 +661,24 @@ pub fn create_bcx<'a, 'cfg>(
                 continue;
             }
 
+            let guidance = if ws.is_ephemeral() {
+                String::new()
+            } else {
+                format!(
+                    "Either upgrade to rustc {} or newer, or use\n\
+                     cargo update --precise -p {}@ver\n\
+                     where `ver` is the latest version of `{}` supporting rustc {}",
+                    version, unit.pkg, unit.pkg, current_version,
+                )
+            };
+
             anyhow::bail!(
                 "package `{}` cannot be built because it requires rustc {} or newer, \
-                 while the currently active rustc version is {}",
+                 while the currently active rustc version is {}\n{}",
                 unit.pkg,
                 version,
                 current_version,
+                guidance,
             );
         }
     }

tests/testsuite/rust_version.rs

@@ -124,7 +124,10 @@ fn rust_version_too_high() {
         .with_status(101)
         .with_stderr(
             "error: package `foo v0.0.1 ([..])` cannot be built because it requires \
-             rustc 1.9876.0 or newer, while the currently active rustc version is [..]",
+             rustc 1.9876.0 or newer, while the currently active rustc version is [..]\n\
+             Either upgrade[..]\n\
+             cargo update[..]\n\
+             where[..]",
         )
         .run();
     p.cargo("build --ignore-rust-version").run();
@@ -159,7 +162,10 @@ fn rust_version_dependency_fails() {
              Downloading crates ...\n  \
              Downloaded bar v0.0.1 (registry `[..]`)\n\
              error: package `bar v0.0.1` cannot be built because it requires \
-             rustc 1.2345.0 or newer, while the currently active rustc version is [..]",
+             rustc 1.2345.0 or newer, while the currently active rustc version is [..]\n\
+             Either upgrade[..]\n\
+             cargo update[..]\n\
+             where[..]",
         )
         .run();
     p.cargo("build --ignore-rust-version").run();